The effect of regucalcin, a regulatory protein in Ca2+ signaling, on nitric oxid (NO) synthase activity in the cytosol of kidney cortex of rats was investigated. The presence of calcium chloride (10 μM) in the enzyme reaction mixture caused a significant increase in NO synthase activity. This increase was significantly prevented by the addition of trifluoperazine (TFP; 20 or 50 μM), an antagonist of calmodulin, supporting the existence of Ca2+/calmodulin-dependent NO synthase in rat kidney cortex cytosol. NO synthase activity was significantly decreased by the addition of regucalcin (10−10-10−8 M) in the reaction mixture in the absence or presence of calcium chloride (10 μM). The regucalcin (10−8 M) effect was not seen in the presence of Nw-nitro-L-argine metylester (NAME; 10−6 or 10−5 M), an inhibitor of NO synthase. Regucalcin significantly reduced NO synthase activity in the presence of TFP (50 μM) or EGTA (1 mM) which has a significant inhibitory effect on the enzyme activity. The presence of anti-regucalcin monoclonal antibody (25 or 50 ng/ml) in the reaction mixture caused a significant increase in NO synthase activity. This increase was completely abolished by the addition of regucalcin (10−7 M). NO synthase activity was not significantly changed in the kidney cortex cytosol of regucalcin transgenic rats overexpressing endogenous regucalcin as compared with that of wild-type rats. However, the effect of calcium chloride (10 μM) in increasing NO synthase activity in the kidney cortex cytosol of wild-type rats was significantly weakened in regucalcin transgenic rats. The present study demonstrates that endogenous regucalcin has a suppressive effect on NO synthase activity in the kidney cortex cytosol of rats.

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