ω-agatoxin IVA blocks nicotinic receptor channels in bovine chromaffin cells

We have studied the contribution of P-type voltage-dependent Ca²⁺ channels to both catacholamine (CA) and ATP secretion from bovine chromaffin cells induced by high K⁺ or nicotine using ω-agatoxin IVA, a selective blocker of P-type voltage-dependent Ca²⁺ channels. We found that high K⁺ (75 mM) induced the release of about 13% of norepinephrine, 5% epinephrine and 11% ATP, and that ω-agatoxin (100 nM) did not affect this secretion. However, both nicotine-induced CA and ATP secretion were significantly blocked (about 50%) by ω-agatoxin IVA (100 nM). In addition, this toxin also reversibly blocked (about 70%) the inward current induced by nicotine in bovine chromaffin cells. The results suggest that, besides its known action of blocking P-type voltage-dependent channels, ω-agatoxin is a potent and reversible blocker of the nicotinic receptor channel in chromaffin cells, and that this action would explain the blockade of nicotine-induced secretion.