Colloid osmotic pressure of the subcapsular interstitial fluid of rat kidneys during hydropenia and volume expansion

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.

Renal interstitial pressure measured in the subcapsular space using an in vivo oncometer

Continuous measurements of renal interstital pressure are of importance for several reasons. The present paper describes an in vivo oncometer developed for this purpose. A piece of dialysis tubing was filled with a 0.15 M NaCl solution containing 5 g albumin 100 ml-1. For detecting the hydrostatic pressure inside the tubing, a thin catheter and a silver wire were inserted into it and both ends of the tubing were sealed with glue. The catheter and the silver wire were connected to a servo-nulling pressure device. The oncometer was then placed under the renal capsule. The pressure inside the dialysis tubing was pi onc+Psc-pi sc, and since pi onc was known, the net interstitial pressure (Pnet, i.e. Psc-pi sc) could be measured continuously in the subcapsular space. Measurements were made during (I) intravenous bolus injection of 2 ml of 0.9% NaCl, (2) saline expansion of 5% of body weight, and (3) elevation of renal venous pressure to 20 mmHg by clamping the renal vein. In the control situation, Pnet averaged-3.3 mmHg, a value in good accordance with findings in earlier studies in which the hydrostatic and oncotic pressure components have been measured separately. Following bolus injection of fluid, Pnet increased transiently by 2.6 mmHg, whereas volume expansion produced a permanent increase in Pnet of almost the same magnitude. During elevated renal venous pressure Pnet was unaffected, except for a minor increase on clamping and a minor decrease on release of the clamp. The results show that reproducible and accurate measurements of Pnet in the renal subcapsular space can be made with an in vivo oncometer.

Chemically induced renal papillary necrosis and upper urothelial carcinoma. Part 1

In the past, renal papillary necrosis (RPN) has been commonly associated with long-term abusive analgesic intake, but over recent years a wide variety of industrially and therapeutically used chemicals have been shown to induce this lesion experimentally or in man. Destruction of the renal papilla may result in: (1) secondary degenerative cortical changes which precede chronic renal failure or (2) a rapidly metastasizing upper urothelial carcinoma, which has a very poor prognosis. This article will briefly review the published data on the morphology, function, and biochemistry of the normal renal medulla and the pathology associated with RPN, together with the secondary changes which give rise to cortical degeneration or epithelial carcinoma. It will then examine in detail those chemicals which have been reported to cause RPN in an attempt to delineate structure-activity relationships. Finally, the many different theories that have been proposed to explain the pathophysiology of RPN will be examined and an hypothesis will be put forward to explain the primary pathogenesis of the lesion and its secondary consequences.

Arterial hypertension--a disease of the juxtaglomerular apparatus?

From a special strain of genetically hypertensive rats, the Milan hypertensive strain (MHS), arterial hypertension can be transplanted with the kidney to the Milan normotensive strain (MNS). During development of hypertension in MHS rats there was an activation of the tubuloglomerular feedback control that reduced glomerular filtration rate, leading to retention of electrolytes and fluid. This increased extracellular fluid volume reduces feedback sensitivity, but in a fashion that gives rise to chronic extracellular fluid expansion and can thereby raise the blood pressure. In a limited sense, arterial hypertension in these animals exists to prevent the kidney from retaining more extracellular fluid volume. The altered function in the juxtaglomerular apparatus of the MHS rats thus may explain the rise in arterial blood pressure.

Chemically induced renal papillary necrosis and upper urothelial carcinoma. Part 2

In the past, renal papillary necrosis (RPN) has been commonly associated with long-term abusive analgesic intake, but over recent years a wide variety of industrially and therapeutically used chemicals have been shown to induce this lesion experimentally or in man. Destruction of the renal papilla may result in: (1) secondary degenerative cortical changes which precede chronic renal failure or (2) a rapidly metastasizing upper urothelial carcinoma, which has a very poor prognosis. This article will briefly review the published data on the morphology, function, and biochemistry of the normal renal medulla and the pathology associated with RPN, together with the secondary changes which give rise to cortical degeneration or epithelial carcinoma. It will then examine in detail those chemicals which have been reported to cause RPN in an attempt to delineate structure-activity relationships. Finally, the many different theories that have been proposed to explain the pathophysiology of RPN will be examined and an hypothesis will be put forward to explain the primary pathogenesis of the lesion and its secondary consequences.

Tubuloglomerular feedback and interstitial pressure in obstructive nephropathy

The possible role of the tubuloglomerular feedback (TGF) mechanism in the altered glomerular hemodynamics and tubular reabsorption which occur with prolonged (24-hr) ureteral obstruction and the changes in renal interstitial hydrostatic and oncotic pressure which may modulate TGF sensitivity were examined. The proximal tubule stop-flow pressure (PSF) response to increased distal tubular flow rates (TGF activity) was determined in rats with sham operation, 24-hr unilateral ureteral obstruction (UUO), or 24-hr bilateral ureteral obstruction (BUO), both before and for 2 hr after relief of obstruction. Subcapsular hydrostatic pressure, lymph flow and oncotic pressure, clearance and excretory data were measured in the second series of animals. During and after release of UUO, TGF sensitivity was increased, as indicated by the marked decrease in the loop perfusion rate at which 50% of the maximum decrease in PSF occurred (the turning point of TGF activation). Interstitial oncotic pressure but not hydrostatic pressure was significantly increased in UUO kidneys. In BUO rats, the turning point for TGF activation was slightly higher than the controls and the change in PSF with maximum loop perfusion rates was reduced, indicating a blunting of the TGF response before and particularly during postobstructive diuresis after release of BUO. Interstitial hydrostatic and oncotic pressures were both slightly increased resulting in no changes in net interstitial Starling forces. We conclude that enhanced TGF sensitivity after release of prolonged UUO, associated with increased interstitial oncotic pressure, may play a role in preventing postobstructive diuresis, while the blunting of TGF sensitivity after BUO may contribute to this phenomenon.

Effect of furosemide administration on glomerular and tubular dynamics in the rat

Furosemide, a potent diuretic, has also been shown (1) to inhibit or reduce tubuloglomerular feedback activity, (2) act as a vasodilatory agent, and (3) exhibit a modest carbonic anhydrase inhibitory effect, which could potentially reduce proximal tubule reabsorption. If furosemide can inhibit tubuloglomerular feedback as well as cause vasodilation, then glomerular filtration rate (GFR) should increase through alterations in the dynamics of glomerular ultrafiltration. The effect of acute furosemide infusion (4 mg/kg of body wt per hour) on glomerular and tubular dynamics was examined in Munich-Wistar rats by two protocols: The first allowed a 3% volume depletion (based on body wt) to occur as a result of furosemide administration (group 1); the second allowed a complete replacement of volume after furosemide administration (group 2). The results demonstrated that when volume status was maintained after furosemide administration, the nephron filtration rate remained constant (35 +/- 3 vs. 33 +/- 2 nl/min, NS) despite a twofold increase in distal flow rate (5 +/- 1 vs. 10 +/- 1 nl/min, P less than 0.01), indicating an inhibition or suppression of the tubuloglomerular feedback system. With either protocol, furosemide administration did not alter total nephron vascular resistance and nephron blood flow (190 +/- 17 vs. 200 +/- 15 ml/min); however, the afferent arteriolar resistance did decrease in rats in which volume status was maintained. Finally, with volume status maintained, we were not able to demonstrate a reduction in absolute proximal fluid reabsorption despite a 7 mm Hg increase in interstitial hydrostatic pressure (4 +/- 1 to 11 +/- 1 mm Hg, P less than 0.01) and no compensatory increase in interstitial oncotic pressure. These data indicate that tubuloglomerular feedback was inhibited but that GFR was not increased. Major changes occurred in interstitial pressures and interstitial volume after furosemide administration, but absolute proximal reabsorption remained constant.

The effect of intraarterial infusion of prostacyclin on the tubuloglomerular feedback control in the rat

Effects of intraarterial prostacyclin (PGI2) infusions on interstitial hydrostatic and oncotic pressures and on the tubuloglomerular feedback (TGF) control of glomerular filtration rate (GFR) were studied in rat kidneys. The hilar lymph flow rate was used as a measure of interstitial hydrostatic pressure and the lymph protein concentration was used for interstitial oncotic pressure estimation. In the micropuncture experiments the stop-flow pressure technique was employed for determining the TGF characteristics, i.e. stop-flow pressure (PSF), maximal reduction of PSF (delta PSF) and turning point (TP), defined as the end-proximal flow rate at which 50% of delta PSF was obtained. Non-hypotensive doses of PGI2 (50 to 100 ng X kg-1 B.W. X min-1) infused in 30 min evoked an increase in urine and lymph flow rates and a decrease in lymph protein concentration, but did not affect GFR. delta PSF was reduced (9.9 +/- 1.0 mmHg versus 4.7 +/- 2.2 mmHg) and TP increased (22 +/- 2 nl/min versus 34 +/- 2 nl/min), but the PSF was unaffected. These changes were seen during the infusion period and during the immediate post-infusion control period of 30 min. Our data indicate that non-hypotensive doses of PGI2 in some way can affect the renal interstitial pressure and the TGF control system.

Physiologic basis for the maintenance of glomerulotubular balance in young growing rats

To examine the physiologic basis of preservation of glomerulotubular balance in young growing animals, we measured the various determinants for fluid transfer across the glomerular and postglomerular capillaries in young (approximately 40 days, N = 8) and adult (N = 8) male Munich-Wistar rats under mild volume expansion. The single nephron (SN) GFR increased by approximately 2.5-fold from young to adult animals. The increase in SNGFR was due to marked rises in both glomerular plasma flow rate (QA) and ultrafiltration coefficient (Kf). The increase in QA was associated with a nearly 60% reduction in afferent and efferent arteriolar resistances. As with SNGFR, the absolute proximal reabsorption rate (APR) increased by some 2.5-fold, indicating preservation of perfect glomerulotubular balance. Of the factors determining peritubular capillary uptake of APR, the mean oncotic pressure difference across the peritubular capillary was similar in young and adult animals. The mean hydraulic pressure difference was also comparable in the two groups. By contrast, the peritubular capillary reabsorption coefficient (Kr) rose markedly and accounted entirely for the increase in peritubular capillary uptake of APR during growth. These results obtained under mild volume expansion indicate that the maintenance of glomerulotubular balance in the growing rat requires harmonious growth of renal microcirculation, that is, glomerular capillary (Kf) and arteriolar (QA) maturation balances the development of peritubular capillary (Kr).

Prevention of interstitial pressure change at unilateral nephrectomy by prostaglandin synthesis inhibition

The mechanisms underlying the adaptive process in the remaining kidney after unilateral nephrectomy are not well known. In the present study adaptive changes in interstitial pressure conditions following uninephrectomy were investigated in male Sprague-Dawley rats. Direct subcapsular hydrostatic pressure recordings were made via microcatheters , renal lymph was collected for lymph flow rate estimations and determinations of lymph protein content and the urine flow rate and urinary excretion of sodium and potassium were measured, before and after contralateral nephrectomy. There was a significant rise in subcapsular hydrostatic pressure by about 50%, and in the lymph flow rate, by more than 100%, within 1 hr after nephrectomy. The protein concentration in collected renal lymph was significantly decreased (from 2.30 +/- 0.25 to 1.45 +/- 0.26 g/dl), compared with control collections in sham-operated animals (from 2.22 +/- 0.23 to 1.78 +/- 0.19 g/dl). These findings indicate an increased hydrostatic and decreased intersititial pressure in response to contralateral nephrectomy. Urine flow rate and electrolyte excretion were significantly elevated (urine flow rate by 92%, sodium excretion from 0.210 +/- 0.035 to 0.352 +/- 0.067 mumoles/min and potassium excretion from 0.306 +/- 0.074 to 1.617 +/- 0.228 mumoles/min). Inhibition of prostaglandin synthesis prior to nephrectomy abolished the lymph flow increase and oncotic pressure decrease and also the diuretic and natriuretic responses. The adaptively increased potassium excretion was, however, only blocked by diclofenac sodium and not by indomethacin. These results demonstrate the important role of the renal interstitium and the prostaglandin system in the renal adaptation to nephron loss.

Blood flow dependence of postglomerular fluid transfer and glomerulotubular balance

The rate of blood flow entering a capillary network can, in some vascular systems, regulate capillary surface area and the rate of fluid and solute transfer. To determine whether such a mechanism exists in the renal peritubular capillary, we performed micropuncture studies in 28 rats during relatively low and high efferent arteriolar blood flow (EABF). High EABF was achieved by intravenous infusion of isoncotic plasma (group 1: from 120 +/- 11 to 301 +/- 49 nl/min [+/- SE]); whole blood with high hematocrit (approximately 75 vol %) (group 2: from 141 +/- 14 to 252 +/- 31 nl/min); or acetylcholine (group 3: from 193 +/- 20 to 266 +/- 26 nl/min). In group 1 rats, plasma infusion caused an increase in single nephron glomerular filtration rate (SNGFR), on average, from 23.2 +/- 2.4 to 45.2 +/- 3.9 nl/min, owing primarily to increased glomerular plasma flow rate (from 63 +/- 5 to 210 +/- 21 nl/min). The rate of fluid uptake by the peritubular capillary, assessed by the absolute rate of proximal fluid reabsorption (APR), also rose significantly, on average from 10.5 +/- 1.2 to 17.5 +/- 2.4 nl/min. This rise in APR was associated with near constancy in mean transcapillary hydraulic (delta Pc) and oncotic (delta IIc) pressure differences, and was therefore attributed to a significant increase in peritubular capillary reabsorption coefficient (Kr), with the mean from 0.017 +/- 0.003 to 0.030 +/- 0.005 nl/(s . mmHg). In group 2 rats, high hematocrit blood infusion led to a significant rise in APR; on average, from 10.7 +/- 0.7 to 15.0 +/- 1.2 nl/min, without changing SNGFR. This rise in APR occurred despite unfavorable changes in the physical forces, namely a significant increase in delta Pc and constancy in delta IIc. Instead, an increase in EABF was again associated with a significant rise in Kr (on average, from 0.016 +/- 0.002 to 0.030 +/- 0.06 nl/[s . mmHg]), which accounted entirely for the rise in APR, independently of SNGFR. In group 3 rats, in which an increase of EABF was induced pharmacologically with acetylcholine, a rise in EABF was also accompanied by a significant increase in Kr, on average, from 0.019 +/- 0.002 to 0.026 +/- 0.004 nl/(s . mmHg). The results indicate that: (a) Kr is modulated by EABF. (b) In view of plasma flow dependence of GFR, blood flow dependence of Kr and APR provides an important basis for glomerulotubular balance.

Modulation of renal tubular function by renal interstitium

Rat micropuncture experiments have been performed to investigate the influence of renal interstitial pressure conditions on proximal tubular fluid absorption and on the tubulo-glomerular feedback mechanism. The feedback mechanism works by sensing the distal tubular fluid flow rate at the macula densa segment and adjusting the tonus of the arterioles with resulting changes in blood flow and glomerular filtration rate. It was found that in situations with high hydrostatic and low oncotic pressure within the interstitium like in saline volume expansion and post-nephrectomy situations, proximal tubular fluid absorption and the sensitivity of the tubulo-glomerular feedback control are reduced giving rise to increased urine production without activating the feedback mechanism. In situations with low hydrostatic and high oncotic interstitial pressures like in dehydration, hypotension and hypovolemia proximal fluid absorption and feedback sensitivity is increased, feedback sensitivity even so much that the mechanism is activated to reduce GFR even though the load to the distal portion of the nephron is reduced below normal. In this way renal interstitial pressure-volume conditions can modulate renal function in response to extracellular fluid needs and blood pressure level.

Hydrostatic and oncotic pressures in the interstitium of dehydrated and volume expanded rats

The pressures in the renal interstitial space seem to have important influence on the setting of the sensitivity of the tubuloglomerular feedback that controls the glomerular filtration rate (GFR), and on the rate of proximal tubular fluid reabsorption. Measurements were made of interstitial pressure conditions, GFR, renal plasma flow (RPF), urinary excretion of sodium and potassium, and plasma renin activities in dehydrated animals and normopenic controls, before and after saline volume expansion (5% of body weight and hour). Colloid osmotic pressure, estimated from the protein concentration in renal hilar lymph, was 7.5 mmHg in the dehydrated animals (controls 2.8 mmHg) and decreased to 3.1 (controls 1.7 mmHg) after volume expansion. The lymph flow rate was increased in both groups of animals after volume expansion. Interstitial hydrostatic pressure, measured in the subcapsular space, was 2-3 mmHg in dehydrated and control animals and increased to 3-4 mmHg after volume expansion. In dehydrated rats GFR and RPF was reduced to 60% of the control values, but after volume expansion they regained control values. After volume expansion, urinary excretion of fluid and electrolytes increased more in controls than in dehydrated rats. Plasma renin activity was decreased in both groups of rats after volume expansion. Thus, in dehydrated animals there was a high colloid osmotic pressure and a low hydrostatic pressure in the renal interstitium, while after volume expansion the oncotic pressure fell and the hydrostatic pressure rose. The effect of volume expansion was found to be dependent on the preceding volume balance situation in the animal.

Modulation of proximal tubular hydraulic conductivity by peritubular capillary oncotic pressure

Fluid absorption from the proximal tubular lumen is probably a multifactorial process. Earlier studies from our laboratory have indicated that a transepithelial hydrostatic and oncotic pressure difference may be the driving force for as much as 30% of the reabsorbed fluid. During saline volume expansion proximal tubular reabsorption declines and the present experiments were undertaken to investigate whether this reduction could be caused by changes in the passively driven flux component. The hydraulic conductivity was therefore determined from the reabsorptive rate in split oil droplets with normal and high hydrostatic pressure gradients across the wall, at the same time as the peritubular capillary net-work was perfused with solutions containing a colloid of high or low concentration. In the reabsorption experiments the split oil droplet radius was measured and in a separate series of experiments the relationship between droplet radius and pressure was determined; this was found to be 7.3 mmHg pressure increase per 1 micrometer increase in radius. The increase in the rate of reabsorption from the droplets due to increased intraluminal hydrostatic pressure was 1.02 +/- 0.13 nl/min/mm tubular length when a solution with a high colloid concentration was perfused through the capillary net-work, compared with 0.41=0.11 nl/min/mm tubular length when a low colloid containing solution was used for perfusion. The hydraulic conductance in the proximal tubular wall at high colloid perfusion was calculated to be 0.54 nl/min.mm.mmHg while at a low capillary colloid oncotic pressure it was significantly lower 0.025 nl/min.mm.mmHg. This drop in hydraulic conductance might be one factor responsible for the decline in fluid absorption in animals exposed to saline volume expansion.

Mechanism of glomerulotubular balance in the setting of heterogeneous glomerular injury. Preservation of a close functional linkage between individual nephrons and surrounding microvasculature

Autologous immune complex nephropathy (AICN), an experimental model for human membranous glomerulopathy, is characterized by marked heterogeneity in function from glomerulus to glomerulus. However, the fraction of the filtered load of fluid reabsorbed by the proximal tubule remains nearly constant from nephron to nephron, despite wide variation in single nephron glomerular filtration rate (SNGFR). To define the physiological mechanisms responsible for this marked variation in SNGFR values within a given kidney and for the remarkable preservation of glomerulotubular balance, the various determinants of fluid exchange across glomerular and peritubular capillary networks were evaluated in Munich-Wistar rats with AICN. For comparison, similar measurements were obtained in rats with the functionally more homogeneous lesion of heterologous immune complex nephropathy. In AICN rats studied approximately 5 mo after injection of renal tubule epithelial antigen (Fx1A), a high degree of glomerulus-proximal tubule balance was found, despite marked variations in SNGFR values within a single kidney. These changes were associated with marked heterogeneity in immunoglobulin and complement deposition within and among glomeruli. Although mean capillary hydraulic pressure and Bowman's space hydraulic pressure ranged widely from glomerulus to glomerulus, the mean glomerular transcapillary hydraulic pressure difference was remarkably uniform among these functionally diverse glomeruli and could not, therefore, be implicated as the cause of the dispersion in SNGFR values. The two remaining determinants of SNGFR, namely, glomerular plasma flow rate (Q(A)) and ultrafiltration coefficient (K(f)), varied markedly from glomerulus to glomerulus, but always in direct proportion to SNGFR, and proved to be responsible for the marked variation in SNGFR. The mean net peritubular capillary reabsorptive force ( P(r)) correlated closely with the absolute rate of fluid reabsorption in adjacent proximal tubules (APR) in AICN. Of the factors determining P(r), peritubular capillary hydraulic pressure was essentially constant in a given AICN kidney, whereas peritubular capillary plasma protein concentration and oncotic pressure varied directly with APR and largely accounted for the observed tight correlation between P(r) and APR. ON THE BASIS OF THESE OBSERVED CORRELATIONS, WE SUGGEST THAT THE CLOSE QUANTITATIVE COUPLING BETWEEN SNGFR AND APR IN INDIVIDUAL NEPHRONS IN AICN IS DUE TO THE FUNCTIONAL RESPONSE OF INDIVIDUAL GLOMERULI: those with the most pronounced declines in SNGFR are characterized by the most pronounced declines in Q(A) and K(f). The resultant low peritubular capillary oncotic pressure favors a decline in APR, thus favoring nearly perfect glomerulotubular balance. In glomeruli with higher SNGFR values, Q(A) and K(f) values are also higher. These changes in K(f) once again are capable of establishing the conditions in downstream peritubular capillaries, this time favoring augmented APR (i.e., high intracapillary oncotic pressure), again leading to nearly perfect glomerulotubular balance.

Importance of efferent arteriolar vascular tone in regulation of proximal tubule fluid reabsorption and glomerulotubular balance in the rat

Micropuncture study was performed in 21 mildly volume-expanded Munich-Wistar rats before and during partial aortic constriction to examine the effects of endogenous prostaglandins (PG) and angiotensin II (AII) on single nephron glomerular filtration rate (SNGFR) and absolute proximal reabsorption rate (APR). Animals received either vehicle (group 1), indomethacin (group 2), or indomethacin plus saralasin (group 3). Before aortic constriction, these inhibitors were without effect on values of SNGFR and APR. In group 1 rats, reduction in mean renal arterial perfusion pressure (RAP) to approximately 65 mm Hg resulted in marked and proportional declines in SNGFR and APR. With equivalent reduction in RAP in group 2 rats, however, SNGFR fell to a lesser extent and APR tended to increase slightly above preconstriction values. Indomethacin administration was therefore associated with disruption of glomerulotubular balance. In view of the roughly equivalent declines in afferent arteriolar resistance measured in groups 1 and 2, the magnitude of increase in efferent arteriolar resistance (R(E)) appeared to be of major importance in determining the observed presence or absence of glomerulotubular balance. Thus, the lesser fall in SNGFR in group 2 than in group 1 was a result of the higher value for glomerular capillary hydraulic pressure in group 2, a consequence of the higher value of R(E). The higher average value for APR during reduced RAP in group 2 than in group 1 is also attributable to this pronounced rise in R(E), the effect of which was to augment the net reabsorptive pressure both by favoring higher postglomerular oncotic pressure and lower downstream (peritubular capillary) hydraulic pressure. Since intrarenal release of AII is enhanced when RAP declines, and because AII is known to raise R(E) selectively, it is likely that endogenous AII brought about the marked increase in R(E) in group 2, which was readily demonstrable only in indomethacin-treated rats, presumably because endogenous PG synthesis was suppressed. In keeping with this conclusion, when the action of endogenous AII was inhibited by saralasin in group 3 rats, reduction in RAP failed to induce a rise in R(E), so that net filtration and reabsorption pressures again declined proportionally, as did SNGFR and APR. The present evidence therefore suggests that glomerulotubular balance is influenced to an important extent by the prevailing vasomotor tone of the efferent arteriole.

Postglomerular vascular hydrostatic and oncotic pressures during acute saline volume expansion in normotensive man

The importance of the peritubular physical factors as mediators of the natriuretic response to saline volume expansion was examined in twenty normotensive, hydropenic indivduals. Intrarenal venous pressure (IRVP), used as a measure of peritubular capillary hydrostatic pressure, and efferent arteriolar colloid osmotic pressure, calculated from arterial colloid osmotic pressure and the filtration fraction, were measured before and during sustained volume expansion with 0.9% NaCl, increasing the body weight by 3% and plasma volume about 20%. During expansion there was a significant increase in urine flow from 1.1 +/0 0.1 to 3.1 +/- 0.4 ml/min and sodium excretion from 161 +/- 12 to 551 +/- 61 microEq/min. Efferent colloid osmotic pressure fell from 31.9 +/- 0.6 to 23.6 +/- 0.5 mmHg (P less than 0.001) while IRVP changed from 24.8 +/- 0.8 to 25.1 +/- 0.9 mmHg (P greater than 0.10). In eight individuals IRVP increased during saline loading but later fell during sustained expansion. Glomerular filtration rate and renal blood flow did not change significantly. It is concluded that increase in peritubular capillary hydrostatic pressure is not necessary either to induce or to maintain the natriuresis of a moderate saline volume expansion. Although the fall in postglomerular vascular colloid osmotic pressure is a possible mediator of the natriuretic response, the change in peritubular transcapillary net driving force produced by a modest saline volume expansion is probably small.

Changes in interstitial pressure during acute interstitial volume depletion in normally hydrated rats

Interstitial fluid pressure was measured in normally hydrated rats during acute interstitial volume depletion by intravenous hyperoncotic bovine serum albumin infusion. Body fluid volumes, systemic arterial and venous pressure and selected blood and urine variables were also measured. The infusion increased plasma volume twice as much as do iso-oncotic infusions which cause comparable increases in mean central venous pressure. The kidneys responded with a diuresis and natriuresis closely resembling those which follow iso-oncotic infusion in normally hydrated rats. At the end of the elevated renal response plasma volume and plasma protein concentration were not restored to pre-infusion values; total interstitial fluid volume was decreased to one half its control value. Interstitial fluid pressure decreased linearly with volume so that effective interstitial compliance was constant at 0.0717 ml/mm Hg per gram dry tissue weight (1.79 ml/mm Hg per 100 g BW). This was not significantly different from the value 0.0704 previously found in normally hydrated rats but very significantly higher than that in dehydrated rats with comparable interstitial depletion. It is concluded that interstitial compliance is normal over a wide range of interstitial fluid volume in normally hydrated rats but that it can be altered in states of chronic body water depletion.

The extravascular pool of albumin in the non-diuretic rat kidney estimated by means of radioactive tracers and quantitative immunochemistry

The extravascular pool of albumin in the non-diuretic rat kidney was determined as the difference between total and intravascular albumin pools. The total pool of albumin was determined in two ways in the same kideny: 1. as the distribution volume of rat endogenous albumin measured immunochemically and 2. as the distribution volume of 125I-labelled rat albumin equilibrated for 4 days. The distribution volume of 131I human albumin after 1 min equilibration was taken as a measure of the intravascular pool. Total albumin pool estimated as rat endogenous albumin volume of distribution averaged 13.4, 25.1, and 43.0 mul per 100 mg tissue in cortex, outer and inner medulla, respectively. Similar figures were obtained using 125I-labelled rat albumin. Intravascular pool of albumin in the 3 zones averaged 7.2, 19.7, and 26.8 mul per 100 mg tissue. The calculated extravascular fraction of albumin averaged 0.44, 0.21, and 0.35 for cortex, outer and inner medulla, respectively.

The hydraulic conductivity of the rat proximal tubular wall determined with colloidal solutions

The hydraulic conductivity of the rat proximal tubular wall was determined using colloidal solutions perfused in short (50--200 mum) (SMP) or long (90--200 mum) (LMP) proximal tubular segments. In SMP human serum albumin (HSA) or polyvinylpyrrolidone (PVP) was added to raffinose solutions. A Lp of 0.019 nl-min-1-mm-1-mm Hg-1 was found when high colloid concentrations were used while values of 0.055--0.092 were found when low colloid concentrations were used. In other experiments, the Lp was determined by perfusing short tubular segments with pure raffinose solutions. A value of 0.015 nl-min-1-mm-1-mm Hg-1 was found. This is twice the value found when raffinose solutions were perfused through long tubular segments and it is concluded that the short microperfusion technique overestimates Lp with a factor of two. When microperfusions of long tubular segments were conducted, PVP was added to an equilibrium solution consisting of NaCl (110 mM) and raffinose (80 mM). Lp was found to be 0.018--0.021 when high colloid concentrations were used, while a value of 0.029 was found when a low colloid concentration was used. As found in both SMP and LMP a decrease in Lp's with increasing colloid concentrations indicates that a significant influence of radial concentration differences is highly probable. It is therefore suggested that the highest Lp derived when using the lowest colloid concentrations represents the best estimate. With this Lp value (0.03--0.05 nl-min-1-mm-1-mm Hg-1) and the existing transtubular hydrostatic and oncotic pressure difference it can be calculated that these passive forces might constitute the driving force for 1/3 of the fluid reabsorbed in the proximal tubule.

Kinetics of the glomerular ultrafiltration in the rat kidney. A theoretical study

The glomerular filtration process was evaluated theoretically from micropuncture data obtained from Sprague-Dawley rats. The hydrostatic pressures in the glomerular capillaries and Bowman's space minus the oncotic pressure in systemic plasma gave the net driving force at the proximal end of the glomerular capillary. From the single nephron filtration fraction the mean net driving force over the glomerular membrane was calculated to be 20 mm Hg during normotension, decreasing to 12 mm Hg during a perfusion pressure of 80 mm Hg. The hydraulic permeability for one glomerulus was 0.7-0.8 nl/min-100 g b.wt. mmHg. The pressures at the distal end of the glomerular capillaries were 13 and 6 mm Hg under the above two conditions, indicating non-equilibrium of the filtration process at the end of the glomerular capillary. It was shown that the glomerular filtration rate is mainly influenced by the driving pressures. During hypotension an increased plasma flow dependency was evident. Brenner et al. found a filtration equilibrium and a plasma flow dependent glomerular filtration rate in a mutant Wistar rat strain. The discrepancy between their results and ours is due to the low glomerular plasma flow and hydrostatic pressures in the Wistar rats. It is concluded from our results that both pre- and postglomerular resistances may influence the glomerular filtration rate and glomerular plasma flow independently.

Control of proximal tubule fluid reabsorption in experimental glomerulonephritis

We have recently shown that in the early autologous phase of nephrotoxic serum nephritis (NSN) single nephron glomerular filtration rate is unchanged from values in normal hydropenic control rats, but that single nephron filtration fraction and efferent arteriolar oncotic pressure (piE) are reduced because of a marked reduction in the glomerular capillary ultrafiltration coefficient. The present study was undertaken to examine the influence of this decline in piE as well as the other known determinants of peritubular capillary fluid exchange on absolute proximal fluid reabsorption (APR) in NSN. The findings indicate that APR and proximal fractional reabsorption are reduced significantly in NSN, relative to values in a separate group of age and weight-matched normal hydropenic control rats studied concurrently. In addition to the measured decline in piE, efferent arteriolar plasma flow (Qe) and peritubular capillary hydraulic pressure (Pc) were found to increase significantly, while interstitial oncotic pressure, estimated from hilar lymph, was not significantly different from values in control rats. Using a mathematical model of peritubular capillary fluid uptake we found that, assuming that the capillary permeability-surface area product and interstitial hydraulic pressure are unchanged in NSN, the observed changes in piE and Pc are sufficient to offset the effect of the increase in QE, yielding a calculated reduction in APR of approximately 4 nl/min, in excellent agreement with the observed mean decline of 4.1 nl/min. These findings suggest that control of APR in NSN is mediated by the same factors that regulate APR under normal physiological conditions, namely, the imbalance of forces governing peritubular capillary uptake of isotonic reabsorbate.

Influence of peritubular hydrostatic and oncotic pressures on fluid reabsorption in proximal tubules of the rat kidney

Proximal tubular reabsorption was measured with the split droplet technique under the following conditions. Group 1. Star vessel perfusions with ultrafiltrate in the presence of human serum albumin (HSA) and with ultrafiltrate alone. Group 2. Capillary perfusions, under locally high flow rates, with ultrafiltrate in the absence or presence of HSA. Group 3. Subcapsular perfusions with ultrafiltrate of high HSA concentrations but under low hydrostatic pressure and ultrafiltrate of low HSA concentrations but under elevated hydrostatic pressure. In Group 1, the presence of HSA increased the reabsorptive rate, whereas ultrafiltrate alone had no provable effect on the reabsorptive rate. In Group 2, a flow dependent decrease of the reabsorptive rate, that was not influenced by the presence of HSA, was observed. Finally, in Group 3, high concentrations of HSA did not alter the reabsorptive rate, whereas elevation of the hydrostatic pressure decreased the reabsorptive rate. The results cannot be explained on the basis of a simple passive mode of action of colloid osmotic and hydrostatic pressure on the reabsorptive rate. The possibility of a direct tubulae capillary transport route is discussed.

The intrarenal distribution of 125I-albumin in the Syrian hamster

The intrarenal distribution of radioiodinated human serum albumin (125RISA) after intravenous injection was studied in Syrian hamsters by scintillation counting and frozen section autoradiography. After 15, 30 and 60 min the virtual plasma albumin space in the renal cortex of the hamster represented 6.49, 7.13, and 8.06 percent respectively of the kidney tissue volume. From the cortex to the renal papilla the albumin space increased to about 30 percent of the tissue volume. In comparison to this the albumin space in the renal cortex of the rat was about 20% and in the renal papilla about 33% (11). Frozen section autoradiography indicated that the distribution of radioalbumin in the renal cortex of the Syrian hamster is limited mainly to the kidney vessels, being especially noticeable in the glomerular capillaries. Toward the papilla increasingly greater (mainly extratubular) activity could be observed not only intravascularly but also interstitially. In the cortex of the rat kidney, on the other hand, radioactive albumin was accumulated (probably by filtration and reabsorption) predominantly in the proximal tubular epithelium. Wtithin 30 min thekidneys of the rat excreted mor than 10 times as much 125I than the hamster kidneys. These results (substantially less cortical accumulation and urinary excretion of radioalbumin in the Syrian hamster) indicate that, in contrast to the rat, obviously much less albumin is filtered (and then accumulated by proximal reabsorption) by the Syrian hamster glomeruli. This suggests that the Syrian hamster kidney is more suitable than the rat kidney for determining the interstitial, cortical, albumin space.

Dependence of saline-induced natriuresis upon exposure of the kidney to the physical effects of extracellular fluid volume expansion

In many previous studies, the natriuresis induced by saline loading has been demonstrated to persist even though glomerular filtration rate (GFR) has been decreased to below pre-expansion levels by a reduction in renal artery pressure. In such studies, however, the kidney has been exposed to the effects of volume expansion for varying periods of time before renal artery pressure was controlled. The present experiments were designed to evaluate whether this period of exposure induces critical changes in intrarenal factors that are responsible for the natriuresis.Experiments were carried out in rats, in which renal artery pressure was decreased to 70 mm Hg either at the onset of saline loading (immediate clamping experiments) or after 45 min of saline loading had elapsed (delayed clamping experiments). In the delayed clamping experiments, consonant with previous studies, mean sodium excretion, 3.2 mueq/min, remained markedly increased above control, despite a reduction in GFR to 91% of the hydropenic control value. In contrast, when renal artery pressure was comparably reduced at the onset of saline loading mean sodium excretion was only trivially increased, 0.4 mueq/min, although GFR increased to 140% of the hydropenic control value. These results exclude an important role for either a circulating hormone or a reduction in plasma oncotic pressure in the natriuretic response to saline loading, and indicate that intrarenal factors are the critical determinants of the natriuresis. We have used the difference in response to saline loading in the immediate and delayed clamping experiments to evaluate the role of two intrarenal factors, interstitial hydrostatic pressure and renal plasma flow. Interstitial pressure changes were estimated from changes in tubular pressure and diameter by using the in situ compliance characteristics of the tubules. In a group of rats saline loaded without aortic clamping, interstitial pressure increased by 4-5 mm Hg and renal plasma flow increased by 2.5 ml/min. During the period of reduced renal artery pressure, however, neither interstitial pressure nor renal plasma flow was detectably increased above control in either the immediate or the delayed clamping experiments. The only noteworthy difference between the experiments in which a natriuresis occurred (unclamped and delayed clamping studies) and the experiments in which no natriuresis occurred is that in the former group the kidney was at least transiently exposed both to an increase in renal plasma flow and interstitial pressure. These findings indicate, first, that extracellular fluid volume expansion can induce a natriuresis only if the kidney has been exposed to at least a transient increase in either interstitial hydrostatic pressure or renal plasma flow (or both); and, second, that a sustained increase in interstitial pressure and renal plasma flow is not required for the natriuresis to persist.