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J Appl Physiol 97: 1387-1394, 2004. First published June 11, 2004; doi:10.1152/japplphysiol.00280.2004
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Fructose feeding and intermittent hypoxia affect ventilatory responsiveness to hypoxia and hypercapnia in rats

Evelyn H. Schlenker ,* Yijiang Shi,* Joni Wipf, Douglas S. Martin, and Curtis K. Kost, Jr.

Basic Biomedical Sciences, University of South Dakota School of Medicine, Vermillion, South Dakota 57069

Submitted 16 March 2004 ; accepted in final form 9 June 2004


   ABSTRACT
TOP
 ABSTRACT
METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
We hypothesized that, in male rats, 10% fructose in drinking water would depress ventilatory responsiveness to acute hypoxia (10% O2 in N2) and hypercapnia (5% CO2 in O2) that would be depressed further by exposure to intermittent hypoxia. Minute ventilation (E) in air and in response to acute hypoxia and hypercapnia was evaluated in 10 rats before fructose feeding (FF), during 6 wk of FF, and after FF was removed for 2 wk. During FF, five rats were exposed to intermittent air and five to intermittent hypoxia for 13 days. Six rats given tap water acted as control and were exposed to intermittent air and subsequently intermittent hypoxia. In FF rats, plasma insulin levels increased threefold in the rats exposed to intermittent hypoxia and during washout returned to levels observed in rats exposed to intermittent air. During FF, ventilatory responsiveness to acute hypoxia was depressed because of decreased tidal volume (VT) responsiveness. During washout, E decreased as a result of decreased VT and frequency of breathing, and the ventilatory responsiveness to hypoxia in intermittent hypoxia rats did not recover. In all rats, the ventilatory responses to hypercapnia were decreased during FF and recovered after washout because of an increased VT responsiveness. In the control group, hypoxic responsiveness was not depressed after intermittent hypoxia and was augmented after washout. Thus FF attenuated the ventilatory responsiveness of conscious rats to hypoxia and hypercapnia. Intermittent hypoxia interacted with FF to increase insulin levels and depress ventilatory responses to acute hypoxia that remained depressed during washout.

insulin; glucose; blood pressure; telemetry

WITHIN THE LAST 20 YEARS, CONSUMPTION of fructose, especially in beverages, has increased sharply in the United States (1). This increase parallels the rise in obesity and has been linked to the epidemic of metabolic syndrome X (a combination of such factors as obesity, insulin resistance, dyslipidemia, and hypertension) in adults and recently in children (1, 5, 11). Administration of fructose (either in drinking water or as part of solid food) to male rats causes elevated systolic blood pressure, insulin insensitivity, downregulation of insulin receptors in liver and skeletal muscle, increased plasma levels of triglycerides, elevated urinary levels of norepinephrine and epinephrine, elevated expression of the angiotensin AT1a receptor in the thoracic aorta, and left ventricular hypertrophy (4, 8, 9, 18, 19). Underlying reasons for these physiological consequences include increased stimulation of the sympathetic nervous system, increased levels of tumor necrosis factor-{alpha}, and stimulation of the renin-angiotensin system (2, 9, 25).

Control of breathing is abnormal in various animal models of Type 2 diabetes, but they may include confounders such as obesity, diabetic neuropathy, and genetic abnormalities (6, 20). In contrast, the fructose-fed model rats are not obese and do not exhibit genetically determined leptin-receptor insensitivity. Moreover, it is a relatively reversible model (4). That is, increases in blood pressure and dyslipidemia as a consequence of fructose feeding are reversed after removal of the fructose (4).

Of further significance is that intermittent hypoxia (IH) has been shown to contribute to the production of insulin insensitivity (3). In patients with insulin insensitivity and obstructive sleep apnea, treatment with constant positive airway pressure reduced their insulin insensitivity (7). In the ob/ob mouse, an animal model of Type 2 diabetes, when exposed to IH insulin levels increased dramatically (14). Whether IH in fructose-fed rats affects minute ventilation (E) and insulin levels has not been evaluated.

The purpose of this study was to determine the effect of fructose feeding on control of breathing in rats. Previous studies in our laboratory noted that an animal model of obesity and Type 2 diabetes, the obese Zucker rat, exhibited altered breathing patterns while breathing air and abnormal responses to hypoxia and hypercapnia relative to control rats (6). We hypothesized that feeding fructose to normal Sprague-Dawley rats would elevate blood pressure and heart rate and depress their E in response to hypoxic and to hypercapnic gas challenges and that these responses would be reversed after washout. To determine whether exposure to IH further compromised E, one-half of the rats were exposed to IH (10% O2 in N2) for 1.5 h per day for 13 days and the other one-half to intermittent air for the same time. This protocol was determined from preliminary studies in rats (Schlenker EH, unpublished observations) to have the same effects on ventilatory parameters in response to hypoxia as 11 exposures of 5 min of hypoxia (10% O2) interspaced with 5 min of air for up to 5 days. To evaluate whether the effects of fructose feeding on control of breathing were specific, a separate set of experiments was carried out in rats not fed fructose but exposed to intermittent air and IH. Fructose feeding elevated blood pressure and heart rate but attenuated the ventilatory responsiveness of conscious rats to hypoxia and hypercapnia. IH exposure interacted with fructose feeding to increase insulin levels and depress ventilatory responses to acute hypoxia.


   METHODS
TOP
 ABSTRACT
 METHODS
RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
For this study, a total of 16 adult male Sprague-Dawley rats (Harlan) were utilized. Ten rats were part of the fructose study, whereas 6 rats served as controls, received only tap water, and were exposed to IH or intermittent air using the same protocol that was used for rats in the fructose intervention. A schematic of both studies is presented in Fig. 1 and explained in more detail below.



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  		Fig. 1. Schematic of the fructose study and control study. The experimental conditions, measurements made, and numbers of rats (n) undergoing each treatment are shown. E, Minute ventilation.

 
Rats were housed individually, and before the commencement of the experiment they received water and food ad libitum with a 12-h on/12-h off lighting schedule. All procedures noted below, except evaluation of blood pressure and heart rate, were done at the same time each day during the light hours. The University of South Dakota Animal Care and Use Committee approved the experimental protocols utilized in this study.

Evaluation of E and metabolism in air and after acute exposure to hypoxia and hypercapnia.   Each rat was weighed and then placed into a clear Plexiglas cylinder measuring 22 cm in length and 15.5 cm in diameter. The front of the chamber was sealed and contained three ports: one leading to a Statham low-pressure transducer, which in turn connected to the Bio-Pac Data Acquisition system; a port allowing air to enter the chamber and measure inspired O2 and CO2; and a port used to measure chamber temperature using a Cole Palmer digital thermometer. The back of the plethysmograph contained two ports: one was used to measure airflow through the chamber by using a Gilmont Rotameter, and another to served as a "leak" to stabilize measurements or when connected to Vacuumed O2 and CO2 analyzers to measure fractional contents of the expired gases.

Before any study, the rat was accustomed to being handled and was placed into the plethsmographic chamber for ~20 min for 2–3 days. On the day of ventilatory and metabolic evaluations, the rat was placed into the plethsmographic chamber for ~30 min of acclimation. Then E and metabolism were evaluated. Subsequently, the rat was exposed to 7 min of 10% O2 in N2, and E was again evaluated. the chamber was then flushed with air for ~10 min, and the ventilatory measurement was repeated. Finally, the rat was exposed to 5% CO2 in O2 and then the chamber was flushed with air. Subsequently, the rat was removed from the plethysmograph, and its body temperature was measured with a thermometer-thermocouple system (Sensortek, Clifton, NJ).

The ventilatory parameters evaluated using the barometric technique (6) included tidal volume (VT) and frequency of breathing (f), and the product of these two parameters, E. CO2 production (CO2) was determined by using the flow-through method and calculated as flow rate multiplied by the difference in the fractional content of expired and inspired CO2. To determine ventilatory equivalent, a measure of how well E and metabolism was matched, the ratio of E to CO2 was calculated. E, CO2, and VT were normalized by body weight. Ventilatory responsiveness to hypoxia or to hypercapnia was determined by subtracting the variable measured during the preceding air exposure from that during the gas challenge. This difference was then divided by the preceding air value and multiplied by 100.

Radiotelemetry implantation.   Four rats that were part of the fructose experiment were instrumented with radiotelemetry devices (model TA11PA-C40, Data Sciences International, St. Paul, MN) to monitor blood pressure and heart rate in conscious rats. The radiotelemetry devices were implanted during isoflurane anesthesia utilizing aseptic surgical techniques as described previously (19). In brief, a portion of the aorta distal to the renal arteries was exposed through a midline abdominal incision, and the catheter of the radiotelemetry device was inserted into the aorta through a puncture wound created with a 21-gauge needle. Medical-grade tissue adhesive and a cellulose fiber patch secured the catheter. The main body of the device, which contains the pressure sensor, radiotransmitter, and the battery, was then sutured into the abdominal wall after the midline incision was closed. Penicillin G procain (5,000 units) and heparin (50 units) were given to the rats in postoperative intramuscular injections. Data acquisition was performed by using the Dataquest LabPRO software package (Data Sciences International) with sampling parameters adjusted to 10-s scan periods at 10-min intervals. Blood pressure and heart rate determinations were made after surgery, and baseline values were obtained 10 days after surgery.

Evaluation of glucose and insulin levels in the fructose experiment.   To determine glucose and insulin levels, 1–1.5 ml of jugular blood were taken after a 6-h fast. One drop of blood was used to determine glucose levels by using the OneTouch Ultra (Lifescan, Milpitas, CA) system that was calibrated twice before and after evaluation of glucose levels in a group of rats. The rest of the blood was placed into heparinized tubes and placed on ice. The tubes were centrifuged at 5,000 rpm at 4°C for 10 min, and the plasma was stored at –80°C. For evaluation of plasma insulin levels, the plasma was thawed and diluted with buffer according to directions for the Biotak rat insulin enzyme immunoassy system (Amersham Biosciences) assay. According to data published by the manufacturer, with use of this this method, normal male rat plasma insulin levels averaged 6–7 ng/ml. Determination of blood glucose and plasma insulin were conducted while the rats were exposed to fructose and then during the washout period. Glucose levels are expressed as millimoles per liter.

Timeline of experiment.   In Fig. 1, the timelines of both studies are presented. For the fructose study, baseline ventilatory and metabolic measurements were conducted in all 10 rats. Then, four rats were implanted with radiotelemetry devices, and baseline blood pressure and heart rate were evaluated 10 days later while the animal was in its home cage. Three days later, all rats received 10% fructose in their drinking water. Measurements of water intake were conducted every 2–3 days, and food intake was measured weekly. One week later, five rats were exposed to IH (10% O2 in N2 for 1.5 h) and five to air for the same period of time in the exposure chamber. This protocol was followed for 13 days. Then E, metabolism, and ventilatory responses to acute hypoxia and hypercapnia were evaluated according to the protocol described above. Finally, fructose was removed from the drinking water for 2 wk to determine whether there was an effect of removing fructose on cardiopulmonary and metabolic factors. Blood samples were taken as described above after the ventilatory measurements were finished during fructose feeding and at the end of the washout period.

For the control experiment, six rats received tap water. E and metabolism in air and E in response to hypoxia and hypercapnia were evaluated at baseline, after 13 days in which rats were exposed to air for 1.5 h per day and 13 days after exposure to hypoxia for 1.5 h per day.

Statistical analysis.   To evaluate the effects of fructose feeding and exposure to IH on ventilatory and metabolic variables, a two-way ANOVA {group (IH or intermittent air) and treatment [baseline, fructose feeding, and removal of fructose (washout)]} with repeated measures was used. To evaluate the effects of intermittent air or hypoxia on hypercapnic and hypoxic responsiveness in the six rats that did not receive fructose, paired Student's t-tests were used. In rats fitted with radiotelemetry devices and exposed to either air or to hypoxia, the effect of treatment and time of day was evaluated by using a two-way ANOVA with repeated measures. Post hoc t-tests (paired or unpaired) with Bonferroni corrections were conducted if the ANOVAs were significant at P < 0.05.


   RESULTS
TOP
 ABSTRACT
 METHODS
 RESULTS
DISCUSSION
 GRANTS
 REFERENCES
 
Body weights and glucose and insulin levels.   Neither body weights nor 6-h fasting glucose values were affected by fructose feeding or exposure to IH (Table 1). In contrast, insulin levels were higher in the fructose-fed and hypoxia-exposed group relative to the fructose-fed and air-exposed group. Removal of hypoxia and fructose resulted in insulin levels similar to those in the group of rats exposed to air [interaction of treatment and gas exposure F(6,15) = 6.02, P = 0.0495, effects of fructose treatment F(1,15) = 8.31, P = 0.028, and gas exposures F(1,15) = 6.91, P = 0.039]. Thus fructose and hypoxia appear to increase plasma insulin values that are "normalized" after removal of both hypoxia and fructose. During fructose feeding, the rats' consumption of liquid was 146.9 ± 10.0 ml/day, which dropped to 38.5 ± 1.6 ml/day after removal of fructose from the drinking water. In contrast, food consumption increased from 72.9 ± 3.6 to 135.7 ± 4.4 g/wk after removal of fructose from drinking water.


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  		Table 1. Body weights and glucose and insulin levels of rats exposed intermittently to air or hypoxia during fructose feeding

 
Effects of fructose feeding on cardiovascular variables.   In four rats, the heart rate responses were evaluated at baseline, before fructose feeding, during fructose feeding, and after washout (Fig. 2). There was a significant increase (P = 0.003) of heart rate during fructose feeding relative to baseline. Moreover, that increase returned to baseline values during washout. Heart rate was increased at all times of the day and night with fructose feeding relative to washout [F(6,24) = 62.84, P < 0.00001].



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  		Fig. 2. Heart rate (A) and systolic blood pressure (B) in 4 rats at baseline, during fructose feeding, and after washout. Values are means ± SE. Note the increases in heart rate and systolic blood pressure during fructose feeding that return to baseline values after washout. *Significant differences between treatments, P < 0.05.

 
Unlike the large increase of heart rate with fructose feeding, systolic blood pressure (Fig. 2) showed a more modest, but significant increase of ~10–15 mmHg (P = 0.01). After washout, systolic blood pressure returned to baseline values. Again, there was an interaction between time of day and treatment [F(6,24) = 39.45, P < 0.0001]. Fructose treatment increased blood pressure throughout the light and dark parts of the cycle. The highest effects were during the active periods (at night). Thus fructose feeding increased both heart rate and systolic blood pressure at all times, and this was reversed after 2 wk of washout when rats were given tap water to drink.

Effects of fructose feeding on metabolism and E in air.   There were no effects of intermittent hypoxic exposure on ventilatory and metabolic variables when rats were breathing air. Thus the data that are presented in Table 2 consist of pooled values from all 10 animals. There were no significant effects of fructose feeding relative to baseline on any of the variables. In contrast, during washout, body weight-corrected CO2, body weight-corrected E, body weight-corrected VT, and f all decreased compared with these variables during fructose feeding. There was a greater decrease of CO2 relative to E. Thus the ventilatory equivalent (E/CO2) increased during washout relative to baseline or during fructose consumption (P < 0.0001).


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  		Table 2. Effects of fructose feeding on ventilatory variables in air, CO2 and E/CO2

 
Effects of fructose feeding on ventilatory responses to hypercapnia and to hypoxia.   There were no effects of IH compared with intermittent air exposure on the hypercapnic response in the fructose-fed rats (data not shown). Thus the data of the 10 rats were combined. Ventilatory responsiveness to acute hypercapnia (Fig. 3) decreased during fructose administration (P = 0.0304), which then returned to baseline values during washout in the 10 animals. Frequency responsiveness to hypercapnia (Table 3) tended to decrease with fructose feeding and further during washout (P = 0.0331). In contrast, the body weight-corrected VT responsiveness to hypercapnia was not altered by fructose administration, but it increased significantly during washout from 40.6 ± 6.5 to 74.8 ± 5.7% (P = 0.009).



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  		Fig. 3. Percent hypercapnic responsiveness in 10 rats at baseline, during fructose feeding, and after washout. This variable was calculated by taking the E in response to hypercapnia, subtracting the air value preceding the hypercapnic exposure from it, dividing the difference by the air value, and multiplying the resultant by 100. Values are means ± SE. Letters A and B denote significant differences (P < 0.05) between treatments, and AB is not different from either A or B. The washout value had a P value of 0.055.

 

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  		Table 3. Effects of fructose feeding on f and VT responsiveness to acute hypercapnia

 
To determine the underlying mechanisms responsible for changes in ventilatory responsiveness, the raw data of the f, body weight-corrected VT, and body weight-corrected E in air and in response to hypercapnia are shown in Table 4. During exposure to air, body weight-corrected VT and consequently E decreased when fructose was removed. In contrast, in rats that received fructose and were exposed to hypercapnia, VT and E were decreased relative to baseline values. During exposure to hypercapnia, there was a significant decrease in f (P = 0.004), VT (P = 0.01), and E (P = 0.0008) after fructose was removed. Thus, because VT in air dropped to a greater extent than the drop during exposure to hypercapnia, the VT responsiveness (Table 3) actually increased.


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  		Table 4. Effects of fructose feeding on f, VT, and E in air and in hypercapnia

 
Data from six control rats that did not receive fructose showed hypercapnic ventilatory responsiveness that was similar at baseline, after intermittent air, after IH, and during washout (see Fig. 5A). These values were comparable for data in the fructose intervention at baseline and during washout, reinforcing the fact that fructose feeding was responsible for the decrease in hypercapnic responsiveness.



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  		Fig. 5. Hypercapnic (A) and hypoxic (B) responsiveness as defined in legends from Figs. 3 and 4 of 6 control rats not given fructose at baseline, after IA, after IH, and during washout. Values are means ± SE. *During washout, hypoxic responsiveness was significantly greater than at other periods, P < 0.05.

 
Ventilatory responsiveness to acute hypoxia was different between rats exposed to IH and rats exposed to intermittent air during fructose feeding (Fig. 4). Whereas the ventilatory responsiveness of the IH group showed no statistically significant effect of either fructose feeding or washout, the group exposed to fructose and intermittent air showed a tendency toward decreased ventilatory responsiveness to acute hypoxia (P = 0.0621) and an increased response to acute hypoxia during washout (70 ± 11 to 128 ± 20%; P = 0.0067, Table 5 and Fig. 4). When the VT and frequency responses of the two groups were analyzed, they showed that a different strategy in response to acute hypoxia was utilized in each group. Thus, in the IH-exposed rats, f responses were significantly decreased with fructose feeding and did not rebound during washout (1-way repeated-measures ANOVA, P = 0.0115). In contrast, the intermittent air exposed rats did not exhibit significant differences between baseline, fructose feeding, and washout on the frequency responsiveness to acute hypoxia.



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  		Fig. 4. Percent hypoxic responsiveness in rats at baseline, during fructose feeding [while 5 were exposed to intermittent air (IA) and 5 to intermittent hypoxia (IH)], and after washout. The percent hypoxic responsiveness was calculated by taking the E in response to hypoxia, subtracting the air value preceding the hypoxic exposure from it, dividing the difference by the air value, and multiplying the resultant by 100. Values are means ± SE. #Significant decrease in ventilation during fructose exposure in the IA group, P < 0.05. ANOVA for E responsiveness in the IH group was P = 0.0527.

 

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  		Table 5. Ventilatory responsiveness to acute hypoxia of rats intermittently exposed to hypoxia or air

 
Weight-corrected VT responsiveness exhibited a significant effect of treatment in the group of rats exposed to intermittent air (P = 0.0059). VT responsiveness decreased (P = 0.0548) during fructose feeding and significantly rebounded during washout [P = 0.0012; 53.7 ± 13.1% (baseline), 15.2 ± 4.8% (fructose), and 58.1 ± 6.4% (washout)]. In contrast, there was no significant effect on VT responses of fructose feeding or washout in the group exposed to IH.

To determine the underlying mechanisms in ventilatory responsiveness to hypoxia in the two groups reported above, the raw data are presented in Table 6. During washout and air exposure, the intermittent air group showed a significant decrease in f (P = 0.03), which was not observed in the IH group. In contrast, during exposure to acute hypoxia, the IH group, but not the intermittent air group, showed a significant overall decrease in frequency [F(4,8) = 8.20, P = 0.0115] that was evident when fructose was given and during removal of fructose. With fructose feeding, only the IH group deceased body weight-corrected E during acute hypoxic exposure, due primarily to a decrease of breathing frequency. During washout both groups displayed significant decreases in body weight-corrected VT and E when exposed to air and acute hypoxia.


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  		Table 6. Effects of fructose feeding on f, VT, and E in air and in hypoxia

 
In the six control rats not exposed to fructose feeding, the hypoxic responsiveness at baseline, after intermittent air, after IH, and during washout is shown in Fig. 5B. After intermittent air exposure, hypoxic responsiveness was 125.3 ± 27.6%, and after IH it was 101.5 ± 20.0% (P > 0.36). These values are also comparable to baseline values shown in Table 4. However, after washout after IH, the ventilatory response of the six control rats to acute hypoxia was 158.3 ± 23.9%. In contrast, during washout in rats that received fructose and IH, ventilatory responsiveness to hypoxia was only 88 ± 9% (Fig. 4). Thus fructose feeding depressed the ventilatory responsiveness to acute hypoxia and did not return to baseline values in the IH group.

Therefore, fructose feeding decreased ventilatory responses to both hypoxia and to hypercapnia. However, IH relative to intermittent air exposure had different effects on the acute ventilatory responses of rats exposed to acute hypoxia during fructose feeding and washout.


   DISCUSSION
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
GRANTS
 REFERENCES
 
Fructose feeding in this study induced an increase in systolic blood pressure and heart rate, which returned to baseline values once fructose was removed from the rats' drinking water. Exposure of fructose-fed rats to IH resulted in an increase in plasma insulin levels that returned to values observed in the intermittent air group during fructose feeding and washout. VT, frequency, and E decreased during washout, as did CO2. Because the decrease in CO2 was greater than that in E, the ventilatory equivalent increased during washout. Ventilatory responsiveness to acute hypercapnia was depressed in the entire group, without exhibiting differences due to IH or air exposure, contrasting to the effects of IH on responsiveness to acute hypoxia. These ventilatory changes were not seen in exposing rats to either intermittent air or IH without fructose. Each of these findings will be discussed below.

Fructose feeding and cardiovascular function.   Administration of fructose to rats either in solid food or in drinking water results in the elevations in systolic blood pressure and heart rate that are dependent on both the concentration of the fructose and the length of time the fructose is given (4, 8, 19). Dai and McNeill (4) evaluated the effects of 5, 10, and 20% fructose in drinking water relative to control on the variables listed above over a 12-wk period. They reported that the optimal concentration of 10% fructose increased systolic blood pressure by ~30 mmHg after 2 wk of treatment and returned to baseline values after a 2-wk washout period. In the present study, systolic blood pressure showed significant, but smaller increases of ~10–15 mmHg. The difference between the present study and several published reports is that to evaluate heart rate and systolic blood pressure most investigators use the tail cuff method that requires restraint and heating of rats. This technique in conjunction with fructose feeding may further activate the autonomic nervous system and confound the blood pressure measurements reported by several investigators (4, 18, 19). Only one other study (8) reported findings comparable to ours. These investigators accustomed their animals to the equipment before the study was commenced, suggesting that the increase in heart rate reflects an increased sympathetic stimulation noted with fructose feeding (9) as seen with increased urinary excretion of epinephrine and norepinephrine. Increased blood pressure with fructose feeding is also maintained by stimulation of the renin-angiotensin system. For example, Shinozaki and colleagues (18) showed that fructose feeding elevated angiotensin II levels in rats. Elevated systolic blood pressure in fructose-fed rats can also be prevented by administration of an angiotensin AT1-receptor blocker, olmesartan, during fructose feeding (18). Thus the elevations of both heart rate and systolic blood pressure during fructose administration in our study may have been caused by several factors that were reversed during washout.

Fructose feeding, glucose, and insulin levels and IH.   Several studies have shown that fructose feeding contributes to elevated levels of cholesterol and triglycerides and to the development of insulin resistance, which may result in elevated levels of insulin with or without increases in plasma glucose levels (4, 22, 23). In the present study, the combination of IH and fructose resulted in marked elevations of plasma insulin levels that during washout became comparable to those in the intermittent air group. The intake of fructose in this group compared with the intermittent air group was not different (data not presented). In a recent study, Polotsky and coworkers (14) reported that exposing genetically obese insulin-resistant mice (ob/ob) to IH elevated fasting plasma insulin levels further. Control mice exposed to IH did not increase plasma insulin levels. Prior infusion of leptin into ob/ob hypoxic-exposed mice prevented the increased insulin resistance. Moreover, the only gene that was upregulated in white adipose tissue in lean IH-exposed mice was leptin. Whether fructose feeding and IH cause the same effect needs to be investigated.

A potential mechanism for the increased levels of insulin in the IH-exposed group may be increased levels of tumor necrosis factor-{alpha} that are elevated during fructose feeding (24). Common pathways that both hypoxia and insulin resistance can act through include p38 mitogen-activated protein kinase and hypoxia-inducible factor-1{alpha} (10, 17). Moreover hypoxia-inducible factor can be further increased by angiotensin II (17). Thus the interaction between IH and elevated levels of insulin on cardiopulmonary function needs to be investigated at the molecular level.

Fructose feeding affects control of breathing.   This is the first study that evaluated the effects of fructose feeding on control of breathing. E during air exposure was not affected while the animals were given fructose, but it decreased below baseline values during washout. One mechanism that may explain this observation is the decrease in CO2 during washout; however, this drop was greater than that of E. Consequently, the ventilatory equivalent increased during washout. Factors responsible for these findings may be due to alterations in central modulators of breathing. Fructose feeding has been shown to have profound effects on brain tryptophan, serotonin (5-HT), and the metabolite of 5-HT, 5-hydroxyindoleacetic acid (5-HIAA). Thibault (23) analyzed tryptophan, 5-HT, and 5-HIAA levels in the hypothalamus, thalamus, raphe nuclei, and brain stem of rats fed a diet high in fructose relative to a control diet. She noted that tryptophan levels were increased in all brain regions. 5-HT and 5-HIAA levels were increased in the hypothalamus but markedly decreased in the thalamus, raphe nuclei, and brain stem with fructose feeding. The effects of washout were not evaluated in that study. Because low serotonin level can depress E (12), this neurotransmitter system may be involved in the results obtained in the present study. Evaluation of serotonin levels and its metabolites, as well as receptor levels in brain stem regions associated with control of breathing during and after fructose feeding, may help determine whether this stipulation is correct.

In several animal models of insulin insensitivity such as in the ob/ob mouse (20), the obese Zucker rat (6), and the Bio 14.6 hamster (16), ventilatory responses to hypercapnia and hypoxia are abnormally low, similar to the response of fructose-fed rats in the present study. However, the underlying mechanisms for the depression of breathing in all these models vary. For example, administration of leptin to the ob/ob mouse improves E (20). When Bio 14.6 hamsters received thyroid hormone supplementation, control of breathing normalized (16). In the present study, ventilatory responsiveness to hypercapnia returned to baseline values after washout, whereas VT responsiveness actually increased and frequency responsiveness remained at the same level as during fructose feeding. This suggests that regulation of VT and frequency in response to hypercapnia is altered not only by fructose feeding but also by washout. Moreover this pattern was not seen while rats were breathing air or in control rats exposed either to intermittent air or IH. Thus, clearly, fructose feeding affects hypercapnic responsiveness. The mechanisms responsible for this finding are not known, but they may involve alterations of neurotransmitter levels such as 5-HT (23), leptin (14), and possibly cytokines such as TNF-{alpha} (15) acting at the level of respiratory muscles (26).

The ventilatory responsiveness of rats to acute hypoxia was modified by both fructose feeding and concomitant exposure to IH. Evidence for this is threefold. 1) The intermittent air-exposed group's responsiveness to hypoxia did not exhibit the same E and frequency pattern as did the group exposed to IH. 2) Rats not given fructose but exposed to either intermittent air or IH demonstrated responses comparable to baseline values in the fructose intervention. 3) The combination of fructose and IH had a more long-lasting effect than the intermittent air and fructose, because the IH group showed a tendency (P = 0.052) to maintain a decrease in E responsiveness during washout, whereas the values for the group exposed to intermittent air increased from the fructose values and were similar to baseline values. These divergent findings suggest that the underlying mechanisms responsible for the results are different.

The results of this study may have several clinical implications. First, excess dietary fructose may contribute to development of insulin insensitivity (11) that contributes to the development of hypertension (1) and to depression of E in response to hypoxia and hypercapnia as seen in patients with sleep apnea (3, 13). Second, the combination of a predisposition or a subclincial state of insulin insensitivity and IH, as seen in sleep apnea, may contribute to the development of further insulin insensitivity and ultimately Type 2 diabetes. A recent study by Harsch and colleagues (7) found that constant positive airway pressure treatment of sleep apnea patients who also had insulin insensitivity resulted in an improvement in insulin sensitivity without any change in body mass index. Moreover, patients who were not obese showed greater improvements than did obese patents. By understanding the underlying mechanisms responsible for the contribution of insulin insensitivity to cardiopulmonary dysfunction and the exacerbating role of IH, treatment of patients who manifest either one or both of these abnormalities may be helped.

In summary, fructose feeding increases heart rate and systolic blood pressure, but it decreases E and ventilatory responses to hypoxia and hypercapnia. Moreover, the ventilatory responses to acute hypoxia are different depending on concurrent exposures to IH that also increase insulin levels compared with rats exposed to fructose and intermittent air.


   GRANTS
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
REFERENCES
 
This project was funded by National Institutes of Health Division of Research Resources Grants P20 RR-15567 and 1 P20 RR-17662.


   FOOTNOTES
 

Address for reprint requests and other correspondence: E. H. Schlenker, Basic Biomedical Sciences, Univ. of South Dakota School of Medicine, Vermillion, SD 57069 (E-mail: eschlenk{at}usd.edu).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

* E. H. Schlenker and Y. Shi contributed equally to this work. Back


   REFERENCES
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 

  1. Bray GA, Neilsen SJ, and Popkin BM. Consumption of high-fructose corn syrup in beverages may play a role in the epidemic of obesity. Am J Clin Nutr 79: 537–543, 2004.[Abstract/Free Full Text]
  2. Catena C, Giacchetti G, Novello M, Colussi G, Cavarape A, and Sechi LA. Cellular mechanisms of insulin resistance in rats with fructose-induced hypertension. Am J Hypertens 16: 973–978, 2003.[CrossRef][Web of Science][Medline]
  3. Couglin S, Calverley P, and Wilding J. Sleep disorder breathing—a new component of syndrome x? Obes Rev 2: 267–274, 2001.[CrossRef][Medline]
  4. Dai S and McNeill JH. Fructose-induced hypertension in rats is concentration- and duration-dependent. J Pharmacol Toxicol Methods 33: 101–107, 1995.[CrossRef][Web of Science][Medline]
  5. Elliot SS, Keim NL, Stern JS, Teff K, and Havel PJ. Fructose, weight gain, and insulin resistance syndrome. Am J Clin Nutr 76: 911–922, 2002.[Abstract/Free Full Text]
  6. Farkas GA and Schlenker EH. Pulmonary ventilation and mechanics in morbidly obese Zucker rats. Am J Respir Crit Care Med 150: 356–362, 1994.[Abstract]
  7. Harsch IA, Schahin SP, Radespiel-Troger M, Weintz O, Jahreiss H, Fuchs FS, Wiest GH, Hahn EG, Lohmann T, Konturek PC, and Ficker JH. Continuous positive airway pressure treatment rapidly improves insulin sensitivity in patients with obstructive sleep apnea syndrome. Am J Respir Crit Care Med 169: 156–162, 2004.[Abstract/Free Full Text]
  8. Higashiura K, Ura N, Takada T, Agata J, Yoshida H, Miyazarki Y, and Shimamoto K. Alteration of muscle fiber composition linking insulin resistance and hypertension in fructose-fed rats. Am J Hypertens 12: 596–602, 1999.[CrossRef][Web of Science][Medline]
  9. Kadide K, Rakugi H, Higaki J, Okamurea A, Nagai M, Morigushi K, Ohishi M, Satoh N, Tuck ML, and Ogihara T. The renin-angiotensin and adrenergic nervous system in cardiac hypertrophy in fructose-fed rats. Am J Hypertens 15: 66–71, 2002.[CrossRef][Web of Science][Medline]
  10. Kietzmann T, Krones-Herzig A, and Jungermann K. Signaling cross talk between hypoxia and glucose via hypoxia-inducible factor 1 and glucose response elements. Biochem Pharmacol 64: 903–911, 2002.[CrossRef][Web of Science][Medline]
  11. Kohen-Avramoglu R, Theriault A, and Adeli K. Emergence of the metabolic syndrome in childhood: an epidemiological overview and mechanistic link to dyslipidemia. Clin Biochem 36: 413–420, 2003.[CrossRef][Web of Science][Medline]
  12. McCrimmon DR, Mitchell GS, and Dekin MS. Glutamate, GABA, and serotonin in ventilatory control. In: Regulation of Breathing (2nd ed.), edited by Dempsey JA and Pack AI. New York: Dekker, 1995, p. 151–218.
  13. Peppard PE, Young T, Palta M, and Skatrud J. Prospective study of the association between sleep-disordered breathing and hypertension. N Engl J Med 342: 1378–1384, 2000.[Abstract/Free Full Text]
  14. Polotsky VY, Li J, Punjabi NM, Rubin AE, Smith PL, Schwartz AR, and O'Donnell CP. Intermittent hypoxia increases insulin resistance in genetically obese mice. J Physiol 552: 253–264, 2003.[Abstract/Free Full Text]
  15. Schlenker EH and Burbach JA. Elevated tumor necrosis factor-alpha like antigen levels (TNFL) in brains of dystrophic hamsters (Abstract). Soc Neurosci Abstr 18: 483, 1992.
  16. Schlenker EH and Burbach JA. Thyroxine affects ventilation, lung morphometry, and necrosis of the diaphragm in dystrophic hamsters. Am J Physiol Regul Integr Comp Physiol 268: R779–R785, 1995.[Abstract/Free Full Text]
  17. Semenza GL. Signal transduction to hypoxia-inducible factor 1. Biochem Pharmacol 64: 993–998, 2002.[CrossRef][Web of Science][Medline]
  18. Shinozaki K Ayajiki K, Nishio Y, Sugaya T, Kashiwagi A, and Okamura T. Evidence for a causal role of the renin-angiotensin system in vascular dysfunction associated with insulin resistance. Hypertension 43: 255–262, 2004.[Abstract/Free Full Text]
  19. Song D, Arikawa E, Galipeau D, Battell M, and McNeill JH. Androgens are necessary for the development of fructose-induced hypertension. Hypertension 43: 1–6, 2004.[Free Full Text]
  20. Tankersley C, Kleeberger S, Russ B, Schwartz AR, and Smith P. Modified control of breathing in genetically obese (ob/ob) mice. J Appl Physiol 81: 716–723, 1996.[Abstract/Free Full Text]
  21. Tasali E and Van Cauter E. Sleep-disorder breathing and the current epidemic of obesity. Am J Respir Crit Care Med 165: 562–563, 2002.[Free Full Text]
  22. Tay A, Özçelikay T, and Altan VM. Effects of L-arginine on blood pressure and metabolic changes in fructose-hypertension rats. Am J Hypertens 15: 72–77, 2002.[CrossRef][Web of Science][Medline]
  23. Thilbault L. Dietary carbohydrates: effects on self-selection, plasma glucose and insulin, and brain indoleaminergic systems in rat. Appetite 23: 275–286, 1994.[Medline]
  24. Tofovic SP, Kost CK Jr, Jackson EK, and Bastacky SI. Long-term caffeine consumption exacerbates renal failure in obese, diabetic, ZSF1 (fa-fa(cp)) rats. Kidney Int 61: 1433–44, 2002.[CrossRef][Web of Science][Medline]
  25. Togashi N, Ura N, Higashiura Murakami H, and Shimamoteo K. Effect of TNF-{alpha}-converting enzyme inhibitor on insulin resistance in fructose-fed rats. Hypertension 39: 578–580, 2002.[Abstract/Free Full Text]
  26. Wilcox P, Milliken C, and Bressler B. High-dose tumor necrosis factor alpha produces an impairment of hamster diaphragm contractility. Attenuation with a prostaglandin inhibitor. Am J Respir Crit Care Med 153: 1611–1615, 1996.[Abstract]




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