R‐Type Calcium Channels Contribute to Colonic Inhibitory Neuromuscular Transmission

Abstract

The inhibitory neurotransmitters, ATP and nitric oxide (NO), relax the colonic smooth muscle which is important for propulsion of colonic content. Voltage‐gated Ca2+ channels (VGCC) control neurotransmitter release and regulate colonic motility. The α1E subunit of the R‐type VGCC is expressed by enteric neurons yet its role in regulating colonic motility is unclear. We investigated the role of R‐type VGCCs in colonic inhibitory neuromuscular transmission using alpha1E knockout (KO) mice. Intracellular recordings of inhibitory junction potentials (IJP) were obtained from circular muscle cells in distal colon. IJPs were evoked by trains of electrical stimulation (1s, 10 Hz, 40‐90 volts). IJPs have a fast ATP‐mediated component and a slow NO‐mediated component. Peak amplitude (ATP component) and area under the curve (AUC; both components) of the IJP were compared between wildtype (WT) and KO mice. Resting membrane potential (RMP) was measured as an index of ongoing inhibitory transmission. There was a voltage dependent increase in IJP amplitude and AUC in WT and KO mice. IJP amplitude was similar in WT and KO mice. The IJP AUC was decreased in KO compared to WT. Nifedipine (1 µM; L‐type VGCC blocker) had no effect on these responses. RMP in KO muscle cells was decreased compared with WT (‐44 ± 2 mV in WT and ‐38 ± 1 mV in KO; P < 0.05). Thus, there was a 5.4 mV RMP depolarization in KO mice. Nifedipine did not change RMP of WT or KO cells (‐39 ± 3 mV, WT and ‐40 ± 1 mV in KO). In conclusion, R‐type VGCCs contribute to colonic inhibitory neuromuscular transmission by controlling NO release. In addition, R‐type VGCCs contribute to ongoing release of NO and ATP which maintain colonic muscle tone.