β-Adrenoceptor and adenylate cyclase regulation in cardiac myocyte growth

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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 Kd 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 Bmax 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.

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