2 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 f rom 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-Agatoxi n IVA caused a biphasic dose-dependent inhibition of secretion with a high-potency component (IC50<1 nM), responsible for 10-15% of catechol amine release evoked by 70 mM K+, and a low-potency component that acc ounted for about 40% of release, with IC50 values of 57 nM and 48 nM f or noradrenaline and adrenaline release, respectively. The release of catecholamines from chromaffin cells was also inhibited dose dependent ly by omega-conotoxin MVIIC with IC50 values of 182 and 218 nM for nor adrenaline and adrenaline release, respectively. The effects of 3 nM o mega-agatoxin IVA and 3 mu M omega-conotoxin MVIIC were additive, indi cating that at the concentrations used the toxins were acting at indep endent sites, presumably, P-and Q-type Ca2+ channels. The blockade of Q-type channels inhibited the release of adrenaline (72 +/- 4.1%) sign ificantly more than the release of noradrenaline (50 +/- 2.7%), sugges ting a higher density or a closer coupling of these channels to exocyt osis in adrenergic chromaffin cells. The blockade of P-type channels c aused a greater inhibition of catecholamine secretion at low levels of K+-evoked depolarization and shorter times of stimulation than that o bserved at higher levels of stimulation. The contribution of Q-type ch annels to catecholamine secretion did not change significantly with th e intensity of stimulation. The data show that two types of omega-agat oxin IVA-sensitive Ca2+ channels are coupled to catecholamine release in chromaffin cells, and that the contribution of P-type channels to s ecretion is larger at low levels of depolarization.