Q‐ and L‐type Ca2+ 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 Ca²⁺ 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. ⁴⁵Ca²⁺ 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 ω-conotoxin GVIA (CTx-GVIA, 1 μM) and ω-agatoxin IVA (Aga-IVA, 0.2 μM) partially inhibited ⁴⁵Ca²⁺ 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.