CLONIDINE-INDUCED RHYTHMIC ACTIVITY IN RABBIT ANOCOCCYGEUS MUSCLE

1. Clonidine 0.5 mM induced an extremely regular rhythmic activity in isolated rabbit anococcygeus muscle, The movements were resistant to t etrodotoxin effect. 2. Prazosin (5 x 10(-8) M-5 x 10(-6) M) and yohimb ine (1.5 x 10(-7) M-5 x 10(-4) M) showed no remarkable effect on cloni dine induced rhythmic activity. 3. The clonidine induced contractions were dependent on extracellular calcium and could be inhibited by the omission of calcium from medium or the introduction of verapamil (IC50 = 1.3 x 10(-7) M) or nifedipine (IC50 = 7.5 x 10(-8) M), 4. Pretreatm ent of animals with reserpine made the preparations 2800-fold more sen sitive to this action of clonidine. 5. It can be concluded from this s tudy that clonidine is able to induce rhythmic activity in rabbit anoc occygeus muscle through a mechanism that increases intracellular conce ntration of Ca++ via membrane calcium channels.