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.