PHARMACOLOGICAL CHARACTERIZATION OF CHICK AND FROG BETA-ADRENERGIC RECEPTORS IN PRIMARY CULTURES OF MYOCARDIAL-CELLS

In membrane preparations derived from primary cultures of chick myocardial cells, beta adrenergic receptors modeled for a single low-affinity site for both betaxolol (beta-1-selective) and ICI 118551 (beta-2-selective) displacement of [I-125]iodocyanopindolol (ICYP), indicating that the chick beta receptor is pharmacologically distinct from both mammalian beta-1 and beta-2 adrenergic receptors with respect to these antagonists. However, the highly beta-1-selective compound CGP 20712A was able to distinguish two binding sites on ICYP competition curves, a high-affinity "beta-1 site" (75%) and a low-affinity "beta-2 site" (25%). Also, in chick heart cell membranes the relative ability of agonists to displace ICYP produced a profile typical of beta-1 adrenergic receptors with a rank order of potency or efficacy of: isoproterenol > epinephrine = norepinephrine. When agonist-mediated adenylyl cyclase stimulation was assessed the order of potency was slightly different, isoproterenol > epinephrine greater-than-or-equal-to norepinephrine. Additionally, antagonism of isoproterenol stimulation of adenylyl cyclase by CGP 20712A yielded a K(b) value (1.6 +/- 0.35 x 10(-7) M) intermediate between the high and low-affinity binding sites of CGP 20712A, suggesting that the low-affinity site is coupled to adenylyl cyclase. In membrane preparations of frog myocardial cells, ICYP/antagonist competition curves modeled for a mixed population of receptors, with subtype percentages varying from 50:50 beta-1:beta-2 to 100% beta-2 depending on the specific antagonist used and the individual cell preparation. For ICYP/agonist competition binding experiments the relative ability to displace ICYP was isoproterenol > epinephrine = norepinephrine, a profile typical of beta-1 adrenergic receptors. However, for stimulation of adenylyl cyclase activity the order of potency was isoproterenol > epinephrine > norepinephrine, typical of beta-2-adrenergic receptors. These data indicate that both chick and frog heart cells cultured in serum-free media contain beta receptors which are similar to mammalian beta-1 and beta-2 subtypes from the standpoint of agonist binding or function. Chick heart cells differ from frog heart cells by having a high proportion of beta-1 subtype receptors and by possessing an atypical antagonist binding profile for certain selective beta receptor antagonists.