Effect of cAMP-Dependent Protein Kinase on Catecholamine Secretion from Bovine Adrenal Chromaffin Cells

Abstract

We examined the role of cAMP-dependent protein kinase in Ca²⁺ -elicited catecholamine secretion from bovine adrenal chromaffin cells. When the digitonin-treated cells were incubated with the catalytic subunit of cAMP-dependent protein kinase, the secretion of catecholamines from the cells occurred in the absence of Ca²⁺. The effect of the catalytic subunit was dependent on its activity (50-100 units/ml) and the presence of ATP-Mg²⁺ in the incubation medium. However, incubation of the cells with the regulatory subunit of cAMP-dependent protein kinase did not affect the secretion. Ca²⁺ (43 nM -10μM) also increased the secretion, which was ATP-Mg²⁺ -dependent. The catalytic subunit (25-200 units/ml) enhanced the Ca²⁺ -evoked secretion at the suboptimal but not optimal Ca²⁺ concentration, which induced maximal secretion. A potent synthetic peptide inhibitor of cAMP-dependent protein kinase abolished the catalytic subunit-induced secretion, but not the Ca²⁺ -evoked secretion. On the other hand, K- 252a, a potent inhibitor of protein kinases, inhibited both the catalytic subunit-induced and the Ca²⁺ -evoked secretion, but not KT5823, a much less potent inhibitor of protein kinases. These results strongly suggest that the catalytic subunit of cAMP-dependent protein kinase produces the secretion of catecholamines via protein phosphorylation. The results further suggest that the cAMP-dependent protein kinase does not participate in an intrinsic process of Ca²⁺ -elicited secretion but it may act as a modulator.