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 tetrodotoxin effect. 2. Prazosin (5 x 10(-8)M-5 x 10(-6)M) and yohimbine (1.5 x 10(-7)M-5 x 10(-4)M) showed no remarkable effect on clonidine-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. Pretreatment of animals with reserpine made the preparations 2800-fold more sensitive to this action of clonidine. 5. It can be concluded from this study that clonidine is able to induce rhythmic activity in rabbit anococcygeus muscle through a mechanism that increases intracellular concentration of Ca++ via membrane calcium channels.

Methoxamine-induced rhythmic activity in rabbit anococcygeus muscle.

1. Methoxamine 0.5 microns induced an extremely regular rhythmic activity in isolated rabbit anococcygeus muscle. 2. Prazosin had an inhibitory effect on methoxamine-induced rhythmic contractions. IC50 of prazosin was 7.94 nM. 3. The methoxamine-induced contractions are dependent on extracellular calcium and can be inhibited by the omission of calcium from media or the introduction of verapamil (IC50 = 0.11 microM) or nifedipine (IC50 = 0.21 microM). 4. Application of reserpine made the preparations 40-fold more sensitive to methoxamine. 5. It can be concluded that rhythmic contractions produced by methoxamine are mediated through stimulatory action of methoxamine on alpha-I adrenoceptors and depend on extracellular calcium. 6. Lithium made the muscle more sensitive to methoxamine action. In preincubated muscles with 1, 3 and 5 mM lithium the initiation of contractions occurred at 1.5 x 10(-7), M, 5 x 10(-8) M and 1.5 x 10(-8) M of methoxamine, respectively.