Role of N-, P- and Q-type voltage-gated calcium channels in transmitter release from sympathetic neurones in the mouse isolated vas deferens

1 N-type voltage-gated calcium channels are known to play an important role in transmitter release from autonomic neurones, and recent studies have demonstrated that non-N-type calcium channels are also involved. The calcium channels coupled to transmitter release from sympathetic neurones in the mouse isolated vas deferens were investigated in the present study. 2 Contractions of the mouse vas deferens were evoked by electrical stimulation at 1-50 Hz. The contractions were entirely nerve-mediated, since they were abolished by tetrodotoxin, and were used as an indirect measure of transmitter release. 3 The N-type calcium channel blocker, omega-conotoxin GVIA, inhibited contractions in a concentration-dependent manner, with a maximal effect at 30 nM. Contractions evoked by stimulation frequencies less than 10 Hz were abolished, and those evoked by 20 and by 50 Hz stimulation were decreased in amplitude by 51.3+/-13.9% and 9.3+/-2.6%, respectively. 4 The N-, P- and Q-type channel blocker, omega-conotoxin MVIIC, inhibited contractions in a concentration-dependent manner and caused greater maximum inhibition than omega-conotoxin GVIA, suggesting an action on P- and/or Q-type channels, in addition to N-type. 5 The P-type channel blocker, omega-agatoxin IVA, alone did not have a significant effect at concentrations up to 300 nM, but inhibited contractions in the presence of omega-conotoxin GVIA. Subsequent addition of omega-conotoxin MVIIC abolished the remaining contractions. Identical results were obtained when the three toxins were tested cumulatively on the purinergic and noradrenergic components of the contraction in the presence of 0.3 mu M prazosin and following desensitization to 10 mu M alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-MeATP), respectively. 6 The results suggest that N-, P- and Q-type channels are involved in the release of noradrenaline and ATP from sympathetic neurones in the mouse vas deferens.