Pharmacological studies of voltage-sensitive Ca²⁺-channels involved in the release of vasoactive intestinal peptide evoked by K⁺ in mouse cerebral cortical slices

Three types of voltage-sensitive Ca²⁺-channels, denominated T, N and L, have recently been identified in the nervous system. This classification is based on both the electrophysiological and pharmacological properties of each type of channel. The increase in free intracellular Ca²⁺ concentration that results from the opening of voltage-sensitive Ca²⁺-channels triggers various cellular processes. One such process is the depolarization-induced release of neurotransmitters. Owing to the rather selective sensitivity of each type of voltage-sensitive Ca²⁺-channel to certain antagonists, attempts have recently been carried out to determine which of the T, N or L channels mediates neurotransmitter release. In the present study we have applied such a strategy to the study of the release of vasoactive intestinal peptide from mouse cerebral cortical slices. The release of vasoactive intestinal peptide evoked by K⁺ 20 mM is inhibited in a concentration-dependent manner by Co²⁺, Ni²⁺ and Mn²⁺ while it remains unaffected by diltiazem 20μM, nifedipine 10μM, ω-conotoxin 1 μM and Cd²⁺ up to 100μM. Such a pharmacological profile indicates that voltage-sensitive Ca²⁺-channels of the N and L types are not involved in the release of vasoactive intestinal peptide. These observations are in contrast to what has been described for amine release in the central nervous system, which appears to be mediated by voltage-sensitive Ca²⁺-channels of the N type. Other pharmacological characteristics of the K⁺-evoked release of vasoactive intestinal peptide are as follows: it is insensitive to tetrodotoxin (2 μM) and it is enhanced by K⁺-channel blockers such as Cs⁺ (1 mM), Ba²⁺ (1 mM) and tetraethylammonium (10 mM). In addition to K⁺, veratridine, an Na⁺-channel agonist, and ouabain, a blocker of the Na⁺/K⁺-ATPase, promote the release of vasoactive intestinal peptide (18-and 4-fold increase over basal level for veratridine 50 μM and ouabain 100/μM respectively). The release of vasoactive intestinal peptide evoked by veratridine 50 μM is completely blocked by tetrodotoxin 2 μM, whereas the stimulatory effect of ouabain is inhibited in a concentration-dependent manner (between 0.1 and 5mM) by amiloride, a blocker of the Na⁺/H⁺ exchange system.