On the mechanism of ?-adrenergic regulation of the Ca channel in the guinea-pig heart

Abstract

Dose-response relations for the increase in the amplitude of Ca current (I _Ca) on external application of isoprenaline (ISP) and internally applied cyclic AMP (cAMP) or catalytic subunit of cAMP-dependent protein kinase (C subunit) were established in single ventricular cells of the guinea pig. An intracellular dialysis technique was used. The threshold concentration was for ISP 10⁻⁹ M, for cAMP 3 μM (pipette concentration to which 10⁻⁵ M 3-isobutyl-1-methylxanthine was added) and for C subunit around 0.4 μM (pipette concentration). The concentrations for the half-maximal effect were 3.7×10⁻⁸ M (ISP), 5.0 μM (cAMP) and 0.95 μM (C subunit) and for the maximum effect 10⁻⁶ M (ISP), 15–20 μM (cAMP) and 3–4 μM (C subunit). For all three agents the maximum increase in the Ca current density was similar (a factor of 3–4), suggesting that they converge on the same site of the Ca channel. Accordingly, the effects of cAMP and C subunit onI _Ca were non-additive to those of ISP. From these data the relationship both between concentrations of ISP and cAMP and between those of cAMP and active C subunit in terms of their effects onI _Ca could be estimated and were compared with those obtained in broken cell preparations. A competitive inhibitor of phosphorylation, 5′-adenylyl-imidodiphosphate (5 mM), greatly reduced the effects of ISP and C subunit onI _Ca. Cell dialysis with 3 mM adenosine-5′-(γ-thio)-triphosphate, which produces a dephosphorylationresistant phosphorylation, markedly potentiated the effects of ISP and cAMP onI _Ca. The results support the hypothesis that phosphorylation of a protein within, or close to, the Ca channel by cAMP-dependent protein kinase is the mechanism of β-adrenergic stimulation.