Regional differences of endogenous ATP release in rabbit arteries

Alkaline Phosphatase Inhibitors Attenuate Renovascular Responses to Norepinephrine

Tissue nonspecific alkaline phosphatase (TNAP) contributes to the production of adenosine by the kidney, and A₁-receptor activation enhances renovascular responses to norepinephrine. Therefore, we hypothesized that TNAP regulates renovascular responsiveness to norepinephrine. In isolated, perfused rat kidneys, the TNAP inhibitor l-p-bromotetramisole (0.1 mmol/L) decreased renal venous levels of 5'-AMP (adenosine precursor) and adenosine by 61% (P<0.0384) and 62% (P=0.0013), respectively, at 1 hour into treatment and caused a 10-fold rightward shift of the concentration-response relationship to exogenous norepinephrine (P<0.0001). Similarly, 2 other TNAP inhibitors, levamisole (1 mmol/L) and 2,5-dimethoxy-N-(quinolin-3-yl)benzenesulfonamide (0.02 mmol/L), also right shifted the concentration-response relationship to norepinephrine. The ability of TNAP inhibition to blunt renovascular responses to norepinephrine was mostly prevented or reversed by restoring A₁-adenosinergic tone with the A₁-receptor agonist 2-chloro-N⁶-cyclopentyladenosine (100 nmol/L). All 3 TNAP inhibitors also attenuated renovascular responses to renal sympathetic nerve stimulation, suggesting that TNAP inhibition attenuates renovascular responses to endogenous norepinephrine. In control propranolol-pretreated rats, acute infusions of norepinephrine (10 μg/kg/min) increased mean arterial blood pressure from 95±5 mm Hg to a peak of 169±4 mm Hg and renovascular resistance from 12±2 mm Hg/mL/min to a peak of 55±12 mm Hg/mL/min; however, in rats also treated with intravenous l-p-bromotetramisole (30 mg/kg), the pressor and renovascular effects of norepinephrine were significantly attenuated (blood pressure: basal and peak, 93±7 and 146±6 mm Hg, respectively; renovascular resistance: basal and peak, 13±2 and 29±5 mm Hg/mL/min, respectively). TNAP inhibitors attenuate renovascular and blood pressure responses to norepinephrine, suggesting that TNAP participates in the regulation of renal function and blood pressure.