The antagonism between acetylcholine and the curare alkaloids, d-tubocurarine, c-curarine-I, c-toxiferine-II and beta-erythroidine in the rectus abdominis of the frog.

Abstract

1) The antagonism of d-tubocurarine, c-curarine-I, c-toxiferine-II, and β-erythroidine to the acetyicholine contracture of the frog's rectus abdominis was studied. 2) The antagonism can be expressed by means of an activity constant derived from the law of mass action. For the three quaternary alkaloids this activity constant (recorded in fig. 4) is independent of the concentration of the curare alkaloid used; for the tertiary amine, β-erythroidine, the activity constant increases when the concentration is increased. 3) The rectus abdominis muscle of the frog reaches an equilibrium faster with acetylcholine than with d-tubocurarine. 4) Since the dose-response curve for acetylcholine is shifted only along the dose scale and the same maximal response is obtained even though the curare alkaloid is present, the antagonism between acetylcholine and the curare alkaloids used is competitive. 5) Variation of temperature (5°-35°C.) and of potassium ion concentration (1-8 mM /L.) does not significantly change the average value of the activity constant of d-tubocurarine.