Q- and L-type Ca 2+ channels dominate the control of secretion in bovine chromaffin cells

Abstract

Potassium-stimulated catecholamine release from superfused bovine adrenal chromaffin cells (70 mM K + in the presence of 2 mM Ca 2+ for 10 s, applied at 5-min intervals) was inhibited by the dihydropyridine furnidipine (3 μM) by 50%. ω-Conotoxin MVIIC (CTx-MVIIC, 3 μM) also reduced the secretory response by about half. Combined CTx-MVIIC plus furnidipine blocked 100% catecholamine release. 45Ca 2+ uptake and cytosolic Ca 2+ concentrations ([Ca 2+] i) in K +-depolarized cells were partially blocked by furnidipine or CTx-MVIIC, and completely inhibited by both agents. The whole cell current through Ca 2+ channels carried by Ba 2+ ( I Ba) was partially blocked by CTx-MVIIC. Although ω-conotoxin GVIA (CTx-GVIA, 1 μM) and ω-agatoxin IVA (Aga-IVA, 0.2 μM) partially inhibited 45Ca 2+ entry, I Ba and the increase in [Ca 2+] i, the combination of both toxins did not affect the K +-evoked secretory response. The results are compatible with the presence in bovine chromaffin cells of a Q-like Ca 2+ channel which has a prominent role in controlling exocytosis. They also suggest that Q- and L-type Ca 2+ channels, but not N- or P-types are localized near exocytotic active sites in the plasmalemma.