LACK OF INVOLVEMENT OF PROTEIN-KINASE-A PHOSPHORYLATION IN VOLTAGE-DEPENDENT FACILITATION OF THE ACTIVITY OF HUMAN CARDIAC L-TYPE CALCIUM CHANNELS

Phosphorylation by protein kinase A is thought to be involved in volta ge-dependent facilitation of calcium channels. Here we have shown that the subunit complex of a cloned human cardiac calcium channel, expres sed in Xenopus oocytes, responds to voltage-dependent facilitation by an approximately 50% increase of the calcium channel peak current. The removal of all protein kinase A consensus sequences by site-directed mutagenesis decreased but did not eliminate the response to prepulse f acilitation. Moreover, Rp-cAMP-S, an inhibitor of protein kinase A, co uld not prevent facilitation of the mild-type calcium channel currents . Similarly, AMP-PNP a nonhydrolyzable analog of ATP, while significan tly decreasing the whole-cell current amplitude, failed to reduce the response to double-pulse facilitation. Therefore, we conclude that the voltage-dependent facilitation of cloned calcium channel currents is not due to enhancement of phosphorylation, but probably to some type o f voltage-induced conformational change in the channel. (C) 1996 Acade mic Press, Inc.