Spectrum of deranged sodium homeostasis in essential hypertension

Reduced nephron endowment due to fetal uninephrectomy impairs renal sodium handling in male sheep

Reduced nephron endowment is associated with development of renal and cardiovascular disease. We hypothesized this may be attributable to impaired sodium homoeostasis by the remaining nephrons. The present study investigated whether a nephron deficit, induced by fetal uninephrectomy at 100 days gestation (term=150 days), resulted in (i) altered renal sodium handling both under basal conditions and in response to an acute 0.9% saline load (50 ml.kg-1 of body weight.30 min-1); (ii) hypertension and (iii) altered expression of renal channels/transporters in male sheep at 6 months of age. Uninephrectomized animals had significantly elevated arterial pressure (90.1+/-1.6 compared with 77.8+/-2.9 mmHg; P<0.001), while glomerular filtration rate and renal blood flow (per g of kidney weight) were 30% lower than that of the sham animals. Total kidney weight was similar between the groups. Renal gene expression of apical NHE3 (type 3 Na+/H+ exchanger), ENaC (epithelium Na+ channel) beta and gamma subunits and basolateral Na+/K+ ATPase beta and gamma subunits were significantly elevated in uninephrectomized animals, while ENaC alpha subunit expression was reduced. Urine flow rate and sodium excretion increased in both groups in response to salt loading, but this increase in sodium excretion was delayed by approximately 90 min in the uninephrectomized animals, while total sodium output was 12% in excess of the infused load (P<0.05). In conclusion, the present study shows that animals with a congenital nephron deficit have alterations in tubular sodium channels/transporters and cannot rapidly correct for variations in sodium intake probably contributing to the development of hypertension. This suggests that people born with a nephron deficit should be monitored for early signs of renal and cardiovascular disease.

Nitric oxide synthesis inhibition does not impair water immersion-induced renal vasodilation in humans

Nitric oxide (NO) is tonically released in the kidney to maintain renal perfusion and adequate sodium and water clearance. Little is known about the role of NO in the renal adaptation to an acute volume challenge. This is important for our understanding of pathophysiologic conditions associated with impaired NO activity. This study examined the effects of NO synthesis inhibition on neurohumoral, renal hemodynamic, and excretory responses to head-out immersion (HOI). Seven healthy men underwent four 7-h clearance studies. One study served as a time control study (placebo infusion), and in one study N(G)-monomethyl-L-arginine (L-NMMA; 3 mg/kg priming dose + 3 mg/kg per h) was infused during hours 2 to 5. In a third and fourth clearance study, HOI was applied from hours 3 to 5, during infusion of either placebo or L-NMMA. To assess the degree of NO synthesis inhibition, the effect of L-NMMA on [(15)N]-arginine-to-[(15)N]-citrulline conversion rate was studied in four others. HOI decreased mean arterial pressure (MAP) from 87 +/- 3 to 76 +/- 2 mmHg and renal vascular resistance (RVR) from 82 +/- 6 to 70 +/- 7 mmHg. min/L, and increased sodium excretion (UNaV) from 110 +/- 27 to 195 +/- 29 micromol/min and flow (UV) from 14.4 +/- 1.4 to 15.8 +/- 1.4 ml/min. L-NMMA caused profound and sustained increases in MAP and RVR and decreases in UNaV and UV. HOI superimposed on L-NMMA infusion decreased the elevated MAP from 93 +/- 4 to 83 +/- 2 mmHg and RVR from 111 +/- 9 to 95 +/- 7 mmHg. min/L, and increased UNaV from 41 +/- 8 to 95 +/- 15 micromol/min and UV from 10.0 +/- 1.1 to 12.7 +/- 1.4 ml/min. The relative changes were not significantly different from the effects of HOI without L-NMMA infusion. HOI decreased plasma renin activity and aldosterone and increased plasma atrial natriuretic peptide and urinary cGMP. L-NMMA decreased urinary cGMP, but did not affect the plasma hormones or the changes induced by HOI. L-NMMA decreased the [(15)N]-arginine-to-[(15)N]-citrulline conversion rate to one-third of baseline. The results indicate that in a state of NO deficiency in humans, the kidney can still respond to an acute volume challenge with vasorelaxation, diuresis, and natriuresis.

Modifications induced by -10 degrees Trendelenburg's posture in sodium tubular handling in patients with essential hypertension

In essential hypertensive patients "exaggerated natriuresis" is a response to acute volume expansion. However, the underlying mechanisms for this remain to be determined. We studied 19 patients with essential hypertension (HP) and 9 normotensive subjects (NS). In all examined subjects the response to acute central volume expansion, without the plasma compositional change that Trendelenburg's position involves, was evaluated during 90 min (period T) after a similar period of deambulation (period D). Mean blood pressure (MBP), tubular sodium handling by the lithium clearance technique, plasma renin activity (PRA), plasma aldosterone (PA), plasma catecholamines and urine prostaglandine E2 and kallikrein were assessed after D and T. MBP was significantly higher in HP than in NS (p = 0.00001). HP showed "exaggerated natriuresis" after T (fractional excretion of sodium increased from 0.55 +/- 0.1% after D to 1.20 +/- 0.2% after T, p < 0.01). This was because of a decrease in their proximal fractional reabsorption of sodium (from 74.96 +/- 1.8% after D to 62.50 +/- 2.8% after T, p < 0.01). Plasma epinephrine and plasma dopamine after T were significantly lower than in standing position in HP (p < 0.01) but no in NS. The decrease in plasma renin activity after T in HP was 53%, and 32% in NS. There were not any significant differences between groups in the other neurohormonal systems studied. We conclude that the major determinant of "exaggerated natriuresis" in hypertensive patients is a higher stimulation of the cardiopulmonary receptors following Trendelenburg's position and consequently stronger reflex inhibition of sympathetic system activity and renin-angiotensin II activity. The "exaggerated natriuresis" after Trendelenburg's position in HP was an expression of abnormal pressure natriuresis.

Calcium and sodium handling during volume expansion in essential hypertension

To evaluate the actual role of extracellular fluid volume (ECFV) expansion per se in modulating the rate of urinary calcium excretion, a thermoneutral water immersion (WI) study was conducted in 10 normal subjects and 30 patients with essential hypertension. Central hypervolemia by 2 hours of WI caused a significant diuretic and natriuretic response (P < .005) in normal subjects; no significant changes were detected in urinary calcium and magnesium excretion. WI provoked either an appropriate or exaggerated natriuresis (P < .001) in 21 hypertensive patients; these subjects also exhibited a highly positive correlation between urinary sodium and calcium excretion during WI (P < .001). In the remaining nine hypertensive patients, WI produced a significant diuretic response, but a barely discernible (P = NS) natriuresis (inappropriate response). These subjects also exhibited a significant reduction of urinary calcium (P < .001) and magnesium (P < .01) excretion. The data indicate that (1) volume expansion per se may have a role in regulating calcium excretion in hypertensive subjects; (2) a calcium leak may be attributable to a close relationship between urinary sodium and calcium metabolism, and causally related to a disturbance of sodium and volume homeostasis in hypertension.

Effect of chronic sodium loading on cardiovascular response in young blacks and whites

The effect of long-term oral sodium loading on blood pressure and on stress-induced cardiovascular response was studied in normotensive and marginally hypertensive young adults. The 121 subjects, 18-23 years old, included 38 whites and 83 blacks. Blood pressure and heart rate response to the stress of mental arithmetic was measured before and after 14 days of sodium load, which consisted of 10 g NaCl/day added to the usual diet. A sodium-sensitive response to sodium load occurred in 18.4% of whites and 37.3% of blacks. Sodium-insensitive subjects had a higher rate of sodium excretion (p less than 0.001). Sodium-sensitive hypertensive subjects had a significantly greater weight gain (p less than 0.001). A significant correlation between blood pressure change and sodium excretion (r = -0.28, p less than 0.01) occurred in the sodium-sensitive group. The high sodium intake did not augment the blood pressure or heart rate response to the beta-adrenergic-mediated stimulus of mental arithmetic in the population when grouped by blood pressure, race, or sodium sensitivity. These results suggest that blood pressure increase in response to sodium load, particularly in blacks, is related to functional changes in peripheral vascular resistance.

Renal nerves in the pathogenesis of hypertension: a review

1. Renal nerve discharge may contribute to renal arteriolar hyperplasia and hypertrophy but the kidney is not the major site of increased vascular resistance in hypertension. 2. Natriuretic and diuretic responses to dietary salt are dissociated in some SS hypertensives. Experiments with perfused rat kidneys indicate that this dissociation is due to circulating catecholamines and not to increased renal nerve activity. 3. Reduced tubular dopamine production may be involved in SS hypertension but decreased dopaminergic nerve activity is not apparent. 4. Increased salt intake initiates a generalized increase in vascular resistance in SS hypertensives. The small amount of salt retained over 5-7 days could not produce the observed increase in blood pressure by altering ECF volumes or transmembrane ion gradients which suggests that a systemic vasoconstrictor, such as renin, is involved. In these patients, inappropriate renal nerve activity could shift the balance between salt intake, blood pressure and renin release since low level renal nerve activity raises the blood pressure required to inhibit renin release. Reversal of SS hypertension with ACE inhibitors is consistent with this hypothesis.

Evidence that dopaminergic sympathetic axons supply the medullary arterioles of human kidney

The ability of the kidney to excrete sodium appears to depend on release of dopamine from intrarenal sources. In the present study, we have used immunohistochemistry to examine the possibility that renal dopaminergic nerves constitute one of these sources. We found that the sympathetic axons supplying cortical structures in human kidney contain tyrosine hydroxylase-like immunoreactivity but lack DOPA decarboxylase-like immunoreactivity. By contrast, the vasa recta arterioles of the renal medulla are supplied by varicose tyrosine hydroxylase-positive nerve fibres, some of which also contain DOPA decarboxylase. As DOPA decarboxylase has been demonstrated in other situations to be a selective marker for dopaminergic terminal axons, our results suggest the innervation of renal medullary blood vessels in man by both noradrenergic and dopaminergic sympathetic nerves.

Role of renal hemodynamics in the exaggerated natriuresis of essential hypertension

Extracellular fluid volume expansion is known to produce exaggerated natriuresis in essential hypertension. In order to assess the role of hemodynamic and intrarenal physical factors upon natriuretic response to central volume expansion, two hour water immersion (WI) experiments were made in six uncomplicated essential hypertensives and six normotensive healthy controls. Before and during WI we measured mean arterial pressure (MAP), urine flow (V/min), sodium (UNaV) and potassium (UKV) excretion, glomerular filtration rate (GFR), effective renal plasma flow (ERPF) and intrarenal (wedged) venous pressure (IRVP). In comparison with normotensive controls, the exaggerated natriuretic response in hypertensives (P less than 0.05 for UNaV during WI) was associated with an enhanced vasodilating response as demonstrated by a greater increase in ERPF (P less than 0.05) and by a more pronounced fall in calculated renal precapillary resistances (P less than 0.05). A more significant increase in IRVP was found in hypertensive group (P less than 0.05). Glomerular filtration rate (GFR) did not change in either group during WI. MAP, unchanged in normotensives, was significantly reduced in hypertensives (P less than 0.05), while remaining in the hypertensive range. These findings suggest that intrarenal physical factors play a major role in determining the exaggerated natriuresis during WI in hypertensive man.

Endogenous renal dopamine and control of blood pressure

Activation of specific receptors for dopamine in the renal vasculature and tubules leads to increases in glomerular filtration, and to diuresis and natriuresis. There is evidence for intrarenal production and release of dopamine, which may originate from two sources: tubular decarboxylation of plasma l-DOPA and a population of dopaminergic sympathetic neurons that innervate the renal cortex. Studies of plasma and urinary catecholamine levels indicate that dopamine is released within the kidney in response to sodium loading and to activation of sensory pathways related to nociception and chemoreception. There is also evidence for deficient renal release of dopamine in patients with renovascular or essential hypertension. Collectively, the available data suggest that intrarenal dopamine has a physiological function in control of blood volume and blood pressure, and that defects in this control may be implicated in the aetiology of some hypertensive states.