The mechanisms underlying the muscle relaxant activity of 1-bebeerine (BB), a tertiary alkaloid isolated from the roots of Chondrodendron platyphyllum, were examined in mammalian and amphibian skeletal muscles. Injections of BB (0.05-1 g/kg, i.p.) in rats caused a dose-related flaccid paralysis and respiratory arrest at high doses. In isolated rat diaphragm and toad sartorius muscles, BB depressed the indirectly elicited muscle twitches (IC50: 228 microM and 5.4 microM, respectively, at 22 degrees C) and blocked the nerve-elicited muscle action potential. The neuromuscular blockade was not reversed by neostigmine (10 microM). High concentrations of BB (170 and 340 microM) caused muscle contracture unrelated to the junctional blockade, and intensified by increasing the bath temperature. Analysis of the contraction properties showed that BB (40 and 80 microM) increased the twitch/tetanus ratio (46% and 125%) and prolonged the relaxation time; the falling phase of the directly elicited action potential in toad sartorius muscle fibers was slower probably by a decreased potassium conductance. BB (0.1-340 microM) reduced the binding of [125l]alpha--bungarotoxin to the junctional ACh receptor of the rat diaphragm (IC50: 47.7 microM, at 37 degrees C. At low concentrations BB (1.5-15 microM) induced either opening or blockade of the ACh receptor-ionic channel. The results showed that BB blocked noncompetitively the neuromuscular transmission through a mechanism that affects the ACh recognition site and the ionic channel properties. The alkaloid also produced muscle contracture and changed the contractile properties through its extra-junctional action at the calcium handling by the sarcoplasmic reticulum or the contractile machinery.