Augmentation of cGMP/PKG pathway and colonic motility by hydrogen sulfide

Hydrogen sulfide (H2S), like nitric oxide (NO), causes smooth muscle relaxation, but unlike NO, does not stimulate soluble guanylyl cyclase (sGC) activity and generate cyclic guanosine 5=-monophosphate (cGMP). The aim of this study was to investigate the interplay between NO and H2S in colonic smooth muscle. In colonic smooth muscle from rabbit, mouse, and human, L-cysteine, substrate of cystathionine-gamma-lyase (CSE), or NaHS, an H2S donor, inhibited phosphodiesterase 5 (PDE5) activity and augmented the increase in cGMP levels, IP3 receptor phosphorylation at Ser(1756) (measured as a proxy for PKG activation), and muscle relaxation in response to NO donor S-nitrosoglutathione (GSNO), suggesting augmentation of cGMP/PKG pathway by H2S. The inhibitory effect of L-cysteine, but not NaHS, on PDE5 activity was blocked in cells transfected with CSE siRNA or treated with CSE inhibitor D, Lpropargylglycine (DL-PPG), suggesting activation of CSE and generation of H2S in response to L-cysteine. H2S levels were increased in response to L-cysteine, and the effect of L-cysteine was augmented by GSNO in a cGMP-dependent protein kinase-sensitive manner, suggesting augmentation of CSE/H2S by cGMP/PKG pathway. As a result, GSNO-induced relaxation was inhibited by DL-PPG. In flatsheet preparation of colon, L-cysteine augmented calcitonin generelated peptide release in response to mucosal stimulation, and in intact segments, L-cysteine increased the velocity of pellet propulsion. These results demonstrate that in colonic smooth muscle, there is a novel interplay between NO and H2S. NO generates H2S via cGMP/ PKG pathway, and H2S, in turn, inhibits PDE5 activity and augments NO-induced cGMP levels. In the intact colon, H2S promotes colonic transit. NEW & NOTEWORTHY Hydrogen sulfide (H2S) and nitric oxide (NO) are important regulators of gastrointestinal motility. The studies herein provide the cross talk between NO and H2S signaling to mediate smooth muscle relaxation and colonic transit. H2S inhibits phosphodiesterase 5 activity to augment cGMP levels in response to NO, which, in turn, via cGMP/PKG pathway, generates H2S. These studies suggest that interventions targeted at restoring NO and H2S homeostasis within the smooth muscle may provide novel therapeutic approaches to mitigate motility disorders.