Subclassification of beta-adrenergic receptors in cultured rat cardiac myoblasts and fibroblasts.

3-Adrenergic receptors in primary cultures of neonatal rat cardiac cells were identified with the radioligand [ l25 I]iodohydroxybenzylpindolol ([ 125 I]IHYP). At the time of cell plating, a differential attachment procedure was employed to separate myocardial (M) cells from fibroblast-like (F) cells. After 3-4 days, the cultures enriched in M cells were still more than 80% pure and the cultures enriched in F cells were more than 95% pure. For binding studies, confluent cell layers were nonenzymatically detached as single cell suspensions. Both M and F cells contained a limited number of fiadrenergic receptors (M, 7600 2100 sites/cell; F, 9000 2400 sites/cell) which had very high affinity (M, K d = 88 33 PM; F, Kd = 71 23 pM) for [ 125 I]IHYP. For each cell type, the binding sites were stereoselective for the /-isomers of agonists and antagonists. Further binding studies on the relative potency of /8-agonists showed that the /{-receptors in M cells could be subclassified as fii (isoproterenol > epinephrine = norepinephrine), whereas the /{-receptors in F cells were more typical of /} 2 (isoproterenol > epinephrine > norepinephrine). The order of potency of these catecholamines in stimulating the adenylate cyclase activity in M and F cells was consistent with the order of potency observed in binding studies. Practolol, a /?i inhibitor, was 30 times more effective as a competitor of [ 125 I]IHYP binding in M cells than in F cells. It thus appears that, whereas M and F cells share a similar number of receptors per cell and similar affinities for [ 125 I]IHYP, the receptors in the two cell types nevertheless can be distinguished on the basis of their subclassification. These results also emphasize the importance of obtaining a homogeneous cell population for studies of the /{-adrenergic receptor in cultured cardiac tissue. Circ Res 47: 41-48, 1980