Evidence against an action of mibefradil at N-type voltage-operated calcium channels.

Abstract

Mibefradil is a tetralol derivative that inhibits cloned and native T-type voltage-operated calcium channels (VOCCs) at an IC50 of 1 microM and with an apparent 15-fold selectivity for T- over L-type VOCCs. Recent electrophysiological studies in Xenopus oocytes and inhibition of noradrenaline release studies in human isolated right atria concluded that mibefradil (0.3-3 microM) can block N-type VOCCs like omega-conotoxin GVIA. We tested this hypothesis in rat and guinea pig isolated driven left atria and rat vas deferens in response to sympathetic nerve stimulation. Mibefradil (3 microM) did not inhibit the inotropic responses to sympathetic electrical field stimulation in atria. In contrast, these responses were blocked by GVIA (10 nM) and tetrodotoxin (TTX 0.1 microM). In rat vas deferens, pretreated with benextramine (BNX 10 microM), contractions considered to be due to transmitter ATP evoked by electrical field stimulation were blocked by: (a) TTX 0.1 microM, (b) P2x receptor desensitisation (alphabeta-MeATP 100 microM), and (c) GVIA (pIC50 9.04+/-0.15, n=5). In contrast, mibefradil (0.3-30 microM) had no effect on these N-type VOCC-sensitive sympathetic nerve responses. Potassium (62 mM K+) and alphabeta-MeATP-induced contractions were unaffected by GVIA (10 nM), but mibefradil inhibited the potassium-induced contractions with a pIC50 6.02+/-0.08 (n=5), consistent with a T- and L-type VOCC blocking action on the postjunctional smooth muscle. We conclude that mibefradil up to 30 microM does not block N-type VOCCs in these isolated intact tissue assays of sympathetic nerve-muscle transmission. Therefore, at least in these tissues, mibefradil can be used to define T- and L-type VOCC activities at submicromolar and micromolar concentrations in sympathetic nerve-mediated responses.