INCREASING MAXIMAL HEART-RATE INCREASES MAXIMAL O-2 UPTAKE IN RATS ACCLIMATIZED TO SIMULATED ALTITUDE

Maximal exercise heart rate (HRmax) is reduced after acclimatization t o hypobaric hypoxia. The low HRmax contributes to reduce maximal cardi ac output ((Q) over dot (max)) and may limit maximal O-2 uptake ((V) o ver dot O-2max) The objective of these experiments was to test the hyp othesis that the reduction in (Q) over dot (max) after acclimatization to hypoxia, due, in part, to the low HRmax, limits (V) over dot O-2ma x. If this hypothesis is correct, an increase in (Q) over dot (max) wo uld result in a proportionate increase in (V) over dot O-2max. Rats ac climatized to hypobaric hypoxia [inspired PO2 (PIO2) = 69.8 +/- 3 Torr for 3 wk] exercised on a treadmill in hypoxic (PIO2 = 71.7 +/- 1.1 To rr) or normoxic conditions (PIO2 = 142.1 +/- 1.1 Torr). Each rat ran t wice: in one bout the rat was allowed to reach its spontaneous HRmax, which was 505 +/- 7 and 501 +/- 5 beats/min in hypoxic and normoxic ex ercise, respectively; in the other exercise bout, HRmax was increased by 20% to the preacclimatization value of 600 beats/min by atrial paci ng. This resulted in an similar to 10% increase in (Q) over dot (max), since the increase in HRmax was offset by a 10% decrease in stroke vo lume, probably due to shortening of diastolic filling time. The increa se in (Q) over dot (max) was accompanied by a proportionate increase i n maximal rate of convective O-2 delivery ((Q) over dot (max) x arteri al O-2 content), maximal work rate, and (V) over dot O-2max in hypoxic and normoxic exercise. The data show that increasing HRmax to preaccl imatization levels increases (V) over dot O-2max, supporting the hypot hesis that the low HRmax tends to limit (V) over dot O-2max after accl imatization to hypoxia.