Journal of Applied Physiology
Vol. 82, No. 1, pp. 317-323, January 1997
CONTROL OF BREATHING, CIRCULATION, AND TEMPERATURE

Ventilation and metabolism among rat strains

Kingman P. Strohl¹, Agnes J. Thomas¹, Pamela St. Jean¹, Evelyn H. Schlenker², Richard J. Koletsky¹, and Nicholas J. Schork¹

¹ Departments of Medicine, Human Genetics, and Epidemiology and Biostatistics, Case Western Reserve University, Cleveland, Ohio 44106; and ² Department of Physiology and Pharmacology, University of South Dakota, Vermillion, South Dakota 57069

ABSTRACT

INTRODUCTION

METHODS

RESULTS

DISCUSSION

ACKNOWLEDGEMENTS

FOOTNOTES

REFERENCES

ABSTRACT

Strohl, Kingman P., Agnes J. Thomas, Pamela St. Jean, Evelyn H. Schlenker, Richard J. Koletsky, and Nicholas J. Schork. Ventilation and metabolism among rat strains. J. Appl. Physiol. 82(1): 317-323, 1997.We examined ventilation and metabolism in four rat strains with variation in traits for body weight and/or blood pressure regulation. Sprague-Dawley [SD; 8 males (M), 8 females (F)], Brown Norway (BN; 10 M, 11 F), and Zucker (Z; 11 M, 12 F) rats were compared with Koletsky (K; 11 M, 11 F) rats. With the use of noninvasive plethysmography, frequency, tidal volume, minute ventilation (E), O₂ consumption, and CO₂ production were derived at rest during normoxia (room air) and during the 5th minute of exposure to each of the following: hyperoxia (100% O₂), hypoxia (10% O₂-balance N₂), and hypercapnia (7% CO₂-balance O₂). Statistical methods probed for strain and sex effects, with covariant analysis by body weight, length, and body mass. During resting breathing, strain effects were found with respect to both frequency (BN, Z > K, SD) and tidal volume (SD > BN, Z) but not to E. Sex influenced frequency (F > M) alone. Z rats had higher values for O₂ consumption, CO₂ production, and respiratory quotient than the other three strains, with no independent effect by sex. During hyperoxia, frequency was greater in BN and Z than in SD or K rats; SD rats had a larger tidal volume than BN or Z rats; Z rats had a greater E than K rats; and M had a larger tidal volume than F. Strain differences persisted during hypercapnia, with Z rats exhibiting the highest frequency and E values. During hypoxic exposure, strain effects were found to influence E (SD > K, Z), frequency (BN > K), and tidal volume (SD > BN, K, Z). Body mass was only a modest predictor of E during normoxia, of both E and tidal volume with hypoxia, hypercapnia, or hyperoxia, and of frequency during hypercapnia. We conclude that strain of rats, more than their body mass or sex, has major and different influences on metabolism, the pattern and level of ventilation during air breathing, and ventilation during acute exposure to hypercapnia or hypoxia.

ventilatory control; genetics; rat strains

INTRODUCTION

VENTILATION IS ACHIEVED through combinations of tidal volume and breathing frequency and is an integrated homeostatic response designed to help maintain cellular metabolism and acid-base balance in the face of environmental and metabolic changes in CO₂ and O₂ levels. Among otherwise healthy individuals, there can exist two- to sixfold variations in ventilation in response to increases in CO₂ and decreases in O₂, respectively; and an individual's response can be modified over time by several factors (1, 5, 6, 9, 11, 20, 26). An inherited basis for ventilation and ventilatory responsiveness is supported by observations in several human populations. Beral and Read (2) observed absent or blunted hypercapnic responses in 12 natives of Papua, New Guinea, in contrast to the two- to fourfold increases in ventilatory responsiveness to CO₂ exhibited by residents of Sydney, Australia. Greater similarities in hypoxic ventilatory responsiveness have been demonstrated between monozygotic twins than between dizygotic twins (5, 10). Saunders et al. (22) found that siblings had similar blunted ventilatory responses to hypercapnia, independent of prior endurance athlete training.

Three animal studies provide evidence for genetic transmission of ventilatory traits. In 1984, Ou and associates (16, 17) reported that differences in resting ventilation and ventilatory responses to hypocapnic hypoxia differed between two lines of Sprague-Dawley (SD) rats, which were obtained from different vendors. Tankersley et al. (25) showed significant interstrain differences with respect to an interaction between the hypercapnic ventilatory responses and ozone exposure in mice. In a follow-up report, this group (24) reported significant interstrain differences in respiratory frequency and responsiveness to hypoxia and hypercapnia in male mice from several different inbred lines. No studies of strain differences have systematically assessed sex effects or metabolic status, factors that have substantial influences on respiratory control.

The present study sought to determine the relative influence of strain, sex, and body habitus on interindividual variations in the control of ventilation in the rat. The specific hypothesis was that lineage does not have as great a relative influence on resting ventilation and ventilation in response to chemical loading as does sex or weight, factors that affect ventilation through both environmental and genetic components. If there are no differences in trait values for ventilation and metabolism among different strains of rats independent of weight, body mass, or sex, the hypothesis would be confirmed. Rejection of the hypothesis would indicate a strong genetic influence on ventilation.

METHODS

Selection of strains. Four strains of rats were chosen to represent a diversity in cardiopulmonary traits, weight, and genetic origin. SD rats, available commercially from several different breeding colonies distributed throughout the United States, are normotensive rats, with a standard biochemical profile, often used in cardiovascular studies. SD animals from different vendors have been shown to differ in response to acute and chronic exposure to hypocapnic hypoxia (16, 17). Brown Norway (BN) rats are commonly used in hematologic and cardiovascular studies (13, 19). BN rats are a normotensive strain successfully inbred in several commercial colonies. The Zucker (Z) rat carries a recessive trait for obesity, insulin resistance, and a variable degree of hypertension (18). Animals from these three strains were obtained from a single vendor (Harlan, Indianapolis, IN). Koletsky (K) rats are a second strain of genetically obese animals, exhibiting autosomal recessive transmission of not only obesity but also hyperinsulinemia, hyperglycemia, and other abnormal biochemical traits (12). In contrast to the Z animals, both lean and obese K animals exhibit high blood pressure. The K strain has been inbred for 40 generations in a colony at Case Western Reserve University.

All animals were housed at Case Western Reserve University for at least 2 wk before testing and were provided food and water ad libitum. Animals were under the direct observation of the veterinary staff and were in good health, gaining weight, and drinking normal amounts of water for at least 1 wk before testing. A total of 82 animals were studied: 21 BN (10 males), 16 SD (8 males), 12 K lean phenotype (6 males), 10 K obese phenotype (5 males), 11 Z lean phenotype (5 males), and 12 Z obese phenotypes (6 males). Adult animals were studied between the age of 16 and 19 wk, and the strain order was randomized. Values for body weight and length (nose to anus, in cm) were measured at the time of testing.

Procedures for testing were given prior approval by the Animal Resource Center, Case Western Reserve University.

Apparatus for testing. Ventilation and metabolism were assessed in a whole body respiratory metabolic chamber, modified for the unanesthetized unrestrained rat (23). A round Plexiglas chamber (diameter of 14 cm, 8.4 liters in volume) contained ports to measure the following parameters: the flow rate of air or test gases going through the chamber (by using a rotameter), swings in chamber pressure (Validyne transducer), temperature, and relative humidity (by using a Digitech thermal digital thermometer and a Palmer hygrometer), and the fractional content of CO₂ and O₂ entering and exiting the chamber (with the use of infrared CO₂ and fuel cell O₂ analyzers). The flow rate through the chamber was set at 600 ml/min. In preliminary studies, flow rates <500 ml/min permitted a buildup of CO₂ and were associated with a fall in metabolic rate, whereas flow rates >700 ml/min limited the accuracy of metabolic measures. Gas mixtures were changed by rapidly (~30 l/min) flushing the chamber with the desired gas mixture. Calibration for tidal volume changes (0.5, 1.0, and 1.5 ml) was performed at the average respiratory frequency, ~2 breaths/s, using double-ground glass syringes. These calibrations were performed while the chamber was empty. O₂ consumption and CO₂ production were determined by the open-circuit method (23). Ventilatory and metabolic parameters were recorded both on strip chart and by an analog-to-digital converter coupled to a computer. Two setups were available, each using the same transducers and monitors. Animals were studied in tandem.

Testing protocol. Tests were performed between 11:00 A.M. and 1:00 P.M. Each rat was weighed and its nose-to-anus length measured before the animal was acclimatized to the testing apparatus for 60 min, with room air flowing through the chamber. Ventilation, O₂ consumption, and CO₂ production were then measured five times over a 15-min period to obtain baseline values. The animal was subsequently exposed to the following test gases: 10% O₂-balance N₂ for 5 min (hypoxia); 100% O₂ for 5 min (hyperoxia); room air for 20 min; and 7% CO₂-balance O₂ (hypercapnia) for 5 min. Ventilatory parameters were measured continuously throughout the testing period. After each complete series of experiments, body temperature was measured rectally to a depth of 2-2.5 cm.

Measurements. Ventilatory parameters were evaluated from an average of 10 consecutive breaths that were determined not to be sniffs or sighs and scored by computer by using a respiratory-based software program (BGPLOT, Cleveland, OH). Values were obtained for inspiratory tidal volume, frequency of breathing, and minute ventilation. Calculations of minute ventilation were made from respiratory frequency and inspiratory tidal volume. Lee index, providing an estimate of body mass (3, 18), was determined by dividing the cubed root of body weight by the nose-to-anus length. O₂ consumption and CO₂ production at rest as well as ventilation and tidal volume were expressed relative to body weight.

Statistical analysis. We employed analysis of covariance (ANCOVA) with correction for multiple comparisons (14). Covariance models were fit with ventilatory and metabolic measures as outcome variables; predictors included strain, sex, weight, length, and Lee index, as well as sex-by-strain interaction. Between-strain or between-sex differences were investigated for those outcome variables for which strain or sex were significant (P < 0.05) predictors from the ANCOVA; sample means were compared after adjustment for weight, length, and Lee index. To adjust for the effect of multiple comparisons, a Bonferroni correction was employed when comparing means between strains, such that P values of 0.0125 (0.05/4) were taken to be significant. For those variables with significant sex-by-strain interactions, ANCOVAs were conducted separately by sex. Statistical analyses were performed by using the SAS statistical package (21).

RESULTS

There were no variations in body temperature [38.13 ± 0.05 (SE) °C], chamber temperature (23.10 ± 0.05°C), chamber humidity (26.25 ± 0.22%), or barometric pressure (742.4 ± 0.3 Torr) that significantly correlated with strain, sex, weight, length, or Lee index.

Table 1 summarizes the variations within and between the four strains, in regard to values obtained during resting breathing. Grouped mean data are shown for each trait variable. Morphometric values and all unadjusted values are shown in boldface. Metabolic measures and ventilatory values are given both as unadjusted and as adjusted values. Differences between strains are given if differences reached previously determined significance levels. Significant strain differences after adjustment for body weight, length, and Lee index were found in metabolism and components of resting ventilation, including frequency and inspiratory tidal volume.

Table 1. Rat strain characteristics with respect to resting ventilation Strain Strain Effect P Value* Significant Strain Differences Sex Effect P Value* Significant Sex Differences BN SD K Z No. of rats 10 M, 11 F 8 M, 8 F 11 M, 11 F 11 M, 12 F Body wt, g 249.67 ± 17.62  364.81 ± 28.25  398.20 ± 25.97  424.35 ± 26.64  0.0001 SD > BN, K > BN, Z > BN 0.0001 M > F Length, cm 19.81 ± 0.48  22.84 ± 0.65   22.77 ± 0.51   24.04 ± 0.36  0.0001 SD > BN, K > BN, Z > BN 0.0001 M > F Lee index  0.314 ± 0.001  0.310 ± 0.001   0.319 ± 0.003    0.309 ± 0.004  0.0558 NS 0.217 NS  O2, ml · min1 · 2.53 ± 0.15  1.79 ± 0.10  1.81 ± 0.15  1.80 ± 0.13  0.006 Z > SD 0.369 NS   100 g1 1.86 ± 0.09 1.84 ± 0.06 2.08 ± 0.06  2.13 ± 0.07   CO2, ml · min1 · 1.74 ± 0.10  1.34 ± 0.10  1.24 ± 0.10  1.48 ± 0.11  0.0001 Z > BN, Z > SD, 0.761 NS   100 g1 1.28 ± 0.07 1.37 ± 0.05 1.43 ± 0.05  1.70 ± 0.06  Z>K RQ 0.69 ± 0.01  0.74 ± 0.02  0.69 ± 0.01  0.82 ± 0.01  0.0001 Z > BN, Z > SD, 0.160 NS 0.70 ± 0.02 0.74 ± 0.01 0.69 ± 0.01  0.81 ± 0.02  Z>K f, breaths/min 104.48 ± 4.49  91.86 ± 5.24  104.36 ± 4.45  129.17 ± 4.54  0.0001 BN > SD, BN > K, 0.002 F > M 122.32 ± 6.25 92.48 ± 4.56 95.48 ± 4.54  119.89 ± 5.04  Z > SD, Z > K VT, ml/100 g 0.31 ± 0.02  0.29 ± 0.02  0.24 ± 0.01  0.20 ± 0.02  0.0001 SD > BN, SD > Z 0.150 NS 0.23 ± 0.01 0.30 ± 0.01 0.27 ± 0.01  0.24 ± 0.01   E, ml · min1 · 32.36 ± 2.15  27.27 ± 2.39  24.12 ± 0.80  25.62 ± 1.98  0.215 NS 0.277 NS  100 g1 27.91 ± 1.87 28.17 ± 1.37 24.65 ± 1.34  28.40 ± 1.51   E/CO2, ml/min 18.59 ± 0.63  20.43 ± 0.89  20.91 ± 1.04  17.91 ± 1.02  0.041 SD > Z 0.267 NS 21.25 ± 1.39 20.53 ± 1.01 19.31 ± 0.99  16.76 ± 1.12  Values are unadjusted means ± SE (boldface) and means ± SE adjusted for sex, weight, length, Lee index, and age (plain typeface). Lee index, 3; O2, O2 consumption, CO2, CO2 production; RQ, respiratory quotient (CO2/O2); f, frequency; VT, tidal volume; E, minute ventilation; BN, Brown Norway; SD, Sprague-Dawley; K, Koletsky; Z, Zucker; M, male; F, female; NS, not significant. * Strain and sex effect P values are from analysis of covariance (ANCOVA). Adjusted means were compared between strains and between sexes; a critical P value of 0.0125 was used when comparing strains with control for multiple comparisons. Length was a significant predictor of O2 (P = 0.002) and CO2 (P = 0.01). Lee index was a significant predictor of O2 (P = 0.007), CO2 (P = 0.002), and E (P = 0.034).

As shown in Table 1, Z rats had significantly higher adjusted values for O₂ consumption compared with SD rats and greater CO₂ production than the other strains; moreover, in the Z rats, the respiratory quotient is higher, suggesting lesser use of fat substrate at rest. Table 1 also indicates that significant differences existed between strains in regard to respiratory frequency and tidal volume at rest, but not in minute ventilation, indicating that different strains exhibit respiratory patterns that are not dependent on weight alone. SD and K rats exhibit slower, deeper breathing patterns than the BN and the Z animals. When ventilation is expressed relative to CO₂ production, SD animals were different from the Z rats, with the former having the higher value.

Once corrected for body weight or mass, the sex of the animal had no independent effect on metabolic measures. Moreover, males and females differed only in regard to breathing frequency, a finding that persisted after adjustment for morphometric values. Differences in tidal volume did not reach statistical significance.

Lee index and length were each significant predictors for metabolic measures, even after accounting for strain differences. Lee index was weakly correlated to minute ventilation. Apart from the effect of length on O₂ consumption and Lee index on minute ventilation, strain effects on metabolism and the components of resting ventilation were more pronounced than body weight, length, or mass.

Table 2 presents values for ventilation and its components while the animal breathed 100% O₂ for 5 min, a maneuver that was intended to unload the carotid body compared with its function during resting breathing. Differences and direction of difference in regard to frequency and tidal volume while animals breathed 100% O₂ were similar to these observed while animals breathed room air. In regard to minute ventilation, the Z animals maintained ventilation at higher values than K animals. Although minute ventilation was similar between sexes, female animals exhibited a higher frequency and lower tidal volume than did male animals, indicating that this response is differentially affected by both lineage and sex.

Table 2. Ventilation on 100% O2 Strain Strain Effect P Value* Significant Strain Differences Sex Effect P Value* Significant Sex Differences BN SD K Z f, breaths/min  94.14 ± 3.62  108.51 ± 5.18  77.47 ± 2.84 77.38 ± 3.78  89.14 ± 4.24 84.87 ± 3.69  113.85 ± 3.79 103.87 ± 4.18  0.0001 BN > SD, BN > K, Z > SD, Z > K 0.006 F > M VT, ml/100 g   0.29 ± 0.01   0.20 ± 0.01   0.27 ± 0.02 0.28 ± 0.01   0.22 ± 0.01  0.25 ± 0.01    0.20 ± 0.02   0.25 ± 0.01  0.0001 K > BN, SD > BN, SD > Z 0.003 M > F  E, ml · min1 · 100 g1  27.43 ± 1.53  22.67 ± 1.43  20.68 ± 1.47 21.54 ± 1.04  18.64 ± 0.64 19.98 ± 1.02   22.18 ± 1.57  24.65 ± 1.16  0.0134 Z > K 0.715 NS Values are unadjusted means ± SE (boldface) and means ± SE adjusted for sex, weight, length, Lee index, and age (plain typeface). * Strain and sex effect P values are from ANCOVA. Adjusted means were compared between strains and between sexes; a critical P value of 0.0125 was used when comparing strains with control for multiple comparisons. Weight was a significant predictor of f (P = 0.015) and E (P = 0.036). Length was a significant predictor of VT (P = 0.005) and E (P = 0.003). Lee index was a significant predictor of VT (P = 0.006) and E (P = 0.0007).

Table 3 summarizes ventilatory parameters on 7% CO₂-93% O₂ inhalation for 5 min, a standard test of chemoreceptor function. As would be expected, both tidal volume and frequency were significantly increased in all animals; however, Z animals exhibited increases in frequency that significantly exceeded those of the other strains. Strain differences in tidal volume observed during resting breathing and on 100% O₂ were more pronounced during 7% CO₂-93%O₂ inhalation. Significant differences in minute ventilation were observed, with greater increases among Z and SD animals than seen among either K or BN animals. Sex effects were not apparent after adjustment for morphometric traits. While morphometric traits were significant predictors of frequency, tidal volume, and minute ventilation, their relative effects on these measures were less pronounced than strain effects. Thus CO₂ chemoreceptor function appears to be more a function of strain than of sex or morphometric measures.

Table 3. Ventilation on 7% CO2-93% O2 Strain Strain Effect P Value* Significant Strain Differences Sex Effect P Value* Significant Sex Differences BN SD K Z f, breaths/min 114.95 ± 2.98 125.03 ± 5.51  131.69 ± 3.07 134.26 ± 4.03  124.45 ± 4.90 117.04 ± 3.93  174.71 ± 3.59 170.38 ± 4.45  0.0001 Z > BN, Z > SD, Z > K, SD > K 0.448 NS VT, ml/100 g   0.39 ± 0.02   0.26 ± 0.02    0.45 ± 0.04   0.46 ± 0.02     0.27 ± 0.02   0.32 ± 0.02    0.31 ± 0.03   0.38 ± 0.02  0.0001 SD > BN, SD > K, SD > Z, Z > BN 0.201 NS  E, ml · min1 · 100 g1  44.75 ± 2.71  29.22 ± 3.34   58.41 ± 4.33  60.23 ± 2.44   31.26 ± 1.81  36.52 ± 2.38   53.88 ± 5.35  62.12 ± 2.70  0.0001 Z > BN, Z > K, SD > BN, SD > K 0.179 NS Values are unadjusted means ± SE (boldface) and means ± SE adjusted for sex, weight, length, Lee index and age (plain typeface). * Strain and sex effect P values are from ANCOVA. Adjusted means were compared between strains and between sexes; a critical P value of 0.0125 was used when comparing strains with control for multiple comparisons. Weight was a significant predictor of f (P = 0.007). Length was a significant predictor of f (P = 0.038), VT (P  = 0.046) and E (P = 0.005). Lee index was a significant predictor of f (P = 0.025), VT (P = 0.015), and E (P = 0.0003).

Presented in Table 4 are results obtained while animals were given a 5-min challenge with 10% O₂. This test has previously been shown to differentiate SD rats obtained from different colonies (16, 17). Differences among strains were in general less pronounced than during CO₂ inhalation. Tidal volume and minute ventilation in the SD animals were the highest of all four strains. Sex effects were apparent only in regard to frequency.

Table 4. Ventilation on 10% O2-balance N2 Strain Strain Effect P Value* Significant Strain Differences Sex Effect P Value* Significant Sex Differences BN SD K Z f, breaths/min 117.00 ± 3.45 142.45 ± 7.51  131.37 ± 5.46 128.37 ± 5.49  115.95 ± 5.22 112.26 ± 5.36  147.00 ± 7.09 127.26 ± 6.07  0.016 BN > K 0.0004 F > M VT, ml/100 g   0.37 ± 0.01   0.28 ± 0.01     0.37 ± 0.02    0.38 ± 0.01    0.27 ± 0.02   0.30 ± 0.01     0.27 ± 0.02    0.31 ± 0.01  0.0001 SD > BN, SD > K, SD > Z 0.073 NS  E, ml · min1 · 100 g1  43.01 ± 1.90  39.54 ± 2.55    48.34 ± 3.14   48.33 ± 1.86   29.51 ± 0.69  32.01 ± 1.81    38.59 ± 2.84   39.01 ± 2.06  0.0001 SD > K, SD > Z 0.071 NS Values are unadjusted means ± SE (boldface) and means ± SE adjusted for sex, weight, length, Lee index, and age (plain typeface). * Strain and sex effect P values are from ANCOVA. Adjusted means were compared between strains and between sexes; a critical P value of 0.0125 was used when comparing strains to control for multiple comparisons. Length was a significant predictor of VT (P = 0.026). Lee index was a significant predictor of VT (P = 0.020). and E (P = 0.008).

Measures of ventilatory responsiveness to chemoreceptor input are approximated by the differences in ventilation between appropriate baseline values. Figure 1 is a histogram showing in panels A-E the relative difference among strains in regard to chemoresponsiveness, by using differences in values adjusted for the effects of weight, length, and Lee index. Figure 1A indicates that, relative to room-air breathing, all strains exhibited increased values for frequency, tidal volume, and minute ventilation under hypoxic conditions. SD animals exhibited the greatest response and were significantly more responsive than K animals with respect to frequency and minute ventilation. Tidal volume was significantly different between the Z and K animals, having the highest and lowest values, respectively. Also, Fig. 1D shows that the four strains differ substantially in regard to responsiveness to CO₂, a central chemoreceptor function. The BN rat has the smallest difference of the four strains; both the SD and the Z animals have significantly greater responses than the K or BN strains. Males had greater changes than females for frequency and ventilation, but sex effects were less striking than lineage effects (data not shown in Fig. 1). Figure 1B shows that the relative fall in ventilation with inhalation of 100% O₂, compared with resting breathing, is similar in the four strains. In Fig. 1C, the difference in frequency and minute ventilation on 100 and 10% O₂ is highest in the SD rats but similar among the other three strains. Finally, in Fig. 1E, the difference in ventilation between 100% O₂ and 7%CO₂-93%O₂ is highest in the SD and Z animals and lowest in the BN animals. The change in frequency was more pronounced in males. Body mass was not a significant predictor for any of the differences, and there were no unique patterns in the strains with obese traits. In summary, these data indicate that differences in chemoreceptor responsiveness appear to be predominantly due to strain effects.

An analysis was performed to examine an effect of sex-by-strain interaction. Traits exhibiting a significant degree of interaction were O₂ consumption (male: Z > BN, SD, K; female: K > SD); CO₂ production (male: Z > BN, SD, K; female: no difference); frequency of breathing on room air (male: SD < BN, Z; female: no difference); change in frequency from room air to 10% O₂ (males: SD > Z; females: no difference); change in ventilation from room air to 10% O₂ (males: SD > Z; females: SD > K); and change in frequency from room air to 7% CO₂ (males: SD > BN, Z and Z > BN; females: no differences). These results suggest that 1) in contrast to the other strains, metabolic regulation in the Z strain exhibits considerable sexual dimorphism; and 2) during gas challenges, a majority of commonly measured ventilatory traits are determined by lineage effects, independent of either sex or weight.

DISCUSSION

These studies describe significant differences in metabolic function and ventilation and its components among four rat lineages, selected to represent a diversity in body habitus, cardiopulmonary traits, weight, and genetic origin. The results are summarized as follows: 1) metabolic activity was greatest in Z rats; 2) during resting breathing, ventilation among strains was comparable, but the particular patterns of frequency and/or tidal volume used to obtain total ventilation were peculiar to individual strains; 3) males and females differed with regard to frequency, and this pattern persisted during exposure to 100% O₂ or to hypoxic conditions, with females showing a greater frequency response than males; 4) Z and SD rats showed a greater response to CO₂ challenge than did the other groups, and no sex effects were apparent; and 5) when responses were expressed as percentage of control values, differences in ventilatory responsiveness appeared predominantly due to strain effects, with smaller or no significant effects of weight, body mass, or sex.

These results reject the hypothesis that morphometric factors predominate in the determination of metabolic rate, ventilation, and ventilatory control in the adult rat. Instead, there appears to be a high degree of lineage-dependent, i.e., genetic, transmission of these physiological measurements. The degree of variance among strains is larger than that reported for two separate colonies of SD animals (16, 17) but is similar to that observed in murine strains (24, 25).

The study design took into account variability and errors expected in measurements of values related to ventilation and metabolism (6, 22). The coefficient of variation in our groups of animals varied from 10 to 18%, and is similar to those reported in the literature in the studies of mice and rats. Frequency has the lowest (10%) coefficient of variation, whereas minute ventilation has the highest (18%) coefficient of variation in our preliminary data. This difference might be accounted for by physiological factors or by the method of measurement. The accuracy of measurement of frequency is nearly 100%, whereas the theoretical accuracy in tidal volume, due to thermal drift and calibration accuracy present even with exacting conditions and expected by theory, is ±10% (6). All animals were studied in the same apparatus, and the strain order was interspersed, making it difficult to assign strain differences in tidal volume to experimental error as a function of time, strain, or technique. Measurements of minute ventilation are also affected by these factors. However, we intentionally utilized conservative thresholds for significance, and experimental errors would increase variance and obscure the differences we observed.

Differences were detected in metabolic rate specifically with the Z rat, even though all animals were housed and fed under the same conditions. This difference in metabolic rate was explained to some degree by differences in CO₂ production, indicating that under the conditions of measurement Z rats relied more on protein and/or carbohydrate substrates, rather than on fat, for metabolism. Moreover, Z animals also had the highest O₂ consumption relative to body mass. This difference could represent a higher activity level in this strain. In contrast to the other strains, Z animals exhibited sex-by-strain interaction in regard to O₂ consumption and CO₂ production; male Z rats consumed more O₂ and produced more CO₂ than did males from other strains, whereas female Z rats were indistinguishable from females of other strains. During testing, there were no observable strain differences in behavior, and all animals were studied at the same time of day; however, it is possible that circadian factors act on this trait and differentiate the Z animals from the other three strains.

The pattern of breathing at rest was influenced more by strain than by weight or body mass. Ventilation was equivalent among the four strains but was achieved through different mechanisms; SD rats had deeper, slower breathing patterns than BN and Z animals. When ventilation was adjusted for CO₂ production, only the SD rats differed from the Z, whereas the SD, K, and BN rats were similar. This finding suggests that, at rest, Z animals may differ in dead-space ventilation (15), possibly related to the size of central airways; however, dead-space differences are unlikely to explain differences among all strains. Environmental effects can alter resting ventilation. Powerful effects include ambient temperature and humidity effects on thermoregulation; however, the animals were studied under similar conditions, in the range of preferred ambient temperature (7), and humidity had no association with strain, sex, or weight values. In any event, the data suggest that the patterning of tidal volume and frequency operates differently among different strains and is not simply a result of strain differences in weight, body mass, or sex-by-strain interaction.

Alterations in chemosensory input were tested by brief (5-min) exposure to different inspired-gas mixtures, spanning a wide range of stimuli. None of these strains are bred specifically for respiratory traits, yet there were strain differences not only in the level of ventilation achieved with the same environmental challenge but also in the change in ventilation with hypoxia or hypercapnia. These differences in ventilation and its components indicate a considerable degree of genetic influence on the control of breathing. Both central and peripheral chemoreceptor functions track with strain and are relatively strong compared with the effects of sex or body habitus. These strain differences in chemoreception provide a choice of animals in which to examine chemoreceptor function and a caution that findings in one strain or lineage of rat are perhaps more limited and less generalizable than expected by conventional wisdom.

It is likely that more than one gene influences these ventilatory parameters and that the influence of these genes on ventilation and its components are determined by genetic interaction and/or interaction with the environment. Our results suggest variations in the rat genome that contribute to variations in ventilation, metabolism, and ventilatory responses to changes in inspired O₂ and CO₂ levels. This flexibility may be useful for successful adaptation to new environments.

ACKNOWLEDGEMENTS

The authors thank Qwendolyn E. Alexander for preparation of the manuscript and Drs. Aravinda Chakravarti and Paul Ernsberger for their advice.

FOOTNOTES

Address for reprint requests: K. P. Strohl, Veterans Affairs Medical Center 111J(W), 10701 East Blvd., Cleveland, OH 44106.

Received 23 April 1996; accepted in final form 13 September 1996.

REFERENCES