Some pharmacological effects of tizanidine on smooth muscle organs and alpha 2-adrenoceptor

Microsomal and crude synaptosomal fractions were prepared from the longitudinal muscle of guinea-pig ileum. A specific binding of [3H]yohimbine (10 nM) to alpha 2-adrenoceptor in the crude synaptosomal fraction was inhibited by tizanidine and clonidine. Tizanidine is about one-third as potent as clonidine. A specific binding of [3H]QNB (0.3 nM) to muscarine receptor in the microsomal fraction was inhibited by atropine (10(-8)-10(-7) M) but not by tizanidine (up to 10(-4) M). Tizanidine inhibited spontaneous movements of guinea pig ileum and rat stomach (in situ) and intestinal transit in mice, and induced mydriasis in mice. These effects induced by tizanidine might be due to activation of alpha 2-adrenoceptor but not to atropine like action.