Mechanism of Rho-kinase-mediated Ca²⁺-independent contraction in aganglionic smooth muscle in a rat model of Hirschsprung's disease

Purpose: Lack of ganglion cells is the main cause of bowel movement disorder in Hirschsprung's disease. Because smooth muscle is the primary organ, the properties of intestinal smooth muscle need to be investigated. We therefore investigated the reactivity of the contractile system and the mechanism of contraction in aganglionic intestinal smooth muscle. Methods: Colonic smooth muscle strips from endothelin-B receptor gene-deficient [EDNRB(-/-)] rats were loaded with the Ca²⁺ indicator dye fura-PE3/AM and changes in fluorescence intensity were monitored. The intracellular calcium concentration ([Ca²⁺]i) and force development in the strips were measured simultaneously. Results: The force induced by 10 μM substance P (SP) was higher than that induced by 60 mM K⁺ depolarization (control), whereas [Ca²⁺]i elevation induced by 10 μM SP was less than that induced by 60 mM K⁺ in all segments. Pretreatment with the Rho-kinase inhibitor Y-27632 inhibited force development more strongly in EDNRB(-/-) aganglionic segments than in EDNRB(+/+) ganglionic segments. However, [Ca ²⁺]i was higher in EDNRB(-/-) aganglionic segments than in EDNRB(+/+) ganglionic segments. Conclusions: The Ca²⁺-independent pathway involving Rho-kinase was hyperactivated in EDNRB(-/-) aganglionic segments. This phenomenon is assumed to compensate for Ca²⁺ channel downregulation and Ca²⁺-dependent contraction. From a clinical point of view, the motility of aganglionic intestine would be controllable with the control of Ca²⁺-independent contraction before definitive operations in Hirschsprung's disease.