Arginine and tetrahydrobiopterin supplementation in rats with salt-induced blood pressure increase: minor hypotensive effect but improvement of renal haemodynamics.

High salt (HS) intake can lead to hypertension, probably the result of the predominance of vasoconstrictor reactive oxygen species over vasodilator nitric oxide (NO). We aimed to examine if the supposed NO deficiency and the resultant blood pressure increase could be corrected by supplementation of L-arginine, the substrate, and tetrahydrobiopterin (BH4), a co-factor of NO synthases. Wistar rats without known genetic background of salt sensitivity were exposed to HS diet (4%Na) for 10 or 26 days, without or with supplementation with oral L-arginine, 1.4 mg/kg b.w. daily, alone or together with intraperitoneal BH4, 10 mg/kg daily. Systolic blood pressure (SBP, tail-cuff method) was measured repeatedly and found to increase ~40 mmHg after 26 days; L-arginine and BH4 did not significantly attenuate this increase. At the end of chronic studies, in anaesthetized rats the diet- and treatment-induced changes in renal haemodynamics were assessed. HS diet selectively decreased (-30%, P < 0.03) the inner medullary blood flow (IMBF, laser-Doppler flux) without changing total or cortical renal perfusion. Arginine supplementation tended to raise all renal circulatory parameters, and distinctly increased IMBF, to 61% above the HS diet level (P < 0.05). In conclusion, unlike in confirmed genetically determined salt-dependent hypertension, L-arginine and BH4 supplementation failed to attenuate the SBP increase observed after exposure to HS diet. On the other hand, arginine increased total and regional renal perfusion, especially IMBF. This suggests that the delivery of arginine increased intrarenal NO synthesis, an action of renoprotective potential which presumably countered the harmful influence of the local tissue oxidative stress.

Effects of renal nerve stimulation on intrarenal blood flow in rats with intact or inactivated NO synthases.

AIM: We studied a possible action of nitric oxide (NO), an intrarenal vasodilator, to buffer a decrease in renal perfusion induced by electrical stimulation of renal nerves (RNS). METHODS: In anaesthetized rats RNS was performed (15 V, 2 ms pulse duration) for 10 s at the frequencies of 2, 3.5, 5 and 7.5 Hz. The total renal blood flow (RBF), an index of cortical perfusion, was measured using a Transonic probe on the renal artery. The outer and inner medullary blood flow (OMBF, IMBF) was measured by laser-Doppler flowmetry. The effect of RNS on RBF, OMBF and IMBF was determined in rats which were either untreated or pre-treated with L-NAME (0.6 or 1.8 mg kg(-1) i.v.), or S-methyl thiocitrulline (SMTC, 20 microg kg(-1) min(-1) i.v.), a selective inhibitor of neuronal NO synthase (nNOS). RESULTS: In untreated rats, RNS decreased IMBF significantly less than RBF and OMBF. High-dose L-NAME treatment significantly enhanced the RNS induced decrease of RBF but not of OMBF or IMBF. SMTC treatment significantly enhanced the decrease of IMBF, without affecting the response of RBF or OMBF. CONCLUSION: At intact NO synthesis the inner medullary circulation is not controlled by renal nerves to the extent observed for the outer medulla or cortex. NO generated by all NOS isoforms present in the kidney buffers partly the intrarenal vasoconstriction triggered by electrical RNS. The NO derived from nNOS seems of particular importance in the control of inner medullary perfusion, interacting with NO generated by endothelial NOS and renal nerves.

Role of prostaglandin cyclooxygenase and cytochrome P450 pathways in the mechanism of natriuresis which follows hypertonic saline infusion in the rat.

AIM: The prostaglandin cyclooxygenase (COX) and P450 cytochrome (CYP450) pathways of arachidonic acid metabolism are functionally interrelated and both engaged in control of sodium excretion; the study focused on their contribution to the natriuresis which follows hypertonic saline infusion in the rat. METHODS: In anaesthetized rats, clearance studies were conducted, supplemented with laser-Doppler measurements of the cortical and medullary blood flow (CBF, MBF), and measurement of medullary tissue admittance (Y), an index of interstitial ion concentration. RESULTS: Indomethacin (Indo), 5 mg kg(-1) i.v. paradoxically enhanced the natriuresis secondary to intra-aortic suprarenal 5% saline load, further increasing sodium excretion by 385 +/- 73% (P < 0.01). After acute clotrimazole, 10 mg kg(-1) i.v. an inhibitor of CYP450 epoxygenase, the increase in natriuresis was smaller and did not differ from that observed after the drug's ethanol solvent. In rats pre-treated with clotrimazole for 3 days, hypertonic saline loading increased sodium excretion (U(Na)V) to 0.94 +/- 0.22 micromol min(-1) , compared with a significantly greater (P < 0.05) increase to 2.76 +/- 0.48 micromol min(-1) measured in untreated controls. Indo increased U(Na)V twofold, similarly in the clotrimazole and in the control group; in the absence or presence of clotrimazole treatment, COX blockade significantly decreased MBF and increased Y. CONCLUSION: The data indicate that blockade of the CYP450 epoxygenase significantly impairs excretion of sodium in rats acutely loaded with hypertonic NaCl solution. The paradoxical post-Indo natriuresis is preserved in clotrimazole treated rats, which speaks against the role of CYP450 pathway in the response.