Low-pressure receptor activity and exaggerated natriuresis in essential hypertension

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.

Naloxone inhibits renal hemodynamic effect of head-out water immersion in humans

Head-out water immersion (HOI) is followed by renal vasodilation and natriuresis, in association with a fall in blood pressure. The latter suggests an exaggerated sympathetic suppression. We studied the role of endogenous opioids on the renal response to HOI. Six healthy subjects underwent four- hour clearance studies during: (1) time control; (2) naloxone 0.1 mg/kg i.v. bolus, followed by 0.1 mg/kg/hr; (3) HOI; and (4) concomitant HOI and naloxone administration. Compared to the time control study, naloxone had no effects on mean arterial pressure (MAP), plasma renin activity (PRA), aldosterone, catecholamines, atrial natriuretic peptide (ANP), glomerular filtration rate (GFR), estimated renal plasma flow (ERPF), and sodium excretion. HOI caused significant decrements of MAP, PRA, aldosterone, and catecholamines, and increased ANP, GFR, from 94 +/- 5 to 102 +/- 5 ml/min (P < 0.01), ERPF, from 529 +/- 30 to 616 +/- 35 ml/min (P < 0.01), and sodium excretion. Renal blood flow increased as well, and calculated renal vascular resistance decreased from 99 +/- 6 to 77 +/- 5 mm Hg.min.liter-1 (P < 0.01). HOI during concomitant naloxone administration had similar effects on MAP and humoral factors, however, caused no change in GFR, ERPF and renal blood flow, and the fall in renal vascular resistance, from 98 +/- 6 to 83 +/- 5 mm Hg.min.liter-1 (P < 0.05) was significantly less than found in the absence of naloxone (P < 0.05). The natriuretic effect was undisturbed. These data suggest that endogenous opioids play a role in the response to HOI, in particular, potentiate the renal vasodilatory response.

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.

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.

Renal hemodynamics and natriuresis during water immersion in normal humans

The renal vascular and functional responses to acute "central hypervolemia" by water immersion to the neck were determined in six normal subjects. During isotonic-isooncotic expansion by water immersion there was a significant increase in urine flow from 1.1 +/- 0.1 to 6.9 +/- 1.0 ml/min (p less than 0.05) and sodium excretion from 99.1 +/- 8.8 to 300 +/- 28 mu Eq/min (p less than 0.05). Glomerular filtration rate did not change while renal blood flow significantly increased during water immersion. Deep intrarenal venous pressure (IRVP) increased from 18.2 +/- 1.4 to 32 +/- 1.7 mmHg (p less than 0.05) while mean arterial pressure was unchanged. This marked natriuresis seen during water immersion was associated with reduced renal vascular resistance and increased deep intrarenal venous pressure demonstrating that continued natriuresis could relate to increased capillary hydrostatic pressure.