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 Ca2+ through neuronal voltage-gated calcium channels. In sympathetic neurones both N- and L-type calcium channels are present. Potassium channel blockade increases Ca2+ 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 ω-conotoxin GVIA (single concentrations: 3×10-10-10-8 M) caused a slowly developing reduction of the stimulation-evoked 3H overflow. At 3×10-8 M, ω-conotoxin GVIA caused an equilibrium block with a rapid (15 min.) onset. After 2 hr exposure to ω-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 ω-conotoxin GVIA (3×10-8 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.