Voltage-gated calcium channel regulation of aortic baroreceptor exocytosis

In aortic baroreceptor neuronal cell bodies, the channel responsible for 70% of the total calcium current has been shown to be a N-type, omega-conotoxin GVIA (w-CgTx) sensitive calcium channel. To examine the role of the L- and N-type calcium channels in vesicle exocytosis from aortic baroreceptor neurons, we examined the effects of nifedipine (5.0 uM) and w-CgTx (2.0 uM) on 90 mM KCl depolarization induced exocytosis. Fluorescently labeled aortic baroreceptor neurons were isolated from 14-day old rat nodose ganglia and cultured for 7-10 days prior to exocytosis measurements. The fluorescent membrane label FM1-43 was used to measure synaptic terminal exocytosis. FMI-43 loaded terminals were depolarized by 90 mM KCl in the absence and presence of 5.0 uM nifedpine. Application of nifedipine had no effect on the rate or extent of vesicle exocytosis (24 regions in 3 cultures). To determine the role of the N-type calcium channel, FM1-43 measured exocytosis was determined in the absence and presence of 2.0 uM w-CgTx. Application of w-CgTx had no significant effect on the initial rate or extent of vesicle exocytosis recorded in the first 60 seconds of destaining. However, w-CgTx did significantly inhibit the late phase of destaining which occurred following 80 seconds of depolarization (control = 85+9% vs 56+ 5% with w-CgTx, n=9, P < .05). These preliminary results may suggest that the w-CgTx insensitive current may be enough to release the docked vesicle pool and the N-type channels may be essential for refilling of the readily releasable synaptic vesicle pool.