The involvement of Ser1898 of the human l-type calcium channel in evoked secretion

A PKA consensus phosphorylation site S1928 at the 11.2 subunit of the rabbit cardiac L-type channel, Ca _V1.2, is involved in the regulation of Ca _V1.2 kinetics and affects catecholamine secretion. This mutation does not alter basal Ca _V1.2 current properties or regulation of Ca _V1.2 current by PKA and the beta-adrenergic receptor, but abolishes Ca _V1.2 phosphorylation by PKA. Here, we test the contribution of the corresponding PKA phosphorylation site of the human α ₁1.2 subunit S1898, to the regulation of catecholamine secretion in bovine chromaffin cells. Chromaffin cells were infected with a Semliki-Forest viral vector containing either the human wt or a mutated S1898A α ₁1.2 subunit. Both subunits harbor a T1036Y mutation conferring nifedipine insensitivity. Secretion evoked by depolarization in the presence of nifedipine was monitored by amperometry. Depolarization-triggered secretion in cells infected with either the wt α ₁1.2 or α ₁1.2/ S1898A mutated subunit was elevated to a similar extent by forskolin. Forskolin, known to directly activate adenylyl-cyclase, increased the rate of secretion in a manner that is largely independent of the presence of S1898. Our results are consistent with the involvement of additional PKA regulatory site(s) at the C-tail of α ₁1.2, the pore forming subunit of Ca _V1.2.