Role of beta-adrenergic and cholinergic systems in acclimatization to hypoxia in the rat

The role of beta-adrenergic and muscarinic cholinergic systems on maximal treadmill exercise performance and systemic O-2 transport during hypoxic exercise (PIO2 similar to 70 Torr) was studied in rats acclimatized to hypobaric hypoxia (PIO2 similar to 70 Torr for 3 weeks, A rats) and in non-acclimatized littermates (NA rats). Untreated A rats had lower resting (fH) and maximal heart rate (fHmax) and cardiac output (Q), and higher maximal O-2 uptake (V-O2max) than NA. The only effect of cholinergic receptor blockade with atropine (Atp) was an increase in pre-exercise fH to comparable levels in A and in NA. beta(1)-adrenergic receptor blockade with atenolol (Aten) lowered pre-exercise fH and (fHmax) to comparable values in A and in NA rats. However, since both pre-exercise fH and fHmax were lower in untreated A, the effect of Aten was relatively smaller in A. Aten reduced maximal exercise cardiac output (Q(max)) in NA; however, tissue O-2 extraction increased such that V-O2max was not affected. Aten did not influence V-O2max or any other parameter of systemic O-2 transport in A. In conclusion the increased cholinergic tone may be responsible for the lower resting fH but not the lower fHmax of A; the integrity of the beta-adrenergic system is not necessary to attain V-O2max in hypoxia either in A or in NA; the decreased response to beta-adrenergic stimulation in A limits the efficacy of this system on the mechanisms of systemic O-2 transport and reduces the effect of its blockade on these mechanisms. (C) 1997 Elsevier Science B.V.