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

Open Access
Publisher Website
Google Scholar
Abstract
To define further the role of nitric oxide (NO) in urinary tract function, we have measured the presence of nitric oxide synthase (NOS) activity, and its relationship with functional NO‐mediated responses to electrical field stimulation (EFS) in the urethra, the detrusor and the ureter from sheep. NOS activity was assayed by the conversion of L‐[14C]‐arginine to L‐[14C]‐citrulline. Endogenous production of citrulline was confirmed by thin layer chromatography. NOS enzymatic activity was detected in the cytosolic fraction from tissue homogenates with the following regional distribution (pmol citrulline mg−1 protein min−1): urethra (33 ± 3.3), detrusor (13.1 ± 1.1) and ureter (1.5 ± 0.2). No activity was detected in the particulate fraction of any region. NOS activity was dependent on Ca2+‐calmodulin and required exogenously added NADPH and tetrahydrobyopterin (BH4) for maximal activity. Exclusion of calmodulin from the incubation mixture did not modify NOS activity, but it was significantly reduced in the presence of the calmodulin antagonist, calmidazolium, suggesting the presence of enough endogenous calmodulin to sustain the observed NOS activity. NOS activity was inhibited to a greater extent by NG‐nitro‐l‐arginine (l‐NOARG) and its methyl ester (l‐NAME) than by NG‐monomethyl‐l‐arginine (l‐NMMA), while 7‐nitroindazole (7‐NI) was a weak inhibitor and L‐cannavine had no effect. Citrulline formation could be inhibited by superoxide dismutase in an oxyhaemoglobin‐sensitive manner, suggesting feedback inhibition of NOS by NO. EFS induced prominent NO‐mediated relaxations in the urethra while minor or no responses were observed in the detrusor and the ureter, respectively. Urethral relaxations to EFS were inhibited by NOS inhibitors with the rank order of potency: L‐NOARG = L‐NAME > 7‐NI > L‐NMMA. In conclusion, we have demonstrated the presence of NO‐synthesizing enzymatic activity in the sheep urinary tract which shows similar characteristics to the constitutive NOS isoform found in brain. We suggest that the enzymatic activity measured in the urethral muscle layer may account for the NO‐mediated urethral relaxation during micturition whereas regulation of detrusor and ureteral motor function by NOS containing nerves is less likely.