EFFECT OF 7-NITRO INDAZOLE ON NEUROTRANSMISSION IN THE RAT VAS-DEFERENS - MECHANISMS UNRELATED TO INHIBITION OF NITRIC-OXIDE SYNTHASE

1 The effect of the nitric oxide synthase (NOS) inhibitor, 7-nitro ind azole (7-NI), on sympathetic and purinergic neurotransmission in the r at isolated vas deferens preparation has been studied. 2 7-NI (50-200 mu M) caused a dose- and frequency-dependent inhibition of the phasic (predominantly purinergic) contractile response of the rat vas deferen s to electrical (field) stimulation (100 V, 0.5 ms). Greatest inhibiti on occurred at lower frequencies of stimulation (0.1-10 Hz). The susta ined tonic contractile response (predominantly noradrenergic) was inhi bited only at a high frequency of stimulation (60 Hz) and only at the highest concentration of 7-NI studied (200 mu M). 3 7-NI (100 mu M) si gnificantly reduced the contractile response of the vas deferens to ex ogenous ATP (20 mu M-5 mM) and the stable P-2X purinoceptor agonist, a lpha,beta-methylene ATP (2.5 and 25.0 mu M) but was without effect on contractions due to noradrenaline (0.1-50 mu M) indicating a lack of a ntagonist effect on post-junctional alpha(1) adrenoceptors. 4 The effe ct of 7-NI (100 mu M) on the phasic contractile response to field stim ulation (0.1 and 2.0 Hz) was unaffected by preincubation of preparatio ns with yohimbine (1.0 mu M) or propranolol (0.01-10.0 mu M) indicatin g the absence of involvement of alpha(2)- or beta-adrenoceptors in thi s response. 5 7-NI (50-600 mu M) caused dose-related inhibition of con tractions elicited by addition of a depolarizing concentration of KCl (64 mM). 6 The effect of 7-NI (100 mu M) on the phasic contractile res ponse to held stimulation (0.1 and 2.0 Hz) was unaffected by preincuba tion of preparations with L-arginine (1 mM). Neither L-arginine (1 mM) nor N-G nitro L-arginine methyl ester (L-NAME, 100 mu M) affected the response of the vas deferens to field stimulation at 0.1 or 2.0 Hz. N itric oxide synthase (NOS) enzyme activity, measured as the conversion of [H-3]-L-arginine to [H-3]-citrulline, was not detectable in rat va s deferens homogenates. 7 7-NI preferentially inhibits the purinergic component of the response of the rat vas deferens to held stimulation. The mechanism of action of 7-NI is not known but is not related to NO S inhibition. It seems likely that 7-NI combines an antagonist action at smooth muscle cell P-2X-purinoceptors with the ability to inhibit t he cellular influx of calcium ions. Although these hitherto unrecorded effects of 7-NI occur at relatively high concentrations, the effects described may contribute to the pharmacological effects of this NOS in hibitor.