Renal functional reserve in treated and untreated hypertensive rats.

Renal functional reserve (renal response to protein loading, RFR) has been suggested as a method to verify the presence of hyperfiltration. This study was designed to evaluate the role of RFR as an indicator of increased glomerular capillary hydrostatic pressure in short-term treated and untreated rats with two-kidney, one-clip Goldblatt hypertension. One month after placing a silver clip, micropuncture studies were performed on the unclipped kidney. Normal rats and three groups of clipped rats [untreated group (HYP), a group treated with captopril (CEI) and a group treated with verapamil (VER) 5 days before the micropuncture studies] were studied. Glomerular hemodynamics and proximal tubular reabsorption were measured in control period and during intravenous administration of glycine (G). In normal rats, G produced afferent and efferent dilation, increases in single nephron plasma flow (SNPF) and single nephron glomerular filtration rate (SNGFR) of 24%. Systemic hypertension in HYP rats was associated with increases in transcapillary pressure gradient (delta P) and SNGFR. In this hyperfiltration state, infusion of G did not modify SNGFR of SNPF defining loss of RFR. The antihypertensive treatment was equally effective in normalizing MAP and delta P in CEI and VER, but only CEI rats responded to G with a 20% increase in SNGFR due to an increase in delta P. The most striking findings were that loss of RFR in both HYP and VER rats was associated with a significant decrease in absolute and proximal fractional reabsorption.(ABSTRACT TRUNCATED AT 250 WORDS)     

Renal functional reserve in experimental chronic glomerulonephritis

Loss of renal functional reserve, that is, absence of the glomerularvasodilatory response to amino-acid infusion, has been interpretedas equivalent to glomerular hyperperfusion/hypertension, andthere fore proposed as a marker of high risk for progressiveglomerular sclerosis. To substantiate the validity of this hypothesiswe evaluated the renal response to glycine and the extent ofglomerular damage 10–12 weeks after induction of anti-glomerularbasement membrane glomerulonephritis with or without superimposedclip hypertension. Untreated rats and rats chronically treatedwith quinapril, a converting-enzyme inhibitor, were studied.In untreated groups, loss of renal functional reserve was demonstratedsince GFR, single-nephron GFR (SNGFR) and plasma flow (SNPF)did not increase during glycine infusion. The absence of renalreserve was associated with glomerular hyperfusion/hypertension,and development of proteinuria and glomerulosclerosis. Quinaprilreduced proteinuria and diffuse sclerosis in anti-glomerularbasement membrane GN, and decreased blood pressure and segmentalglomeruloscierosis in anti glomerular basement membrane GN withsuperimposed clip hypertension. Both treated groups demonstrateda restoration of renal functional reserve, as depicted by increasesin GFR, SNGFR, and SNPF after glycine, despite persistence ofglomerular hyperperfusion/hypertension. These data demonstrthat renal functional reserve testing, although it does notdetect glomerular hyperperfusion/hypertension, can provide informationon the progression of glomerular damage.     

Converting enzyme inhibition and the glomerular hemodynamic response to glycine in diabetic rats.

GFR normally increases during glycine infusion. This response is absent in humans and rats with established diabetes mellitus. In diabetic patients, angiotensin-converting enzyme inhibition (ACEI) restores the effect of glycine on GFR. To ascertain the glomerular hemodynamic basis for this effect of ACEI, micropuncture studies were performed in male Wistar-Froemter rats after 5 to 6 wk of insulin-treated streptozotocin diabetes. The determinants of single-nephron GFR (SNGFR) were assessed in each rat before and during glycine infusion. Studies were performed in diabetics, diabetics after 5 d of ACEI (enalapril in the drinking water), and weight-matched controls. Diabetic rats manifest renal hypertrophy and glomerular hyperfiltration but not glomerular capillary hypertension. ACEI reduced glomerular capillary pressure, increased glomerular ultrafiltration coefficient, and did not mitigate hyperfiltration. In controls, glycine increased SNGFR by 30% due to increased nephron plasma flow. In diabetics, glycine had no effect on any determinant of SNGFR. In ACEI-treated diabetics, the SNGFR response to glycine was indistinguishable from nondiabetics, but the effect of glycine was mediated by greater ultrafiltration pressure rather than by greater plasma flow. These findings demonstrate that: (1) The absent response to glycine in established diabetes does not indicate that renal functional reserve is exhausted by hyperfiltration; and (2) ACEI restores the GFR response to glycine in established diabetes, but this response is mediated by increased ultrafiltration pressure rather than by increased nephron plasma flow.     

Early events in ischemic renal failure in the rat: effects of antioxidant therapy

Renal ischemia followed by two hours of reperfusion produces a complex form of acute renal failure characterized by a reduction in nephron filtration rate (SNGFR) and moderate proximal tubular damage. We have examined glomerular hemodynamics, SNGFR and histologic changes after renal ischemia and two hours of reperfusion in untreated (I) rats and rats pretreated with the antioxidant, probucol (IP). SNGFR decreased significantly by 47% in I rats. Reduction in SNGFR was primarily the result of a major decrease in the glomerular capillary hydrostatic pressure gradient, delta P, and a decrease in nephron plasma flow (SNPF). The glomerular ultrafiltration coefficient remained equal to control valves. In IP rats SNGFR was improved to values 89% of control rats due to higher values of delta P and SNPF. Histologic evidence of modest damage to cells of the proximal tubule was equal in both untreated and probucol treated ischemic animals. These studies demonstrate that: (a) primary reductions in nephron filtration rate at the glomerulus result from decreases in delta P and nephron plasma flow; b) pretreatment with the antioxidant, probucol, increases nephron plasma flow and SNGFR, and maintains more normal values for delta P; and c) tubular damage was equivalent in I and IP rats in spite of differences in SNGFR.     

Gentamicin-induced glomerulotoxicity in the pregnant rat

Ten daily injections of gentamicin, 40 mg/kg/d intraperitoneal (IP), produced a marked reduction in the glomerular capillary ultrafiltration coefficient (Kf) of virgin female Munich-Wistar rats without eliciting significant reductions in glomerular filtration rate (GFR) or single nephron (SN)GFR. A similar gentamicin-induced decrease in Kf was seen in midterm pregnant (day 12) rats and the normal gestational increase in GFR and SNGFR was blunted by gentamicin. Concomitant converting enzyme inhibition (CEI) (MK 421, 50 mg/L drinking water) completely prevented the gentamicin-induced decrease in Kf in virgin females, thus confirming that the gentamicin-induced decline in Kf is mediated by angiotensin II (ANG II). These studies indicate that pregnancy neither exacerbates nor ameliorates gentamicin-induced glomerulotoxicity and also that glomerular ANG II responsivity is not diminished in midterm pregnant rats.     

Salt-sensitivity of proximal reabsorption alters macula densa salt and explains the paradoxical effect of dietary salt on glomerular filtration rate in diabetes mellitus

GFR varies inversely with dietary NaCl in patients with early type I diabetes and in streptozotocin (STZ)-diabetic rats. To explain this paradox within the laws of physiology, it was hypothesized that it results from heightened sensitivity of the diabetic proximal tubule to dietary salt because changes in proximal reabsorption (Jprox) elicit reciprocal adjustments in GFR through the normal actions of tubuloglomerular feedback (TGF). Micropuncture was done in rats after 5 wk of moderately hyperglycemic STZ-diabetes and 1 wk of different NaCl diets. First, single-nephron GFR (SNGFR) and early distal tubular Na(+), Cl(-) and K(+) concentration (representing the TGF signal) were measured by collecting from early distal nephrons. In nondiabetics, dietary salt did not affect SNGFR or the TGF signal. In diabetics, the TGF signal varied directly with dietary salt while SNGFR varied inversely with dietary salt. Next, Jprox was measured by collecting from late proximal tubules. To control for different SNGFR, SNGFR was manipulated by perfusing Henle's loop to alter TGF activity. Controlling for SNGFR, dietary salt did not affect Jprox in nondiabetics but exerted a major inverse impact on Jprox in diabetics. In conclusion, normal rats acclimate to dietary NaCl by primarily adjusting transport downstream of the macula densa. In contrast, diabetes renders reabsorption in the proximal tubule sensitive to dietary NaCl with subsequent effects on the TGF signal. This explains the paradoxical effect of dietary NaCl on GFR in early diabetes.     

Glomerular hyperfiltration in experimental diabetes mellitus: potential role of tubular reabsorption

An increase in Na+/glucose cotransport upstream to the macula densa might contribute to the increase in single nephron GFR (SNGFR) in early diabetes mellitus by lowering the signal of the tubuloglomerular feedback, i.e., the luminal Na+, Cl-, and K+ concentration sensed by the macula densa. To examine this issue, micropuncture experiments were performed in nephrons with superficial glomeruli of streptozotocin-induced diabetes mellitus in rats. First, in nondiabetic control rats, ambient early distal tubular concentrations of Na+, Cl-, and K+ were about 21, 20, and 1.2 mM, respectively, suggesting collection sites relatively close to the macula densa. Second, glomerular hyperfiltration in diabetic rats was associated with a reduction in ambient early distal tubular concentrations of Na+, Cl-, and K+ by 20 to 28%, reflecting an increase in fractional reabsorption of these ions up to the early distal tubule. Third, in diabetic rats, early proximal tubular application of phlorizin, an inhibitor of Na+/glucose cotransport, elicited (1) a greater reduction in absolute and fractional reabsorption of Na+, Cl-, and K+ up to the early distal tubule, and (2) a greater increase in early distal tubular concentration of these ions, which was associated with a more pronounced reduction in SNGFR. These findings support the concept that stimulation of tubular Na+/glucose cotransport by reducing the tubuloglomerular feedback signal at the macula densa may contribute to glomerular hyperfiltration in diabetic rats. Glomerular hyperfiltration in diabetic rats serves to compensate for the rise in fractional tubular reabsorption to partly restore the electrolyte load to the distal nephron.     

Comparison of glomerular and mesangial prostaglandin synthesis and glomerular contraction in two rat models of diabetes mellitus

Enhanced prostaglandin production is postulated to contribute to altered vascular reactivity and glomerular hyperfiltration in early insulin-deficient diabetes mellitus. Rats with streptozocin-induced diabetes (STZ-D) show glomerular hyperfiltration and develop renal disease. BB rats with genetic diabetes (BB-D) also hyperfilter but have only minor renal lesions. We therefore compared glomerular and mesangial prostaglandin E2 (PGE2) production and glomerular contractility in response to pressors as a reflection of in vitro vascular reactivity in these models. Glomeruli isolated from rats with 3 wk of STZ-D produced significantly more PGE2 under basal and ionophore A23187-stimulated conditions than those from control rats. Glomeruli from BB-D rats under basal and stimulated conditions, however, generated amounts of PGE2 that were comparable to either those of nondiabetic littermates or of normal Wistar rats. Mesangial cells cultured from glomeruli of STZ-D, BB-D, and control rats all had identical prostaglandin profiles judged by conversion of [14 C]arachidonic acid. They also produced comparable amounts of PGE2 under basal conditions and after stimulation with angiotensin II or A23187, as determined by radioimmunoassay. Planar surface area of glomeruli isolated from control rats showed a dose-dependent decrease in response to angiotensin II (10(-11)-10(-9) M). This response to angiotensin II was at least as great in glomeruli from STZ-D rats. Contraction of glomeruli from control and STZ-D rats was also comparable after vasopressin or norepinephrine. Similarly, glomeruli from BB-D and BB control rats contracted in a comparable fashion to angiotensin II and norepinephrine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of endothelin 1 on proximal reabsorption and tubuloglomerular feedback

Endothelin 1 (ET-1) is a powerful constrictor of the afferent glomerular artery, implicated in the occurrence of both functional and acute renal failure. Besides being produced by the endothelium, ET-1 is also secreted by proximal tubular cells, suggesting that it may act as an endoluminal messenger. The present study is intended to verify whether ET-1 may play a role in the tubuloglomerular feedback system. The experiments were performed in rat superficial glomeruli. In 25 nephrons we measured by the total collection technique the single-nephron glomerular filtration rate (SNGFR; nl/min) and reabsorption rates before (control) and during microinjection (MIJ) of ET-1 10(-7) M into the first proximal convolution or Bowman's space. The SNGFR rose from 27+/-3 to 61+/-11 nl/min (p<0.01), the percent proximal reabsorption rose from 43 to 74%, and the absolute reabsorption rose from 13+/-2 to 46+/-11 nl/min (p<0.01). In additional 23 nephrons the collections were performed at the earliest distal convolution accessible on the renal surface, while MIJ was performed in the last proximal convolution of the same nephrons. The SNGFR rose during MIJ from 22+/-3 to 40+/-6 nl/min (p<0.01), the percent reabsorption rose from 61 to 66% (p>0. 77), and the absolute reabsorption rose from 12+/-2 to 26+/-4 nl/min, (p<0.003). Exposure of the macula densa to intraluminally injected ET-1 causes an abrupt increase in SNGFR of the experimental nephron, in the absence of changes in systemic and renal hemodynamics. During proximal MIJ, ET-1 may have reached the macula densa during the time preceding the beginning of collection and interruption of the delivery of fluid to the distal nephron. ET-1 directly stimulates fractional and absolute volume reabsorption along the proximal tubule. Proximal secretion and/or filtration of ET-1 could represent a physiological mechanism to activate the tubuloglomerular feedback, eliciting a response opposite to that triggered by systemic and intrarenal infusion.     

Natriuretic and diuretic actions of a highly selective adenosine A1 receptor antagonist

The natriuretic and diuretic action of a highly selective adenosine A1 receptor (A1AdoR) antagonist, 1,3-dipropyl-8-[2-(5,6-epoxy)norbornyl]xanthine (CVT-124), was investigated in anesthetized rats. CVT-124 (0.1 to 1 mg/kg) caused dose-dependent increases in urine flow and fractional and absolute sodium excretion of by six- to 10-fold and, at 0.1 mg/kg, increased the GFR (1.6+/-0.1 to 2.5+/-0.2 ml/min; P<0.01). There were no changes in BP or heart rate. CVT-124 reduced absolute proximal reabsorption (26+/-3 to 20+/-2 nl/min; P<0.05) despite unchanged proximally measured, single-nephron GFR (SNGFR) (42+/-5 to 44+/-4 nl/min; NS) and thereby decreased fractional proximal reabsorption (60+/-3 to 46+/-4%; P<0.05). Despite increasing distal tubular fluid flow rate (5.4+/-0.7 to 9.7+/-0.9 nl/min; P<0.001), it reduced the proximal-distal difference in SNGFR (before: 9.4+/-1.0 versus during CVT-124: 4.6+/-1.5 nl/min; P<0.01), suggesting that it had blunted the effects of the macula densa on SNGFR. Direct measurements of maximal tubuloglomerular feedback (TGF) responses were made from proximal stop flow pressure (PSF) during orthograde loop perfusion from the proximal tubule with artificial tubular fluid at 40 nl/min. TGF was blunted by intravenous CVT-124 (0.5 mg/kg; deltaPSF with vehicle: 8.3+/-0.6 versus CVT-124: 6.5+/-0.3 mm Hg; n = 9; P<0.01). In conclusion, A1AdoR blockade reduces proximal reabsorption and uncouples it from glomerular filtration. It increases distal delivery of fluid yet does not activate a macula densa-dependent fall in SNGFR because it blunts the TGF response. Natriuresis accompanied by blockade of proximal glomerulotubular balance and TGF characterizes a new class of diuretic drugs.     

Serial glomerular and tubular dynamics in thyroidectomized rats with remnant kidneys

Serial measurements were performed in Munich-Wistar rats with five-sixths nephrectomy that had undergone prior selective thyroidectomy (Tx group) or thyroidectomy with thyroxine replacement (TxT4 group) to determine the effects of Tx on glomerular and tubular dynamics in relation to Tx attenuation of renal failure progression. At 1 week, inulin clearance rates (Cin) in TxT4 and Tx rats were 0.367 +/- 0.171 and 0.120 +/- 0.036 mL/min, respectively, different at P less than 0.01. Corresponding single-nephron filtration rate (SNGFR), glomerular plasma flow (QA), glomerular transcapillary hydraulic pressure (delta P), and proximal tubular reabsorption (Jv) were all reduced in Tx compared with TxT4 rats (P less than 0.01). Protein excretion (UPROT) was 151 +/- 40 in TxT4 rats, and 9 +/- 5 mg/d in Tx animals. Glomerular mesangial matrix expansion and focal tubulointerstitial changes were more frequent in TxT4 than Tx rats. By 4 weeks, Cin, SNGFR, QA, glomerular ultrafiltration coefficient (Kf) and Jv were similar in Tx and TxT4. Only glomerular capillary pressure (PGC) remained lower in Tx rats (35 +/- 3 v 50 +/- 3 mm Hg in TxT4, P less than 0.001). UPROT was 161 +/- 24 in TxT4 and 17 +/- 12 mg/d in Tx rats. While 7% +/- 4% of glomeruli showed focal sclerosis in TxT4 rats, there was none in the Tx group. Maximal glomerular planar area increased between 1 and 4 weeks in the TxT4 group, but not in the Tx group. However, this measurement was not significantly different between TxT4 and Tx glomeruli at 1 or 4 weeks. Minimal focal tubulointerstitial changes were found in TxT4, but there were not progressive from those observed at 1 week. The reduced PGC at 1 week was the result of a disproportionately greater increase in afferent (RA) than efferent arteriolar resistance (RE) in Tx rats (P less than 0.025); however, at 4 weeks, both RA and RE had decreased to values identical to those in TxT4 animals and the lower PGC in Tx rats was the result of a reduced mean arterial pressure. In conclusion, a reduced PGC was the sole functional correlate of decreased proteinuria and glomerulosclerosis afforded by Tx in this partial nephrectomy model. Suppression of either nephrectomy-related hypertrophy or tubulointerstitial injury by Tx could not be excluded as at least partially protective factors.     

Nephron responses to converting enzyme inhibition in non-clipped kidney of Goldblatt hypertensive rat at normotensive pressures

To evaluate the effects of angiotensin converting enzyme inhibition (SQ 20881, CEI) on superficial nephron function of the non-clipped kidney in Goldblatt hypertensive rats in the absence of alterations in renal arterial pressure, control renal arterial pressure (RAP) was reduced first to the range generally obtained during CEI (124 +/- 4 mm Hg). RAP was maintained during the CEI period by adjustment of a suprarenal aortic clamp. At the reduced RAP, whole kidney and single nephron glomerular filtration rates (GFR) were reduced from the hypertensive levels and were lower than the measurements in normotensive control rats. During CEI, whole kidney GFR and single nephron GFR increased by 55 and 42%, respectively. There were decreases in absolute as well as fractional proximal reabsorption rates. In the intermediate nephron segment, fractional reabsorption was decreased, but absolute fluid reabsorption increased in proportion to the increased delivery rate. Proximal tubule and peritubular capillary hydrostatic pressures increased significantly during CEI also. These results indicate that an increased activity of the renin-angiotensin system occurring in Goldblatt hypertensive rats subjected to aortic constriction exerts effects to lower GFR and increase proximal reabsorption rate. The concomitant superficial nephron and whole kidney GFR responses to CEI when arterial pressure was maintained suggests that the pre-existing levels of angiotensin exerted similar influences on the total nephron population.     

Effect of acute iNOS inhibition on glomerular function in tubulointerstitial nephritis

BACKGROUND: Tubulointerstitial nephritis (TIN) is characterized by progressive inflammatory infiltrate of the renal interstitium, induction of cortical tubular inducible nitric oxide synthase (iNOS) and reductions in glomerular filtration rate (GFR). These studies were designed to examine the changes in glomerular hemodynamics 7 and 21 days after induction of TIN and to evaluate the effect of acute iNOS blockade on glomerular function in the early stages of this model. METHODS: TIN was induced by immunizing Brown Norway rats with renal tubular antigen in complete Freund's adjuvant (RTA/CFA). Control rats were immunized with CFA alone. Micropuncture and morphologic studies were performed 7 and 21 days after immunization. RESULTS: Histology revealed minimal peritubular and interstitial inflammation in the RTA/CFA group one week after immunization while extensive interstitial inflammation with few preserved superficial nephron was observed three weeks after RTA/CFA immunization. Micropuncture studies on day 7 in the RTA/CFA group revealed a significant reduction in single nephron GFR due to a profound reduction in nephron plasma flow and in the ultrafiltration coefficient. Studies performed on day 21 revealed that single nephron GFR (SNGFR), nephron plasma flow (SNPF) and the ultrafiltration coefficient had returned to the normal baseline value despite the severe reduction in GFR. To assess the role of increased nitric oxide production secondary to iNOS induction on the glomerular hemodynamic changes observed in the early stages of the disease, the iNOS blocker (l-N(6)-iminoethyl lysine, L-NIL) was administered IV (1 mg/h) in RTA/CFA rats and CFA rats. L-NIL had no effect in CFA rats but produced significant increases in GFR, SNGFR and SNPF in RTA/CFA rats.CONCLUSIONS: These results demonstrate that TIN is associated with a progressive reduction in GFR, which is likely the result of functional vasoconstriction and decreases in the ultrafiltration coefficient in the early stages of the disease and on a significant reduction in the number of functioning nephron in the later stages. Induction of iNOS with increased NO production actively participates in the functional changes observed in the early stages of the disease most likely by inhibiting normal endothelial NOS activity.     

Effect of mesangial cell lysis and proliferation on glomerular hemodynamics in the rat.

To elucidate an involvement of mesangial cells in the regulation of glomerular hemodynamics, renal micropuncture techniques and glomerular morphometry were employed in Munich-Wistar rats with mesangial cell lytic or proliferative lesions induced by administration of an antibody reactive with Thy-1.1-like antigens on the mesangial cell surface. The antibody-induced mesangial cell lysis at day 1 resulted in a significant decrease in glomerular ultrafiltration coefficient, leading to reduction in single nephron glomerular filtration rate (SNGFR) in spite of a significant increase in both glomerular hydrostatic pressure and single nephron plasma flow (SNPF). During the antibody-induced proliferative lesion at day 6, glomerular ultrafiltration coefficient and SNGFR remained reduced; however, SNPF was now decreased. Morphometric analysis showed the enlargement of capillary luminal volume and the development of new open space in the mesangium accessible for blood flow in the mesangial cell-lytic glomeruli at day 1. An increase in mesangial cell volume was found in the proliferative glomeruli at day 6. The total area of peripheral glomerular basement membrane, presumed as the probable filtration area, was unchanged in these glomeruli. These results indicate that mesangial lesions decrease glomerular ultrafiltration coefficient, and suggest that mesangial cells participate in regulation of glomerular filtration rate.     

A micropuncture study of renal sodium retention in nephrotic syndrome in rats: evidence for increased resistance to tubular fluid flow

Micropuncture studies were carried out in surface nephrons of rats with nephrotoxic-serum (NTS)-induced nephrotic syndrome during a period of active sodium and water retention. It was found that hydrostatic pressure and tubular diameter were increased in the proximal tubules (13.4 +/- 0.2 vs. 10.4 +/- 0.2 mm Hg; 31.3 +/- 0.9 vs. 18.4 +/- 0.7 mu), whereas pressure and tubular diameter were normal in the distal tubules. Single nephron glomerular filtration rate (SNGFR) was decreased and fractional reabsorption of fluid was markedly increased in the proximal tubules (74.1 vs. 61.7%). The increased pressure gradient between the proximal and distal tubules suggests a condition of increased resistance to flow between the proximal and distal tubules. Microinfusion of proximal tubules with an isotonic "equilibrium" solution led to little or no rise in intratubular pressure in normal rats but it led to a significant rise in nephrotic rats. When proximal tubules of normal rats were infused with a solution containing 100 mg/100 ml albumin, pressure rose to levels observed in nephrotic rats. The mechanism of the increased resistance to flow appeared to be related, therfore, to the presence of protein in the tubular fluid. Sodium retention in the nephrotic animals might be attributed to the reduction in GFR. In other types of renal disease in animals and man with comparable or greater reductions in GFR, sodium retention does not occur, however, and fractional excretion of sodium in the urine is increased in proportion to the reduction in GFR. Thus, the rise in proximal fractional reabsorption secondary to impaired fluid flow could be an important factor in the sodium retention of this disease.     

Mechanisms of glomerular immune injury: effects of antioxidant treatment

Significant glomerular vasoconstriction and production of reactive oxygen species has been known to occur with exposure to anti-glomerular basement membrane antibody (AGBM-Ab) in the rat model. Previously published studies have demonstrated that such effects can be reduced by therapy with phentolamine, an alpha-adrenergic antagonist. It was hypothesized that antioxidant pretreatment with water-soluble probucol would improve glomerular hemodynamics 60 to 90 min after the administration of AGBM-Ab. These relationships were examined with both in vivo renal micropuncture and in vitro studies in rats. Single-nephron GFR (SNGFR) decreased markedly in untreated rats after AGBM-Ab as a result of afferent and efferent arteriolar vasoconstriction with consequent reductions in nephron plasma flow (SNPF) and decreases in the glomerular ultrafiltration coefficient (LpA). Basal SNGFR was increased, and SNGFR was significantly higher after AGBM-Ab in probucol-treated versus untreated rats. This finding was due solely to higher values for SNPF and prevention of afferent arteriolar constriction. A reduction in LpA after AGBM-Ab was not prevented by probucol treatment. In vitro analyses of glomeruli revealed reduced myeloperoxidase activity in antioxidant-treated rats. Lipoxygenase activity and leukotriene products, however, were not changed by antioxidant therapy, yet vasoconstriction was prevented. H(2)O(2) generation before and after formyl-methionyl-leucyl-phenylalanine stimulation was significantly reduced before and after AGBM-Ab in glomeruli harvested from rats that were treated with the antioxidant. Antioxidant therapy in this model of AGBM-Ab injury did not prevent reductions in LpA, an index of glomerular membrane damage, but did prevent afferent arteriolar vasoconstriction. Reactive oxygen species generation was reduced by probucol. The specific mechanisms whereby antioxidant therapy ameliorates glomerular hemodynamic effects will be defined in additional studies and is likely to involve either enhanced vasodilator or diminished vasoconstrictor activity.     

Effects of the anaphylatoxin, C5a, on renal and glomerular hemodynamics in the rat

The effects of intrarenal infusion of the complement-derived anaphylatoxin, C5a, upon glomerular hemodynamics were examined in the Munich-Wistar rat, a strain with glomeruli on the kidney surface. Human C5a (1.5 micrograms/min) or vehicle was infused into the left renal artery for 12 min, and glomerular capillary (PG) and Bowman's space pressures, nephron plasma flow (SNPF) afferent and efferent arteriolar protein concentrations, nephron filtration rate (SNGFR) and the glomerular ultrafiltration coefficient (LpA) determined. Human C5a infusion resulted in a reduction in SNPF due to increased efferent arteriolar resistance, and PG increased which maintained SNGFR constant. LpA was numerically lower but not significantly decreased. Infusion of porcine C5ades Arg decreased glomerular filtration rate and renal blood flow. No polymorphonuclear leukocytes were observed within glomerular capillaries of C5a infused rats, and rat leukocytes did not exhibit receptors for human C5a infused. Renal artery infusion of either human C5a or porcine C5a resulted in renal hemodynamic alterations and, as documented for human C5a, effects of C5a upon renal vascular resistance can be added to the known effect of C5a on the polymorphonuclear leukocyte.     

Absence of glomerular injury or nephron loss in a normotensive rat remnant kidney model

Severe reduction in renal mass (greater than 50%) in the rat uniformly results in progressive glomerular injury and loss of remnant nephrons postulated to be due to increases in glomerular function (hyperfiltration) and/or size (hypertrophy). Reduction in renal mass in the rat also leads to the development of systemic and/or glomerular hypertension. To examine the independent contributions of systemic hypertension and glomerular hyperfiltration and/or hypertrophy to progressive glomerular injury, a normotensive rat remnant kidney model was developed in the Wistar-Kyoto (WKY) strain. Of the 34 WKY rats that underwent 5/6 nephrectomy, 25 remained normotensive and without evidence of morphologic glomerular injury and/or nephron loss for up to 14 to 16 weeks, despite glomerular hyperfiltration and hypertrophy comparable to that previously observed in other rat strains. Micropuncture studies at approximately six weeks after reduction in renal mass demonstrated markedly increased SNGFR in remnant nephrons of normotensive rats as compared to controls (66 +/- 7 vs. 25 +/- 4 nl/min, P less than 0.01), but glomerular capillary pressures (PGC) estimated from stop flow pressures were only slightly increased (52.7 +/- 1 vs. 47.3 +/- 1 mm Hg, P less than 0.01). These data indicate that compensatory glomerular hyperfiltration and hypertrophy after 5/6 nephrectomy may not lead to progressive glomerular injury provided hypertension does not develop. These data further suggest that in the absence of systemic hypertension, increases in PGC required for adaptive hyperfiltration, may not be sufficient to initiate progressive glomerular injury and nephron loss.     

High protein intake accelerates glomerulosclerosis independent of effects on glomerular hemodynamics

It is unclear whether glomerular hemodynamic changes always accompany alterations in dietary protein, and whether nonhemodynamic factors associated with a high protein intake can influence glomerular injury. In the present study, uninephrectomized (UNx) male rats were fed either standard (20%) or high (40%) protein diets. Body growth was comparable in the two diet groups. At 30 weeks of age, UNx rats fed high protein had increased albuminuria (36.7 +/- 12.6 mg/24 hr), compared to UNx rats fed standard protein (10.2 +/- 6.2 mg/24 hr; P less than 0.05). The incidence of focal glomerulosclerosis (FGS) at 36 weeks was markedly increased in the high protein rats (19.0 +/- 7.7%), compared to the standard protein rats (3.2 +/- 2.0%; P less than 0.05). Moreover, FGS in the high protein rats correlated with the serum cholesterol level measured at 18, 24, and 30 weeks of age. Glomerular hemodynamics were assessed before glomerular injury developed in separate groups of UNx rats fed either standard or high protein. Single nephron GFR (SNGFR) and single nephron plasma flow (SNPF) were increased in UNx rats, compared to two-kidney rats fed standard protein. SNGFR and SNPF, however, were not different between UNx rats fed standard or high protein. Glomerular capillary pressure (PGC) was not different between UNx rats fed standard protein (51.7 +/- 1.3 mm Hg) and two-kidney controls (53.1 +/- 1.5 mm Hg). Moreover, PGC was not altered in UNx rats fed high protein (51.8 +/- 1.6 mm Hg). We conclude that high dietary protein can accelerate the development of glomerular injury independent of effects on glomerular hemodynamics.     

Combined converting enzyme inhibition and angiotensin receptor blockade reduce proteinuria greater than converting enzyme inhibition alone: insights into mechanism

Patients with various renal diseases receiving an angiotensin-converting enzyme inhibitor (CEI) were enrolled in a protocol to determine whether adding an angiotensin type 1 receptor blocker (ARB) reduces urinary protein excretion (UPE). All patients had significant proteinuria (range 517-8,562 mg/24 h) despite administration of CEI for at least 4 weeks. Following baseline measurements, losartan (50 mg/d) was started and testing was repeated at 1 month. Compared with CEI alone, combined CEI plus ARB reduced UPE by 45 +/- 8% (p < 0.005). Compared with CEI alone, CEI + ARB lowered UPE in each patient independent of baseline protein excretion or renal diagnosis. Reduction in proteinuria occurred independent of changes in mean arterial blood pressure (MAP), suggesting that the mechanism involved local changes in glomerular dynamics. If renal angiotensin II (ANG II) formation occurred despite CEI, the ANG II formed would suppress plasma renin activity (PRA), and adding an ARB would cause PRA to rise. In 7 of 10 subjects, addition of ARB to CEI increased PRA (p < 0.03) suggesting that intrarenal ANG II formation occurred in CEI-treated subjects. As a second marker of ANG II tissue activity, we measured the effects adding ARB on plasma aldosterone (ALDO). In 9 of 10 subjects, ALDO was acutely lowered (p < 0.009) suggesting that ANG II levels were incompletely blocked by CEI. We conclude that: combined CEI and ARB reduces UPE greater than CEI alone; reduction in proteinuria is independent of changes in MAP or renal diagnosis; and the additive effects of CEI and ARB are due at least in part to greater inhibition ofANG II action at the tissue level in the kidneys and adrenal glomerulosa.