Mechanisms involved in carbachol-induced Ca2+ sensitization of contractile elements in rat proximal and distal colon

1 Mechanisms involved in Ca2+ sensitization of contractile elements induced by the activation of muscarinic receptors in membrane-permeabilized preparations of the rat proximal and distal colon were studied. 2 In alpha-toxin-permeabilized preparations from the rat proximal and distal colon, Ca2+ induced a rapid phasic and subsequent tonic component. After Ca2+-induced contraction reached a plateau, guanosine 5'-triphosphate (GTP) and carbachol (CCh) in the presence of GTP further contracted preparations of both the proximal and distal colon (Ca2+ sensitization). 3 Y-27632, a rho-kinase inhibitor, inhibited GTP plus CCh-induced Ca2+ sensitization more significantly in the proximal colon than in the distal colon. Y-27632 at 10 muM had no effect on Ca2+ induced contraction or slightly inhibited phorbol-12,13-dibutyrate-induced Ca2+ sensitization in either proximal or distal colon. Chelerythrine, a protein kinase C inhibitor, inhibited GTP plus CCh-induced Ca2+ sensitization in the distal colon, but not in the proximal colon. The component of Ca2+ sensitization that persisted after the chelerythrine treatment was completely inhibited by Y-27632. 4 In beta-escin-permeabilized preparations of the proximal colon, C3 exoenzyme completely inhibited GTP plus CCh-induced Ca2+ sensitization, but PKC(19-31) did not. In the distal colon, C3 exoenzyme abolished GTP-induced Ca2+ sensitization. It inhibited CCh-induced sensitization by 50 % and the remaining component was inhibited by PKC(19-31). 5 These results suggest that both protein kinase C and rho pathways in parallel mediate the Ca2+ sensitization coupled to activation of muscarinic receptors in the rat distal colon, whereas the rho pathway alone mediates this action in the proximal colon. British Journal of Pharmacology (2004).