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 presynaptic release of ATP, which acts postsynaptically on non-adrenergic receptors to contract smooth muscle cells. These responses were fully blocked by nanomolar concentrations of the omega-conotoxins GVIA, MVIIA, and MVIIC, most likely by inhibiting Ca2+ entry through presynaptic N-type Ca2+ channels controlling the release of ATP. Repeated washout of the toxins allowed the recovery of contractions, except for omega-conotoxin GVIA, whose inhibitory effects remained unchanged for at least 60 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 GVIA. At low extracellular Ca2+ concentrations (1.5 mM), 20 nM of omega-conotoxin GVIA or MVIIA inhibited completely the twitch contractions in 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 microM) inhibited completely the twitch contraction after 10 min. In 5 mM Ca2+ blockade developed very slowly and was very poor after 30 min, omega-conotoxin MVIIC depressed the response by only 20%. These results are compatible with the idea that the three omega-conotoxins block the purinergic neurotransmission of the vas deferens by acting on presynaptic N-type voltage-dependent Ca2+ channels. However, omega-conotoxin MVIIC seems to bind to sites different from those recognised by omega-conotoxin GVIA and MVIIA, which are markedly differentiated by their Ca2+ requirements for binding to their receptors.
Hirata
H
H
Departamento de Farmacología, Facultad de Medicina, Universidad Autónoma de Madrid, Spain.
Albillos
A
A
Fernández
F
F
Medrano
J
J
Jurkiewicz
A
A
García
A G
AG
D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism; D002121:Calcium Channel Blockers / Q000378:metabolism / Q000633:toxicity*; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism*; D004558:Electric Stimulation; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects; D009498:Neurotoxins / Q000378:metabolism* / Q000633:toxicity*; D010455:Peptides / Q000378:metabolism / Q000633:toxicity; D051381:Rats; D017208:Rats, Wistar; D013111:Spider Venoms / Q000378:metabolism / Q000633:toxicity; D009435:Synaptic Transmission / Q000187:drug effects*; D014649:Vas Deferens / Q000187:drug effects / Q000294:innervation* / Q000378:metabolism; D020867:omega-Agatoxin IVA; D020866:omega-Conotoxin GVIA; D020960:omega-Conotoxins
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D009498:Neurotoxins; D010455:Peptides; D013111:Spider Venoms; D020867:omega-Agatoxin IVA; D020960:omega-Conotoxins; C075851:omega-conotoxin-MVIIC; C078452:ziconotide; D020866:omega-Conotoxin GVIA; D002118:Calcium