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The effect of 7-nitro indazole (7-NI) and a range of substituted indazole derivatives on nitric oxide synthase (NOS) enzyme activity in homogenates of rat cerebellum, bovine endothelial cells and lung from endotoxin-pretreated rats was investigated. 3-bromo 7-nitro indazole was either equipotent (IC50, 0.86 +/- 0.05 microM c.f. 0.78 +/- 0.2 microM, n = 6, P > 0.05) or approximately 4x (IC50, 0.17 +/- 0.01 microM c.f. 0.71 +/- 0.01 microM, n = 6, P < 0.05) or 20x (IC50, 0.29 +/- 0.01 microM c.f. 5.8 +/- 0.4 microM, n = 6, P < 0.05) more potent than 7-NI as an inhibitor of bovine endothelial, rat cerebellar and rat lung NOS enzyme activity respectively. 2,7-dinitro indazole also inhibited NOS in all three tissue sources with a potency similar to that of 7-NI. These results suggest that 3-bromo 7-NI and 2,7-dinitro indazole may prove to be useful additional tools with which to examine the biological properties of nitric oxide (NO).

1. 7-Nitro indazole (7-NI) produces potent inhibition of rat cerebellar nitric oxide synthase (NOS) with an IC50 of 0.9 +/- 0.1 microM (n = 6). NOS activity is dependent on the presence of both exogenous CaCl2 and NADPH. The inhibitory potency of 7-NI remained unaltered in the presence of different concentrations of either CaCl2 (0.75-7.5 mM) or NADPH (0.05-5.0 mM). 2. Kinetic (Lineweaver-Burke) analysis of the effect of 7-NI on rat cerebellar NOS revealed that inhibition was of a competitive nature with a Ki value of 5.6 microM. The Km of of cerebellar NOS with respect to L-arginine was 2.5 microM. 3. The following indazole derivatives (IC50 values shown in parentheses, all n = 6) caused concentration-related inhibition of rat cerebellar NOS in vitro: 6-nitro indazole (31.6 +/- 3.4 microM), 5-nitro indazole (47.3 +/- 2.3 microM), 3-chloro indazole (100.0 +/- 5.5 microM), 3-chloro 5-nitro indazole (158.4 +/- 2.1 microM) and indazole (177.8 +/- 2.1 microM). The IC50 values for 5-amino indazole, 6-amino indazole and 6-sulphanilimido indazole were in excess of 1 mM; 3-indazolinone was inactive. 4. 7-NI (10 mg kg-1) administered i.p. to rats produced 60 min thereafter a significant inhibition of NOS activity in cerebellum (31.1 +/- 3.2%, n = 6), cerebral cortex (38.2 +/- 5.6%, n = 6), hippocampus (37.0 +/- 2.8%, n = 6) and adrenal gland (23.7 +/- 3.0%, n = 6). NOS activity in olfactory bulb and stomach fundus were unchanged. 5. These results indicate that 7-NI is a potent and competitive inhibitor of rat brain NOS in vitro and also inhibits NOS in different brain regions and in the adrenal gland in vivo. Inhibition of NOS is a characteristic property of the indazole nucleus. Nitration of the indazole ring at positions 5, 6 and 7 results in a graded increase in inhibitory potency. Indazole-based inhibitors of NOS may prove useful tools with which to evaluate the biological roles of nitric oxide in the central nervous system.