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

Abstract

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 Mockers, suggesting the existence of a third type of Ca²⁺ channel. In the present paper, ω‐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 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 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.