Are different gap junctions involved in neuromuscular interaction in mouse proximal colon?

Abstract

Aims: Gap junctions have been observed using electron microscopy between circular muscle cells of the circular muscle of colon in the mammalian gastrointestinal tract. They are also present between interstitial cells of Cajal (ICC) in the myenteric plexus of the gastrointestinal tract. Gap junctions furthermore couple ICCs to smooth muscle and may transmit nerve activity, but no functional evidence exists. Aim: We tested the hypothesis that gap junctions are involved in the neuromuscular interaction in mouse colon by using different types of gap junction blockers, namely carbenoxolone, heptanol, octanol and GAP–27. Methods: Intracellular recordings of smooth muscle cells of the circular muscle of murine colon, following electrical neural stimulation, were performed using standard microelectrode techniques. Results: Electrical field stimulation (EFS) of the myenteric plexus elicited TTX-sensitive and hexamethonium independent inhibitory junction potentials (IJPs) in circular smooth muscle cells which can be differentiated in a fast (fIJP) and a slow (sIJP) component. The IJPs and the resting membrane potential (RMP) showed different changes to incubation with the different gap junction blockers. Carbenoxolone (10–4M) time dependently depolarized the RMP and had no significant effect on fIJP or sIJP. Heptanol (3×10–4M) time dependently hyperpolarized the RMP and significantly reduced the fIJP and sIJP. Octanol (3×10–4M) had no effect on the RMP and abolished the fIJP and sIJP. The newly described gap junction blocker GAP 27 had no effect on RMP, fIJP or sIJP. Conclusion: We conclude that gap junctions are involved in neuromuscular transmission. The effects of the different gap junction blockers can not be explained by a single class of gap junctions, thus further research is indicated since selective gap junction blockade might become a powerful pharmacological tool.