Q-TYPE AND L-TYPE CA2+ CHANNELS DOMINATE THE CONTROL OF SECRETION IN BOVINE CHROMAFFIN CELLS

Potassium-stimulated catecholamine release from superfused bovine adrenal chromaffin cells (70 mM K+ in the presence of 2 mM Ca2+ for 10 s, applied at 5-min intervals) was inhibited by the dihydropyridine furnidipine (3 mu M) by 50%. omega-Conotoxin MVIIC (CTx-MVIIC, 3 mu M) also reduced the secretory response by about half. Combined CTx-MVIIC plus furnidipine blocked 100% catecholamine release. Ca-45(2+) uptake and cytosolic Ca2+ concentrations ([Ca2+](i)) in K+-depolarized cells were partially blocked by furnidipine or CTx-MVIIC, and completely inhibited by both agents. The whole cell current through Ca2+ channels carried by Ba2+ (I-Ba) was partially blocked by CTx-MVIIC. Although omega-conotoxin GVIA (CTx-GVIA, 1 mu M) and omega-agatoxin IVA (Aga-IVA, 0.2 mu M) partially inhibited Ca-45(2+) entry, I-Ba and the increase in [Ca2+](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 Ca2+ channel which has a prominent role in controlling exocytosis. They also suggest that Q- and L-type Ca2+ channels, but not N- or P-types are localized near exocytotic active sites in the plasmalemma.