Two types of omega-agatoxin IVA-sensitive Ca2+ channels are coupled to adrenaline and noradrenaline release in bovine adrenal chromaffin cells.

To clarify the role of P-type Ca2+ channels in catecholamine release from adrenal chromaffin cells we examined the concentration dependence of the effect of omega-agatoxin IVA on the release both of adrenaline and noradrenaline induced by a K(+)-evoked depolarization. omega-Agatoxin IVA caused a biphasic dose-dependent inhibition of secretion with a high-potency component (IC50 < 1 nM), responsible for 10-15% of catecholamine release evoked by 70 mM K+, and a low-potency component that accounted for about 40% of release, with IC50 values of 57 nM and 48 nM for noradrenaline and adrenaline release, respectively. The release of catecholamines from chromaffin cells was also inhibited dose dependently by omega-conotoxin MVIIC with IC50 values of 182 and 218 nM for noradrenaline and adrenaline release, respectively. The effects of 3 nM omega-agatoxin IVA and 3 microM omega-conotoxin MVIIC were additive, indicating that at the concentrations used the toxins were acting at independent sites, presumably, P- and Q-type Ca2+ channels. The blockade of Q-type channels inhibited the release of adrenaline (72 +/- 4.1%) significantly more than the release of noradrenaline (50 +/- 2.7%), suggesting a higher density or a closer coupling of these channels to exocytosis in adrenergic chromaffin cells. The blockade of P-type channels caused a greater inhibition of catecholamine secretion at low levels of K(+)-evoked depolarization and shorter times of stimulation than that observed at higher levels of stimulation. The contribution of Q-type channels to catecholamine secretion did not change significantly with the intensity of stimulation. The data show that two types of omega-agatoxin IVA-sensitive Ca2+ channels are coupled to catecholamine release in chromaffin cells, and that the contribution of P-type channels to secretion is larger at low levels of depolarization.