RELATIONSHIP BETWEEN TYRAMINE POTENTIATION AND SELECTIVE INHIBITION OF MONOAMINE OXIDASE TYPES A AND B IN THE RAT VAS DEFERENS

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Abstract
The degree of selective monoamine oxidase (MAO) inhibition produced by (−)‐(deprenyl, clorgyline, LY51641 and tranylcypromine was examined in relation to modification of tyramine and noradrenaline contractile responses of the rat isolated vas deferens. All inhibitors possessed reversible α‐adrenoceptor blocking activity, determined against noradrenaline on the denervated vas deferens. For LY51641 and tranylcypromine, antagonism was competitive, with pA2 values of 6.17 and 5.26. Clorgyline, LY51641 and (−)‐deprenyl (10−5m) inhibited the tyramine response while present in the organ bath: LY51641, which was the most potent as an α‐adrenoceptor blocker, produced this effect at 10−6m. Responses to tyramine and noradrenaline were potentiated on washing out the inhibitors, but noradrenaline potentiation was seen only when tyramine had been present in the system. Tranylcypromine (10−6m) potentiated responses to noradrenaline and tyramine while present in the organ bath. Potentiation of tyramine responses by clorgyline and LY51641 occurred at 91% and 64% inhibition of MAO type A respectively, although full potentiation of the tyramine response was elicited only when substantial inhibition of both enzyme types occurred. Selective inhibition of MAO type B by 67% (with deprenyl) was not associated with tyramine potentiation.