Localization of tubular uptake segment of filtered Cystatin C and Aprotinin in the rat kidney.

AIM: The renal tubular uptake of 125I-Aprotinin (*Ap) is on average located more superficially than its filtration site, causing transfer of some of *Ap filtered in deep to more superficial cortical zones. 125I-Cystatin C (*Cy) showed less uptake in deep cortical zones than Ap, suggesting a longer and/or a more superficial tubular uptake site. To test that hypothesis and to quantify the outward transfer of the filtered polypeptides, we estimated the tubular uptake pattern of the tracers in perfusion fixed rat kidneys after intravenous injection of *Cy and *Ap. METHODS: Autoradiographs were made from 10 mum thick slices of Microfil nephron casts from outer (OC), middle (MC) and inner (IC) cortical zones to quantify cortical border-crossing *Ap transfer. Single nephron glomerular filtration rate (snGFR) was estimated as the zonal uptake of *Ap corrected for *Ap transfer, divided by its time-integrated plasma concentration and the zonal number of glomeruli. RESULTS: *Ap and *Cy uptake fell exponentially along the proximal convoluted tubule (PCT), indicating an uptake proportional to luminal concentration. Uptake in IC exceeded that in MC and OC nephrons. The per cent PCT length with *Cy uptake (67.2 +/- 1.6) exceeded that of *Ap (54.6 +/- 1.8). The zonal border-crossing PCT length (29-34% of total PCT) from deep to more superficial cortical zones transferred 4-6% more *Cy than *Ap. CONCLUSION: Greater tubular uptake length of *Cy than of *Ap causes more cortical border-crossing of *Cy. The zonal snGFR estimated from Aprotinin uptake corrected for border-crossing agreed well with that obtained with the Hanssen ferrocyanide technique.

Arnold Heller and the lymph pump.

This article reviews studies on lymph propulsion in the lymph vessels by active contraction of the vessels, first described by Arnold Heller in 1869 in German language, and here translated into English. His observations were first confirmed by Beatrice Carrier (1926) and Howard Flory et al. (1927), and several groups were active up to World war II. Few publications appeared in the period 1940--1960, followed by increasing activity and development of new experimental techniques for use both in various experimental animals and in humans. Recently it has been shown that passive lymph flow may add to active propulsion. Both mechanisms depend on lymph formation, i.e. the uptake of interstitial fluid by the initial lymph vessels which is still not well understood.

Influence of iothalamate on renal medullary perfusion and oxygenation in the rat.

PURPOSE: To evaluate controversial results regarding the effect of the contrast medium (CM) iothalamate on renal medullary blood flow by applying two different methods simultaneously. MATERIAL AND METHODS: The outer medullary blood flow (OMBF) response was estimated using laser-Doppler flowmetry and hydrogen gas wash-out (microelectrodes) simultaneously. Outer medullary oxygen tension (PO2) was measured using Clark type microelectrodes. Iothalamate was injected i.v. at 1600 mg I/kg body weight for 2 min. RESULTS: CM induced a transient 28% decrease in OMBF as measured with the laser Doppler. The hydrogen gas wash-out rate was reduced by 50%, indicating a reduced perfusion. CM induced a transient 60% reduction in PO2, while renal fluid and electrolyte excretion increased several fold. CONCLUSION: The CM iothalamate reduces outer medullary perfusion as estimated by two different techniques applied simultaneously. The PO2 in the same region was also reduced. Previous controversies regarding the effect of iothalamate on OMBF can be explained by extreme dosage and injection rates greatly exceeding clinical relevance.

The effect of AVP-V receptor stimulation on local GFR in the rat kidney.

AIM: Arginine vasopressin (AVP) might influence urinary concentration ability by altering the intrarenal distribution of glomerular filtration rate (GFR). METHODS: To study this possibility we have measured the intracortical distribution of GFR following acute AVP-V1 receptor stimulation in anaesthetized female Sprague-Dawley (SPD) rats during euvolemia and water diuresis by the aprotinin method, allowing two consecutive measurements of zonal GFR in the same kidney. RESULTS: Acute i.v. bolus injection of 50 ng V1 receptor agonist ([Phe2, Ile3, Orn8]-vasopressin) followed by a continuous infusion of 5 ng min(-1) in euvolemic rats reduced GFR by 25% in outer cortex (OC), 20% in middle cortex (MC) and 19% in inner cortex (IC) relative to vehicle infusion (all P < 0.05). In water diuretic rats V1 receptor agonist reduced GFR by 22% in OC, 10% in MC and 11% in IC relative to vehicle infusion (P < 0.05). GFR decreased slightly more in OC than in MC and IC in both euvolemic and water diuretic rats (P < 0.05) indicating a distribution of GFR towards MC and IC. Acute infusion of the selective non-peptide V1 receptor antagonist OPC-21268 in euvolemic rats reduced GFR by 14% in OC, 13% in MC and 11% in IC relative to vehicle infusion (P < 0.05), with no significant difference between the layers. CONCLUSIONS: The change in distribution of GFR not only between OC and IC, but also between OC and MC suggests that the afferent/efferent arterioles and not the medullary vasa recta is the main site of resistance change. We conclude that acute i.v. infusion of V1 receptor agonist in high doses reduces GFR more in superficial than in deep cortex in both euvolemic and water diuretic rats and that this may be of some importance for water conservation, adding to the V2- receptor effect on water permeability of the collecting ducts.

Drainage of plasma proteins from the renal medullary interstitium in rats.

1. Lymph vessels are scarce or lacking in the renal inner medulla, raising the question of whether plasma proteins entering the medullary interstitium are removed by diffusion through the interstitium to lymphatics in the outer medulla or cortex, or by convection into the vasa recta. 2. Using micropipettes, we infused 125I-albumin into the papilla of anaesthetized rats and watched its disappearance from the injection site as well as the uptake in the thoracic duct and plasma. 3. Tracer infused into the renal cortex appeared almost immediately in the thoracic duct lymph, and rose to a sevenfold higher concentration than in plasma, whereas tracer infused into the papilla appeared first and increased more sharply in plasma than in the lymph. No spread from the papillary injection site was observed. Tracer injected in renal hilar lymphatics was quantitatively recovered in the thoracic duct. 4. The plasma concentration pattern following papillary infusion was similar to that obtained by intravenous injection, indicating uptake in blood and subsequent distribution to extracellular fluid and lymph from all organs. 5. We conclude that plasma proteins normally diffusing out from the vasa recta are brought back through water flux (1) from the collecting ducts due to the high sodium chloride concentration in the papillary interstitium and (2) from the interstitium into the vasa recta driven by plasma protein osmotic pressure. Accordingly, there is no need for lymph vessels in the inner medulla.

Distribution spaces for hyaluronan and albumin in rat tail tendons.

A low concentration of hyaluronan (HA) in lymph compared with tissue suggests a large bound fraction. To investigate the distribution and mobility of HA and serum albumin (Alb), we eluted the rat tail tendon with a series of l5 successive centrifugations, each preceded by the addition of 0.15 M NaCl (15% of initial wet wt). The eluate concentration fell exponentially versus the accumulated eluate, allowing estimation of the maximal elutable amount (E(HA) and E(Alb)). Alb elution was practically complete from a space of approximately 28% of wet wt at all centrifugation rates. Twenty percent of HA was elutable at 500 rpm, apparently from the same space as Alb, increasing to 40% at >4,000 rpm. This pattern was not significantly influenced by using 2 M NaCl or by the addition of plasma or metabolic inhibitors. Without prehydration and centrifugation at high revolutions per minute, both Alb and HA concentrations fell rapidly toward zero, presumably in part reflecting mobilization of HA- and Alb-free fluid from the collagen intrafibrillar space (3). We conclude that with prehydration the fibrils swell, increasing the intramolecular spaces to become "penetrable" to HA and allowing removal of HA-containing fluid when the fibrils are compressed by the next centrifugation at high revolutions per minute, increasing E(HA) from 23 to 45%. Chemical binding presumably explains the unelutable 55% of tendon HA. Intrafibrillar HA may act to stabilize the fibrillar volume.

Odd E. Hanssen and the Hanssen method for measurement of single-nephron glomerular filtration rate.

In the middle of the twentieth century, the suspicion that deep and superficial nephrons might serve different functions created a demand for measurement of single-nephron glomerular filtration rate (SNGFR). Rather unexpectedly, the answer came from Odd E. Hanssen (1917-1964), a Norwegian physician working on his own in the Department of Pathological Anatomy, University of Oslo, with minimal support and no interaction with renal physiologists. In 1963, after nearly 10 years of work, he presented the ferrocyanide method, allowing simultaneous estimates of SNGFR in a large number of nephrons in all layers of the kidney. This review first describes his early visions of the method and the elaborate and extremely time-consuming studies in mice to verify the technique. As a byproduct came valuable information on the relationship between nephron size and SNGFR, glomerular intermittency, and the emptying of the tubules on filtration stop. Hanssen died from a cerebral hemorrhage in 1964, and for several years the method seemed entirely forgotten. Fortunately, Andrew Baines took up the use of ferrocyanide in 1963-1964 while working on his thesis in Toronto, but his first publication came in 1969 from Saclay, France, in collaboration with Christian de Rouffignac. Modifications allowing determination of absolute SNGFR were worked out by de Rouffignac and by Jaime Coehlo in New York. Thereafter, the "Hanssen method" spread rapidly, and in the early 1980s about 50 reports had been published from 17 laboratories in 9 countries. The distribution of SNGFR in mammals, birds, and fish was described, as well as the response to water and salt loads, vasoactive substances, hormones, varying perfusion pressure, blood loss, etc. Finally, after mentioning two recent methods inspired by the Hanssen technique but using other filtration markers, the review concludes that most of our present knowledge on SNGFR distribution and regulation has been obtained by the method developed by Hanssen 40 years ago.

Distribution of interstitial fluid pressure and fluid volumes in hind-limb skin of rats: relation to meridians?

To determine the distribution of interstitial fluid pressure (Pi) and volume (Vi), and to relate the distribution of these parameters to the distribution of potential meridians located by measurement of electrical impedance, we measured Pi, extracellular fluid (Ve) and plasma volumes (Vp) in 14 pre-defined skin areas, 2 x 2 mm, and in concave and convex regions on the hind-limb and groin of rats in control conditions. Pi was measured with sharpened glass capillaries connected to a servo-controlled counter-pressure system, while Ve and Vp were determined as the extravascular distribution spaces of 51Cr-EDTA and 125I-human serum albumin, respectively. Vi was calculated as Ve - Vp, and Vw as the difference between skin wet and dry weight. Grand mean Pi averaged -0.81 mmHg (SD 0.83, n=95). Pi in skin was significantly higher in lateral and medial parts of the medial aspect of hind-limb compared to pressures in the intermediate area (P<0.05). Pressures in the concave groin and the convex knee area were more negative and positive, respectively, than in the flat intermediate central hind-limb area. There was a significantly higher Vi (P<0.05) and Vw (P<0.05) in the lateral side than that in the medial side. Vp was higher medially and laterally than in the intermediate area (P<0.05 for both comparisons), and correlated positively and significantly with Pi (r=0.66, P<0.05). No correlation was found between Pi and electrical impedance. The study suggests that the distribution of Pi, Vi, Vp and Vw is heterogeneous in hind-limb skin at a macroscopic level without obvious relations to potential meridians.

The effect of AVP-V2 receptor stimulation on local GFR in the rat kidney.

The effect of AVP-V2 receptor agonist desmopressin, dDAVP, its non-peptide antagonist OPC-31260 and vehicle infusion on glomerular filtration rate (GFR) in the outer, middle and inner cortex was studied in both hydropenic and water diuretic Inactin anaesthetized female Sprague-Dawley rats using the aprotinin method. Two subsequent GFR measurements were carried out in the same kidney by injection of 125I- and 131I-labelled aprotinin before and after i.v. infusion of dDAVP, OPC-31260 or the vehicle. Acute infusion of dDAVP in hydropenic rats increased total GFR by 14% relative to vehicle infusion, whereas in water diuretic rats it had no effect relative to vehicle. No significant changes in arterial pressure (Pa) or renal blood flow (RBF) were recorded. Infusion of OPC-31260 reduced total GFR by 11% compared with vehicle. These results are consistent with the findings that a presensitization of the vasculature by high plasma levels of AVP is necessary for the renal vascular effects mediated by the V2 or V2-like receptors to occur. The ratio between inner and outer cortex GFR remained unchanged from control to experimental condition as follows: dDAVP infusion in hydropenic rats, 0.504 vs. 0.494 in control; vehicle infusion in hydropenic rats, 0. 393 vs. 0.392; OPC-31260 infusion in hydropenic rats, 0.517 vs. 0. 523; dDAVP in water diuretic rats, 0.547 vs. 0.543; vehicle in water diuretic rats, 0.413 vs. 0.417. Thus no significant difference in the GFR response was observed between superficial and deep cortical layers of the rat kidney.

Effect of L-NAME on glomerular filtration rate in deep and superficial layers of rat kidneys.

The effect of the NO synthase blocker N(G)-nitro-L-arginine methyl ester (L-NAME) on glomerular filtration rate (GFR) in outer (OC), middle (MC), and inner cortex (IC) was studied in anesthetized male Sprague-Dawley rats by the aprotinin method. The filtered amount of 125I- and 131I-labeled aprotinin injected before and after L-NAME injection was measured in the same cortical tissue samples after excising the kidney. Arterial pressure increased on average by 43 mmHg, whereas renal blood flow fell by 26% after L-NAME, giving an increase in renal resistance of 92%. At constant renal arterial pressure, resistance rose by only 39%, revealing that autoregulation was responsible for about one-half of the resistance increase. Total and zonal GFR showed a small, statistically insignificant increase after L-NAME, regardless of whether the renal pressure was allowed to rise or not. The response varied considerably among animals, but in each animal the GFR varied proportionately in OC, MC, and IC. We conclude that the vasodilator tone of NO is predominantly located in postglomerular resistance vessels and is similar in the three cortical layers.

Autoregulation of total and zonal glomerular filtration rate in spontaneously hypertensive rats with mesangiolysis.

In this study we tested the hypothesis that mesangial cells participate in autoregulation of the glomerular filtration rate (GFR) in normotensive and hypertensive rats. Mesangial cell lesions were induced by intravenous administration of antithymocyte (anti-Thy 1.1) antibodies in spontaneously hypertensive rats (SHR) and in Wistar-Kyoto rats (WKY). Normal murine serum was injected in control rats. Hemodynamic measurements were performed 24 h after the infusion of the anti-Thy 1.1 antibodies. Renal blood flow (RBF) was measured by a transit time flowmeter (Transonic) and the GFR was measured as the uptake of 125 iodine-labeled aprotinin ([125]I-Ap) by proximal tubular cells at the control renal arterial pressure and (131)I-Ap at a pressure reduction close to the lower pressure limit of RBF autoregulation. RBF was unaltered and the autoregulatory capability was maintained in SHR and WKY after mesangial cell lesions. Mesangiolysis significantly reduced the total GFR in normotensive, but not in hypertensive animals. The fractional compensation of the GFR was attenuated in the outer cortical layer (p<0.05) in normotensive WKY. In SHRs the fractional compensation of the GFR was impaired in all cortical layers after mesangiolysis, slightly more in the outer than in the inner cortex. We conclude that mesangial cells may contribute to the autoregulation of GFR in hypertensive rats, but to a lesser extent in normotensive rats.

Acute effects of angiotensin II receptor antagonist on autoregulation of zonal glomerular filtration rate in renovascular hypertensive rats.

This study was designed to assess the renal capability to autoregulate total blood flow, glomerular filtration rate (GFR) and local GFR in outer, middle and inner cortical layers (OC, MC, IC) in the two-kidney, one-clip (2K-1C) renovascular hypertensive rat, with or without acute infusion of the angiotensin II receptor antagonist losartan (5 mg/kg, i.v.). Age-matched, sham-operated Wistar rats were used as controls. The hemodynamic study in all animals was performed 4 weeks after clipping. The clipping increased blood pressure significantly, whereas losartan reduced the renal arterial pressure (RAP) from 165+/-8 to 125+/-6 mm Hg (p < 0.01) in 2K-1C hypertensive rats and reduced the RAP from 107+/-2 to 101+/-1 mm Hg (p < 0.05) in normotensive animals. Renal blood flow (RBF), total and local GFR were decreased in the nonclipped kidney of 2K-1C hypertensive rats compared with sham-operated rats, but losartan significantly increased the RBF and GFR. RBF was well maintained in response to reduction in RAP in the nonclipped kidneys with and without losartan treatment. The capability of total GFR autoregulation was impaired in untreated 2K-1C hypertensive rats and losartan-treated sham-operated rats, whereas losartan completely abolished GFR autoregulation in the nonclipped kidney of 2K-1C hypertensive rats. Losartan impaired autoregulation of zonal GFR to the same extent in all three cortical layers of sham-operated rats, whereas in the nonclipped kidney of 2K-1C hypertensive rats losartan had a more pronounced effect on the superficial GFR autoregulation than in middle and inner cortex, indicating that angiotensin II plays a major role in regulating the GFR response to the acute changes of renal arterial pressure.

Interstitial exclusion of macromolecules studied by graded centrifugation of rat tail tendon.

Mechanical compression of cartilage and tendon has been shown to expel fluid both from collagen fibrils and from the extrafibrillar space. As reported previously, albumin (Alb) concentration and colloid osmotic pressure in tendon fluid (TF) expelled by repeated centrifugations fell progressively at increasing centrifugation force (G = 600, 2,400, and 13,100), suggesting either molecular sieving in compressed tendon or mobilization of protein-free (excluded) fluid. The present experiments, including analysis of 51Cr-EDTA, aprotinin (Ap), Alb, immunoglobulin G (IgG), and hyaluronan (hyaluronic acid; HA) with molecular weight (MW) ranging from 341 to 5 x 10(6), strongly favored the exclusion hypothesis; the fraction of Alb, IgG, and HA-free fluid (excluded) was already 0.23-0.36 in the first centrifugate, increasing to 0.73-0.82 in the third. The corresponding numbers were, respectively, 0.11 and 0.43 for Ap (MW 6,500), and 0 and 0.08 for 51Cr-EDTA. These data, combined with calculated exclusion by collagen fibrils, proteoglycans, and HA, indicated that the first centrifugate was mainly derived from the extrafibrillar space, with increasing addition of macromolecular free intrafibrillar fluid in the second and third centrifugates, with each space contributing about equally to the total centrifugate volume. The calculations also indicated that Alb-, IgG-, and Ap-free fluid was mobilized from extrafibrillar space by increasing overlap of excluded territories. An excess of HA in tendon compared with that estimated from centrifugate concentrations suggests a large bound or immobilized HA fraction.

Autoregulation of total and zonal glomerular filtration rate in spontaneously hypertensive rats during antihypertensive therapy.

The effects of angiotensin II type 1 receptor antagonist (losartan), angiotensin 1-converting enzyme (ACE) inhibitor (enalapril), and calcium channel blocker (nifedipine) on autoregulation of total and zonal glomerular filtration rate (GFR) were studied in spontaneously hypertensive rats (SHRs), 10 and 40 weeks of age, and 10-week-old Wistar-Kyoto (WKY) rats. Untreated animals in each group served as controls. Renal blood flow (RBF) was measured by a transit-time flow probe (Transonic) on the left renal artery. Total and zonal GFR (outer, middle, and inner cortex) were estimated from tubular uptake of 125iodine-labeled aprotinin (125IAp) injected i.v. at control renal arterial pressure (RAP), and 131IAp injected at a RAP reduced to the lower limit of RBF autoregulation. Autoregulation of RBF was reset to higher pressure levels in untreated hypertensive rats. Enalapril normalized this resetting in 10-week-old SHRs, but not in aged SHRs 40 weeks. Losartan did not completely normalize this resetting in either 10-or 40-week-old SHRs, whereas nifedipine impaired RBF autoregulation in both WKYs and SHRs. A decreased autoregulatory compensation of GFR after pressure reduction was observed in losartan-treated 10-week-old SHRs and after all drug regimens in 40-week SHRs. GFR autoregulation in outer, middle, and inner cortex was impaired in losartan-treated 10-and 40-week-old SHRs. With all treatments, the autoregulation in 10- and 40-week-old SHRs was better preserved in the inner than in the outer cortex. The impaired autoregulation may indicate that a part of the dilatory capacity of preglomerular vessels has already been taken out by hypotensive treatment. Renal vascular abnormalities may have an additional effect.

Renal handling of radiolabelled human cystatin C in the rat.

Serum cystatin C concentration correlates negatively with glomerular filtration rate as well as or better than that of serum creatinine, suggesting a constant formation, and elimination from extracellular fluid mainly by glomerular filtration. It is not known, however, how well the renal plasma clearance of this 13-kDa basic polypeptide matches the glomerular filtration rate. This was investigated in rats during control conditions and after reduced renal perfusion pressure. 125I-cystatin C and an indicator for glomerular filtration (51Cr-EDTA or 131I-aprotinin) were injected intravenously. The renal accumulation and urinary excretion of the tracers were recorded in periods of 2.5 to 20.0 min. The renal plasma clearance of 125I-cystatin C (Ccy) based on the renal content of 125I correlated well with the glomerular filtration rate (CCr-EDTA) in periods up to 6 min; i.e. Ccy = 0.94 x CCr-EDTA, r = 0.99. Less than 0.5% of the filtered amount appeared in the urine. During more prolonged periods, Ccy increasingly underestimated glomerular filtration rate, reaching about 0.4 x CCr-EDTA in a 20-min period. Free 125I relative to total plasma 125I activity increased from about 2% at 5 min to about 70% at 20 min. In nephrectomized rats, free 125I accumulated in plasma at a slower rate, accounting for about 15% of the total activity 20 min after injection of 125I-cystatin C. We conclude that cystatin C is (a) mainly removed from the extracellular fluid by the kidneys, (b) practically freely filtered in the glomeruli, and (c) completely absorbed and rapidly broken down by the proximal tubular cells.

Autoregulation of zonal glomerular filtration rate and renal blood flow in spontaneously hypertensive rats.

Autoregulation of total and zonal glomerular filtration rate (GFR) in outer, middle and inner cortex was estimated in spontaneously hypertensive rats (SHR), from the tubular uptake of 125I-labeled aprotinin (125I-Ap), injected at control renal arterial pressure (RAP), and 131I-Ap, injected at reduced RAP in left kidney. Normotensive Wistar-Kyoto (WKY) rats were used as controls. Renal blood flow (RBF) autoregulation was reset to higher pressure levels in SHR. When RAP was lowered close to the lower pressure limit of RBF autoregulation, total GFR was reduced to 89.5 +/- 3.1 and 88.1 +/- 3.3% of control in 10- and 40-wk WKY and to 87.7 +/- 2.3 and 88.0 +/- 2.2% in 10- and 40-wk SHR. In WKY, the fall of GFR in the three cortical layers was not different during RAP reduction. In 10- and 40-wk-old SHR, however, GFR fell significantly less in inner than in middle and outer cortex (P < 0.05). We conclude that autoregulation of GFR is most efficient in the inner cortex of SHR. In all animals, GFR was less well autoregulated than RBF.

Glomerular filtration and tubular absorption of the basic polypeptide aprotinin.

The basic polypeptide aprotinin (Ap), mol. wt 6500, pI 10.5, is filtered in the glomeruli, virtually completely taken up by the proximal tubular cells and retained there for many hours. This process was studied in rats by determining the renal plasma clearance (CAp) as the amount of [125I]Ap accumulated in the kidney plus that excreted in the urine per unit of time divided by the integrated plasma concentration. In periods lasting 4-20 min after i.v. bolus injection or infusion to constant plasma concentration, CAp was 65% of glomerular filtration rate (GFR) estimated as kidney plus urinary clearance of [51Cr]EDTA (Ccr-EDTA). Less than 0.8% of the filtered Ap appeared in the urine. CAp varied inversely with plasma protein concentration in mg ml-1: CAp/Ccr-EDTA = 0.98-0.0058 x Ppr, corresponding to a glomerular Gibbs-Donnan distribution for a net molecular charge of +6, in agreement with the amino acid composition of Ap. CAp (kidney + urinary) was not altered by inhibiting tubular uptake of [125I]Ap by maleate or by exceeding the uptake capacity with large doses of unlabelled Ap. Neutralized Ap (malonylated) did not accumulate in the kidney, but showed a urinary clearance indistinguishable from that of [51Cr]EDTA. Both CAp and Ccr-EDTA were reduced to 0.04 ml min-1 when glomerular filtration pressure was lowered by ureteral stasis and increased Ppr (80-90 mg ml-1). These findings indicate: (1) no steric or charge restriction to filtration of Ap in the glomerular membrane, (2) the Gibbs-Donnan equilibrium should be considered when estimating glomerular sieving of charged polypeptides in intact animals (3) charge dependent tubular uptake, (4) little or no transtubular transport of intact Ap, (5) no appreciable tubular uptake of Ap from the peritubular side and (6) local renal accumulation of Ap in a period of up to 20 min may be used to estimate local glomerular filtration and/or local proximal tubular reabsorption rates. Model analysis based on the appearance of 125I in plasma, the time course of renal Ap content, and literature data on subcellular Ap distribution are consistent with two populations of endosomes, transporting Ap at widely different rates from the proximal tubular brush border to the lysosomes where breakdown occurs at a high rate.

Repeatable measurement of local and zonal GFR in the rat kidney with aprotinin.

The basic polypeptide aprotinin (Ap), mol. wt 6513, is freely filtered in glomeruli and completely reabsorbed by the proximal tubules. Cellular processing is slow with return to plasma of breakdown products beginning after 20-30 min. When corrected for Gibbs-Donnan distribution of Ap between glomerular filtrate and plasma (i.e. 0.65 at a plasma protein concentration of 50 mg ml-1), the renal clearance of [125I]Ap, estimated as the ratio of kidney uptake and integrated non-protein bound plasma 125I concentration, equals that of [51Cr]EDTA (urine + kidney content). Zonal GFR per gram tissue was obtained from uptake in three to six samples from outer and inner cortex (OC, IC) and the cortico-medullary border zone, 5-30 min after i.v. injection in rats. Control GFR in OC was 2.05 (SD 0.39) ml g-1 min-1 and the IC/OC ratio 0.66 (SD 0.14). Repeated local clearances (CI and CII) were obtained by injecting a second tracer (i.e. [131I]Ap) 15 min after the first injection (i.e. [125I]Ap), which by then had a low plasma concentration. The kidneys were removed at 30 min, frozen and dissected. During control conditions CII/CI averaged 1.06 in OC and IC, and the coefficient of variation (CV) between CII/CI ratios of individual tissue samples was 2% in both zones. Lowering left renal arterial pressure before the second injection reduced GFR proportionally in both zones (34 and 37%) with a CV of intersample CII/CI ratios of 5%. We conclude that the method allows precise and repeatable measurements of local and zonal GFR.

Renal cortical interstitium and fluid absorption by peritubular capillaries.

Every minute, the cortical peritubular capillaries in a 1-g rat kidney take up more than 0.5 ml tubular reabsorbate. Studies of renal lymph and measurements of pressure in capillaries (Pc) and interstitium (Pi) indicate that normally the protein colloid osmotic pressure of peritubular capillary plasma (COPp) provides the necessary absorptive force, keeping Pi at 2-4 mmHg, i.e., 8-10 mmHg lower than Pc. At reduced COPp, continued delivery of fluid from the tubules automatically raises Pi to maintain capillary fluid uptake. The transient Pi response to sudden exposure of the kidney to subatmospheric pressure shows that such adjustment of forces may take place in only 5 s. Most remarkable, adjustment of forces may take place in only 5 s. Most remarkable, reabsorption continues during protein-free perfusion of the isolated rat kidney, apparently effected by a Pi exceeding Pc. A relative retardation of interstitial uptake of ferritin from plasma in this case suggests fluid reabsorption through both small and large pores in the capillary wall. Collapse of the capillaries is presumably prevented by tight tethering to the capillary wall, giving the narrow interstitium a very low compliance.