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EFFECTS OF TREK-1 CHANNEL BLOCKERS ON NITRERGIC INHIBITORY JUNCTION POTENTIALS (IJPS) IN MURINE LOWER ESOPHAGEAL SPHINCTER (LES)
Y Zhang, WG Paterson
GI Diseases Research Unit, Queen’s University, Kingston, Ontario
BACKGROUND: The ionic mechanisms underlying nitrergic IJPs in intestinal smooth muscle remain controversial. Recently, it has been reported that opening of TREK-1 K+ channels contributes to nitrergic IJPs in colonic smooth muscle. We investigated the effects of the TREK-1 channel blockers, theophylline and L-methionine, on the nitrergic IJPs in mice LES clasp fibres.
METHODS: Conventional intracellular recordings and pharmacological techniques were employed in smooth muscle strips from CD1 mouse LES clasp and colonic circular smooth muscle as control tissue.
RESULTS: Smooth muscle of LES clasp displayed a resting membrane potential (RMP) of –41.9 ± 1.3 mV and unitary membrane potentials of 0.18 ± 0.02 mV2/Hz (n = 12). In the presence of atropine, guanethidine and Substance P to isolate non-adrenergic, non-cholinergic inhibitory innervation, transmural nerve stimulation produced a biphasic IJP consisting of an initial brief IJP, followed by a long-lasting slow IJP (LSIJP). Our previous studies have demonstrated that the entire LSIJP and a component of the initial brief IJP are nitrergic. L-methionine (1 mM) depolarized the RMP, but had no effect on the IJP. Cumulative application of theophylline to 1 mM maximally hyperpolarized the RMP to –70.7 ± 3.9 mV, partially inhibited the initial brief IJP and abolished the unitary potentials and LSIJP. The theophylline-induced hyperpolarization was prevented by pre-application of caffeine (5 mM) or cyclopiazonic acid, but not by ODQ (20 µM), a guanylate cyclase inhibitor. Membrane permeable 8-Br-cAMP significantly hyperpolarized the RMP and increased the unitary potentials and biphasic nitrergic IJPs. The effects of L-methionine and theophylline on RMP and nitrergic IJPs in the colonic smooth muscle were identical to that in the LES.
CONCLUSIONS: These data indicate that theophylline abolishes the nitrergic IJP, but this is likely due to interruption of sarcoplasmic reticulum-dependent Ca2+ releasing pathways, rather than via TREK-1 K+ channel blockade or intracellular accumulation of either cAMP or cGMP. This finding, combined with the lack of effect of L-methionine on the nitrergic IJP suggest that TREK-1 channels are not involved in the nitrergic IJP in gut smooth muscle.
Supported by CIHR