Renal response to amino acid infusion in essential hypertension

Metabolites and Hypertension: Insights into Hypertension as a Metabolic Disorder: 2019 Harriet Dustan Award

For over 100 years, essential hypertension has been researched from different perspectives ranging from genetics, physiology, and immunology to more recent ones encompassing microbiology (microbiota) as a previously underappreciated field of study contributing to the cause of hypertension. Each field of study in isolation has uniquely contributed to a variety of underlying mechanisms of blood pressure regulation. Even so, clinical management of essential hypertension has remained somewhat static. We, therefore, asked if there are any converging lines of evidence from these individual fields that could be amenable for a better clinical prognosis. Accordingly, here we present converging evidence which support the view that metabolic dysfunction underlies essential hypertension.

Decreases in renal functional reserve and proximal tubular fluid output in conscious oophorectomized rats: normalization with sex hormone substitution

Age-dependent glomerulosclerosis with reduced GFR develops earlier among men than among women. Therefore, whether female sex hormones could prevent the age-dependent decrease in GFR was investigated. The kidney function in oophorectomized rats treated with placebo (OOX group), estrogen (OOX+E(2) group), or estrogen plus progesterone (OOX+E(2)+P group) for 5 mo and in sham-operated rats (sham group) was examined. The rats were 13 mo of age at the time of the investigation. They were conscious and chronically instrumented. The results demonstrated that estrogen, alone or in combination with progesterone, was without effect on the baseline GFR and effective renal plasma flow but prevented severe decreases in the renal functional reserve and fractional proximal tubular fluid output (lithium clearance technique, fractional lithium excretion), which were observed in the OOX group. The renal functional reserve (estimated by stimulation with glycine) was -223 +/- 151, 483 +/- 129, 675 +/- 76, and 208 +/- 140 micro l/min in the OOX, OOX+E(2), OOX+E(2)+P, and sham groups, respectively. Fractional lithium excretion was 12.4 +/- 3.1, 26.8 +/- 2.0, 31.8 +/- 2.5, and 23.6 +/- 3.3% in the OOX, OOX+E(2), OOX+E(2)+P, and sham groups, respectively. In conclusion, oophorectomy at the age of 8 mo did not produce a decrease in baseline GFR in female rats within a period of 5 mo. However, oophorectomy led to severe decreases in the renal functional reserve and fractional proximal tubular fluid output. Both effects were prevented with administration of estrogen. Sham-operated rats demonstrated values for renal functional reserve and fractional lithium excretion that were between those observed for the OOX group and the groups treated with sex hormones.

Impairment of renal vasodilation with l-arginine is related to more severe disease in untreated hypertensive patients

Data remain insufficient to place the decreased response to L-arginine in hypertensive patients within a consistent pathophysiological sequence. The aim of the present study in patients with essential hypertension was to assess the relationships between the response to L-arginine and a set of relevant clinical and laboratory parameters. In this prospective, interventional study, we administered L-arginine to untreated hypertensive individuals and healthy control subjects and measured the clearance of inulin and of para-aminohippurate and a set of biochemical and clinical variables. L-Arginine infusion revealed major differences between control subjects and 1 subgroup (group B) of hypertensive individuals. Group B hypertensives (n=18) had no increase in inulin clearance and no decrease in renal vascular resistance with L-arginine; however, in another subset of hypertensive patients (group A, n=27), the insulin clearance increased and renal vascular resistance decreased similar to the control group (group C, n=11). The ambulatory blood pressure monitoring in group B showed both an increased mean diastolic pressure and a "nondipper" pattern in the nocturnal regulation of arterial pressure. These findings in group B were accompanied by significant alterations in optic fundus and left ventricle hypertrophy and increased microalbuminuria (all, P<0.05). Furthermore, group B individuals had significantly lower values of HDL cholesterol and a higher baseline atherogenic index, plasma insulin level, and glucose/insulin index. We disclose a previously undescribed relationship between end organ repercussion and decreased renal hemodynamic response to L-arginine. Our results may help to understand the mechanisms that lead to target organ damage in hypertension.

Effects of oral administration of L-arginine on renal function in patients with heart failure

OBJECTIVES

Although the beneficial effects of L-arginine on systemic haemodynamics have been reported in patients with heart failure, its effect on renal function has not been examined. We evaluated the effects of oral administration of L-arginine on renal haemodynamics, sodium and water handling, and various hormonal factors in patients with chronic heart failure.

SUBJECTS AND METHODS

A double-blind crossover trial was performed in 17 patients with chronic congestive heart failure (NYHA II-III, 56 +/- 12 years of age) who were randomly assigned to receive oral L-arginine (15 g/day) and placebo or placebo and arginine sequentially for 5 days each. Twenty-four hour creatinine clearance (Ccr), and 24-h urinary cyclic guanosine 5-monophosphate (GMP) excretion were determined. Saline loading was performed on day 5 of each treatment Renal blood flow, glomerular filtration rate (GFR), and urinary sodium excretion rate (UNa) were assessed before and after saline loading.

RESULTS

Twenty-four hour GMP excretion (1.4 +/- 1.1 versus 0.8 +/- 0.5 micromol/day, P < 0.01) and Ccr (150 +/- 43 versus 125 +/- 42 ml/min, P < 0.05) were higher and plasma endothelin level (2.5 +/- 0.6 versus 3.1 +/- 0.8 pg/ml, P < 0.05) was lower with L-arginine treatment compared to placebo treatment In addition, the relative increase of UNa and GFR after saline loading were significantly higher in L-arginine treatment (UNa, 47 +/- 12%; GFR, 44 +/- 31%) than in placebo treatment (UNa, 34 +/- 9%; GFR, 22 +/- 29%) (P < 0.05).

CONCLUSIONS

Oral administration of L-arginine has beneficial effects on glomerular filtration rate, natriuresis, and plasma endothelin level in patients with chronic congestive heart failure.

Abnormal renal vasodilation to an amino acid infusion in congestive heart failure: normalization by enalapril

In congestive heart failure (CHF), the neurohormonal mechanisms that cause renal vasoconstriction, particularly those depending on the renin-angiotensin system, could interfere with renal vasodilating mechanisms. To elucidate this issue, we studied the kidney response to an amino acid infusion (known to cause renal vasodilation in healthy individuals) in eight patients with CHF. We found that the amino acid infusion (0.7 mL/kg/h of a 10% solution) elicited no renal hemodynamic response, in marked contrast to healthy subjects. We next hypothesized that the renin-angiotensin system (known to be activated in heart failure) has a role in the lack of response to the amino acid infusion. To test this hypothesis, we repeated the study after two 5-mg doses of enalapril, an inhibitor of the angiotensin-converting enzyme, administered 12 hours apart. After enalapril treatment, the amino acid infusion caused a 45% increase in mean renal blood flow (RBF) from 383 +/- 55 to 557 +/- 51 mL/min at the fifth hour (P < 0.05). This normalization of the renal response to the amino acid infusion occurred without changes in cardiac output or in systemic vascular resistance. Hence, the renal fraction of the cardiac output increased during the amino acid infusion. The recovery of the renal vascular response was not accompanied by an increase in glomerular filtration rate (GFR; filtration fraction decreased), suggesting a predominant efferent arteriole dilatation. Our study shows that, in heart failure, the kidney loses its ability to increase RBF in response to an amino acid load. This lack of renal vascular response can be restored by inhibiting the renin-angiotensin system and is unrelated to changes in systemic hemodynamics.

Early impairment of renal hemodynamic reserve in patients with asymptomatic heart failure is restored by angiotensin II antagonism

BACKGROUND

The early/asymptomatic stages of heart failure (HF) are characterized by sodium retention secondary to derangement of sodium reabsorption at the proximal nephron level. Because this phenomenon is reversed by ACE inhibition, abnormalities of renal sodium handling may depend on intrarenal changes of angiotensin II (AII)/nitric oxide (NO) levels. Renal hemodynamic reserve (ie, the glomerular vasodilatory response to amino acid infusion) has been proposed as a reliable test to assess in vivo AII/NO balance.

METHODS AND RESULTS

In this study, the effects of 6 weeks of treatment with 5 mg/d of enalapril or with 50 mg/d of losartan on systemic hemodynamics and renal function were assessed, at baseline and after amino acid infusion (AA), in patients with mild HF (NYHA class I) and in healthy volunteers. Untreated HF patients showed a basal renal function comparable to that of healthy subjects. After AA, glomerular filtration rate and renal plasma flow significantly increased in healthy subjects (+29.0% and +30.4%, respectively), whereas no vasodilatory response was observed in HF. Although they did not affect basal renal hemodynamics, both enalapril and losartan restored a normal response to AA in HF patients. Blood pressure and heart rate were comparable in HF subjects and healthy subjects at baseline and were not modified by either treatment. Left ventricular ejection fraction was depressed in HF but did not change after either drug. Urinary excretions of cGMP and nitrate (indexes of NO activity in the kidney), comparable in healthy subjects and in HF patients, were unchanged by either enalapril or losartan and did not correlate with renal reserve.

CONCLUSIONS

(1) Renal functional reserve is absent in patients with early/asymptomatic HF and normal renal function and (2) both enalapril and losartan restore a normal vasodilatory response to AA in these patients without affecting basal systemic and renal hemodynamics. These data suggest a major role of AII in the development of early abnormalities in patients with HF.

Impaired renal haemodynamic response to amino acid infusion in essential hypertension during angiotensin converting enzyme inhibitor treatment

OBJECTIVE

To determine whether hyperfiltration induced by amino acid infusion can be influenced by angiotensin converting enzyme (ACE) inhibition.

DESIGN

We studied the acute effects of ramipril in 12 healthy control subjects and in 14 patients with essential hypertension. We studied also the effects of 2 months' treatment with ramipril inn 12 patients with essential hypertension and performed a time-control study without amino acids infusion with 12 control subjects. The glomerular filtration rate (GFR), renal plasma flow (RPF), fractional excretion of sodium (FENa) and fractional excretion of lithium (FELi) were determined during 6 clearance periods of 30 min each and amino acids infusion was administered during the last four periods. Plasma concentrations of angiotensin II, aldosterone, atrial natriuretic peptide (ANP), arginine vasopressin, insulin and glucagon were determined.

RESULTS

Both the GFR and the RPF increased markedly in healthy subjects after amino acid infusion both with (GFR 7%, RPF 7%) and without ramipril (GFR 7%), RPF 8%), both P < 0.05. Ramipril administered acutely to essential hypertensives prevented the amino acid-induced increase in RPF [with ramipril 5% (NS), without ramipril 9% (P < 0.05)]. The GFR increased equally with (5%) and without (8%) ramipril (P < 0.20). ACE inhibition after 2 months' treatment of essential hypertension blunted the amino acid-induced increase both in GFR and in RPF [with ramipril GFR 5% and RPF 3% (NS), without ramipril GFR 12%, RPF 11% (P < 0.05)]. The FENa did not change in all four experiments. The FELi, insulin and glucagon increased to the same extent in the first three experiments. ANP increased (P < 0.05) in control subjects both with and without ramipril; angiotensin II and aldosterone decreased significantly in control subjects without ramipril.

CONCLUSIONS

The renal haemodynamic response both after acute and after short-term ACE inhibition is attenuated in essential hypertension. Presumably, this treatment makes the arterioles at the glomeruli unresponsive to subsequent amino acid infusion. This inhibition of hyperfiltration might be an important mechanism for the renal protective effect of ACE inhibition in some renal diseases.