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NEUROREGULATION AND MOTILITY

Mechanism of active repolarization of inhibitory junction potential in murine colon

Department of Physiology and Cell Biology, University of Nevada School of Medicine, Reno, Nevada 89557

Submitted 12 March 2003 ; accepted in final form 24 June 2003

Enteric inhibitory responses in gastrointestinal (GI) smooth muscles involve membrane hyperpolarization that transiently reduce the excitability of GI muscles. We examined the possibility that an active repolarization mechanism participates in the restoration of resting membrane potential after fast inhibitory junction potentials (IJPs) in the murine colon. Previously, we showed these cells express a voltage-dependent nonselective cation conductance (NSCC) that might participate in active repolarization of IJPs. Colonic smooth muscle cells were impaled with micro-electrodes and voltage responses to nerve-evoked IJPs, and locally applied ATP were recorded. Ba²⁺ (500 µM), a blocker of the NSCC, slowed the rate of repolarization of IJPs. We also tested the effects of Ba²⁺, Ni²⁺, and mibefradil, all blockers of the NSCC, on responses to locally applied ATP. Spritzes of ATP caused transient hyperpolarization, and the durations of these responses were significantly increased by the blockers of the NSCC. We considered whether NSCC blockers might affect ATP metabolism and found that Ni²⁺ decreased ATP breakdown in colonic muscles. Mibefradil had no effect on ATP metabolism. Because both Ni²⁺ and mibefradil had similar effects on prolonging responses to ATP, it appears that restoration of resting membrane potential after ATP spritzes is not primarily due to ATP metabolism. Neurally released enteric inhibitory transmitter and locally applied ATP resulted in transient hyperpolarizations of murine colonic muscles. Recovery of membrane potential after these responses appears to involve an active repolarization mechanism due to activation of the voltage-dependent NSCC expressed by these cells.

gastrointestinal motility; adenosine 5'-triphosphate; voltage-dependent nonselective cation current

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