Control of Sodium Excretion: An Integrative Approach

The kidney, hypertension, and obesity

This paper provides a personal perspective of the role of abnormal renal-pressure natriuresis in the pathogenesis of hypertension. Direct support for a major role of renal-pressure natriuresis in long-term control of arterial pressure and sodium balance comes from studies demonstrating that (1) pressure natriuresis is impaired in all forms of chronic hypertension and (2) prevention of pressure natriuresis from operating, by servo-control of renal perfusion pressure, also prevents the maintenance of sodium balance hypertension. Although the precise mechanisms of impaired pressure natriuresis in essential hypertension have remained elusive, recent evidence suggests that obesity and overweight may play a major role. Obesity increases renal sodium reabsorption and impairs pressure natriuresis by activation of the renin-angiotensin and sympathetic nervous systems and by altered intrarenal physical forces. Chronic obesity also causes marked structural changes in the kidneys that eventually lead to a loss of nephron function, further increases in arterial pressure, and severe renal injury in some cases. Although there are many unanswered questions about the mechanisms of obesity hypertension and renal disease, this is one of the most promising areas for future research, especially in view of the growing, worldwide "epidemic" of obesity.

Role of the angiotensin type 2 receptor in the regulation of blood pressure and renal function

The renin-angiotensin system is a major physiological regulator of body fluid volume, electrolyte balance, and arterial pressure. Virtually all of the biological actions of the principle effector peptide angiotensin II (ANG II) have been attributed to an action at the type 1 (AT(1)) ANG receptor. Until recently, the functional role of the type 2 (AT(2)) receptor, if any, has been unknown, possibly because the AT(2) receptor has a low degree of expression compared with that of the AT(1) receptor. Evidence has now accumulated that the AT(2) receptor opposes functions mediated by the AT(1) receptor. Whereas the AT(1) receptor stimulates cell proliferation, the AT(2) receptor inhibits proliferation and promotes cell differentiation. These differences in growth responses have been ascribed to different cell signaling pathways in which the AT(1) receptor stimulates protein phosphorylation and the AT(2) receptor dephosphorylation. During the past 5 years, studies have demonstrated that the AT(2) receptor is responsible for vasodilation and natriuresis, thus opposing the vasoconstrictor and antinatriuretic effects of ANG II mediated through the AT(1) receptor. Work from our laboratory and others indicates that the AT(2) receptor stimulates vasodilation and natriuresis by an autocrine cascade including bradykinin, nitric oxide, and cyclic GMP. The AT(2) receptor also has been found to control vasodilator prostaglandins, which have a role in blood pressure regulation. The AT(2) receptor appears to play a counterregulatory protective role in the regulation of blood pressure and sodium excretion that opposes the AT(1) receptor.

Effects of acute AV3V lesions on renal and hindlimb vasodilation induced by volume expansion

The role of the anteroventral third ventricle (AV3V) region in the cardiovascular adjustments to volume expansion (VE) with 4% Ficoll (1% body weight, 1.4 mL/min) was studied in urethane-anesthetized rats. In sham-lesioned animals, VE produced a transitory (</=20 minutes) increase in mean arterial pressure, which peaked at 10 minutes (10+/-3 mm Hg), and sustained increases of renal (123+/-10% and 127+/-6% of baseline, respectively, 10 and 40 minutes after VE) and hindlimb vascular (157+/-19% and 153+/-9% of baseline) conductance. After AV3V lesions, VE induced a sustained increase in mean arterial pressure. Although renal blood flow increased in response to VE, renal vascular conductance was unaffected, indicating that renal vasodilation was abolished. On the other hand, after AV3V lesions, the increases in hindlimb blood flow and vascular conductance were higher than those observed in sham-lesioned rats. Results obtained demonstrated that the AV3V region is essential for the renal vasodilation induced by VE.

Effect of renal perfusion pressure on renal interstitial hydrostatic pressure and sodium excretion. Role of vasopressin V1 and V2 receptors

Renal interstitial hydrostatic pressure (RIHP) has recently been cited as an important mediator of pressure natriuresis. Our objective was to determine the roles of vasopressin V1 and V2 receptors in mediating the effects of renal perfusion pressure (RPP) on RIHP and sodium excretion (UNaV). The effects of RPP on renal hemodynamics, RIHP, and UNaV were assessed in control Wistar rats (n = 10) and in rats pretreated with intravenous infusion of the specific nonpeptide vasopressin V1 antagonist OPC-21268 (100 micrograms.kg-1.min-1; n = 8) and the V2 antagonist OPC-31260 (40 micrograms.kg-1.min-1; n = 10). Increasing RPP from 95 to 118 mm Hg in control rats increased RIHP (6.4 +/- 1.0 to 9.9 +/- 1.3 mm Hg), UNaV (0.29 +/- 0.03 to 0.52 +/- 0.05 muEq.min-1.g-1), urine flow rate (UFR) (5.2 +/- 0.3 to 7.6 +/- 0.6 microL.min-1.g-1), and the fractional excretion of sodium (FENa). In rats pretreated with V1 antagonist, similar results were obtained for urine osmolality and the responses of RIHP, UNaV, UFR, and FENa to RPP. V2 antagonist reduced urine osmolality (392 +/- 47 compared with 979 +/- 88 mOsm.kg-1 in control rats) and enhanced the responses of UNaV (0.43 +/- 0.08 to 1.32 +/- 0.32 microEq.min-1), UFR (17.8 +/- 3.2 to 29.2 +/- 3.8 microL.min-1.g-1), and FENa to RPP, but the RIHP response resembled that observed in the control and V1 antagonist groups. Renal blood flow and glomerular filtration rate did not differ among the three groups.(ABSTRACT TRUNCATED AT 250 WORDS)