The effects of several Ca2+channel blockers were evaluated on inhibitory junction potential (IJP) evoked in rat colonic circular muscle by electrical field stimulation (EFS). Glass microelecrodes were used to record membrane potential of smooth muscle cells. IJPs were tetrodotoxin-sensitive (1 μM) and disappeared in Ca2+-free solution. L-type calcium channels blockers, such as nifedipine (1 μM) or verapamil (1 μM), did not affect IJPs. IJPs were significantly reduced by ω-conotoxin GVIA (300 nM), an N-type Ca2+channel blocker. IJPs were resistant to ω-agatoxin IVA (50 nM), a P-type Ca2+channel blocker, and ω-conotoxin MVIIC (1 μM), which blocks both N-and Q-type Ca2+channels at micromolar concentrations. We conclude that the release of NANC neurotransmitter-mediating IJPs in the rat colon evoked by EFS involves N-type Ca2+channels. The fact that ω-conotoxin GVIA does not abolish the IJPs suggests a putative role for L-, P-or Q-type Ca2+channels.
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