Interaction of Adenosine and Acetylcholine on the Bullfrog Atrium

As adenosine has a potent stabilizing action on catechol-amine stimulation in the myocardium, the mode of interaction of adeno-sine and acetylcholine (ACh) was studied with regard to the membrane potential, current and tension components of the bullfrog atrium, using the single or double-sucrose gap method. Adenosine (10-4-3×10-3 M) augmented the twitch contraction in the presence of ACh (10-9-5×10-7m) by lengthening the duration of the action potential. The dose-tension response curve for ACh was modified by adenosine, producing a rise of the inhibitory threshold of ACh, and the modification showed a non competitive interaction of these compounds. Under the voltage clamp, ACh-induced steady current (IACh) was inhibited by adenosine non-competitively. The known inhibition of slow inward current (Is) by ACh was enhanced by adenosine, while the delayed outward current (Ix) was markedly suppressed. Is-dependent and -independent tension com-ponents were both inhibited by adenosine, thereby suggesting a decrease in intracellular concentrations of calcium. The potent suppression of IACh and Ix induced by adenosine, however, appeared to mitigate the inhibitory action of ACh on the action potential and twitch contraction.