Wang T, Kendig DM, Smolock EM, Moreland RS. Carbachol-induced rabbit bladder smooth muscle contraction: roles of protein kinase C and Rho kinase. Am J Physiol Renal Physiol 297: F1534-F1542, 2009. First published September 30, 2009 doi:10.1152/ajprenal.00095.2009.-Smooth muscle contraction is regulated by phosphorylation of the myosin light chain (MLC) catalyzed by MLC kinase and dephosphorylation catalyzed by MLC phosphatase. Agonist stimulation of smooth muscle results in the inhibition of MLC phosphatase activity and a net increase in MLC phosphorylation and therefore force. The two pathways believed to be primarily important for inhibition of MLC phosphatase activity are protein kinase C (PKC)-catalyzed CPI-17 phosphorylation and Rho kinase (ROCK)-catalyzed myosin phosphatase-targeting subunit (MYPT1) phosphorylation. The goal of this study was to determine the roles of PKC and ROCK and their downstream effectors in regulating MLC phosphorylation levels and force during the phasic and sustained phases of carbachol-stimulated contraction in intact bladder smooth muscle. These studies were performed in the presence and absence of the PKC inhibitor bisindolylmaleimide-1 (Bis) or the ROCK inhibitor H-1152. Phosphorylation levels of Thr 38 -CPI-17 and Thr 696 /Thr 850 -MYPT1 were measured at different times during carbachol stimulation using site-specific antibodies. Thr 38 -CPI-17 phosphorylation increased concurrently with carbachol-stimulated force generation. This increase was reduced by inhibition of PKC during the entire contraction but was only reduced by ROCK inhibition during the sustained phase of contraction. MYPT1 showed high basal phosphorylation levels at both sites; however, only Thr 850 phosphorylation increased with carbachol stimulation; the increase was abolished by the inhibition of either ROCK or PKC. Our results suggest that during agonist stimulation, PKC regulates MLC phosphatase activity through phosphorylation of CPI-17. In contrast, ROCK phosphorylates both Thr 850 -MYPT1 and CPI-17, possibly through cross talk with a PKC pathway, but is only significant during the sustained phase of contraction. Last, our results demonstrate that there is a constitutively activate pool of ROCK that phosphorylates MYPT1 in the basal state, which may account for the high resting levels of MLC phosphorylation measured in rabbit bladder smooth muscle.

Background Colonic microbiota digest resistant starches producing short chain fatty acids (SCFAs). The main SCFAs produced are acetate, propionate, and butyrate. Both excitatory and inhibitory effects of SCFA on motility have been reported. We hypothesized that the effect of SCFAs on colonic motility varies with chain length and aimed to determine the effects of SCFAs on propagating and non-propagating contractions of guinea pig proximal and distal colon. Methods In isolated proximal colonic segments, Krebs solution alone or containing 10–100 mM acetate, propionate, or butyrate was injected into the lumen, motility was videorecorded over 10 min, and spaciotemporal maps created. In distal colon, the lumen was perfused with the same solutions of SCFAs at 0.1 mL min−1, the movement of artificial fecal pellets videorecorded, and velocity of propulsion calculated. Key Results In proximal colon butyrate increased the frequency of full length propagations, decreased short propagations and had a biphasic effect on non-propagating contractions. Propionate blocked full and short propagations and had a biphasic effect on non-propagating contractions. Acetate decreased short and total propagations In distal colon, butyrate increased and propionate decreased velocity of propulsion. Conclusions & Inferences The data suggest that luminal SCFAs have differing effects on proximal and distal colonic motility depending on chain length. Thus the net effect of SCFAs on colonic motility would depend on the balance of SCFAs produced by microbial digestion of resistant starches.

The role of serotonin (5-HT) in gastrointestinal motility has been studied for over 50 years. Most of the 5-HT in the body resides in the gut wall, where it is located in subsets of mucosal cells (enterochromaffin cells) and neurons (descending interneurons). Many studies suggest that 5-HT is important to normal and dysfunctional gut motility and drugs affecting 5-HT receptors, especially 5-HT3 and 5-HT4 receptors, have been used clinically to treat motility disorders. However, cardiovascular side effects have limited the use of these drugs. Recently studies have questioned the importance and necessity of 5-HT in general and mucosal 5-HT in particular for colonic motility. A paper published in this issue of Neurogastroenterology and Motility examines the importance of 5-HT3 and 5-HT4 receptors to initiation and generation of one of the key colonic motility patterns, the colonic migrating motor complex (CMMC), in rat. The findings suggest that 5-HT3 and 5-HT4 receptors are differentially involved in two different types of rat CMMCs: the long distance contraction (LDC) and the rhythmic propulsive motor complex (RPMC). The understanding of the role of serotonin in colonic motility has been influenced by the specific motility pattern studied, the stimulus used to initiate the motility (spontaneous vs. induced), and the route of administration of drugs. All of these considerations contribute to the understanding as well as the controversy that continues to surround the role of serotonin in the gut.

Kendig DM, Hurst NR, Bradley ZL, Mahavadi S, Kuemmerle JF, Lyall V, DeSimone J, Murthy KS, Grider JR. Activation of the umami taste receptor (T1R1/T1R3) initiates the peristaltic reflex and pellet propulsion in the distal colon. Am J Physiol Gastrointest Liver Physiol 307: G1100 -G1107, 2014. First published October 16, 2014; doi:10.1152/ajpgi.00251.2014.-Intraluminal nutrients in the gut affect the peristaltic reflex, although the mechanism is not well defined. Recent evidence supports the presence of taste receptors and their signaling components in enteroendocrine cells, although their function is unclear. This study aimed to determine if nutrients modify colonic motility through activation of taste receptors. Colonic sections were immunostained for the umami taste receptor T1R1/T1R3, which mediates the response to umami ligands, such as monosodium glutamate (MSG), in taste cells. Ascending contraction, descending relaxation, and calcitonin gene-related peptide release were measured in three-chamber flat-sheet preparations of rat colon in response to MSG alone or with inosine 5=-monophosphate (IMP). Velocity of artificial fecal pellet propulsion was measured by video recording in guinea pig distal colon. T1R1/T1R3 receptors were present in enteroendocrine cells of colonic sections from human, rat, mouse, and guinea pig. MSG initiated ascending contraction and descending relaxation components of the peristaltic reflex and calcitonin gene-related peptide release in flat-sheet preparations. IMP augmented the MSG-induced effects, suggesting activation of T1R1/T1R3 receptors. In T1R1 Ϫ/Ϫ mice, mucosal stroking, but not MSG, elicited a peristaltic reflex. Intraluminal perfusion of MSG enhanced the velocity of artificial fecal pellet propulsion, which was also augmented by IMP. Propulsion was also increased by L-cysteine, but not L-tryptophan, supporting a role of T1R1/T1R3 receptors. We conclude that T1R1/T1R3 activation by luminal MSG or L-cysteine elicits a peristaltic reflex and CGRP release and increases the velocity of pellet propulsion in distal colon. This mechanism may explain how nutrients regulate colonic propulsion. monosodium glutamate, T1R1/T1R3; motility; human colon; chemosensation THE ABILITY OF INTRALUMINAL nutrients to affect motility of the gastrointestinal (GI) tract has been known for decades (14). However, some of the receptors responsible for sensing intraluminal nutrients have only been discovered very recently. These receptors are the same as those responsible for the detection of taste qualities in the lingual epithelium. Multiple studies show the presence of taste receptors and the associated intracellular signaling molecules in the upper GI tract; however, there is less evidence for the existence of these receptors in the distal gut, specifically, the distal colon (13,37,48). While absorption of nutrients occurs almost completely in the small intestine, amino acids from the diet, dead mucosal cells, digested luminal enzymes, and amino acids generated by bacteria are present in the lumin...