Abstract
The effects of nifedipine, omega-conotoxin GVIA (omega-CgTx) and omega-agatoxin IVA (omega-AgTx) on Ca2+ currents, a 60-mM-K+-induced increase in intracellular Ca2+ concentration (-Ca2+-i) and catecholamine secretion were examined to clarify the subtypes of Ca2+ channels in cultured adrenal chromaffin cells from the pig. Nifedipine, omega-CgTx, and omega-AgTx inhibited Ca2+ currents in a dose-dependent manner, suggesting the presence of L-, N- and P-type Ca2+ channels. The maximal doses of nifedipine (10 microM), omega-CgTx (1 microM), and omega-AgTx (0.1 microM) inhibited Ca2+ currents to 85%, 22%, and 94% of control currents, respectively. The inhibitory effects of these three blockers were observed in the same cell, indicating that at least three subtypes of Ca2+ channels are present in porcine chromaffin cells. The increase in -Ca2+-i and catecholamine secretion induced by 60 mM K+ were inhibited equally by nifedipine (10 microM) and omega-CgTx (1 microM), but not by omega-AgTx (0.1 microM). These results suggest that L-, N- and P-type Ca2+ channels are present in porcine adrenal chromaffin cells, and that the major pathways of Ca2+ entry evoked by a high concentration of K+ are L- and N-type Ca2+ channels.
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