Effects of betacarbolines on the automaticity of the guinea pig sinus node

The effects of betacarbolines on guinea-pig isolated sinus nodes superfused with Tyrode's solution at 35 degrees C were analyzed. All analogs depressed the automaticity. The phase 4 of transitional fibers was depressed, in the absence of any change in maximum diastolic potential. The threshold for harmaline action was 10(-7)M. Dehydrogeneration of harmaline into harmine increased the potency. Removal of the methoxy group (harmane) did not modify the potency but accelerated the recovery. Substitution of the methoxy group by a hydroxy group (harmalol and harmol) reduced markedly the potency of harmaline and harmine, respectively.