Sha, Lei

ABSTRACT Gastric and small intestinal circular smooth muscle layers have a transwall resting membrane potential (RMP) gradient that is dependent on release of carbon monoxide (CO) from interstitial cells of Cajal (ICCs). Our aim was to determine whether a RMP gradient exists in the mouse colon and whether the gradient is CO dependent. Microelectrodes were used to record RMPs from muscle cells at different depths of the circular muscle layer from wild‐type and heme oxygenase‐2‐knockout (HO‐2‐KO) mice. A transwall RMP gradient was present in wild‐type mice. The CO scavenger oxyhemoglobin (20 µM) and the heme oxygenase inhibitor chromium mesoporphyrin IX (CrMP, 5 µM) abolished the transwall gradient. The gradient was absent in HO‐2‐KO mice. Tetrodotoxin (1 µM) caused a significant depolarization in circular smooth muscle cells throughout the circular muscle layer and abolished the transwall gradient. Removal of the submucosal neurons abolished the gradient. The majority of submucosal neurons contained HO‐2 immunoreactivity (HO‐2‐IR), while ICCs did not. These data show for the first time that a transwall gradient exists across the circular smooth muscle layer of the mouse colon, that the gradient is due to CO, and that the source of CO is the submucosal neurons.—Sha, L., Farrugia, G., Linden, D. R., Szurszewski, J. H. The transwall gradient across the mouse colonic circular muscle layer is carbon monoxide dependent. FASEB J. 24, 3840–3849 (2010). www.fasebj.org

Antispasmodics are used clinically to treat a variety of gastrointestinal disorders by inhibition of smooth muscle contraction. The main pathway for smooth muscle Ca2+entry is through L-type channels; however, there is increasing evidence that T-type Ca2+channels also play a role in regulating contractility. Otilonium bromide, an antispasmodic, has previously been shown to inhibit L-type Ca2+channels and colonic contractile activity. The objective of this study was to determine whether otilonium bromide also inhibits T-type Ca2+channels. Whole cell currents were recorded by patch-clamp technique from HEK293 cells transfected with cDNAs encoding the T-type Ca2+channels, CaV3.1 (α1G), CaV3.2 (α1H), or CaV3.3 (α1I) alpha subunits. Extracellular solution was exchanged with otilonium bromide (10−8to 10−5M). Otilonium bromide reversibly blocked all T-type Ca2+channels with a significantly greater affinity for CaV3.3 than CaV3.1 or CaV3.2. Additionally, the drug slowed inactivation in CaV3.1 and CaV3.3. Inhibition of T-type Ca2+channels may contribute to inhibition of contractility by otilonium bromide. This may represent a new mechanism of action for antispasmodics and may contribute to the observed increased clinical effectiveness of antispasmodics compared with selective L-type Ca2+channel blockers.

Neuron activity and insulin release were measured simultaneously from 33 preparations of intrapancreatic canine ganglia and pancreatic parenchyma adjacent to the ganglia. The electrical activity of single neurons of the ganglia was recorded with intracellular microelectrodes, and insulin release from the attached islets was determined with an enzyme-linked immunosorbent assay. Insulin release was 62 ± 18 fmol preparation/min in the presence of 10 mmol/l glucose and pulsatile (3.7 ± 0.4 min/pulse). Corresponding measurements of neuronal electrical activity showed a stable membrane potential of -53.5 ± 0.6 mV. Short, high-frequency (20 Hz) preganglionic nerve stimulation evoked action potentials and, in 46% of the preparations, a threefold rise in the insulin secretory rate associated with increased amplitude of the insulin pulses. The effects were blocked by 10 μmol/l tetrodotoxin (TTX). In other preparations, continuous low-frequency (0.05-0.5 Hz) preganglionic nerve stimulation evoked action potentials and, in 50% of the preparations, a gradual increase of insulin release associated with augmentation of insulin pulse amplitude without alteration of the duration. The effects were blocked by 50 μmol/l hexamethonium (HEX). In the remaining preparations, no change in insulin release was observed during nerve stimulation. In the absence of stimulation, neither TTX nor HEX affected the membrane potential or insulin secretion. These first simultaneous measurements of intrapancreatic ganglion activity and insulin secretion are consistent with amplitude modulation of pulsatile insulin secretion induced by changes in electrical activity in a population of intrapancreatic ganglion neurons.

Carbon monoxide (CO) is proposed as a physiological messenger. CO activates cGMP and has a direct effect on potassium channels. Both actions of CO lead to hyperpolarization of a cell's resting membrane potential, suggesting that CO may function as a hyperpolarizing factor, although direct evidence is still lacking. Here we take advantage of the known membrane potential gradient that exists in the muscle layers of the gastrointestinal tract to determine whether CO is an endogenous hyperpolarizing factor. We find that heme oxygenase-2-null mice have depolarized smooth muscle cells and that the membrane potential gradient in the gut is abolished. Exogenous CO hyperpolarizes the membrane potential. Regions of the canine gastrointestinal tract that are more hyperpolarized generate more CO and have higher heme oxygenase activity than more depolarized regions. Our results suggest that CO is a critical hyperpolarizing factor required for the maintenance of intestinal smooth muscle membrane potential and gradient.