[Harman and 3-methylharman--blockaders of presynaptic alpha 2-adrenoreceptors]

It has been shown in experiments on rat vas deferens that melipramine, harmane and 3-methylharman in low concentrations of the order of 10(-8)--10(-6) M increase the contraction of the vas deference induced by noradrenaline and transmural stimulation. Meanwhile in higher concentrations (10(-6)--4 X 10(-5) M) they have been discovered to decrease this effect (except for harmane). In low concentrations harmane and 3-methylharman competitively remove the inhibitory effect of clonidine on the transmural stimulation-induced contractions. It is inferred that harmane and methylharman are competitive blockers of presynaptic alpha 2-adrenoceptors in adrenergic synapses.