EFFECTS OF L-N-G-NITRO-ARGININE ON NORADRENALINE-INDUCED CONTRACTION IN THE RAT ANOCOCCYGEUS MUSCLE

1 The influence of L-N-G-nitro-arginine (L-NOARG, 30 mu M) on contract ile responses to exogenous noradrenaline was studied in the rat anococ cygeus muscle. 2 Noradrenaline (0.1-100 mu M) contracted the muscle in a concentration-dependent manner. L-NOARG (30 mu M) had no effect on noradrenaline responses. 3 Phenoxybenzamine (Pbz 0.1 mu M) depressed b y 46% (P<0.001) the maximum response and shifted to the right (P<0.001 ) the E/[A] curve to noradrenaline ((p)EC(50) control: 6.92+/-0.09, pE C(50) Pbz: 5.30+/-0.10; n=20). 4 The nested hyperbolic null method of analysing noradrenaline responses after phenoxybenzamine showed that o nly 0.61% of the receptors need to be occupied to elicit 50% of the ma ximum response, indicating a very high functional receptor reserve. 5 Contractile responses to noradrenaline after partial alpha(1)-adrenoce ptor alkylation with phenoxybenzamine (0.1 mu M) were clearly enhanced by L-NOARG. 6 The potentiating effect of L-NOARG on noradrenaline res ponses after phenoxybenzamine was reversed by (100 mu M) L-arginine bu t not by (100 mu M) D-arginine. 7 These results indicate that spontane ous release of NO by nitrergic nerves can influence the alpha(1)-adren oceptor-mediated response to exogenous noradrenaline.