AN ABNORMAL RATE OF ACTIN MYOSIN CROSS-BRIDGE CYCLING IN COLONIC SMOOTH-MUSCLE ASSOCIATED WITH EXPERIMENTAL COLITIS

Previous studies showed that colonic smooth muscle develops less contractile force to neurohumoral stimulation when associated with mucosal inflammation. This study evaluated 1) the Ca2+ dependence for colonic smooth muscle contraction, 2) the maximum velocity of muscle shortening (V(max)), and 3) changes in 20-kDa myosin light-chain (MLC) phosphorylation in distal circular colonic muscle from healthy rabbits and from rabbits with experimental colitis, induced by Formalin and immune complexes. The isometric tension of unskinned muscle stimulated with bethanechol or KCl was less (P < 0.05) in animals with colitis compared with the control group. In saponin-skinned muscle, the amplitude of the maximal tension at [Ca2+] of 3 x 10(-7) M was decreased (P < 0.05) in colitis animals (4.3 +/- 0.9 x 10(4) N/m2, n = 7) compared with healthy animals (10.5 +/- 2.4 x 10(4) N/m2, n = 6). However, the ED50 for Ca2+ stimulation was similar (P < 0.05) in both groups. When MLC was thiophosphorylated with ATP-gamma-S, the tension development was decreased in colitis (2.1 +/- 0.3 x 10(4) N/m2, n = 5; P < 0.01) compared with normals (5.0 +/- 1.4 x 10(4) N/m2, n = 5). In healthy animals, phosphorylation of 20-kDa MLC increased rapidly to 51.2 +/- 3.1% within 15 s after stimulation and subsequently declined to 19.0 +/- 2.1% at 5 min. V(max) was maximal (0.14 L(o)/s) 13 s after stimulation and declined before maximal active isometric stress. In colitis animals, the 20-kDa MLC phosphorylation (P < 0.05) and the V(max) (P < 0.01) were decreased. These studies suggest that 1) the Ca2+ sensitivity for contraction is not decreased in colitis muscle and 2) 20-kDa MLC phosphorylation and V(max) are decreased in colitis, which suggests an abnormal rate of actin myosin cross-bridge cycling in colitis.