Alpha 2 adrenoceptor-mediated contraction of frog rectus abdominis muscle

Effect of various concentrations of clonidine (1.875 X 10(-11) to 1.875 X 10(-6) M) was investigated on rectus abdominis muscle of frog. In lower concentrations it neither had any effect per se nor did it alter the contractile effect of acetylcholine. However, a higher concentration of clonidine (1.875 X 10(-6) M) evoked characteristic slow but sustained contractions. Such an effect was not observed with epinephrine, norepinephrine or isoprenaline. Clonidine-induced sustained contractions were resistant to d-tubocurarine (2.545 X 10(-9) M) but susceptible to blockade by yohimbine (5.116 X 10(-8) M), an alpha 2 antagonist, and verapamil (2.04 X 10(-8) M), a calcium channel blocker. Probably activation of post-synaptic alpha 2 adrenoceptors by clonidine was responsible for the observed effect. This is further supported by the observation that guanfacine, a more specific alpha 2 adrenoceptor agonist, also evoked identical, slow and sustained contractions which could be readily reversed by yohimbine.