Abstract
The effects of several Ca 2+ 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 Ca 2+-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 Ca 2+ channel blocker. IJPs were resistant to ω-agatoxin IVA (50 nM), a P-type Ca 2+ channel blocker, and ω-conotoxin MVIIC (1 μM), which blocks both N- and Q-type Ca 2+ channels at micromolar concentrations. We conclude that the release of NANC neurotransmitter-mediating IJPs in the rat colon evoked by EFS involves N-type Ca 2+ channels. The fact that ω-conotoxin GVIA does not abolish the IJPs suggests a putative role for L-, P- or Q-type Ca 2+ channels.
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