Renal Excretion of Sodium in Arterial Hypertension

The exaggerated natriuresis of essential hypertension occurs independently of changes in renal medullary blood flow

AIMS

In patients with essential hypertension, abnormal renal sodium handling includes exaggerated natriuresis in response to extracellular volume expansion. We tested the hypothesis that exaggerated natriuresis is associated with increases in medullary and/or cortical renal blood flow.

METHODS

Patients with mild essential hypertension, but no signs of end organ damage, and control subjects were studied after 4 days of dietary standardization (<60 mmol Na⁺  day^-1 ) preceded in patients by a 14-day drug washout period. On the study day, subjects received a 4-hour intravenous volume expansion with saline (2.1% of body mass). Renal medullary and cortical blood flows were measured by PET scanning using H₂ ¹⁵ O as tracer; anatomical regions of interest were defined by contrast-enhanced CT scanning.

RESULTS

In patients, arterial blood pressure increased during volume expansion (107 ± 2-114 ± 3 mm Hg, P < 0.05) in contrast to the control group (92 ± 2-92 ± 2 mm Hg). Renal sodium excretion increased more in patients than in controls (+133 ± 31 µmol min^-1 vs +61 ± 14 µmol min^-1 , respectively, P < 0.05) confirming exaggerated natriuresis. During volume expansion, renal medullary blood flow did not change significantly in patients (2.8 ± 0.4-2.5 ± 0.5 mL (g tissue)^-1  min^-1 ) or in controls (3.2 ± 0.3-3.1 ± 0.2 mL (g tissue)^-1 min^-1 ). In control subjects, renal cortical blood flow fell during volume expansion (4.1 ± 0.3-3.7 ± 0.2 mL (g tissue)^-1  min^-1 , P < 0.05) in contrast to patients in which deviations remained insignificant.

CONCLUSION

Exaggerated natriuresis, a hallmark of essential hypertension, is not mediated by increases in regional, renal blood flow.

Effect of sodium loading and exercise on renal haemodynamics and urinary sodium excretion in young patients with essential hypertension before and during propranolol treatment

The effects on renal plasma flow (RPF) and glomerular filtration rate (GFR) of an i.v. sodium load and exercise have been measured in 14 young patients with essential hypertension before and after 3-4 months' treatment with propranolol as well as in 10 normotensive control subjects. In the untreated hypertensive patients, RPF and GFR were significantly reduced during sodium loading and exercise. After propranolol treatment, RPF and GFR were unaffected by sodium loading but decreased during exercise. In the normotensive control group, GFR and RPF were unchanged during sodium loading; RPF decreased during exercise, wherease GFR was not significantly reduced. RPF was significantly lower in the untreated hypertensive patients than in the normotensive control subjects during sodium loading and exercise. Propranolol treatment induced a significant reduction of BP and heart rate. RPF and GFR were not altered by propranolol treatment during sodium loading. During exercise, however, RPF was significantly lower after treatment than before. Urinary sodium excretion during sodium loading was significantly higher in the hypertensive patients than in the control subjects and decreased significantly during propranolol treatment. The reason for the abnormal reduction in renal haemodynamics during sodium loading in patients with essential hypertension is not clarified, but may possibly be related to functional or structural alterations in the renal vascular bed. The lower RPF during exercise after treatment with propranolol is most likely caused by an inhibition of the cardiac output induced by beta-adrenergic blockade. It is conceivable that the reduced urinary sodium excretion during propranolol treatment is attributable to the reduction of BP.

Plasma renin activity and ischemic heart disease

BACKGROUND

An earlier prospective study reported an association between high levels of plasma renin activity (as measured by the renin-sodium profile) and the incidence of myocardial infarction in patients with hypertension. We have investigated the relation between plasma renin activity and ischemic heart disease in the Northwick Park Heart Study.

METHODS

The study included 803 white men 40 to 64 years of age selected from industrial workers in London. Plasma renin activity and established risk factors for ischemic heart disease were measured at entry, which was between 1972 and 1978. Ascertainment of the primary clinical end points of fatal or nonfatal myocardial infarction and sudden death from coronary causes was carried out until the end of 1991.

RESULTS

In an analysis of the 86 first coronary events, we found an independent relation between higher systolic blood pressure and coronary end points (relative risk per 1 SD increase in blood pressure, 1.47; 95 percent confidence interval, 1.16 to 1.85; P < 0.001), but no relation between plasma renin activity and coronary end points (relative risk per 1 SD increase in the level of plasma renin activity, 1.04; 95 percent confidence interval, 0.84 to 1.30). In the 242 men who had hypertension of a degree similar to that of the subjects in the earlier prospective study of the renin profile, and in whom 44 of the 86 coronary events occurred, the relative risk of those in the highest as compared with the lowest third for plasma renin activity was 1.26 (95 percent confidence interval, 0.63 to 2.56).

CONCLUSIONS

Our results suggest that there is no association between plasma renin activity and myocardial infarction or sudden death from coronary causes, at least in normotensive men.

Water loading and restriction in essential hypertension

Blood pressure, plasma arginine vasopressin (AVP), and renal excretory responses to short-term water loading (oral load of 20 ml/kg body weight over 30-45 minutes) were compared in 10 normotensive and 13 mild to moderately essential hypertensive subjects. In addition, we examined the renal concentrating ability of an additional group of 10 normotensive subjects and 12 hypertensive subjects in response to a 24-hour water restriction and intranasal administration of 10 micrograms of [1-deamino,8-D-arginine]vasopression. The hypertensive subjects exhibited both an exaggerated diuresis and natriuresis to the water load. At 20- and 60-minutes after water loading, hypertensive subjects had excreted 34 and 55% of the load, respectively, compared with 15 and 35% in normotensive subjects. Mean blood pressure rose significantly in both groups and hypertensive subjects exhibited a greater rise of systolic blood pressure (16 mm Hg) than normotensive subjects (8 mm Hg) 20 minutes after water loading. The maximum diuresis and natriuresis corresponded to the period in which the rise of blood pressure was greatest. The hypertensive subjects diluted and concentrated their urine as well as normotensive subjects did, indicating normal renal responsiveness to AVP. Plasma Na, osmolality, and AVP decreased similarly in both groups after water loading and rose similarly in the two groups after water restriction. This finding suggests that osmotic responsiveness of AVP is not altered in hypertensive subjects. In conclusion, the data suggest that the exaggerated renal response to water loading could be explained by the greater rise of blood pressure in hypertensive subjects rather than by altered AVP responses.

Effects of diet on exaggerated natriuresis in hypertension

The effects of high and low salt diet on exaggerated natriuresis after volume expansion were examined in two groups of hypertensive patients, 6 with labile and 6 with fixed uncomplicated essential hypertension. Fixed hypertensives eliminated the administered salt-load faster than the labile hypertensives, and diet had no effect on the exaggerated natriuresis of both groups. No association was observed between plasma renin activity and natriuresis in both groups of patients. We conclude that, 1) diet did not affect the saline-induced natriuresis in hypertensive patients, 2) there was no apparent association between plasma renin activity and exaggerated natriuresis in hypertension, 3) other factors such as the capacitance system and vasopressin may play an important role in volume expansion natriuresis, especially in fixed hypertensives.

Late follow-up in women with nephrosclerosis diagnosed at pregnancy

Thirteen nephrosclerotic women were followed for 2 to 7 years from the time a diagnosis was made following a pregnancy complicated by hypertension. Ten patients developed sustained hypertension. Twelve women who were examined responded with a hypertensive pattern to acute salt load. Of the 10 patients who were examined, seven had a reduced renal plasma flow (RPF) demonstrated by the phenolsulfonphthalein (PSP) excretion test. The present observations support the view that the vascular lesion in the kidneys precedes and persists independently of pregnancy. The pregnant state brings the hypertensive disease to clinical expression.

Renal function of conscious spontaneously hypertensive rats

Renal clearance studies were performed in conscious 13-week-old spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) before and during volume expansion by intravenous infusion of isotonic saline. Mean arterial pressure and filtration fraction were greater in SHR, whereas fractional and absolute excretion of sodium and water, glomerular filtration rate, and renal plasma flow in SHR and WKY were not statistically different. This was the case during hydropenia and volume expansion. We did not observe as exaggerated natriuresis after intravenous loading when unanesthetized SHR were compared with the response of WKY. These observations suggest that the kidneys of genetically hypertensive rats of the Okamoto-Aoki strain have adapted to an elevated renal perfusion pressure or that hypertension is required to normalize renal function so that excretion is appropriately matched with intake.

Natriuretic response to saline infusion in normotensive and hypertensive man. The role of renin suppression in exaggerated natriuresis

Previous studies have reported an exaggerated natriuresis in hypertensive man; however, a systematic appraisal of this response in various forms of hypertension has not been made. We measured fractional excretion of sodium (FENa) during a four hour intravenous infusion of 2 liters normal saline in 162 normal subjects and 120 hypertensives. Of these, 13 had primary aldosteronism (ALDO), 19 high renin (HRH), 30 low renin (LRH), and 57 normal renin (NRH) essential hypertension. FENa for normals (1.42%), NRH (1.57%), and HRH (1.46%) was similar. That for LRH (2.56%) and ALDO (4.18%) was elevated compared to the other three subgroups (P less than 0.001). Although the four hour FENa during saline infusion was associated with mean atrterial blood pressure (MABP) within the entire hypertensive population (r = 0.51), when the subgroups of the hypertensive patients were considered separately no association between FENa and MABP was identified. Moreover, the MABP of subjects with HRH was greater (P less than 0.05) than in those with NRH, although the FENa of the two subgroups was similar. Patients with ALDO and LRH have a greater natriuretic response to a salt load than do other subgroups of essential hypertension or normal subjects. The exaggerated natriuresis appears to be a feature of hypertension with renin suppression. The degree of exaggerated natriuresis in not solely a function of an elevated mean arterial blood pressure.

Renal excretion of sodium during oral water administration in patients with systemic hypertension

The renal excretion of sodium and water was studied in 10 hypertensive subjects following an oral sustained 20 ml/kg water load. The study was performed under conditions controlled for sodium content of the diet, time of the day, and posture; the urine was collected by spontaneous voiding. The results were compared with those obtained from 10 normotensive subjects studied under similar conditions.

The increased urine flow following the water load in the hypertensive subjects was not accompanied by increased excretion of sodium, a pattern similar to the one observed in the normotensive subjects.

Comparing the group of hypertensive subjects with previously studied normotensives revealed the following additional information. The hypertensive subjects responded to water administration by increasing volume and C H H2 O more rapidly than the normotensives; however, the maximal response was not significantly different. The fractional reabsorption of sodium was less in the hypertensive subjects despite a lower filtered load of sodium. This suggests a difference in the renal tubular handling of sodium between hypertensive and normotensive subjects.

Effects of five per cent dextros-water infusions in normal and hypertensive man. Evidence for increased proximal and distal tubular sodium rejection by hypertensive patients and its relation to renal hemodynamics

During infusions of 5% dextrose in water urinary sodium excretion and the renal tubular rejection of filtered sodium (E/F Na %) were significantly higher in hypertensive than in normotensive subjects. Increased E/F Na % did not result from alterations in plasma sodium, in filtered sodium, or from an osmotic diuresis.

Fractional sodium reabsorption in proximal (isosmotic) portions of the nephron was diminished in the hypertensive patients. Free water formation (C H H2 o rose with increased "distal" sodium delivery (V) in both groups; however, fractional sodium reabsorption in the "distal" nephron was significantly impaired in the hypertensive patients and urinary osmolality was increased.

Mean arterial blood pressure and E/F Na % were related in curvilinear fashion in the 31 studies, and there was a direct relation between E/F Na % and the magnitude of renal vascular resistance. The data suggested that alterations of renal arterial pressure and vascular resistance in hypertensive disease modify sodium transport in proximal and diluting segments of the nephron and determine to a major extent the increased natriuresis exhibited by hypertensive subjects during infusions that expand extracellular fluid volume.

Effect of a simple deconditioning procedure on the diuretic and natriuretic response of hypertensive patients to a hypertonic salt load

The increased diuretic and natriuretic response of hypertensive patients to an intravenous salt load was modified toward normal by a deconditioning procedure which consisted in repeated exposure to the external situation of the test without, however, infusing any fluid into the vein. After this deconditioning, ratios between amounts of urine and sodium excreted in the first and the second 90-minute period of the test and the total 3-hour quantity of urine excreted were significantly reduced, but blood pressure was not changed. The response of normotensive patients to the same deconditioning was modified in the same direction but to a much smaller extent. It is suggested that the increased diuretic and natriuretic responses of hypertensive patients to an intravenous salt load are similar to the hyperreactive pressor and diuretic responses of these patients when exposed to stimuli that seem threatening to them, and that these responses are caused (at least in part) by hyperreactivity of the central nervous system to the external situation of the test.

Studies on the Mechanism of the Abnormal Sodium Excretion in Arterial Hypertension

Individuals with uncomplicated essential hypertension exhibit an abnormally high excretion of sodium and an associated impairment of urinary diluting and concentrating ability. Various possibilities have been considered to explain these related excretory changes including a solute diuresis resulting from diminished proximal tubular reabsorption of sodium, a reduction in sodium reabsorption from the ascending limb of the loop of Henle, and an increase in medullary blood flow. Evidence has been presented that does not support a defect in the sodium transport mechanism. On the other hand, the observed excretory abnormalities, when considered in terms of the reported pressure-dependent nature of the medullary circulation, could result from an increase in blood flow through the vasa recta secondary to the arterial hypertension.