CAMP-DEPENDENT PHOSPHORYLATION OF THE CARDIAC L-TYPE CA CHANNEL - A MISSING LINK

Cardiac inotropic effects of beta adrenergic agonists occur mainly thr ough an increase in L-type (class C) calcium channel activity. This re sponse has been attributed to phosphorylation of the L-type Ca channel , or a closely associated protein, by the cAMP-dependent protein kinas e A (PKA). Among the three subunits forming the cardiac L-type Ca chan nel (alpha 1, beta and alpha 2-delta), biochemical studies have reveal ed that two subunits, alpha 1 and beta, are phosphorylated in vitro by protein kinase A, the alpha 1 subunit being the primary target. Howev er, attempts to reconstitute the cAMP-dependent regulation of the expr essed class C Ca channel, either in Xenopus oocytes or in cell lines, have provided contradictory results, We were unable to detect cAMP-dep endent modulation of class C alpha 1 subunit Ca channels expressed in Xenopus oocytes, even when coinjected with auxiliary subunits beta and alpha 2-delta. Nevertheless, activity of Ca channels recorded from ca rdiac-mRNA injected oocytes was potentiated by injection of cAMP or PK A, even when expression of the beta subunit was suppressed using antis ense oligonucleotide. Taken together, these results indicate that cAMP -dependent regulation does not exclusively involve the alpha 1 and the beta subunits of the Ca channel and suggest that unidentified protein (s), expressed in cardiac tissue, are most likely necessary.