Calcium channels at the adrenergic neuroeffector junction in the rabbit ear artery

Summary Neurotransmitter release is dependent on influx of Ca²⁺ through voltage-operated calcium channels (VOCCs). These channels may be divided into L, N, T and P subtypes. To investigate the subtypes of VOCC involved in transmitter release from adrenergic nerves in the isolated rabbit ear artery, the effects of some subtype selective VOCC antagonists were examined on contractile responses induced by electrical field stimulation (EFS), and exposure to an isosmolar (low Na⁺, normal Cl^– content) or a hyperosmolar (normal Na^]+, high Cl^– or 60 mM K+ solution). Tetrodotoxin (TTX) and the L channel blocker nimodipine were present in the latter experiments to inhibit sodium-dependent action potential discharge and the direct contractile effect of K⁺ depolarization on the smooth muscle cells. Prazosin abolished the contractile effect of EFS, indicating that the response was elicited by activation of adrenergic nerves. The EFS-induced contractions were concentration-dependently inhibited by the N channel blocker -conotoxin (PIC₅₀ = 9.0) and the proposed L channel blocker T-cadinol (pIC₅₀ = 4.5), while nimodipine and the T channel blocker tetramethrin had no effect. The isosmolar and hyperosmolar K⁺ solutions induced a prazosin-sensitive contraction, amounting to 46% and 10% of the response to 10^–5 M noradrenaline (NA), respectively. -Conotoxin inhibited the contractile response to the hyperosmolar K⁺ solution, but not that to the isosmolar K⁺ solution. T-cadinol preferentially inhibited the response to the hyperosmolar K⁺ solution. Tetramethrin had no effect on contractions induced by either type of K⁺ solution. The contractile response to exogenous NA was unaffected by -conotoxin and tetramethrin, whereas the response was partially inhibited by both nimodipine and T-cadinol. These results suggest that NA release from adrenergic nerves in the rabbit ear artery, depend on Ca²⁺ influx through VOCCs of the N type, whereas L and T channels seem to be of minor importance. Calcium influx into the nerve terminals via a tentative Na⁺/Ca²⁺ exchange mechanism may explain the failure of -conotoxin to inhibit the adrenergic response to the isosmolar K⁺ solution.Correspondence to P. Zygmunt at the above address