Stimulation of unitary T-type Ca2+ channel currents by calmodulin-dependent protein kinase II

The effect of Ca2+/calmodulin-dependent protein kinase II (CaMKII) stimulation on unitary low voltage-activated (LVA) T-type Ca2+ channel currents in isolated bovine adrenal glomerulosa (AG) cells was measured using the patch-clamp technique. In cell-attached and inside-out patches, LVA channel activity was identified by voltage-dependent inactivation and a single-channel conductance of similar to9 pS in 110 mM BaCl2 or CaCl2. In the cell-attached patch, elevation of bath Ca2+ from 150 nM to 1 muM raised intracellular Ca2+ in K+-depolarized (140 mM) cells and evoked an increase in the LVA Ca2+ channel probability of opening (NPo) by two-to sixfold. This augmentation was associated with an increase in the number of nonblank sweeps, a rise in the frequency of channel opening in nonblank sweeps, and a 30% reduction in first latency. No apparent changes in the single-channel open-time distribution, burst lengths, or openings/burst were apparent. Preincubation of AG cells with lipophilic or peptide inhibitors of CaMKII in the cell-attached or excised (inside-out) configurations prevented the rise in NPo elicited by elevated Ca2+ concentration. Furthermore, administration of a mutant recombinant CaMKII alpha exhibiting cofactor-independent activity in the absence of elevated Ca2+ produced a threefold elevation in LVA channel NPo. These data indicate that CaMKII activity is both necessary and sufficient for LVA channel activation by Ca2+.