Clofibrate relaxes the longitudinal smooth muscle of the mouse distal colon through calcium-mediated desensitisation of contractile machinery

Peroxisome proliferator-activated receptor α (PPAR-α) is a ligand-activated transcription factor that exerts strong effects on metabolic pathways. Our aim was to elucidate the effect of clofibrate, a PPAR-α agonist, on the longitudinal muscle of the mouse distal colon. We initially found that clofibrate induced a relaxation response in this muscle. Notably, the PPAR-α antagonists GW9662 and T0070907 did not attenuate this clofibrate-induced relaxation. The structurally related PPAR-α agonists fenofibrate and bezafibrate induced relaxation in the distal colon as effectively as clofibrate. In contrast, wy-14643, which activates PPAR-α more selectively than clofibrate, had no effect. Furthermore, clofibrate-induced relaxation was not affected by N-nitro-L-arginine, an NO synthase inhibitor, 1H-[1,2,4]-oxadiazolo-[4,3- a]quinoxaline-1-one, a soluble guanylate cyclase inhibitor, or H89, a protein kinase A inhibitor. Tetrodotoxin, an Na ⁺ channel blocker, and glibenclamide, apamin, charybdotoxin and XE991, various K ⁺ channel blockers, had no effect on clofibrate-induced relaxation. Importantly, clofibrate induced a relaxation response that was not accompanied by any alteration in the cytoplasmic Ca ²⁺ concentration in the longitudinal muscle of the mouse distal colon. Moreover, calyculin A, a myosin light-chain phosphatase (MLCP) inhibitor, attenuated clofibrate-induced relaxation. Our findings indicate that clofibrate relaxes the longitudinal smooth muscle of the mouse distal colon by regulating MLCP activity.