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

1 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. 2 All inhibitors possessed reversible α‐adrenoceptor blocking activity, determined against noradrenaline on the denervated vas deferens. For LY51641 and tranylcypromine, antagonism was competitive, with pA 2 values of 6.17 and 5.26. 3 Clorgyline, LY51641 and (−)‐deprenyl (10 −5 m ) 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 −6 m . 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. 4 Tranylcypromine (10 −6 m ) potentiated responses to noradrenaline and tyramine while present in the organ bath. 5 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.