Autoradiographic characterization of beta adrenergic receptors in coronary blood vessels and myocytes in normal and ischemic myocardium of the canine heart.

This light microscopic autoradiographic study was performed to test the hypotheses that (a) the density of beta adrenergic receptors (BAR) may differ in various components of the heart and (b) BAR in certain components of the heart may exhibit a selective response to pharmacologic and pathological stimuli. Blocks of canine left ventricle were frozen and tissue sections cut and incubated in (-)[3H]dihydroalprenolol (DHA) to label the BAR. For total and nonspecific binding, serial sections were incubated with and without 10(-5) M (+/-)propranolol. Scintillation spectrometry of sections demonstrated rapid binding, saturability, stereospecificity, a dissociation constant (KD) of 3.2 +/- 0.5 nM (SD) (n = 3), and a maximal binding of 31.3 +/- 3.1 fmol/mg of tissue protein. Isoproterenol was 12.5 times more effective than norepinephrine in displacing DHA. Sections incubated with 10(-5) - 10(-8) M metoprolol, a beta one selective antagonist, demonstrated a KD of 0.7 X 10(-6) M. For autoradiography, emulsion-coated coverslips were attached to the slides. After exposure, the slides were developed and stained, and grain density quantified. Specific BAR binding (n = 4 dogs) was 1,047 +/- 131 (SEM) grains/10(-2) mm2 for myocardial arterioles, 219 +/- 30 for myocardial arteries, 31 +/- 12 for the proximal left anterior descending coronary artery (LAD), and 231 +/- 34 for cardiac myocytes. Specific binding in the presence of 10(-5) M metoprolol was reduced approximately 75% for both arterioles and myocytes. However, at 10(-6) M metoprolol, the percent reduction in specific DHA binding was greater for myocytes (50%) than for arterioles (0%), and at 10(-7) M metoprolol, the percent reduction in specific DHA binding was 17% for myocytes with no reduction over arterioles. After 1 h of LAD occlusion, a selective increase (18%) in BAR density occurred over cardiac myocytes, but not over blood vessels in the ischemic myocardium. Thus, (a) specific BAR binding was five times greater in arterioles than in small arteries and myocardium and 34 times greater than in the proximal LAD; (b) BAR of myocytes were more sensitive than those of arterioles to displacement by the beta one selective antagonist, metoprolol; and (c) a selective increase in BAR occurs in cardiac myocytes but not in blood vessels after 1 h of ischemia in this experimental model.

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