β-Adrenoceptor and adenylate cyclase regulation in cardiac myocyte growth

Abstract

We studied the effect of growth on β-adrenergic receptor properties of neonatal rat heart myocytes cultured in serum-free medium with transferrin and insulin. Growth was induced by addition of 1 μM (−)-norepinephrine for two days, 200 nM of the tumor-promoting phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) for two days, or 30 nM T₃ for six days. The K_d values for β-receptor binding (¹²⁵I-ICYP) were unaffected by growth. The maximum number of β-receptor binding sites calculated as sites/cell was increased 1.47-fold by T₃ (p<.005), but was decreased to 54% of control values by (−)-norepinephrine (p<.005); TPA had no effect on either Kd or B_max values. (−)-Isoproterenol-stimulated adenylate cyclase activity was augmented only in membranes from T₃-treated cells and was reduced by 69% in membranes from (−)-norepinephrine treated cells. TPA had no effect on (−)-isoproterenol-stimulated adenylate cyclase activity. We conclude that the mechanisms controlling β-adrenergic receptor number may be distinct from those controlling growth, since receptor number does not correlate with cell enlargement. Furthermore, in (−)-norepinephrine-stimulated growth, which we have shown previously is an α₁-adrenoceptor mediated response, β-adrenergic signal transduction is modulated in a directionally opposite fashion.