Impaired autoregulation of blood flow and glomerular filtration rate in the isolated dog kidney depleted of renin

Pentosan polysulfate preserves renal microvascular P2X1 receptor reactivity and autoregulatory behavior in DOCA-salt hypertensive rats

Inflammation contributes to ANG II-associated impairment of renal autoregulation and microvascular P2X1 receptor signaling, but its role in renal autoregulation in mineralocorticoid-induced hypertension is unknown. Autoregulatory behavior was assessed using the blood-perfused juxtamedullary nephron preparation. Hypertension was induced in uninephrectomized control rats (UNx) by subcutaneous implantation of a DOCA pellet plus administration of 1% NaCl in the drinking water (DOCA-salt) for 3 wk. DOCA-salt rats developed hypertension that was unaltered by anti-inflammatory treatment with pentosan polysulfate (DOCA-salt+PPS) but was suppressed with "triple therapy" (hydrochlorothiazide, hydralazine, and reserpine; DOCA-salt+TTx). Baseline arteriolar diameters were similar across all groups. UNx rats exhibited pressure-dependent vasoconstriction with diameters declining to 69 ± 2% of control at 170 mmHg, indicating intact autoregulation. DOCA-salt treatment significantly blunted this pressure-mediated vasoconstriction. Diameters remained between 91 ± 4 and 98 ± 3% of control over 65-170 mmHg, indicating impaired autoregulation. In contrast, pressure-mediated vasoconstriction was preserved in DOCA-salt+PPS and DOCA-salt+TTx rats, reaching 77 ± 7 and 75 ± 3% of control at 170 mmHg, respectively. ATP is required for autoregulation via P2X1 receptor activation. ATP- and β,γ-methylene ATP (P2X1 receptor agonist)-mediated vasoconstriction were markedly attenuated in DOCA-salt rats compared with UNx (P < 0.05), but significantly improved by PPS or TTx (P < 0.05 vs. DOCA-salt) treatment. Arteriolar responses to adenosine and UTP (P2Y2 receptor agonist) were unaffected by DOCA-salt treatment. PPS and TTx significantly reduced MCP-1 and protein excretion in DOCA-salt rats. These results support the hypothesis that hypertension triggers inflammatory cascades but anti-inflammatory treatment preserves renal autoregulation in DOCA-salt rats, most likely by normalizing renal microvascular reactivity to P2X1 receptor activation.

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.

Impaired myogenic responsiveness of renal microvessels in Dahl salt-sensitive rats

The mechanisms mediating abnormal renal autoregulation in Dahl salt-sensitive (DS) rats have not been fully defined. In the present study, we assessed myogenic responsiveness of interlobular arteries (ILAs), afferent arterioles (AAs), and efferent arterioles in isolated perfused hydronephrotic Dahl rat kidneys. Dahl rats were divided into four groups according to strain (Dahl salt-resistant [DR] or DS rats) and dietary sodium manipulation (rats fed low or high salt diets). Systolic blood pressure was elevated only in DS rats fed the high salt diet (202 +/- 4 mm Hg, p less than 0.05). Myogenic responses were obtained by stepwise elevation of renal arterial pressure. Vessel diameters were determined by computer-assisted videomicroscopy. Preglomerular microvessels of DS and DR rats responded differently to changes in renal arterial pressure. AAs and ILAs manifested diminished myogenic responsiveness to increasing renal arterial pressure in DS rats compared with DR rats (p less than 0.05). Both AAs and ILAs in DS rats manifested a higher threshold pressure for eliciting myogenic responses and a decrease in maximal pressure-induced vasoconstriction. The sensitivity of the AA myogenic response to nifedipine was enhanced in DS rats compared with DR rats (p less than 0.05). For rats fed the high salt diet, preglomerular vessels exhibited reduced myogenic responsiveness in both strains. In contrast to preglomerular microvessels, efferent arterioles from all four groups of rats failed to exhibit pressure-induced vasoconstriction. Our data suggest that diminished myogenic responsiveness of AAs and ILAs in DS rats contributes to impaired renal autoregulation in this strain.

Sodium balance and renin regulation in rats: role of intrinsic renal mechanisms

Renin secretion was compared in vivo and in vitro among five groups of rats, each group subjected to a different sodium balance for 10 to 14 days. The state of the renin-angiotensin system in vivo was evaluated by measuring the renal renin (RR) and the plasma renin concentration (PRC) in both anesthetized and nonanesthetized animals. The in vitro renin secretion rate (RSR) was determined in isolated perfused kidneys. RR was reduced (-48%) by sodium loading and deoxycorticosterone (DOCA) and increased (+27%) by sodium deprivation and furosemide. Sodium loading and DOCA reduced both the PRC and the RSR to less than 20% of control values. By contrast, sodium deprivation and furosemide induced a more than fourfold rise in the PRC but only a small increase in the RSR (+37%). These results indicate that changes in fractional renin release are induced by sodium balance variation, and these changes are preserved in vitro only in sodium-loaded states. The inability of sodium-deprived kidneys to maintain high renin release in vitro suggests that high plasma renin levels in these rats depend on mechanisms that are not preserved in vitro. There was no evidence supporting the participation of inactive renin secretion in the regulation of fractional renin release under varying sodium balance.

A comparison of the ability of two angiotensin II receptor blocking drugs, 1-Sar; 8-Ala angiotensin II and 1-Sar, 8-Ile angiotensin II, to modify the regulation of glomerular filtration rate in the cat

1 Modest stimulation of the renal nerves in the anaesthetized unilaterally nephrectomized cat resulted in a 15% fall in renal blood flow, no change in glomerular filtration rate and significant falls in both the absolute and fractional rates of sodium excretion.2 The haemodynamic responses to nerve stimulation were not modified by angiotensin II blockade with 1-Sar, 8-Ala angiotensin II although the fall in absolute, but not fractional sodium excretion was significantly larger. In contrast, stimulation of renal nerves following administration of 1-Sar, 8-Ileangiotensin II caused a significant fall in glomerular filtration rate. The reductions in both absolute and fractional sodium were of the same magnitude as in the absence of drug.3 Both renal blood flow and glomerular filtration rate were autoregulated during the reduction of renal perfusion pressure and this was associated with reductions in both absolute and fractional sodium excretions.4 In the presence of 1-Sar, 8-Ala angiotensin II, the haemodynamic and sodium excretory responses to reductions in renal perfusion pressure were not significantly different from those recorded in the absence of drug. However, following administration of 1-Sar, 8-Ile angiotensin II, renal blood flow but not glomerular filtration rate, was autoregulated during reduction in renal perfusion pressure. The falls in absolute and fractional sodium excretions caused by this manoeuvre were of similar magnitude to those obtained in the absence of drug.5 The results obtained using the 1-Sar, 8-Ile angiotensin II are consistent with angiotensin II having an important intra-renal site of action to regulate glomerular filtration rate, possibly via an action at the efferent arteriole. Administration of 1-Sar, 8-Ala angiotensin II was without effect on the regulation of renal haemodynamics which it is suggested reflects a limitation in the use of this particular compound as an intrarenal angiotensin II antagonist.

Renal blood flow distribution at varying perfusion pressure in the alloperfused dog kidney

Tissue blood flow (TBF), its percent distribution and glomerular blood flow (GBF) were measured using labelled microspheres 15 micrometer in diameter (M) and chicken red blood cells (CRBC) at perfusion pressures (PP) of 17.3, 12.8 and 8.0 kPa (130, 95 and 60 mm Hg) in isolated alloperfused dog kidneys. Renal blood flow (RBF) was never interrupted during the isolation. Experiments with M showed a marked inequality of the tissue blood flow in different parts of the renal cortex at a constant PP of 17.3 kPa. TBF was highest in the outermost quarter and lowest in the juxtamedullary one. Using CRBC, a homogeneous TBF was observed in the outer 3/4 of the renal cortex with a lower flow in the innermost quarter. With M, a typical percent "redistribution" of TBF and GBF into the inner cortical regions was indicated during PP reduction. With CRBC, this phenomenon was observed only at PP below the range of RBF autoregulation (8.0 kPa) and was much less conspicuous than with M. The smaller size and higher elasticity of CRBC as compared with M, may result in a more realistic reflection of cortical blood flow distribution. The GBF of outermost superficial glomeruli decreases, even with CRBC, with each PP reduction, the difference exhibiting only a 5% significance level. The lower limit of BF autoregulation in these glomeruli seems to be somewhat higher than that of total RBF autoregulation.

Reversal of renal hypertension: effects on renin, salt and water balance

The effect of removal of one renal artery stenosis on renal sodium and fluid excretion and on the activity of the renin-angiotensin system (RAS) has been investigated in three types of renal hypertension of rats. Blood pressure fell in all experimental models after declamping, independently of changes in urinary sodium and water excretion or plasma angiotensin II (ANG II). Plasma concentrations of ANG II did not rise in response to salt and fluid loss induced by declamping when the contralateral kidney had been removed or when it was depleted from renin. A high renin content of the declamped kidney prevented major salt and fluid loss, whereas renin depletion of this kidney was accompanied by an exaggerated natriuresis and diuresis. Besides this tubular modulation of renal salt and water handling by the local RAS, glomerular filtration rate could be reduced by a stimulated activity of this system in plasma, indicated by a close relationship between serum urea and plasma ANG II levels.

Regulation of renin release and intrarenal formation of angiotensin. Studies in the isolated perfused rat kidney

1. Isolated rat kidneys were perfused at a constant pressure of 90 mmHg in a single-pass system with either a cell-free medium or a suspension of washed bovine red blood cells, free of the components of the renin-angiotensin system. In red blood cell perfused kidneys renal haemodynamics and sodium reabsorption corresponded closer to values observed in the intact rat than in cell-free perfused kidneys. 2. In red blood cell-perfused kidneys in the absence of plasma renin substrate autoregulation of renal blood flow was almost complete at pressures above 90 mmHg, provided that perfusion pressure was changed rapidly. 3. Renin release varied inversely with perfusion pressure within a pressure range from 50 to 150 mmHg; the greatest changes of renin release occurred, when perfusion pressure was reduced from 90 to 70 mmHg; maximal stimulation of renin release was observed at 50 mmHg. After reduction of perfusion pressure, renin release immediately started to rise and reached a new level within 5 min. Local reduction of perfusion pressure in small arteries and arterioles by the injection of microspheres induced a short-lasting decrease in renal plasma flow and a transient stimulation of renin release. 4. High concentrations of furosemide stimulated renin release by a direct intrarenal mechanism. 5. Isoproterenol stimulated renin release in low concentrations without a concomitant vasodilation, whereas high concentrations induced an increase in both renal plasma flow and renin release. The effects of isoproterenol were completely blocked by propranolol. 6. Sodium nitroprusside induced similar increases in renal plasma flow, as did high concentrations of isoproterenol, but only a small and slow increase in renin release was observed. 7. Angiotensin II (AII) suppressed renin release in concentrations corresponding to plasma levels measured in the intact rat independently of its vasoconstrictor effects, whereas vasopressin in antidiuretic concentrations did not affect renin release. 8. AII, AI, synthetic tetradecapeptide renin substrate (TDP), crude and purified rat plasma renin substrate induced a dose-dependent reduction in renal plasma flow. SQ 20 881, a competitive inhibitor of converting enzyme, and low doses of 1-Sar-8-Ala-AII (saralasin), a competitive antagonist of AII, did not change renal plasma flow, whereas high concentrations of saralasin had a vasoconstrictor effect on their own. 9. Saralasin inhibited the vasoconstrictor effects of AII and TDP to a similar degree. SQ 20 881 inhibited the vasoconstrictor effects of AI and purified renin substrate, but did not influence the actions of TDP and the crude renin substrate preparation. 10. From these data it is concluded, that AI is converted into AII within the kidney at a rate of 1-2%. The vasoconstriction induced by the crude renin substrate probably does not involve the AII receptors. TDP may act by itself on the AII receptors or via the direct intrarenal formation of AII...

Relationship between perfusion pressure and renin release in the isolated rat kidney

Isolated rat kidneys were perfused with either a modified Krebs-Henseleit solution containing a gelatine preparation (Haemaccel, 35 g/1) or with a suspension of washed bovine red blood cells (RBC). Wh en perfusion pressure (PP) was varied repeatedly in the range between 30 and 210 mm Hg autoregulation of renal plama flow (RPF) was almost complete in RBC perfused kidneys. Changes of PP by steps of 20 mm Hg at intervals of 5 min resulted in an incomplete autoregulation of RPF and glomerular filtration rate (GFR). Renin release (RR) was inversely related to PP in the range between 50 and 150 mm Hg, while perfusion at a pressure below or above that range had no further effect on RR. The most marked increase in RR was obtained, when PP was reduced from 90 to 70 mm Hg. After reduction of PP, an increase in RR was measurable within 1 min, and a maximum was reached after 5 min. In kidneys perfused with a cell-free medium at a PP of 45 mm Hg for up to 30 min, RR remained elevated for the entire period of pressure reduction. Injection of microspheres into the renal artery resulted in a prompt decrease of RPF, GFR and urinary sodium excretion, but the values returned towards control levels within 15 min; RR increased only transiently after a short initial fall.