One hour of myocardial ischemia in conscious dogs increases beta-adrenergic receptors, but decreases adenylate cyclase activity.

An increased myocardial beta-adrenergic receptor density has been reported following myocardial ischemia. However, it is not clear whether these receptors are effectively coupled to adenylate cyclase which would be necessary for enhanced physiological responsiveness. We, therefore, examined the effects of myocardial ischemia in six conscious dogs (4 intact and 2 with posterior wall denervation) in which the left circumflex coronary artery was occluded. Ischemia was verified by measurement of regional blood flow by radioactive microspheres. After 1 h of coronary artery occlusion, the dogs were anesthetized with pentobarbital and the left ventricle was divided into normal, intermediate and ischemic regions. A crude membrane fraction was prepared from each region. beta-Adrenergic receptors were quantitated with 125I-cyanopindolol binding and adenylate cyclase activity was measured. In all six animals studied, beta-adrenergic receptor density increased progressively and adenylate cyclase activity decreased progressively, when the ischemic myocardium was compared to the intermediate and the non-ischemic myocardium. Since adenylate cyclase activity declined, these results do not support the concept that the increased beta-receptor density induced by myocardial ischemia is causally related to enhanced beta-adrenergic sensitivity.