Role of PKC‐mediated calcium‐sensitization in attenuated carbachol‐induced contraction of the rat colon in TNBS‐induced colitis

Abstract

Contractile agonists increase cytosolic concentration of Ca2+ by promoting influx of extracellular Ca2+ and by releasing Ca2+from intracellular stores. However, the degree of contraction and the level of MLC phosphorylation are not always proportional to Ca2+ concentration suggesting that an additional mechanism, Ca2+-sensitization, contributes to smooth muscle contraction. The objective of this study was to investigate the role of this pathway in the reduced reactivity of inflamed colonic segments to carbachol. Myeloperoxidase activity was greater in inflamed colon segments confirming inflammation in these animals. Carbachol induced concentration-dependent contractions of the colon. The maximum contraction but not sensitivity was attenuated in inflamed colon segments. CaCl2-induced contraction of permeabilized colon strips was significantly greater (P < 0.05) in inflamed tissues compared to controls, indicating greater sensitization of the contractile myofilaments to Ca2+ in colon segments from colitic rats. Carbachol-induced contractions were however greater in colon segments from control rats. Western blot analyses showed that PKC was expressed in the rat colon but the expression was significantly reduced in inflamed colon segments. GF 109203X, a selective PKC inhibitor, concentration-dependently inhibited carbachol-induced contractions of the colon in both groups. However, the effect of GF was significantly less in colon segments from TNBS-treated rats. These results would confirm a role for Ca2+ sensitization involving PKC in carbachol-induced contraction of the rat colon and that the reduced carbachol-induced contraction observed in colitis could be linked to defective PKC-mediated Ca2+ sensitization.