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.

High salt intake increases blood pressure in normal rats: putative role of 20-HETE and no evidence on changes in renal vascular reactivity

Background/Aims

High salt (HS) intake may elevate blood pressure (BP), also in animals without genetic salt sensitivity. The development of salt-dependent hypertension could be mediated by endogenous vasoactive agents; here we examined the role of vasodilator epoxyeicosatrienoic acids (EETs) and vasoconstrictor 20-hydroxyeicosatetraenoic acid (20-HETE).

Methods

In conscious Wistar rats on HS diet systolic BP (SBP) was examined after chronic elevation of EETs using 4-[4-(3-adamantan-1-yl-ureido)-cyclohexyloxy]-benzoic acid (c-AUCB), a blocker of soluble epoxide hydrolase, or after inhibition of 20-HETE with 1-aminobenzotriazole (ABT). Thereafter, in acute experiments the responses of renal artery blood flow (Transonic probe) and renal regional perfusion (laser-Doppler) to intrarenal acetylcholine (ACh) or norepinephrine were determined.

Results

HS diet increased urinary 20-HETE excretion. The SBP increase was not reduced by c-AUCB but prevented by ABT until day 5 of HS exposure. Renal vasomotor responses to ACh or norepinephrine were similar on standard and HS diet. ABT but not c-AUCB abolished the responses to ACh.

Conclusions

20-HETE seems to mediate the early-phase HS diet-induced BP increase while EETs are not engaged in the process. Since HS exposure did not alter renal vasodilator responses to Ach, endothelial dysfunction is not a critical factor in the mechanism of salt-induced blood pressure elevation.

Interaction of nitric oxide and the cytochrome P-450 system on blood pressure and renal function in the rat: dependence on sodium intake

AIM

Interaction was examined of nitric oxide (NO) and cytochrome P-450 (CYP-450)-dependent arachidonic acid derivatives, 20-HETE and EETs, in control of arterial pressure (MABP) and renal function. Modification of this interaction by changing sodium intake was also studied.

METHODS

On low, standard or high Na diet (LS, STD and HS rats respectively) effects of sequential blockade of NO synthases (NOS) and CYP-450 enzyme activity on MABP, renal blood flow (RBF, Transonic probe), renal medullary perfusion (MBF, laser-Doppler technique), medullary tissue NO (selective electrode) and renal excretion were examined in anaesthetized rats. All NOS were blocked with N(ϖ) -nitro-l-arginine methyl ester (l-NAME), the neuronal NOS with S-methyl-l-thiocitrulline (SMTC), and CYP-450 with 1-aminobenzotriazole (ABT).

RESULTS

In each diet group the baseline MABP was highest in rats pre-treated with l-NAME. CYP-450 inhibition significantly decreased MABP only in LS (-9%) and HS rats (-22%) pre-treated with l-NAME. This MABP decrease correlated directly with the dietary sodium content (r = 0.644, P < 0.001). CYP-450 inhibition decreased RBF in LS and HS rats (not in HS pre-treated with l-NAME). Acute exclusion of CYP-450 significantly increased MBF only in STD, SMTC pre-treated rats; in HS group it significantly increased medullary tissue NO by about 1.0 nA. The post-ABT changes in renal excretion occurred in LS and HS rats, irrespective of the status of NO synthesis.

CONCLUSIONS

Both NO- and CYP-450-dependent agents contribute to blood pressure and kidney function control, however, the role of 20-HETE and EETs becomes crucial only under conditions of high sodium intake or after NOS inhibition.

Cytochrome P-450 metabolites in renal circulation and excretion--interaction with the nitric oxide (NO) system

The role of CYP-450 dependent arachidonic acid (AA) metabolites (vasoconstrictor 20-HETE and vasodilator EETs) and NO in control of blood pressure (MABP) and kidney function remains unclear. NO affects the activity of heme-containing enzymes, like CYP-450 related monooxygenases, moreover, their activity depends on Na(+) intake. The focus of this review and underlying studies is on the role of high sodium intake (pro-hypertensive factor) in interrelation between CYP-450 and NOS. The acute vs. chronic non-selective inhibition of CYP-450 AA metabolites (ABT), and selective inhibition of 20-HETE (HET 0016) has also been tested. The renal artery flow (RBF, Transonic probe), medullary blood flow (MBF, laser-Doppler flux), renal excretion, and medullary tissue NO (selective electrode) were measured in male anaesthetized Wistar rats. We conclude that on standard Na(+) intake, opposed effects of 20-HETE and EETs are almost in equilibrium; however, in the renal circulation the vasodilator EETs influence slightly prevails. High sodium intake stimulates NOS, which limits CYP-450 impact on MABP and kidney function. However, this protection disappears after prolonged sodium intake. Long-lasting high sodium intake lowers NO bioavailability and promotes systemic and intrarenal vasoconstrictor activity of 20-HETE. Opposed effects of NO and AA metabolites of CYP-450 on water and solute excretion are also described.

Effects of ATP on rat renal haemodynamics and excretion: role of sodium intake, nitric oxide and cytochrome P450

AIM

Adenosine-5'-triphosphate (ATP) affects intrarenal vascular tone and tubular transport via P2 receptors; however, the actual role of the system in regulation of renal perfusion and excretion remains unclear and is the subject of this whole-kidney study.

METHODS

Effects of suprarenal aortic ATP infusion, 0.6-1.2 mg kg(-1) h(-1), were examined in anaesthetised rats maintained on low- (LS) or high-sodium (HS) diet. Renal artery blood flow (RBF, transonic flow probe) and the perfusion (laser-Doppler flux) of the superficial cortex (CBF) and outer and inner medulla (OM-BF, IM-BF) were measured, together with sodium and water excretion and urine osmolality.

RESULTS

Adenosine-5'-triphosphate did not change arterial pressure, RBF or CBF while the effects on medullary perfusion depended on sodium intake. In LS rats ATP increased IM-BF 19 +/- 6%, the effect was prevented by inhibition of nitric oxide (NO) with N-nitro-l-arginine methyl ester. In HS rats ATP decreased OM-BF 16 +/- 3% and IM-BF (7 +/- 4%, not significant); previous inhibition of cytochrome P450 with 1-aminobenzotriazol blunted the OM-BF decrease and reversed the previous decrease of IM-BF to a 13 +/- 8% increase. Inhibition of P2 receptors with pyridoxal derivative (PPADS) abolished medullary vascular responses to ATP. In HS rats pre-treated with PPADS, ATP increased tubular reabsorption, probably via adenosine formation and stimulation of P1 receptors.

CONCLUSION

The data indicate a potential role of ATP in the selective control of renal medullary perfusion, different in sodium depleted and sodium replete rats. The action of ATP appears to be mediated by the NO system and the cytochrome P450 dependent vasoactive metabolites.

Cytochrome P-450 monooxygenases in control of renal haemodynamics and arterial pressure in anaesthetized rats

The renal regulatory role of cytochrome P450 dependent metabolites of arachidonic acid (AA), vasodilator epoxyeicosatrienoic acids (EETs) and vasoconstrictor 20-hydroxyeicosatetraenoic acid (20-HETE), was examined in anaesthetised rats. We measured renal artery flow (RBF), cortical (CBF) and medullary (MBF) perfusion (laser-Doppler) and medullary tissue nitric oxide (NO, selective electrode), after non-selective inhibition of CYP-450 pathway with 1-aminobenzotriazole (ABT, 10 mg/kg i.v.) or after selective inhibition of 20-HETE synthesis with HET0016 (Taisho Co, Yoshino-cho, Japan), infused into renal artery at 0.3 mg/kg/h or into renal medulla at rates increasing from 0.15 to 1.5 mg/kg/h. ABT caused significant (by 13.7%) decrease in RBF without changing MBF. Renal arterial HET0016 increased MBF (not RBF or CBF) from 152+/-12 to 174+/-12 perfusion units (+16%, P<0.001), while medullary tissue nitric oxide was significantly increased (P<0.001). After renal medullary HET0016, renal perfusion indices were significantly higher than after HET0016 solvent (beta-cyclodextrin). Total renal blood flow seems to be under vasodilator control of EETs whereas renal medullary perfusion under tonic suppression by 20-HETE. The data document, for the first in the whole kidney studies, the functional antagonism of 20-HETE and NO.