Calcium channels involved in noradrenaline release from sympathetic neurones in rabbit carotid artery.

Abstract

Transmitter release from nerve terminals is dependent on the entry of Ca(2+) through neuronal voltage-gated calcium channels. In sympathetic neurones both N- and L-type calcium channels are present. Potassium channel blockade increases Ca(2+) entry into sympathetic neurones. We examined the participation of N- and L-type calcium channels in the stimulation-evoked release of noradrenaline from vascular sympathetic neurones. Rings of rabbit carotid artery were preincubated with [3H]-noradrenaline. Electrical field stimulation was used to evoke 3H overflow. The selective N-type calcium channel blocking agent omega-conotoxin GVIA (single concentrations: 3 x 10(-10)-10(-8) M) caused a slowly developing reduction of the stimulation-evoked 3H overflow. At 3 x 10(-8) M, omega-conotoxin GVIA caused an equilibrium block with a rapid (15 min.) onset. After 2 hr exposure to omega-conotoxin the inhibition was steady (pIC50 (-log M): 9.43; Emax: 91%). The selective L-type calcium blocking agents nifedipine (10(-7)-10(-5) M) and nimodipine (10(-8)-10(-5) M) had no effect on the stimulation-evoked 3H overflow. The calcium channel opener Bay K 8644 (10-6 M) likewise had no effect. The potassium channel blocking agent 4-aminopyridine (10-5-10-3 M) enhanced the stimulation-evoked 3H overflow up to 5 times. 4-Aminopyridine (10(-4) M) did not alter the inhibitory effect of omega-conotoxin GVIA (3 x 10(-8) M). In the presence of 4-aminopyridine (10(-4) M), nifedipine (10(-5) M) and nimodipine (10(-6) M) enhanced the 3H overflow. We conclude that the stimulation-evoked release of noradrenaline from sympathetic neurones in rabbit carotid artery is mediated by N-type calcium channels and that L-type channels are not involved even when potassium channels are blocked by 4-aminopyridine.