OMEGA-AGATOXIN-IVA-SENSITIVE CALCIUM CHANNELS IN BOVINE CHROMAFFIN CELLS

A large component of the whole-cell currents through Ca2+ channels in bovine adrenomedullary chromaffin cells has been shown to be insensitive to both L-type and N-type Ca2+ channel blockers, suggesting the existence of a third type of Ca2+ channel. In the present paper, omega-agatoxin-IVA (AgTx), a selective blocker of P-type Ca2+ channels in mammalian neurons, has been used to investigate the presence of this subtype of Ca2+ channel in bovine chromaffin cells. Barium currents (I(Ba)) through Ca2+ 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 muM), AgTx inhibited I(Ba) by 49.5 +/- 3%. Such a blockade was also present when bovine chromaffin cells were pretreated with 10 muM furnidipine, a novel 1,4-dihydropyridine L-type channel blocker, and after treatment with 1 muM of the N-type channel blocker, omega-conotoxin GVIA (CgTx). A combination of these three types of Ca2+ channel blockers suppressed the macroscopic Ba2+ currents by 88%. We conclude that bovine chromaffin cells, in addition to N- and L-type Ca2+ channels, possess a P-like component in their whole-cell currents through the Ca2+ channels.