DIFFERENCES IN BETA-ADRENERGIC-RECEPTOR AGONIST AFFINITY BETWEEN CARDIAC MYOCYTES AND CORONARY ARTERIOLES IN CANINE HEART

Beta adrenergic receptors (beta AR) are localized in several tissue co mpartments of the heart, including cardiac myocytes, coronary arteries and coronary arterioles, but it is unclear whether there are differen ces between tissues in beta AR coupling to G protein. The goal of thes e studies was to use receptor autoradiography to analyze beta receptor agonist binding characteristics in different tissue compartments of d og heart, including ventricular myocytes (predominantly beta-1) and co ronary arterioles (predominantly beta-2). Frozen sections were incubat ed in [I-125]-pindolol with the beta agonist isoproterenol in the abse nce and presence of 0.1 mM sodium 5'-guanylylimidodiphosphate (GppNHp, a nonhydrolyzable GTP analog) and analyzed by gamma counting or autor adiography. Nonlinear curve-fitting analyses of ventricular section ra dioactivity indicated that in the absence of GppNHp, the data were con sistent with a two-site fit, with 88% of the receptors in the high-aff inity state. In autoradiographic analyses, GppNHp displaced the agonis t binding curve to a greater extent in arterioles (approximately 100-f old) than in myocytes (approximately 10-fold). This suggests that beta receptors on arterioles are more tightly coupled to G protein than ar e beta receptors on myocytes. Thus these studies suggest that 1) beta AR on arterioles are coupled more tightly to G protein than are beta A R on myocytes, possibly because of differences in beta receptor subtyp e, and 2) more beta AR are in the high-affinity state than has been re ported previously in more traditional analyses on membrane preparation s.