OMEGA-CONOTOXINS BLOCK NEUROTRANSMISSION IN THE RAT VAS-DEFERENS BY BINDING TO DIFFERENT PRESYNAPTIC SITES ON THE N-TYPE CA2+ CHANNEL

Electrically-induced twitch responses of the prostatic segment of vas deferens (0.1 Hz, 65 V, 1 ms) are mainly due to the transient presynap tic release of ATP, which acts postsynaptically on non-adrenergic rece ptors to contract smooth muscle cells. These responses were fully bloc ked by nanomolar concentrations of the omega-conotoxins GVIA, MVIIA, a nd MVIIC, most likely by inhibiting Ca2+ entry through presynaptic N-t ype Ca2+ channels controlling the release of ATP. Repeated washout of the toxins allowed the recovery of contractions, except for omega-cono toxin GVIA, whose inhibitory effects remained unchanged for at least 6 0 min. In addition, micromolar concentrations of omega-conotoxin MVIIC were unable to protect against the irreversible inhibition of twitch contractions induced by nanomolar concentrations of omega-conotoxin GV IA, At low extracellular Ca2+ concentrations (1.5 mM), 20 nM of omega- conotoxin GVIA or MVIIA inhibited completely the twitch contractions i n about 10 min. In 5 mM Ca2+ the blockade of twitch contractions after 10 min was 70% for both toxins. In 1.5 mM Ca2+ omega-conotoxin MVIIC (1 mu M) inhibited completely the twitch contraction after 10 min. In 5 mM Ca2+ blockade developed very slowly and was very poor after 30 mi n, omega-conotoxin MVIIC depressed the response by only 20%. These res ults are compatible with the idea that the three omega-conotoxins bloc k the purinergic neurotransmission of the vas deferens by acting on pr esynaptic N-type voltage-dependent Ca2+ channels. However, omega-conot oxin MVIIC seems to bind to sites different from those recognised by o mega-conotoxin GVIA and MVIIA, which are markedly differentiated by th eir Ca2+ requirements for binding to their receptors.