ω-Agatoxin-IVA-sensitive calcium channels in bovine chromaffin cells

Abstract

A large component of the whole-cell currents through Ca 2+ channels in bovine adrenomedullary chromaffin cells has been shown to be insensitive to both L-type and N-type Ca 2+ channel Mockers, suggesting the existence of a third type of Ca 2+ channel. In the present paper, ω-agatoxin-IVA (AgTx), a selective blocker of P-type Ca 2+ channels in mammalian neurons, has been used to investigate the presence of this subtype of Ca 2+ channel in bovine chromaffin cells. Barium currents ( I Ba) through Ca 2+ channels were recorded in whole-cell patch-clamped bovine chromaffin cells. I Ba was blocked by AgTx in a dose-dependent and irreversible manner. At the maximal concentration used (1 μM), AgTx inhibited T Ba by 49.5 ± 3%. Such a blockade was also present when bovine chromaffin cells were pretreated with 10 μM fumidipine, a novel 1,4-dihydropyridine L-type channel blocker, and after treatment with 1 μM of the N-type channel blocker, ω-conotoxin GVIA (CgTx). A combination of these three types of Ca 2+ channel blockers suppressed the macroscopic Ba 2+ currents by 88%. We conclude that bovine chromaffin cells, in addition to N- and L-type Ca 2+ channels, possess a P-like component in their whole-cell currents through the Ca 2+ channels.