EFFECT OF NARINGIN AND NARINGENIN ON CONTRACTIONS INDUCED BY NORADRENALINE IN RAT VAS-DEFERENS .1. EVIDENCE FOR POSTSYNAPTIC ALPHA-2-ADRENERGIC RECEPTOR

1. Naringin at all doses (2 x 10(-6), 5 x 10(-7), and 1 x 10(-7) M) significantly increased contractions induced by noradrenaline in rat vas deferens but the increments of maximal contraction were not concentration-dependent. 2. In a medium containing 1 X 10(-6) M yohimbine (a selective blocker of alpha2-adrenoceptor) and naringin, the curve constructed with noradrenaline decreased below the control curve. 3. Naringenin (aglycone of naringin) (2 x 10(-6) and 1 x 10(-7) M) increased the contractile effect of noradrenaline and the maximal effect evoked was related to the maximal dose of naringenin. 4. The alpha2 antagonism produced by yohimbine in the naringenin-noradrenaline association were retained at two doses of naringenin tested and we noticed a similar behaviour when we used clonidine-noradrenaline.