Renal interstitial hydrostatic pressure and PGE2 in pressure natriuresis

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

Inhibition of the formation of EETs and 20-HETE with 1-aminobenzotriazole attenuates pressure natriuresis

This study examined the effects of chronic blockade of the renal formation of epoxyeicosatrienoic acids and 20-hydroxyeicosatetraenoic acid with 1-aminobenzotriazole (ABT; 50 mg·kg−1· day−1ip for 5 days) on pressure natriuresis and the inhibitory effects of elevations in renal perfusion pressure (RPP) on Na+-K+-ATPase activity and the distribution of the sodium/hydrogen exchanger (NHE)-3 in the proximal tubule of rats. In control rats ( n = 15), sodium excretion rose from 2.3 ± 0.4 to 19.4 ± 1.8 μeq·min−1·g kidney weight−1when RPP was increased from 114 ± 1 to 156 ± 2 mmHg. Fractional excretion of lithium rose from 28 ± 3 to 43 ± 3% of the filtered load. Chronic treatment of the rats with ABT for 5 days ( n = 8) blunted the natriuretic response to elevations in RPP by 75% and attenuated the increase in fractional excretion of lithium by 45%. In vehicle-treated rats, renal Na+-K+-ATPase activity fell from 31 ± 5 to 19 ± 2 μmol Pi·mg protein−1·h−1and NHE-3 protein was internalized from the brush border of the proximal tubule after an elevation in RPP. In contrast, Na+-K+-ATPase activity and the distribution of NHE-3 protein remained unaltered in rats treated with ABT. These results suggest that cytochrome P-450 metabolites of arachidonic acid contribute to pressure natriuresis by inhibiting Na+-K+-ATPase activity and promoting internalization of NHE-3 protein from the brush border of the proximal tubule.

Mechanisms of pressure natriuresis

A central component of the feedback system for long-term control of arterial pressure is the pressure-natriuresis mechanism, whereby increases in renal perfusion pressure lead to decreases in sodium reabsorption and increases in sodium excretion. The specific intrarenal mechanism for the decrease in tubular reabsorption in response to increases in renal perfusion pressure appears to be related to increases in hemodynamic factors such as medullary blood flow and renal interstitial hydrostatic pressure (RIHP), and renal autocoids such as nitric oxide, prostaglandins, kinins, and angiotensin II. Increases in renal perfusion pressure are associated with significant increases in RIHP, nitric oxide, prostaglandin E2, and kinins, and decreases in angiotensin II. The mechanism whereby RIHP increases in the absence of discernible changes in whole kidney renal blood flow and peritubular capillary hydrostatic and/or oncotic pressures may be related to increases in renal medullary flow as a result of nitric oxide-induced reductions in renal medullary vascular resistance. Several lines of investigation support an important quantitative role for RIHP in mediating pressure natriuresis. Preventing RIHP from increasing in response to increases in renal perfusion pressure markedly attenuates pressure natriuresis. Furthermore, direct increases in RIHP, comparable to increases measured in response to increases in renal perfusion pressure, have been shown to significantly decrease tubular reabsorption of sodium in the proximal tubule and increase sodium excretion. The exact mechanism whereby RIHP influences tubular reabsorption is unknown, but may be related to alterations in tight junctional permeability to sodium in proximal tubules, redistribution of apical sodium transporters, and/or release of renal autacoids such as prostaglandin E2.

Decreased sensitivity to renal interstitial hydrostatic pressure in Dahl salt-sensitive rats

The ability of Dahl salt-sensitive (DS) rats to excrete a sodium load is significantly lower than Dahl salt-resistant (DR) rats. Because renal interstitial hydrostatic pressure (RIHP) is a major mediator of natriuresis in response to a sodium load, we proposed that the renal tubules of DS rats are less responsive to increases in RIHP than those of DR rats. To test this hypothesis, we determined the effect of direct increases in RIHP on renal excretory function in prehypertensive DS and DR rats. RIHP was directly increased by renal interstitial volume expansion via injection of 50 microL of a 2% albumin and saline solution into the renal interstitium through a chronically implanted renal interstitial catheter. RIHP, mean arterial pressure, glomerular filtration rate, urine flow rate, urinary sodium excretion, and fractional excretions of sodium, potassium, and lithium (an indicator of proximal tubule sodium handling) were measured before and after direct increases in RIHP in DS (n = 8) and DR (n = 8) rats. Baseline urine flow rate; urinary sodium excretion; fractional excretions of sodium, potassium, and lithium; RIHP; mean arterial pressure; and glomerular filtration rate were not different between DS and DR rats. Renal interstitial volume expansion in DS rats significantly increased RIHP (delta 4.7 +/- 0.8 mm Hg), urine flow rate (delta 14.5 +/- 3.4 microL/min), urinary sodium excretion (delta 2.62 +/- 0.62 mumol/min), and fractional excretions of sodium (delta 1.54 +/- 0.37%), potassium (delta 17.84 +/- 2.90%), and lithium (delta 19.68 +/- 3.52%).(ABSTRACT TRUNCATED AT 250 WORDS)