Redistribution of renal blood flow in acute and chronic reduction of renal mass

Evidence for acute renal cortical vasoconstriction after uninephrectomy

The rate of progression of chronic renal failure (CRF) is similar for many diseases, suggesting a common, perhaps intrinsic, renal signal for its progression. The remnant nephron hypothesis of Bricker suggests that CRF may be the result of persistent compensatory renal growth (CRG). Normally, CRG after unilateral nephrectomy (uniNx) ceases within 1 week. Knowledge of the signals that initiate CRG may therefore shed light on the signals responsible for ongoing CRF. The signals responsible for the initiation of compensatory renal growth after uniNx are unknown. Hemodynamic changes in the remaining renal artery have been observed, but there are as yet no data for the main renal compartment which undergoes hypertrophy, the superficial renal cortex. The noninvasive technique of laser-Doppler flowmetry allows the continuous and independent monitoring of blood velocity and blood volume. The product of the two signals is proportional to tissue blood flow per unit volume of the tissue observed. Under controlled conditions in adult male Sprague-Dawley rats, renal cortical blood velocity increased by 22% within 5 min after uniNx and remained elevated at this level for 60 min. Renal cortical blood volume decreased throughout the experiment. Their product, renal cortical blood flow, increased briefly by 14% 5 min after uniNx but decreased over the time of observation in parallel with renal cortical blood volume. The simultaneous increase in blood velocity and decrease in blood volume in the superficial renal cortex acutely after uniNx suggest that vasoconstriction is an early event in compensatory renal growth.

Mechanism of impaired urinary concentration in chronic primary glomerulonephritis

To define the role of medullary damage and the influence of solute load and blood pressure (BP) in impairing urinary concentration, patients with chronic glomerulonephritis were investigated by histological and functional studies. In 59 biopsy specimens, the degree of medullary fibrosis was correlated inversely with urinary specific gravity and was significantly greater in hypertensive than in normotensive subjects. The following clearance studies were carried out in patients with a GFR of 15 to 40 ml/min in maximal antidiuresis: (1) Eight patients were studied while receiving a high sodium and protein diet and then after 1 week of low sodium, low protein diet; (2) ten patients were loaded with hypertonic saline (3%) to increase urine volume up to 25 to 30% of GFR; (3) the concentrating ability was compared in 15 normotensives and 15 hypertensives with comparable GFR; (4) the concentrating ability was studied in nine hypertensive patients before and after drug-induced normalization of BP. In (1) no change occurred in maximal urine osmolality (UOsm) even if fractional sodium excretion and filtered load of urea were reduced. In (2), values of UOsm fell below those of plasma osmolality. In (3), UOsm and negative free-water generation were lower in hypertensive than in normotensive subjects. In (4), normalization of BP was not associated with any change in UOsm. These results indicate that osmotic diuresis does not play a critical role in reducing urinary concentration. This defect is better accounted for by an intrinsic medullary damage, enhanced in hypertensive patients, which may impair the permeability of collecting ducts to water.

Renal vasodilation after unilateral renal ablation

Renal blood flow was studied in rats 120 minutes after unilateral renal ablation. The influence of endogenous prostaglandin formation was evaluated by indomethacin treatment prior to the ablation. Radioactive microspheres were used for estimation of the total renal and cortical blood flow, and the renal medullary blood flow was determined with the 86-Rb chloride extraction method. The total blood flow in the remaining kidney was increased by 80% following contralateral ablation, with augmentation in all areas, particularly in the deep medullary region. Indomethacin treatment in intact rats evoked increased blood flow as compared with the indomethacin control group. The results indicated that the renal blood vessels respond to ablation of the contralateral kidney with dilation in all kidney regions, and that this vascular dilation may be prostaglandin-mediated.

Influence of indomethacin on cation excretion after acute unilateral nephrectomy in dogs

Acute unilateral nephrectomy (AUN) causes functional changes in the remaining kidney. Since renal prostaglandins (PGs) may participate in this response, we investigated the effect of the PG synthetase inhibitor, indomethacin (INDO), on the function of the remaining kidney after AUN. Glomerular filtration rate (GFR), effective renal plasma flow (ERPF), filtration fraction (FF), sodium excretion, and potassium excretion were measured for 1 hr prior to and 3 hr after AUN in dogs anesthetized with sodium pentobarbital. Group I and II animals underwent AUN. A third group was sham-operated (Group III). Group I received INDO (2 mg/kg) before and 2 hr after AUN, while Groups II and III received a buffered saline vehicle. AUN alone (Group II) had no effect on GFR, ERPF, FF, or sodium excretion, while potassium excretion was increased. In animals administered INDO prior to AUN (Group I), the increase in potassium excretion was abolished, FF rose and sodium excretion, while potassium excretion was increased. In animals administered INDO prior to AUN (Group I), the increase in potassium excretion was abolished, FF rose and sodifore and 2 hr after AUN, while Groups II and III received a buffered saline vehicle. AUN alone (Group II) had no effect on GFR, ERPF, FF, or sodium excretion, while potassium excretion was increased. In animals administered INDO prior to AUN (Group I), the increase in potassium excretion was abolished, FF rose and sodium excretion was decreased. Results suggest that renal PGs participate in the compensatory response of the remaining kidney to AUN and may specifically play a role in increased cation excretion following AUN.

Functional profile of the isolated uremic nephron. Role of compensatory hypertrophy in the control of fluid reabsorption by the proximal straight tubule

An in vitro approach to the study of single nephron function in uremia has been employed in evaluating the control of fluid reabsorption by the renal superficial proximal straight tubule (PST). Isolated segments of PSTs from the remnant kidneys of uremic rabbits (stage III) were perfused in vitro and their rate of fluid reabsorption compared with normal PSTs and with PSTs derived from the remnant kidneys of nonuremic rabbits (stage II). All segments were exposed to a peritubular bathing medium of both normal and uremic rabbit serum thereby permitting a differentiation to be made between adaptations in function which are intrinsic to the tubular epithelium and those which are dependent upon a uremic milieu.Compared with normal and stage II PSTs, there was significant hypertrophy of the stage III tubules as evidenced by an increase in length and internal diameter, and a twofold increase in the dry weight per unit length. Fluid reabsorption per unit length of tubule was 70% greater in stage III than in normal and stage II PSTs, and was closely correlated with the increase in dry weight. Substitutions between normal and uremic rabbit serum in the peritubular bathing medium did not affect fluid reabsorption significantly in any of the three groups of PSTs. Perfusion of the tubules with an ultrafiltrate of normal vs. uremic serum likewise failed to influence the rate of net fluid reabsorption. It has previously been observed that net fluid secretion may occur in nonperfused or stop-flow perfused normal rabbit PSTs exposed to human uremic serum. Additional studies were thus performed on normal and stage III PSTs to evaluate whether net secretion occurs in the presence of rabbit uremic serum. No evidence for net secretion was found. These studies demonstrate that fluid reabsorption is greatly increased in the superficial PST of the uremic remnant kidney and that this functional adaptation is closely correlated with compensatory hypertrophy of the segment. Humoral factors in the peritubular environment do not appear to be important mediators of the enhanced fluid reabsorption.

A study of the intrarenal recycling of urea in the rat with chronic experimental pyelonephritis

The concentrating ability of the kidney was studied by clearance and micropuncture techniques and tissue slice analyses in normal rats with two intact kidneys (intact controls), normal rats with a solitary kidney (uninephrectomized controls), and uremic rats with a single pyelonephritic kidney. Urinary osmolality after water deprivation for 24 h and administration of antidiuretic hormone was 2,501+/-217 and 2,874+/-392 mosmol/kg H2O in intact and uninephrectomized control rats, respectively, and 929+/-130 mosmol/kg H2O in pyelonephritic rats (P less than 0.001 compared to each control group). Fractional water reabsorption and concentrating ability were significantly decreased in the pyelonephritic group, and, to achieve an equivalent fractional excretion of urea, a greater fractional excretion of water was required in the pyelonephritic rats than in the control rats. Whole animal glomerular filtration rate was 1.57+/-0.19 ml/min and 1.39+/-0.18 ml/min in intact and in uninephrectomized controls, respectively, and 0.30+/-0.07 ml/min in pyelonephritic rats (P less than 0.001 compared to each control group). Single nephron glomerular filtration rate was 35.6+/-3.8 nl/min in intact control rats and was significantly increased (P less than 0.05) in both uninephrectomized (88.0+/-10.8 nl/min) and pyelonephritic rats (71.5+/-14.4 nl/min). In all groups fractional water delivery and fractional sodium delivery were closely comparable at the end of the proximal convoluted tubule and at the beginning of the distal convoluted tubule. In contrast, fractional urea delivery out of the proximal tubule was greater in the intact control group (73+/-8%) than in either the uninephrectomized (52+/-2%) or the pyelonephritic group (53+/-3%) (P less than 0.005). Fractional urea delivery at the early part of the distal tubule increased significantly to 137+/-11% and 93+/-6% of the filtered load in intact control and uninephrectomized control rats, respectively (P less than 0.001 compared to the late proximal values of each group), but failed to increase significantly in pyelonephritic rats (65+/-13%), indicating interruption of the normal recycling of urea in the latter group. Analysis of tissue slices demonstrated a rising corticopapillary gradient for total tissue water solute concentration as well as for tissue water urea concentration in both groups of control rats. In contrast, the pyelonephritic animals exhibited no similar gradients from cortex to papilla. These data indicate that the pyelonephritic kidney fails to recycle urea and accumulate interstitial solute. The latter must inevitably lead to a concentrating defect.

Effects of nephrectomy on renal salt and water transport in the remaining kidney

Fluid, sodium, and potassium transport was studied in proximal and distal tubules in rats in which one kidney had been removed two weeks after a suprarenal aortic clamp had been placed to prevent adaptive changes in glomerular filtration rate (GFR) in the experimental kidney. Free-flow micropuncture techniques were used and tubular fluid (TF) samples analyzed for inulin, sodium and potassium. In addition, peritubular total protein concentrations and luminal and peritubular hydrostatic pressures were measured. The following changes were observed 15 hr after unilateral nephrectomy: (1) a significant increase in single nephron GFR; (2) unchanged absolute proximal tubular reabsorption rates of fluid and sodium; (3) increased delivery of fluid into distal tubules; (4) increased distal tubular reabsorption of sodium, but of insufficient magnitude to prevent natriuresis; and (5) an augmentation of distal tubular potassium secretion. Reduction of single nephron GRF to control levels by aortic clamping abolished the natriuresis following nephrectomy.

Compensatory adaptation of bicarbonate excretion following acute contralateral kidney exclusion in the dog

Changes in the excretion of bicarbonate, sodium and potassium in one kidney after exclusion (complete sudden ligation of renal pedicle) of its partner have been studied in 16 dogs undergoing bicarbonate diuresis. Fluid balance, haematocrit, plasma electrolyte and protein concentrations were maintained constant throughout the experiment. Acute contralateral renal pedicle ligation lead to an immediate increase in bicarbonate as well as water, sodium and potassium excretion by the remaining kidney. The rapid and immediate increase in the fractional and absolute rates of bicarbonate excretion was observed at varying levels of bicarbonate loading, with the greatest response occurring at the highest infusion rate. Sodium, potassium and water excretion also increased in parallel with urinary bicarbonate loss. The increase in bicarbonate exposition, glomerular filtration rate, effective renal plasma flow, aldosterone and vasopressin. In 8 sham-operated animals, no abrupt increase in sodium and bicarbonate excretion occurred despite similar continued infusion of sodium bicarbonate. It was concluded that exclusion of one kidney induces immediate adaptive excretory changes for sodium and bicarbonate in the remaining kidney, and that these changes are not accounted for by any of the known factors normally regulating sodium and bicarbonate excretion.

Increased sodium reabsorption postuninephrectomy: evidence for a humoral factor

Natriuresis occurs in the immediate postnephrectomy period. Yet in the days and weeks that follow, increased Na-K-ATPase (sodium-potassium activated adenosine triphosphate) activity, oxygen consumption and sodium reabsorption occur. Increased sodium reabsorption probably occurs early postuninephrectomy but is obscured by passive hemodynamic and physical-chemical factors that induce natriuresis. These factors were controlled in isolated canine kidneys perfused with normal and twenty-four-hour postuninephrectomized blood. A substance that increases sodium reabsorption and glomerular filtration rate in the kidneys perfused with uninephrectomized blood was demonstrated. The relationship between this and a hormonal substance that induces compensatory renal hypertrophy is unknown. The use of this hormone for treatment of renal failure is discussed.

Renal function following branch-artery ligation in the canine kidney

Viable kidney mass has been reduced in dogs by branch-artery ligation combined with uninephrectomy. 1. When viable kidney remnants amount to about 1/4 of the original kidney mass dogs survive in apparent good health with moderate azotaemia; with 1/8 to 1/10 kidney remnants azotaemia is progressive and uraemic death supervenes within some days. 2. Reduction of renal blood flow and of glomerular filtration rate to about one-fourth of their respective control values seems to be compatible with survival; with further reduction death ensues. An inverse and linear relationship between NPN and GFR on a log-log basis has been found. 3. Mean nephron blood flow is equally enhanced in both the more and the less severely infarcted kidneys. On the other hand, mean nephron glomerular filtration is increased when about one half of the kidney is viable, but decreased with more extensive infarction. 4. Mean nephron vascular resistance is diminished; considering the pronounced decrease in Ein, i.e. filtration fraction, predominance of postglomerular vasodilatation is assumed. The formation of non-filtering, actively perfused nephrons is taken into consideration.

Functional and hemodynamic adaptation to progressive renal ablation

Removal of renal tissue stimulates functional and anatomical adaptation in the remaining renal parenchyma. Since recent studies have demonstrated no apparent limitation in compensatory growth following progressive surgical ablation, experiments were performed to determine the changes in glomerular filtration rate and renal blood flow. After removal of 50% of the renal mass mean nephron glomerular filtration rate increased 60%, and after ablation of 75% of the renal tissue it increased 150%. These changes paralleled the increases in renal growth under the same conditions. In comparison, mean glomerular blood flow rose 90% and 240% after 50% and 75% nephrectomy, respectively; these changes in relation to the changes in glomerular filtration rate resulted in a progressive fall in the filtration fraction. Intrarenal blood flow distribution was examined with labeled microspheres. The marked increase in renal blood flow after surgical ablation was characterized by a disproportionate rise in blood flow to the inner cortex. The present investigation, therefore, described the remarkable functional changes that occur as overall glomerular filtration rate declines and provides further insight into the mechanism responsible for maintaining water and electrolyte homeostasis after loss of functioning renal mass.

The effect of ischemia on renal blood flow in the dog

Renal blood flow (RBF) and the distribution of cortical blood flow (microspheres) were measured in the dog after 90 min of total unilateral renal ischemia. RBF was 21% greater than control 2 min after release of the renal artery occlusion, and returned toward control 60 min later. At 2 min after release there was a small but significant increment in deep cortical blood flow which reverted to control by 60 min. When renal artery occlusion was maintained for 180 min, return of blood flow was blunted at 2 min after release of the occlusion, but was not significantly different from control within 10 min after release. Clearance rates of inulin and para-aminohippurate (Cin and Cpah) were 81 and 82% below control after release of occlusion. These data demonstrate that in the dog there is prompt and complete return of blood flow to or above control levels after complete renal artery occlusion. There was no evidence for the "no-reflow" phenomenon.