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.
Baltazar
G
G
Center for Neurosciences of Coimbra, Portugal.
Ladeira
I
I
Carvalho
A P
AP
Duarte
E P
EP
D000311:Adrenal Glands / Q000166:cytology / Q000378:metabolism*; D000818:Animals; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D015220:Calcium Channels / Q000187:drug effects*; D002417:Cattle; D019439:Chromaffin Cells / Q000378:metabolism*; D004594:Electrophysiology; D004837:Epinephrine / Q000009:adverse effects / Q000378:metabolism*; D009638:Norepinephrine / Q000009:adverse effects / Q000378:metabolism*; D010455:Peptides / Q000494:pharmacology; D011188:Potassium / Q000502:physiology; D013111:Spider Venoms / Q000494:pharmacology*; D020867:omega-Agatoxin IVA; D020960:omega-Conotoxins
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D010455:Peptides; D013111:Spider Venoms; D020867:omega-Agatoxin IVA; D020960:omega-Conotoxins; C075851:omega-conotoxin-MVIIC; D011188:Potassium; D009638:Norepinephrine; D004837:Epinephrine