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

Mibefradil is a tetralol derivative that inhibits cloned and native T-type voltage-operated calcium channels (VOCCs) at an IC50, of 1 muM and with an apparent 15-fold selectivity for T- over L-type VOCCs. Recent electrophysio logical studies in. Xenopus oocytes, and inhibition of noradrenaline releas e studies, in human isolated right atria concluded that mibefradil (01.3-3 muM) can block N-type VOCCs like omega -conotoxin GVIA. We tested this hypo thesis in rat and guinea pig isolated driven left atria and rat vas deferen s in response to sympathetic nerve stimulation. Mibefradil (3 muM) did not inhibit the inotropic responses to sympathetic electrical field stimulation in atria. In contrast, these responses were blocked by GVIA (10 muM) and t etrodotoxin (TTX 0.1 muM), In rat vas deferens, pretreated with benextramin e (BNX 10 muM), contractions considered to be due to transmitter ATP evoked by electrical field stimulation were blocked by: (a) TTX 0.1 muM, (b) P-2x receptor desensitisation (alpha beta -MeATP 100 muM), and (c) GVIA (pIC(50 ) 9.04 +/-0.15, n=5). In contrast, mibefradil (0.3-30 RM) had no effect on these N-type VOCC-sensitive sympathetic nerve responses. Potassium (62 mM K +) and alpha beta -MeATP-induced contractions were unaffected by GVIA (10 n M), but mibefradil inhibited the potassium-induced contractions with a pIC( 50) 6.02 +/-0.08 (n=5), consistent with a TL and L-type VOCC blocking actio n on the postjunctional smooth muscle. We conclude that mibefradil up to 30 muM 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 submicr omolar and micromolar concentrations in sympathetic nerve-mediated response s.