Effects of hypoxia on atrial muscarinic cholinergic receptors and cardiac parasympathetic responsiveness.

Chronic exposure of rats to hypoxia resulted in a lower resting heart rate and a supranormal increase in heart rate in response to parasympathetic blockade by atropine. The density of muscarinic cholinergic receptors labeled by the antagonist [3H]quinuclidinyl benzilate was elevated significantly in the atria of animals kept hypoxic for 2-4 weeks. Chronic hypoxia did not change the affinity of the receptor for [3H]quinuclidinyl benzilate, the weight of the atria, or the amount of protein per atrial pair. Thus, the decrease in resting heart rate may be explained by the increase in the density of atrial muscarinic cholinergic receptors.