External transport of beta-adrenergic binding sites in ischemic myocardium

The properties of beta-adrenergic receptors were studied in normal and in flow restricted regions of the dog heart. Purified cardiac membrane preparations and papillary muscle preparations were isolated from control and ischemic areas and tested a) following chronic beta-receptor blockade with metipranolol or exaprolol, and b) after acute regional myocardial ischemia. A significant reduction in the sensitivity of the heart muscle preparations from compromised heart for isoprenaline resulting in a reduced affinity of beta-adrenergic receptors to exaprolol was observed. Quantitative ligand binding data showed higher numbers of (3H) dihydroalprenolol/(3H) DHA/binding sites in the membrane fraction obtained from compromised compared to control myocardium. The ratio of intra- to extracellular beta-adrenergic receptors decreased from 1.35 to 0.55 in the membrane fractions obtained from the compromised hearts. Pretreatment of experimental animals with metipranolol or propranolol attenuated the observed increase in the total number of beta-adrenergic receptor sites in myocardial membrane fractions from ischemic hearts. These data suggest preferential distribution of beta-adrenergic binding sites from intracellular to membrane fractions in flow restricted regions of the dog heart after coronary occlusion.