ON THE REGULATION OF THE EXPRESSED L-TYPE CALCIUM-CHANNEL BY CAMP-DEPENDENT PHOSPHORYLATION

The Ca2+ channel subunits alpha(1C-a) and alpha(1C-b) were stably expressed in Chinese hamster ovary (CHO) and human embryonic kidney (HEK) 293 cells. The peak Ba2+ current (I-Ba) of these cells was not affected significantly by internal dialysis with 0.1 mM cAMP-dependent protein kinase inhibitor peptide (mPKI), 25 mu M cAMP-dependent protein kinase catalytic subunit (PKA), or a combination of 25 mu M PKA and 1 mu M okadaic acid. The activity of the alpha(1C-b) channel subunit expressed stably in HEK 293 cells was depressed by 1 mu M H 89 and was not increased by superfusion with 5 mu M forskolin plus 20 mu M isobutyl-methylxanthine (IBMX). The alpha(1C-b), beta(2), alpha(2)/delta complex was transiently expressed in HEK 293 cells; it was inhibited by internal dialysis of the cells with 1 mu M H 89, but was not affected by internal dialysis with mPKI, PKA or microcystin. Internal dialysis of cells expressing the alpha(1C-a), beta(2), alpha(2)/delta channel with 10 mu M PKA did not induce facilitation after a 150-ms prepulse to +50 mV. The Ca2+ current (I-Ca) of cardiac myocytes increased threefold during internal dialysis with 5 mu M forskolin plus 50 mu M IBMX. These results indicate that the L-type Ca2+ channel expressed is not modulated by cAMP-dependent phosphorylation to the same extent as in native cardiac myocytes.