Pressure-diuresis in volume-expanded rats. Tubular reabsorption in superficial and deep nephrons

Micropuncture experiments were performed in volume-expanded rats to better define the nephron segments in which changes in renal perfusion pressure inhibit tubular reabsorption. Neural influences on the kidney were eliminated by renal denervation, and plasma levels of vasopressin, aldosterone, corticosterone, and norepinephrine were maintained at fixed levels by i.v. infusion. Fractional excretion of sodium, chloride, and water increased markedly after renal perfusion pressure was elevated from 110 to 150 mm Hg. Renal blood flow, glomerular filtration rate, and single nephron glomerular filtration rate measured from deep and superficial nephrons were unsaltered. Reabsorption of chloride and water in the proximal tubule of superficial nephrons decreased by 10% after renal perfusion pressure was elevated and contributed to the pressure-diuretic response. Changes in renal perfusion pressure also altered the reabsorption of water and chloride in juxtamedullary nephrons. The percentage of the filtered water load reaching the tip of the loop of Henle increased from 19.8 +/- 2.9 to 38.1 +/- 3.0% after renal perfusion pressure was elevated. Chloride delivery rose from 34.2 +/- 4.3 to 65.2 +/- 4.8% of the filtered load. These results support the view that alterations in medullary hemodynamics participate in the pressure-natriuretic response by inhibiting tubular reabsorption in the proximal tubule or the thin descending limb of the loop of Henle (or both) of juxtamedullary nephrons.

Role of angiotensin II in experimental membranous nephropathy

Glomerular hemodynamics measurements in rats with experimental membranous nephropathy [passive Heymann nephritis (PHN)] have demonstrated that the appearance of proteinuria 5 days after administration of anti-Fx1A antibody is temporally related to changes in the glomerular ultrafiltration coefficient (LpA). Previous studies in other models of glomerular injury have suggested a significant role for angiotensin II (ANG II) in the glomerular hemodynamic abnormalities. To evaluate the possible role of ANG II in the LpA decrease, converting enzyme inhibitor (CEI) was administered acutely or chronically (5 days before and after induction of PHN) to rats with PHN. Acute ANG II blockade produced a fall in mean arterial pressure (MAP), single-nephron glomerular filtration rate (SNGFR), absolute proximal reabsorption (APR), single-nephron plasma flow, single-nephron blood flow, and glomerular capillary hydrostatic pressure (PG); however, no changes in LpA were detected. Chronic administration of CEI (MK421, 5 mg.kg-1.day-1) in the drinking water was associated with a fall in MAP; however, both SNGFR and APR increased. PG and the transcapillary hydrostatic pressure gradient were unchanged, and LpA remained depressed. These results suggest that reduction of LpA in rats with PHN is ANG II independent and that other mechanisms are required to explain these changes in glomerular hemodynamics.

Serial micropuncture analysis of single nephron function in subtotal renal ablation

We developed a series of methodology to analyze function-structure relationship at single nephron level in animal models of chronic renal disease. Micropuncture measurements were repeated to measure single nephron GFR (SNGFR) and glomerular capillary hydraulic pressure (PGC) in the same nephrons, and subsequently examine the morphology of these glomeruli by serial section histological analysis. Using this approach, a potential causal link between early functional pattern and late structural abnormalities was examined in glomeruli of 10 Munich-Wistar rats up to six weeks after surgical removal of 5/6 total renal mass. After two weeks all SNGFR and PGC values increased uniformly but to varying degrees within each remnant kidney. Thereafter, values for SNGFR were highly variable, many declining while other increased in the same kidney. PGC showed an initial increase with subsequent decrease by four to six weeks. Serial section histological analysis of these same glomeruli revealed that the extent of glomerular sclerosis positively correlated with the functional deterioration, that is, the degree of reduction in SNGFR (P less than 0.01). However, the degree of sclerosis had no tendency to correlate with the levels of SNGFR or PGC recorded in early stage. These studies indicate that pathophysiologic mechanisms other than, or in addition to, early hyperfunction are involved in determining the extent of glomerular structural damage in this model of chronic renal failure.

Pharmacologic treatment of hyperlipidemia reduces glomerular injury in rat 5/6 nephrectomy model of chronic renal failure

The role of lipid abnormalities in the pathogenesis of focal glomerulosclerosis was investigated in the rat remnant kidney model of chronic renal failure. Rats subjected to right nephrectomy and two-thirds segmental infarction of the left kidney (5/6 nephrectomy) were treated for 10 weeks with the lipid-lowering agent clofibric acid. Both serum cholesterol and urine albumin excretion were significantly reduced by clofibric acid. At 10 weeks, the percent of glomeruli with focal glomerulosclerosis was 5 +/- 2% in clofibric acid-treated and 24 +/- 5% in untreated 5/6 nephrectomy rats (p less than 0.01). Inulin clearance was greater in clofibric acid-treated than in untreated 5/6 nephrectomy rats (0.28 +/- 0.02 versus 0.22 +/- 0.02 ml/min 100 g body wt, p less than 0.05). Body weight, kidney weight, and systemic blood pressure were not significantly altered by clofibric acid. Micropuncture studies, performed in separate groups of clofibric acid-treated and untreated 5/6 nephrectomy rats, demonstrated elevated single nephron glomerular filtration rates and glomerular capillary pressures 4 weeks after surgery. However, clofibric acid did not significantly alter single nephron glomerular filtration rates (95 +/- 2.1 nl/min in treated versus 97.0 +/- 6.2 nl/min in untreated, p greater than 0.05) or glomerular capillary pressures (56.6 +/- 1.5 mm Hg in treated versus 57.8 +/- 0.8 mm Hg in untreated, p greater than 0.05) in 5/6 nephrectomy rats. In a separate set of experiments, 5/6 nephrectomy rats were treated with the specific cholesterol synthesis inhibitor, mevinolin. Mevinolin improved serum lipid levels and reduced albuminuria in 5/6 nephrectomy rats without causing significant alterations in blood pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Tubuloglomerular feedback in animals with unilateral, partial ureteral occlusion

Previous experiments have shown that the sensitivity of the tubuloglomerular feedback system (TGF) is reduced by volume expansion in normal rats. This reduction in sensitivity is probably mediated by changes in the renal interstitial pressure. The present study was designed to investigate the TGF control system during volume expansion in rats with chronic, partial ureteral occlusion--hydronephrosis. Hydronephrosis was induced on the left or right side according to the method described by Ulm and Miller, in weanling Sprague-Dawley rats three weeks old. Three to six weeks later the rats were prepared for whole kidney and micropuncture experiments. Sham-operated animals were used as controls. Net interstitial pressure (that is, interstitial hydrostatic pressure minus interstitial oncotic pressure) was higher in the hydronephrotic, volume expanded animals than in the volume expanded controls. From findings in earlier investigations this increase in interstitial pressure would have been expected to reduce TGF sensitivity but this sensitivity was increased in the hydronephrotic kidneys, as indicated by a reduction in the turning point, the tubular flow rate at which 50% of the maximal stop-flow pressure response was observed (14.4 nl/min, sham-operated control 33.4 nl/min). Strong activity of the TGF mechanism was also indicated by a large proximal-distal difference in the single-nephron glomerular filtration rate (11.9 nl/min versus 3.3 nl/min in sham-operated controls) in the hydronephrotic kidney during volume expansion. Thus, in hydronephrotic kidneys in the latter condition the TGF mechanism was highly sensitive and activated to reduce the glomerular filtration rate. This mechanism may protect the diseased kidney from high intrapelvic pressures which otherwise could damage the kidney during saline volume expansion.

Glomerular hemodynamics in mercury-induced acute renal failure

As manifest by tubular collapse and the virtual absence of flow into the glomerulotubular junction (GTJ), filtration in most nephrons (SNGFR) of rats poisoned with 9 mg/kg body wt HgCl2 16 to 28 hours earlier was virtually absent. Arterial colloid osmotic pressure (COPA) and Bowman's space pressure (PBS) were modestly depressed (P less than 0.05 or below), and mean blood pressure was reduced from 115 +/- 2 mm Hg (SEM) to 97 +/- 1 mm Hg (P less than 0.001). Glomerular capillary hydraulic pressure (Pg), 25.6 +/- 1.3 mm Hg was some 24 mm Hg lower than control (P less than 0.001) and yielded a net afferent effective filtration pressure (Pnet) of 4.1 +/- 1.2 mm Hg. Excluding three rats with values greater than 10 mm Hg, Pnet averaged 2.0 +/- 0.9 mm Hg (N = 17 rats) versus 20.0 +/- 1.8 mm Hg in controls (N = 10, P less than 0.001), the former being statistically almost indistinguishable from 0 mm Hg and barely able to support any filtration. This decrease in Pg was caused by a major increase in preglomerular resistance (RA) and a reciprocal fall in efferent arteriolar resistance (RE), the RA/RE ratio of 7.2 +/- 0.8 being fourfold higher than control (P less than 0.001). Renocortical blood flow was not different from control (P greater than 0.2). A wide spread of Pg values in individual glomeruli and the absence of tubular flow despite the appearance of i.v. injected lissamine green in a quadrant of surface glomeruli suggested the possibility of a greatly increased, glomerular capillary resistance. It is concluded that reciprocal changes in RA and RE are the immediate cause of filtration failure in this form of ARF and that, in the virtual absence of filtration, tubular leakage can play no important role. Since PBS was depressed in both the developmental and established phases of ARF, tubular obstruction appears to play no direct role in the pathogenesis of this particular model of murine acute renal failure.

Pathogenesis and treatment of acute renal failure

This article reviews the current understanding of the pathophysiologic sequence of events that culminate in an acute renal insult. The use of urinary indices to differentiate the physiologic causes of oliguria, namely, diminished intravascular volume or renal perfusion, from an established acute renal failure, is discussed for children and adolescents, as well as for neonates. Treatment modalities for the support of children with acute renal failure are described in detail.

Strict metabolic control and renal function in the streptozotocin diabetic rat

Micropuncture studies were made on insulin-treated streptozotocin diabetic rats two weeks after the induction of diabetes and on age-matched control rats. Kidney size, GFR and single nephron GFR were higher in poorly controlled diabetic rats than in normal animals. Single nephron GFR rose as a result of an increase in the hydraulic pressure difference across the glomerular capillary wall caused mainly by a rise in the glomerular capillary pressure due to a diminished ratio of afferent to efferent arteriolar hydraulic resistances. Furthermore, the intratubular pressure was reduced as a result of a decrease in hydraulic resistance in the loop of Henle. Strict metabolic control prevented these changes. In conclusion, the increase in renal function in experimental diabetes is determined by the degree of metabolic control excluding a potential nephrotoxic effect of streptozotocin per se.

Analysis of renal function in the two-kidney Goldblatt model

We analyzed the renal functional responses in all rats 4-6 wk after application of a clip to one renal artery and maintained on either a normal NaCl-intake or a NaCl-depletion protocol utilizing both clearance methods and micropuncture evaluations of glomerular hemodynamics. Although mean arterial pressure (MAP) rose significantly, the response was quite variable and the frequency and degree of hypertension independent of NaCl dietary protocol (142 +/- 7 vs. 137 +/- 6 mmHg, NS). Glomerular filtration rate (GFR) of the unclipped kidney was well maintained regardless of absolute MAP, however, GFR in the clipped kidney was somewhat MAP dependent. The glomerular ultrafiltration coefficient (LpA) was reduced in all clipped rats independent of MAP and dietary NaCl (0.037 +/- 0.002 vs. 0.063 +/- 0.009 nl X s-1 X mmHg-1, P less than 0.05). Glomerular capillary hydrostatic (PG) and glomerular capillary hydrostatic gradient (delta P) were elevated in all clipped rats, regardless of MAP, and to a greater extent in clipped rats maintained on the NaCl-depletion protocol (PG, 59.6 vs. 53.2, P less than 0.05). Among clipped rats, multiple regression analysis revealed no correlation between PG or delta P and the degree of reduction in LpA. The unclipped kidney of the two-kidney, one-clip model is characterized by uniform reductions in LpA independent of the MAP and awake blood pressure produced. Although the increase in PG and delta P may contribute to the reduction in LpA observed, the degree of LpA reduction is independent of the magnitude of elevation in PG and delta P in clipped rats.

Kidney function in rats with corticomedullary nephrocalcinosis: effects of alterations in dietary calcium and magnesium

Single-nephron and whole-kidney function were studied in female rats with corticomedullary nephrocalcinosis, and in animals where the lesion had been prevented either by a dietary magnesium supplement or by using a diet with a calcium:phosphorus ratio in excess of 1. At the single-nephron level, rats with nephrocalcinosis had prolonged tubular fluid transit times. Proximal transit time was 19.42 +/- 1.98 (mean +/- S.E. of mean) vs. 11.58 +/- 0.19 s for controls; distal transit time was 62.64 +/- 9.16 vs. 31.50 +/- 1.03 s for controls. Although single-nephron function is altered in nephrocalcinosis, data obtained from rats in metabolism cages indicate that whole-kidney function is largely unaffected by the lesion.

[Experimental animal studies of compensatory renal adaptation of residual nephrons in physical stress caused by daily swim training]

The influence of a regular physical training after unilateral nephrectomy carried out on the experimental animal rat on the function of the residual kidney was investigated. A significant decrease of the glomerular filtration rate by 22% and an increase of TmPAH by 36% in the loaded animals could be proved 12 weeks after unilateral nephrectomy and two-month load by swimming training. These findings allow the interpretation that physical conditioning for adapting nephrons may be regarded as progression as well as protection factor. Additional long-term investigations are necessary to clarify this problem and finally to prove present therapeutic recommendations for the physical conditioning with regard to prove protective reactions to the renal compensatory adaptation of the residual nephrons without any doubt.

Increased tubuloglomerular feedback activity in Milan hypertensive rats

Studies of whole-kidney function and micropuncture measurements in superficial nephrons were performed to investigate the role of the tubuloglomerular feedback (TGF) in the excretion of salt and water in hydropenic and volume-expanded rats of the spontaneously hypertensive Milan strain (MHS). The rats were 3.5-5 and 5-7 wk old, and age-matched animals from the Milan normotensive strain (MNS) served as controls. There was no difference in mean arterial blood pressure (Pa) between the 3.5- to 5-wk-old prehypertensive MHS (MHSp) and MNS rats, but the glomerular filtration rate (GFR) was higher in MHSp than in MNS [1.35 vs. 0.80 ml X min-1 X g kidney wt (KW)-1, P less than 0.01]. The distal single-nephron glomerular filtration rate (SNGFR) was also higher in MHSp than in MNS (28.6 vs. 20.2 nl X min-1 X g KW-1, P less than 0.05). TGF was determined from both stop-flow pressure response and proximal and distal SNGFR. It was found that MHSp exhibited essentially no TGF response. During development of hypertension 5- to 7-wk-old MHS (MHSd) had a higher Pa than MNS (120 vs. 98 mmHg, P less than 0.01). Normally GFR and SNGFR increase with age, and such was the case with MNS (0.8 to 1.02 ml X min-1 X g KW-1 and 20.2 to 23.4 nl X min-1 X g KW-1), but in MHSd there was a decrease in both GFR and SNGFR with age (1.35 to 1.10 ml X min-1 X g KW-1 and 28.3 to 18.3 nl X min-1 X g KW-1).(ABSTRACT TRUNCATED AT 250 WORDS)

Renal function in the obese Zucker rat

The hyperphagic, genetically obese Zucker rat (fa/fa) exhibits both a greater kidney size and a progressive, premature glomerular sclerosis. In the present study, glomerular filtration rate (GFR), effective renal plasma flow (ERPF) and renal tubular function were evaluated during study 1 in lean Zucker (FA/-), fa/fa, and lean Sprague-Dawley (S-D) rats. The GFR as measured by renal inulin clearance (ClIN) was not significantly different (P greater than 0.05) between S-D (1.36 +/- 0.18 ml/min) vs FA/- (1.36 +/- 0.33 ml/min) and FA/- vs fa/fa (1.25 +/- 0.42 ml/min). The ERPF as measured by renal p-aminohippurate (PAH) clearance (ClPAH) also was not significantly different between S-D (3.98 +/- 0.80 ml/min) vs Fa/- (3.71 +/- 0.81 ml/min) and Fa/- vs fa/fa (3.34 +/- 1.60 ml/min). There was a significant difference (P less than 0.05) in the renal tubular transport maximum (Tm) of PAH between S-D (2.23 +/- 0.40 mg/min) and Fa/- (1.64 +/- 0.63 mg/min) groups but not between Fa/- and fa/fa (1.29 +/- 0.61 mg/min) groups, indicating a strain effect in organic anionic renal transport. The Fa/- vs fa/fa comparisons were significant when GFR, ERPF and Tm were corrected for total body or kidney weight. In a second group of animals (study 2), GFR (as reflected by creatinine clearance [Clcr]) and histologic studies were performed in Fa/- and fa/fa rats. Clcr values were significantly higher in the fa/fa (2.10 +/- 0.44 ml/min) vs Fa/- (1.68 +/- 0.17 ml/min). Histologic studies in group 2 demonstrated no remarkable differences between Fa/- and fa/fa rats. These results suggest wide interanimal variation in obesity associated changes in renal function and possibly pathology in the fa/fa rat.

Nephron site of resistance to phosphaturic effect of PTH during respiratory alkalosis

This study was performed to evaluate the nephron site(s) responsible for the blunted phosphaturic effect of parathyroid hormone during respiratory alkalosis. In normocapnic thyroparathyroidectomized rats, parathyroid hormone administration markedly increased the fractional excretion of phosphate (FEp) from 2.1 +/- 0.5 to 36.6 +/- 5.0%. However, in the respiratory alkalotic rats, parathyroid hormone administration did not significantly increase the FEp (1.4 +/- 0.9 to 5.9 +/- 2.2%). This blunted phosphaturic response to parathyroid hormone was not due to a blunted inhibition of phosphate reabsorption by the superficial proximal tubule, since parathyroid hormone administration significantly increased the fractional delivery of phosphate (FDp) at the superficial late proximal tubule in both normal (25.3 +/- 3.0 to 36.2 +/- 3.8%, delta 10.9 +/- 3.2%) and respiratory alkalotic rats (12.2 +/- 3.1 to 30.3 +/- 4.9%, delta 18.0 +/- 4.7%). Parathyroid hormone administration significantly increased the FDp at the superficial early distal tubule from 9.3 +/- 3.9 to 38.7 +/- 7.4% (delta 29.4 +/- 5.1%) in normal rats and from 4.5 +/- 1.7 to 12.9 +/- 3.4% (delta 8.5 +/- 3.2%) in the respiratory alkalotic rats. We conclude that the blunted phosphaturic response to parathyroid hormone in respiratory alkalotic rats is not due to a blunted inhibition of phosphate reabsorption by the proximal convoluted tubule but is primarily due to enhanced reabsorption by the pars recta segment of the proximal tubule.

Single nephron function during prolactin-induced pseudopregnancy in the rat

Whole kidney and renal micropuncture techniques were used to investigate the effects of chronic prolactin treatment on kidney function in anaesthetized female rats. At the whole kidney level, glomerular filtration rate (GFR) and fluid reabsorption were both significantly (P less than 0.02) increased in the hormone-treated group. At the single nephron level, GFR and proximal fluid reabsorption were also increased (P less than 0.05) by prolactin treatment. Fractional reabsorption was also enhanced at the proximal tubular level in hormone-treated animals. Such changes in renal function are similar to those seen in rat pregnancy and cervically stimulated pseudopregnancy. Since circulating prolactin concentrations are increased in both reproductive states, the hormone may play an important role in establishing the characteristic renal changes seen therein.