INTRODUCTION

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ACUTE EXPOSURE OF HUMANS or animals to environmental hypoxia results in a decrease in the maximal rate of O₂ uptake (O_{2 max}), which is proportional to the decrease in convective O₂ delivery that follows the reduction in blood O₂ content (8, 13). As acclimatization proceeds, blood O₂ content increases because of the stimulation of red blood cell production; however, this is not accompanied by a proportionate increase in O_{2 max}, which shows no change (1, 2, 17) or a modest increase (4, 6, 10, 13) during the course of acclimatization. The increase in arterial blood O₂ content (Ca_O2) due to the polycythemia is offset by a concomitant decrease in maximal cardiac output (_max) (1, 2, 4, 6, 8, 14), such that the maximal rate of convective tissue O₂ delivery (O_{2 max}), i.e., _max × Ca_O2, remains relatively unaffected by acclimatization (3, 4). The decrease in _max is the result of reductions in maximal heart rate (HR_max) and stroke volume (SV_max) (1, 4, 6, 8, 14).

Because the rate of convective tissue O₂ delivery is a major determinant of O_{2 max}, it is likely that the lack of a major change in O_{2 max} after acclimatization is due to the modest effect on O_{2 max}. However, it is uncertain whether a more substantial increase in _max and O_{2 max} after acclimatization would result in a proportionate increase in O_{2 max}. On one hand, it is possible that an increase in cardiac output, by shortening capillary transit time, may result in incomplete O₂ diffusion equilibration at the pulmonary or the tissue level. The former could limit arterial blood oxygenation and offset the effect of the increased blood flow on the rate of convective O₂ delivery to the tissues. The latter would limit the rate of O₂ transfer from capillary to mitochondrion. Second, the increase in myocardial O₂ consumption (mO₂) secondary to an increase in heart rate could exceed the rate of O₂ supply to the myocardium, thereby compromising cardiac function.

The present experiments were performed to test the hypothesis that the modest increase in O_{2 max} with acclimatization is due, at least in part, to the decrease in _max offsetting the elevated Ca_O2 and limiting convective O₂ delivery to the tissues. If this hypothesis is correct, an increase in _max after acclimatization should be accompanied by corresponding increases in O_{2 max} and O_{2 max}. To test this hypothesis, _max of rats acclimatized to simulated altitude was increased by increasing HR_max by right atrial pacing.

	METHODS

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Animal model. The animal model of acclimatization to simulated altitude has been described previously (4, 6). Briefly, male Sprague-Dawley rats (247 ± 7 g) were placed for 3 wk in a chamber where air was circulated at a pressure of 375 ± 8 Torr, which resulted in a PO₂ of moist inspired air (PI_O2) of 69.8 ± 3.0 Torr. The chamber was opened three times a week for ~30 min to replace the animal cages and feed and water the animals. At 3 wk the animals were removed from the chamber and anesthetized with pentobarbital sodium (35 mg/kg ip), and a polyethylene catheter (PE-50) was placed in the aortic arch through the left carotid artery. A second PE-50 catheter was placed in the right atrium through the right jugular vein. A pediatric pacing catheter (1 mm OD) was then placed in the superior vena cava through the opening used to insert the atrial catheter. Adequate positioning of the catheter tips was verified at necropsy. All catheters were tunneled subcutaneously and exteriorized at the back of the neck. The arterial and right atrial catheters were cut ~4 cm from the skin and occluded with metal plugs. The animals were allowed to recover from anesthesia at ambient PI_O2 for ~1 h, then they were placed in a sampling chamber where PI_O2 was maintained at 70 Torr by mixing O₂ and N₂ at ambient pressure. The animals remained at this PI_O2 for an additional 4-5 h.

Exercise protocol. The animals were removed from the sampling chamber, the rectal temperature was measured, and the animals were transferred to a treadmill enclosed in a Lucite box. PI_O2 of the box could be adjusted to the desired level by mixing O₂ and N₂ at ambient barometric pressure. The catheters were connected, through sampling ports located at the top of the box, to pressure transducers. Blood samples were obtained through stopcocks. After 20-30 min in the treadmill, arterial and venous blood samples (0.4 ml each) were obtained, and the blood was replaced with homologous fresh blood obtained from a donor. The treadmill was placed at an angle of 10°, and the speed was set at 10 m/min. The speed was increased by 4 m/min every 90 s until O_{2 max} was reached. O_{2 max} was defined as the O₂ uptake (O₂) after which an increase in speed did not result in a further increase (±5%) in O₂. Arterial and venous blood samples were obtained during the last 30-45 s of exercise, and then rectal temperature was measured within 30 s of termination of exercise.

Gas-exchange measurements. Gas enters and leaves the box enclosing the treadmill through separate inflow and outflow tubes. The desired PI_O2 was obtained by mixing O₂ and N₂ from CO₂-free cylinders. Inflowing volume flow was maintained constant at ~20 l/min by using a high-precision gas mixing pump. Inflowing and outflowing gas O₂ concentrations were monitored continuously and simultaneously, and the difference between inflowing and outflowing concentration was recorded continuously. Outflowing CO₂ concentration was also monitored continuously. O₂ and CO₂ output (ml STPD · min¹ · kg¹) were calculated from inflowing minus outflowing O₂ concentration, outflowing CO₂ concentration, and outflowing volume flow, by using standard gas by exchange equations.

Gas-transport measurements. Arterial and mixed venous blood samples were analyzed for pH, PO₂, and PCO₂ with appropriate electrodes at 38°C and for hemoglobin (Hb) concentration and O₂ saturation and corrected to the rectal temperature by using temperature-correction factors for rat blood (5).

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	RESULTS

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Figure 1A shows that heart rate was successfully maintained at 600 beats/min in each of the rats. Pacing increased HR_max by ~20% above the nonpaced run. In the nonpaced run, at a given submaximal work rate, heart rate was higher in hypoxic exercise. However, HR_max was not significantly different between hypoxic and normoxic runs (Fig. 1, Table 1). The higher HR_max in the paced run was translated into a 10% increase in _max (Table 1); the smaller proportional increase in _max than in HR_max was the result of a decrease in SV_max of ~10% in hypoxic and normoxic exercise and was associated with a decrease in CVP in both cases (Table 1). The increase in _max produced by pacing was translated into a proportionate increase in exercise performance: the work rate and O_{2 max} were ~10% higher in the paced than in the nonpaced run (Fig. 1, Table 1); this was true in hypoxic as well as normoxic exercise. The increase in O_{2 max} was associated with a proportionate increase in O_{2 max} in hypoxic and normoxic exercise. Pacing was not accompanied by significant changes in MABP or SVR (Table 1), although both were significantly higher in normoxia than in hypoxia (Table 1). This has been shown previously (4) and is probably the result of the removal of the vasodilatory effect of hypoxia. As reported elsewhere (4), the increased SVR and MABP of normoxic exercise were accompanied by a reduction in _max and SV_max relative to those observed in hypoxic exercise, probably as a result of the increased left ventricular afterload. The increased HR_max did not result in significant changes in arterial or venous blood oxygenation, the O₂ extraction ratio, or the alveolar-arterial PO₂ difference (Table 2).

View larger version (16K): [in this window] [in a new window]   Fig. 1.   A: heart rate [HR, beats (b)/min)] as a function of work rate (kg · m1 · min1). Circles, normoxic exercise; triangles, hypoxic exercise; filled symbols, paced run; open symbols, nonpaced run. Error bars, SE. SE for HR data is zero, since in all cases HR during pacing was 600 beats/min. B: O2 uptake (O2, ml STPD · min1 · kg1) as a function of work rate. Symbols as in A.

	DISCUSSION

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The main finding of the present study is that when HR_max of rats acclimatized to hypoxia was increased to the level observed before acclimatization, maximal exercise performance, as represented by O_{2 max} and the maximal work rate, was also increased.

The O_{2 max} attained in these experiments during the nonpaced run was essentially the same as that observed previously in acclimatized rats that had undergone catheter placement 7-10 days before the exercise test (4). This indicates that the surgical procedures performed on the day of the experiment, as was the case in the present study, did not influence exercise performance. These data also suggest that the effect of a previous maximal run on the same day was minimized by the alternating order of paced and nonpaced runs followed in these studies.

HR_max before acclimatization in this model is ~600 beats/min and is reduced 15-20% by acclimatization (4). In humans the magnitude of the reduction in HR_max afer acclimatization is strongly correlated to the severity of hypoxia (for review see Ref. 8). At levels of hypoxia comparable to those observed in the present experiments, HR_max is reduced by 17-35% from the sea-level value (3, 14, 15). The reduction in HR_max observed in humans (8, 16) and rats (4) acclimatized to hypoxia occurs despite a continued increased level of sympathetic activity and is thought to be due to downregulation of myocardial -adrenergic receptors (11), probably as a result of increased agonist activity; in addition, upregulation of M₂ muscarinic receptors (12, 20) and downregulation of adenosinergic receptors (12) contribute to reduce HR_max. Although the mechanism of the reduction in HR_max is fairly well understood, its functional relevance with respect to maximal exercise capacity is not clear. The present data show that under the present experimental conditions the increase in HR_max did result in an increase in _max that was translated into proportionate increases in the rate of convective O₂ delivery to the tissues and O_{2 max} (Fig. 2). This suggests that the reduced HR_max is one of the factors contributing to limit O_{2 max} after acclimatization.

View larger version (17K): [in this window] [in a new window]   Fig. 2.   Maximal O2 (O2 max) as a function of maximal rate of convective O2 delivery (O2 max = max × CaO2, where max is maximum cardiac output and CaO2 is arterial blood O2 content) for 4 groups of experiments. , Nonpaced run; , paced run. Error bars, SE.

The fact that the increase in O_{2 max} was commensurate with the increase in convective O₂ delivery suggests that the latter was the major determinant of the increase in O_{2 max}. Within the past few years, evidence has accumulated supporting the idea that, in addition to the rate at which O₂ is delivered to the capillaries of the exercising muscles, O_{2 max} is determined by the rate at which O₂ diffuses from the capillaries to the cells (9, 18). Within this context, it is possible that an increase in _max could compromise O₂ diffusion at the tissue level by shortening the capillary transit time. This does not appear to have occurred in these experiments: the O₂ extraction ratio, i.e., O_{2 max}/ O_{2 max}, did not change with pacing (Table 2, Fig. 2). This argues against mechanisms other than O_{2 max} contributing to influence O_{2 max} in these conditions.

An increase in _max could also decrease the pulmonary capillary transit time and limit alveolocapillary O₂ diffusion. Incomplete alveolocapillary PO₂ equilibration is one of the major factors responsible for a decrease in arterial PO₂ in humans exercising in hypoxia (19). The decrease in arterial blood oxygenation offsets the effect of the increased cardiac output and limits the increase in the rate of convective O₂ delivery that would otherwise occur, as shown in humans in whom the acclimatization-induced decrease in cardiac output was prevented (7). The fact that pacing was not associated with changes in arterial PO₂ or the alveolar-arterial PO₂ difference (Table 2) suggests that alveolocapillary O₂ diffusion was not compromised by the increase in _max observed in these experiments. In contrast to the observations in humans, rats exercising in acute or chronic hypoxia routinely show an increase in arterial PO₂ without significant changes in the alveolar-arterial PO₂ difference from the values observed at rest (4, 5). Factors that appear to contribute to the greater efficacy of alveolocapillary O₂ equilibration of rats are a higher pulmonary diffusing capacity, a lower O₂ affinity of Hb, and a smaller increase in cardiac output during maximal exercise in hypoxia than those observed in similar conditions in humans (4, 5). Also, the changes in _max observed in the present experiments are relatively small; accordingly, the effect of larger increases in _max on diffusive O₂ conductance, at the pulmonary or tissue level, may be different from that observed here.

The increase in _max was proportionately smaller than the increase in HR_max produced by pacing because of a decrease in SV_max. Several factors could lead to a decrease in SV_max, including a decrease in myocardial performance that could result from myocardial O₂ supply being inadequate to satisfy the increased mO₂ due to the elevated HR_max. If the decrease in SV_max were a result of a decrease in contractile performance, an increase in CVP would be expected; however, this was not observed in the present experiments, since pacing was accompanied by a decrease in CVP (Table 1). Accordingly, the most likely explanation is that SV_max decreased as a result of a reduction in filling time associated with the increase in heart rate.

The reduction in HR_max observed during acclimatization would tend to reduce mO₂ in maximal exercise, since heart rate, together with ventricular afterload, is the major determinant of mO₂. Accordingly, a lower HR_max would help reduce the myocardial O₂ cost of hypoxic exercise and may represent a myocardial protective mechanism during maximal exercise in hypoxia. Although this may be the case, the present results indicate that HR_max of acclimatized rats can be elevated to the preacclimatization levels for the elapsed time in these experiments, without apparent deleterious effects and with the increase being translated into an increase in exercise performance. Whether the HR_max attained in these experiments can be maintained for more prolonged periods cannot be ascertained.

In summary, these studies show that increasing HR_max of rats acclimatized to hypoxia results in an increase in cardiac output and proportionate increases in convective O₂ delivery and O₂ during maximal exercise. These data suggest that the reduced HR_max contributes to limit O_{2 max} after acclimatization, since this limitation can be overcome by an increase in HR_max. Furthermore, elevation of HR_max to preacclimatization levels can be sustained, within the time frame of the present experiments, without apparent deleterious effects.