Myocardial adrenergic and cholinergic receptor function in hypoxia:: correlation with O2 transport in exercise

The time course of changes in rat myocardial alpha (1)- and beta -adrenoceptors and of muscarinic cholinergic (M-Ach) receptor characteristics was studied parallel with the changes in exercise systemic O-2 transport during a 21-day period of hypoxia (barometric pressure 380 Torr) to assess the effects of receptor modification during acclimatization on maximal exercise capacity. Hypoxia resulted in polycythemia, pulmonary hypertension, right ventricular hypertrophy, and transient left ventricular weight loss. Maximal O-2 consumption at 30 min of hypoxia was reduced to 60% of the normoxic value and remained unchanged. This was partly due to a gradual decrease in maximal cardiac output and heart rate (HRmax), which offset the increase in blood O-2 content. HRmax correlated positively (r = 0.994) with beta -adrenoceptor density and negatively (r = -0.964) with M-Ach-receptor density, suggesting that HRmax reduction results from intrinsic changes in myocardial receptor characteristics leading to reduced responses to adrenergic stimulation and elevated responses to cholinergic stimulation. alpha -Adrenoceptor density in both ventricles increased initially to eventually fall below normoxic values. The dissociation between the different patterns of right and left ventricular weight and the similar pattern of alpha -adrenoceptor change in both ventricles do not support a role for these receptors on right ventricular myocardial hypertrophy.