Derek M. Kendig

Serotonin and colonic motility.

The role of serotonin (5‐hydroxytryptamine [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. Recent evidence suggests the importance of 5‐HT3 and 5‐HT4 receptors for 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(s) 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 and the controversy that continues to surround the role of serotonin in the gut.

The short chain fatty acids, butyrate and propionate, have differential effects on the motility of the guinea pig colon.

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 SCFAs 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.

Inhibition of NMDAR Reduces Bladder Hypertrophy and Improves Bladder Function in Cyclophosphamide Induced Cystitis

PURPOSE We examined the role of NMDAR in the regulation of bladder hypertrophy and function in a rat model of cyclophosphamide induced cystitis. MATERIALS AND METHODS Cystitis was induced by intraperitoneal injection of cyclophosphamide (150 mg/kg body weight). NMDAR phosphorylation (activity) and signal transduction pathways were examined by direct measurement and by specific inhibitors in vivo. Bladder hypertrophy was measured by bladder weight/body weight and type I collagen expression. Bladder function was examined by metabolic recording, conscious cystometry and detrusor muscle strip contractility in response to carbachol. RESULTS NMDAR activity measured by the phosphorylation level of the NMDAR1 (NR1) subunit was expressed in the spinal cord but not in the bladder at 48 hours of cystitis. NMDAR inhibition with dizocilpine (MK-801) reduced the cystitis induced increment of bladder weight and type I collagen up-regulation in the bladder. NMDAR regulated type I collagen up-regulation was mediated by the PI3K/Akt pathway. NMDAR inhibition also attenuated cystitis induced urinary frequency measured by metabolic cage and cystometry. Cystitis decreased the responsiveness of detrusor muscle strips to carbachol, which was reversed by MK-801 in vivo. Unlike MK-801 the NMDAR antagonist D-AP5, which could not block central NMDAR activity, had no effect on bladder hypertrophy, type I collagen up-regulation or Akt activation caused by cystitis in the bladder. CONCLUSIONS Findings suggest that NMDAR activity has a role in cystitis induced bladder hypertrophy and overactivity. NMDAR mediated Akt activation may underlie the mechanism of bladder dysfunction.

Role of various kinases in muscarinic M3 receptor-mediated contraction of longitudinal muscle of rat colon

The longitudinal muscle layer in gut is the functional opponent to the circular muscle layer during peristalsis. Differences in innervation of the layers allow for the contraction of one layer concurrently with the relaxation of the other, enabling the passage of gut contents in a controlled fashion. Differences in development have given the cells of the two layers differences in receptor populations, membrane lipid handling, and calcium handling profiles/behaviors. The contractile activity of the longitudinal muscle is largely mediated by cholinergic neural input from myenteric plexus. Activation of muscarinic receptors leads to rapid activation of several kinases including MLC kinase, ERK1/2, CaMKII and Rho kinase. Phosphorylation of myosin light chain (MLC20) by MLC kinase (MLCK) is a prerequisite for contraction in both circular and longitudinal muscle cells. In rat colonic longitudinal muscle strips, we measured muscarinic receptor-mediated contraction following incubation with kinase inhibitors. Basal tension was differentially regulated by Rho kinase, ERK1/2, CaMKII and CaMKK. Selective inhibitors of Rho kinase, ERK1/2, CaMKK/AMPK, and CaMKII each reduced carbachol-induced contraction in the innervated muscle strips. These inhibitors had no direct effect on MLCK activity. Thus unlike previously reported for isolated muscle cells where CaMKII and ERK1/2 are not involved in contraction, we conclude that the regulation of carbachol-induced contraction in innervated longitudinal muscle strips involves the interplay of Rho kinase, ERK1/2, CaMKK/AMPK, and CAMKII.