DOUBLE-PULSE FACILITATION OF SMOOTH-MUSCLE ALPHA-1-SUBUNIT CA2+ CHANNELS EXPRESSED IN CHO CELLS

Frequent strong depolarizations facilitate Ca2+ channels in various cell types by shifting their gating behavior towards mode 2, which is characterized by long openings and high probability of being open. In cardiac cells, the same type of gating behavior is potentiated by beta-adrenoceptors presumably acting via phosphorylation of a protein identical to or associated with the channel. Voltage-dependent phosphorylation has also been reported to underlie Ca2+ channel facilitation in chromaffin adrenal medulla and in skeletal muscle cells. We studied a possible voltage-dependent facilitation of the principal channel forming alpha(1)-subunit of the dihydropyridine-sensitive smooth muscle Ca2+ channel. Single channel and whole-cell Ca2+ currents were recorded in Chinese hamster ovary cells stably expressing the class C-b Ca2+ channel alpha(1)-subunit. Strong depolarizing voltage-clamp steps preceding the test pulse resulted in a 2- to 3-fold increase of the single Ca2+ channel activity and induction of mode 2-like gating behavior. Accordingly we observed a significant potentiation of the whole-cell current by similar to 50%. In contrast to the previous suggestions we found no experimental evidence for involvement of channel phosphorylation by protein kinases (cAMP-dependent protein kinase, protein kinase C and other protein kinases utilizing ATP gamma S) in the control and facilitated current. The data demonstrate that the L-type Ca2+ channel alpha(1)-subunit solely expressed in Chinese hamster ovary cells is subject to a voltage-dependent facilitation but not to phosphorylation. We suggest that this newly identified type of voltage-dependent facilitation of Ca2+ channels is due to a direct voltage-dependent conformational change inducing the same type of gating behavior as otherwise induced by phosphorylation.