Effect of 7‐nitro indazole on neurotransmission in the rat vas deferens: mechanisms unrelated to inhibition of nitric oxide synthase

Abstract

1 The effect of the nitric oxide synthase (NOS) inhibitor, 7-nitro indazole (7-NI), on sympathetic and purinergic neurotransmission in the rat isolated vas deferens preparation has been studied.2 7-NI (50-200 JLM) caused a dose-and frequency-dependent inhibition of the phasic (predominantly purinergic) contractile response of the rat vas deferens to electrical (field) stimulation (100 V, 0.5 ms). Greatest inhibition occurred at lower frequencies of stimulation (0.1-1O Hz). The sustained tonic contractile response (predominantly noradrenergic) was inhibited only at a high frequency of stimulation (60 Hz) and only at the highest concentration of 7-NI studied (200 J.M). indicating the absence of involvement of a2-or ,-adrenoceptors in this response.5 7-NI (50-600 LM) caused dose-related inhibition of contractions elicited by addition of a depolarizing concentration of KCI (64 mM). 6 The effect of 7-NI (100IM) on the phasic contractile response to field stimulation (0.1 and 2.0 Hz) was unaffected by preincubation of preparations with L-arginine (1 mM). Neither L-arginine (1 mM) nor NC nitro L-arginine methyl ester (L-NAME, 100 LM) affected the response of the vas deferens to field stimulation at 0.1 or 2.0 Hz. Nitric oxide synthase (NOS) enzyme activity, measured as the conversion of [3H]-L-arginine to [3H]-citrulline, was not detectable in rat vas deferens homogenates. 7 7-Nr preferentially inhibits the purinergic component of the response of the rat vas deferens to field stimulation. The mechanism of action of 7-NI is not known but is not related to NOS inhibition. It seems likely that 7-NI combines an antagonist action at smooth muscle cell P2X-purinoceptors with the ability to inhibit the 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 inhibitor.

NOS inhibitors exhibit antinociceptive properties in the rat formalin test

“…Furthermore, our data suggest that neuronal and inducible NOS isoforms induced indirect inhibitory pain behaviours responses likely through a non-specific NO pathway. Although our findings parallelled those reported by Allawi et al 23 suggesting that 7-nitro indazole exerted effects in the periphery which were unrelated to neuronal NOS blockade, they contrast data previously reported by Hao et al 24 in spinally injured rats. The reason for the discrepancy between those two studies is not clear although it may reflect differences in nociceptive stimulus or in the NOS inhibitor used.…”

Characterization of nitric oxide synthase activity in sheep urinary tract: functional implications

“…Non-specific effects of 7-NI has also been described, suggesting that its use as selective inhibitor of NOS must be carefully considered. Thus, 7-nitroindazole induced smooth muscle relaxations (Medhurst et al, 1994) and inhibited the purinergic component of the EFSinduced response in the vas deferens (Allawi et al, 1994), through NOS-independent mechanisms.…”

Activities of 7-Nitroindazole and 1-(2-(Trifluoromethylphenyl)-imidazole Independent of Neuronal Nitric-Oxide Synthase Inhibition

“…Although 7-NI inhibited carrageenan-induced thermal hyperalgesia in the early phase of inflammation in wild-type mice, the hyperalgesia persisted in the early phase in nNOS-deficient mice (Tao et al, 2004). Purinergic neurotransmission in the rat vas deferens was inhibited by 7-NI, whereas no NOS activity was detected in the same tissue (Allawi et al, 1994) and a very high concentration (1 mM) of L-NAME had no effect on purinergic neurotransmission (Ventura and Burnstock, 1997). 7-NI at a dose that altered neither NOS activity nor the NO concentration inhibited picrotoxin-induced convulsions (Paul and Ekambaram, 2003).…”