Abstract

AHR-2666, 3-(m-chlorophenoxy)-1-methylearbamoylpyrrolidine, a skeletal muscle relaxant, produces a dose-related, reversible decline in isometric twitch tension. While AHR-2666 (pK_a = 0.19 ± 0.04) does not bring about contracture in concentrations of 0.4 to 1.2 mM, it antagonizes both the potassium contracture and contractures produced by the addition of caffeine (7.5-50 mM) to the Ringer's solution. AHR-2666 appears to act as a competitive inhibitor of caffeine in frog sartorius muscle. The increased ⁴⁵Ca efflux produced by 10 mM caffeine is completely blocked by AHR-2666 (0.4 mM): higher concentrations of caffeine are less completely antagonized. By itself, AHR-2666 has no effect on either the uptake or release of ⁴⁵Ca from resting muscle. The muscle action potential was inhibited parallel with that of the twitch. The data suggest that the peripheral muscle relaxant effects of AHR-2666 are similar to those of the local anesthetics.