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 Iiodohydroxybenzylpindolol (125 IIHYP). 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 IIHYP. 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 IIHYP 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 IIHYP, 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