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

Summary To determine the colloid osmotic pressure of subcapsular interstitial fluid in rat kidneys two different methods were used. Collection of subcapsular fluid with glass pipettes or implanted microcatheters and subsequent protein analysis resulted in a protein concentration of 1.8g%±0.6 and 2.0g%±0.8, respectively. Lymph protein concentration was not significantly different from that of subcapsular fluid samples. During extracellular volume expansion both subcapsular and lymph protein concentration fell to 0.42g%±0.23 and 0.7g%±0.5. Application of anin vivo oncometric method resulted in an effective oncotic pressure about twice that estimated from protein determinations. Using average values for intratubular and intracapillary oncotic and hydrostatic pressures a tubulo-interstitial net driving force of 20 mm Hg and an interstitial-capillary net driving force of 13 mm Hg is estimated in hydropenic animals. During volume expansion net transtubular pressure gradient is reduced to about 60–70% of control while the transcapillary gradient is virtually unchanged.     

Balance between tubular flow rate and net fluid reabsorption in the proximal convolution of the rat kidney

Summary In rats with hereditary Diabetes insipidus (Brattleboro strain) proximal fluid reabsorption was studied during spontaneous variations of GFR. Independent of kidney weight and arterial blood pressure total kidney GFR varied between 0.35 ml/min×gKW and 1.8 ml/min×gKW, while single nephron filtrate showed a variation between 18 nl/min×gKW and 66 nl/min×gKW. End-proximal TF/P inulin of 78 fluid samples did not correlate with filtration rate and averaged 2.38±0.55 S.D. Also, proximal transit time was independent of filtration rate with a mean of 9.7 sec±1.1 S.D. These results imply a direct correlation between mean luminal radius and filtration rate, which was demonstrated by microphotography of the renal surface.Our data show that complete glomerulo-tubular balance in the proximal tubule exists during spontaneous variations of glomerular filtration rate. In support of the hypothesis ofGertz [4,6] the lateral net fluid flux was found to be linearly dependent upon tubular surface area. In addition, intratubular volume changed in direct proportion to single nephron filtrate. The present results are interpreted to further strengthen the assumption of a critical influence of tubular geometry on proximal fluid reabsorption during spontaneous variations of GFR.Supported by the Deutsche Forschungsgemeinschaft and the U.S. Department of the Army, through its European Research Office.     

Micropuncture study of fluid handling in the cat kidney

Summary Superficial proximal and distal tubules of both kidneys of pentobarbital-anesthetized cats, infused with 2.5% (w/v) polyfructosan in Ringer solution at 0.45 ml/min, were micropunctured. Whole one-kidney GFR averaged 4.1 ml/min. SNGFR averaged 15 nl/min in 51 determinations and was proportional to whole-kidney GFR in individual cats. Absolute fluid reabsorption along the length of the accessible portion of the proximal tubule was 1.6 nl/min·mm. The length of the whole proximal tubule was 6 mm as measured from the glomerulum to the descending thin limb of Henle's loop. The length of the distal tubule was 3.5 mm from the macula densa to the first branching of the collecting duct. Proximal fluid was isoosmolar with plasma whereas fluid entering the distal tubule was markedly hyposmolar. Late distal tubular fluid samples were also isosmolar. The cat appears. to be well suited for micropuncture experiments in terms of freedom from respiratory movements and the presence of surface distal tubules.Supported by the Swiss National Science Foundation, Grant 3.3730.74Preliminary communication: Experientia32, 779 (1976)Performed during tenure as an International Exchange Fellow of the Swiss National Science Foundation, Grant 880.292.74     

Micropuncture method for the determination of nephron filtration rate

Summary Two possible artifacts may explain the phenomenon that nephron GFR (N-GFR) measured by distal tubular puncture is smaller than that measured by proximal tubular puncture: a loss of the inulin-like substance used in this laboratory (polyfructosan) from the tubular lumen or unreliable distal punctures. To test these possibilities (a) known amounts of polyfructosan were injected into the proximal tubule and the percentage recovery from the distal tubule measured, (b) N-GFR was measured by distal puncture, subsequently by recollection from the same site and finally by a proximal puncture.On the average, 98.5±7.5% of the proximally injected polyfructosan was recovered from the distal tubule. This is not significantly different from 100% (p>0.1) and demonstrates that proximal tubule and loop of Henle are impermeable to polyfructosan. The ratio between the N-GFR measured by a distal puncture and that measured by subsequent recollection was 1.016±0.096 and not significantly different from 1.000 (p<0.20), demonstrating the reliability of distal tubular puncture. The mean distal N-GFR of 27.9±5.3 nl/min was significantly smaller (p<0.001) than the proximal N-GFR of 35.1±8.0 nl/min. The existence of the proximal-distal N-GFR difference thus is confirmed and two possible artifacts eliminated. The best explanation remains the operation of a tubulo-glomerular feedback mechanism.A current point of dispute is the effect of alterations in intratubular pressure (ITP) on N-GFR. Collection of tubular fluid at ITPs below the previously measured free flow pressure (FFP) resulted in a change of N-GFR of 0.45 nl/min· cm H₂O. In contrast, fluid collection at ITPs greater than the FFP resulted in a change of N-GFR of 1.48 nl/min· cm H₂O. We conclude that although N-GFR is sensitive to ITP changes in both directions, pressure decreases are of little practical importance for the determination of N-GFR whereas intratubular pressure increases are to be avoided.This work was supported by the Deutsche Forschungsgemeinschaft.On leave from the University of Melbourne as a recipient of a scholarship from the Deutscher Akademischer Austauschdienst.     

Changes in interstitial pressure during and after blood volume expansion in rats

Summary Interstitial fluid pressure was measured via a chronically implanted capsule before, during and after acute isotonic, iso-oncotic blood volume expansion in normal or in 48-h dehydrated rats. At the same time, the patterns of body fluid distribution, of selected renal responses and of mean arterial and mean central venous pressure responses were studied. Dry tissue weight (DTW) was subsequently determined by freeze drying of the shaved carcass.Dehydration decreased plasma volume and interstitial fluid volume significantly below normal values. The initial intracapsular pressure in dehydrated animals (–3.7±0.6 mm Hg) was not significantly different from that in normal rats (–2.5±0.5), but dehydrated rats showed initially a very significantly lower effective interstitial compliance (0.0005 ml/mm Hg per gram DTW) than did the normal group (0.0704).In the course of the renal response to the volume load, effective interstitial complicance increased to 0.0350 in dehydrated rats but showed no change in normal rats. Neither group completely corrected its elevated blood volume; both returned their central venous pressures to pre-infusion levels; both decreased their interstitial fluid volumes below preinfusion levels and both decreased their intracapsular fluid pressures 1 mm Hg below the level prevailing in non-infused animals at that time.It is concluded that a reduction in interstitial, hydrostatic pressure can be a functionally important influence in the apparent control of central venous pressure following acute blood volume expansion.This study was supported by the Canadian Heart Foundation.     

Baroreflex sympathetic activation increases threshold pressure for the pressure-dependent renin release in conscious dogs

Stimulus-response curves relating renal-venous-arterial plasma renin activity difference (P.R.A.-difference) to mean renal artery pressure (R.A.P.) were studied in seven chronically instrumented conscious foxhounds with a daily sodium intake of 6.1 mmol/kg. R.A.P. was reduced in steps and maintained constant for 5 min using an inflatable renal artery cuff and a pressure control system.The stimulus-response curve obtained during control conditions (C) or during common carotid artery occlusion (C.C.O.) could be approximated by two linear sections: a rather flat section or plateau-level of P.R.A.-difference at normal blood pressure or above, and a very steep section between a distinct threshold pressure and 65–70 mm Hg. While the parameters of the curves varied from dog to dog, the curves kept their inique shape in the individual dog for at least 1 week. C.C.O. had no effect on the plateau-level of the P.R.A.-difference (C:0.98±0.14,C.C.O.:0.99±0.14 ng Al·ml^–1·h^–1) and on the slope of the curve below threshold pressure (C:–0.379±0.041,C.C.O: –0.416±0.082 ng Al·ml^–1·h^–1·mm Hg^–1) but shifted the stimulus-response curve to the right and increased threshold pressure (C:92.7±2.8,C.C.O.:109.7±4.1 mm Hg;P<0.05).Renal blood flow, which was measured simultaneously in three of the dogs, showed good autoregulation down to 70 mm Hg under resting conditions and was not affected by C.C.O. except for a 30% reduction of renal blood flow at the lowest pressure step (70 mm Hg).-Adrenergic blockade in 4 of the dogs reduced the plateau-level of the P.R.A.-difference from 0.86±0.19 to 0.36±0.05 ng AI·ml^–1·h^–1 (P<0.05) but had no effect on the increase of threshold pressure elicited by C.C.O.It is concluded that the stimulus-response curve for the pressure-dependent renin release has a remarkable long-term stability in the individual dog. The curve is shifted to the right by a moderate carotid baroreflex increase of renal sympathetic nerve discharge which leaves total renal blood flow largely unchanged. It is suggested that the increase in threshold pressure is independent of -adrenergic effects.This study was supported by the German Research Foundation within the SFB 90, HeidelbergA priliminary part of this investigation has been presented to the meeting of the German Physiological Society, Dortmund, March 1984 [Pflügers Arch (1984) suppl 400:R11,41]     

Technical problems in the micropuncture determination of nephron filtration rate and their functional implications

Summary The theory of a functional coupling between distal tubular fluid composition and glomerular filtration rate implies that the blockade of flow at a proximal site should lead to a marked increase of GFR. This potential alteration of steady state GFR was studied by comparing the influence of sampling from distal or proximal sites on the filtration rate of identical nephrons. During antidiuresis an average GFR of 25.2 nl/min±7.5 S.D. was found in distal collections, while proximally collected samples gave an average GFR of 34.5 nl/min±8.4 S.D. This difference of 9.3 nl/min is highly significant (p<0.001). During saline diuresis a mean nephron GFR of 41.6 nl/min±5.0 was found by distal sampling and of 45.3 nl/min±5.4 by proximal sampling (p>0.05). The proximal-distal difference in nephron GFR is interpreted to indicate the operation of a tubulo-glomerular feedback control system. Thus, a true steady-state GFR probably cannot be obtained by proximal fluid collection.Even in the presence of high intratubular pressures and unusually short oil blocks no evidence of sample contamination by retrograde fluid flow past an injected oil block was obtained.The application of a counter-pressure to the sampling pipette which has been recommended by Gertzet al. [5] as a means to standardize fluid collections, was found to lead to abnormally high intratubular pressures. The reason for this finding appears to be an unexpectedly high and inconstant tip resistance to flow during fluid flow into the pipette.     

Continuous recording of hydrostatic pressure in renal tubules and blood capillaries by use of a new pressure transducer

Summary Using a new ultra miniature pressure sensor hydrostatic pressure in renal tubules and renal blood capillaries was continuously recorded. The transducer was mounted in a special plexiglas device which was fixed on a micromanipulator. With a glass capillary attached to one end of the plexiglas block tubules and capillaries were punctured using conventional micropuncture techniques. The volume displacement of this transducer is about 1 nl/10 mm Hg. The recorded signal was found to be linear up to 300 mm Hg. The frequency of the system was highly dependent upon the capillary tip diameter. Examples of recorded pressures in renal tubules and blood capillaries are given.Supported by the Deutsche Forschungsgemeinschaft.     

Effect of lower-body positive pressure on postural fluid shifts in men

Summary To quantify the effect of 60 mm Hg lower-body positive pressure (LBPP) on orthostatic blood-volume shifts, the mass densities (±0.1 g· l^–1) of antecubital venous blood and plasma were measured in five men (27–42 years) during combined tilt table/antigravity suit inflation and deflation experiments. The densities of erythrocytes, whole-body blood, and of the shifted fluid were computed and the magnitude of fluid and protein shifts were calculated during head-up tilt (60°) with and without application of LBPP. During 30-min head-up tilt with LBPP, blood density (BD) and plasma density (PD) increased by 1.6±0.3 g · l^–1, and by 0.8±0.2 g · l^–1 (±SD) (N=9), respectively. In the subsequent period of tilt without LBPP, BD and PD increased further to +3.6±0.9 g · l^–1, and to +2.0±0.7 g · l^–1 (N=7) compared to supine control. The density increases in both periods were significant (p<0.05). Erythrocyte density remained unaltered with changes in body position and pressure suit inflation/deflation. Calculated shifted-fluid densities (FD) during tilt with LBPP (1006.0±1.1 g · l^–1,N=9), and for subsequent tilt after deflation (1002.8±4.1 g · l^–1,N=7) were different from each other (p<0.03). The plasma volume decreased by 6.0±1.2% in the tilt-LBPP period, and by an additional 6.4±2.7% of the supine control level in the subsequent postdeflation tilt period. The corresponding blood volume changes were 3.7±0.7% (p<0.01), and 3.5±2.1% (p<0.05), respectively. Thus, about half of the postural hemoconcentration occurring during passive head-up tilt was prevented by application of 60 mm Hg LBPP.H. Hinghofer-Szalkay was a European Space Agency fellow on leave from the Physiological Institute, Karl-Franzens-University, A-8010 Graz, Austria.     

A simple method for multiple fluid exchange

A simple device for fluid exchange is described, which allows the exchange of an unlimited number of solutions at low (10–1000 nl/min) constant perfusion rate. The applicability of the system has been tested in microperfusion experiments of rat distal tubules. At a luminal perfusion rate of 40 nl/min, the lag time was some 20 sec and 80 % fluid exchange time some 3 sec. Simple modification allows further reduction of the lag time. Under control conditions, the potential difference across the late distal tubule (PDte) approaches –19.4±2.5 mV (n = 27). Increase of luminal potassium concentration from 5.4 to 40 mmol/l hyperpolanzes PDte to –29.9±4.3 mV (n = 8). Amiloride (10 mol/l) leads to a reversible depolarization to –3.2±1.0 mV (n = 19), barium (1 mmol/l) to a reversible hyperpolarization to –25.8±2.6 mV (n = 19). As expected, PDte is largely created by amiloride sensitive sodium channels and is partially blunted by barium sensitive potassium channels.     

Filtration pressure response to infusion of atrial natriuretic peptides

The present experiments were undertaken to assess the effect of an atrial extract (ANF) and of the synthetic atriopeptin II (APII) on filtration pressure of rat kidneys. Continuous recordings of stop flow pressure (SFP) were made to obtain an index of the change of glomerular capillary pressure produced by atrial peptides and its time course. Short-term infusion of ANF or APII increased SFP from 40.6±0.99 to 50.7±1.42 mm Hg (p<0.001) and from 44.0±1.28 to 52.7±1.75 mm Hg (p<0.001) respectively. The maximum response was achieved promptly. Return of SFP to control was slow: 20 minutes after termination of the infusion SFP was still elevated by 4.9±1.27 mm Hg (p<0.01). Tubule and stellate vessel pressures increased less than 2mm Hg, changes that were not significant. Arterial pressure fell 6 mm Hg (p<0.05). When arterial pressure was reduced by an aortic clamp to 85–90 mmHg prior to administration of APII the response of SFP was markedly blunted (from a mean increase of 9.0±1.07 mm Hg to 4.5±0.53 mm Hg). The increase of SFP probably reflects an increase of glomerular capillary pressure. The finding suggests that atrial peptides increase glomerular filtration rate at least in part by increasing filtration pressure.     

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 μm 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.     

Tubuloglomerular feedback control in kidneys of adrenalectomized rats

The technique of orthograde tubular microperfusion has been used to test the functional integrity of tubuloglomerular feedback control in adrenalectomized rats. Kidneys of these animals display high renin contents, as well as an impairment of electrolyte transport in the loop of Henle, which was demonstrated by monitoring the electrical conductivity of fluid entering the distal tubule over a perfusion range of 10–50 nl/min. The calculated electrolyte concentration of the perfusion fluid leaving the loop of Henle was significantly higher in adrenalectomized rats than in controls. Intratubular stop flow pressure (SFP) during perfusion with a modified endproximal Ringer's solution was recorded continuously in early proximal segments at rates of 0–40 nl/min. Direct pressure transmission from perfused endproximal tubular segments to the pressure recording capillary was prevented by paraffin blockade of the intermediate proximal segment. An intact feedback response in each of the 32 tubules of the adrenalectomized rats studied was observed, which was reversible and could be demonstrated repetitively in individual tubules. Thus SFP at 40 nl/min was –11.2±1.3 mm Hg in adx rats as compared to –8.1±1.3 in control rats. An increased freedback sensitivity was observed in dexamethasone treated adrenalectomized rats despite normal renal renin content. Here SFP at 40 nl/min was –15±1.1. Thus, the feedback signal can be mediated in the apparent absence of adrenal steroid hormones and a high sensitivity of tubuloglomerular feedback control is not necessarily paralleled by high renin content of renal tissue.Index of Abbreviations adx adrenalectomized - AI angiotensin I - b.w. body weight - k conductivity (Siemens cm^–1) - EPFR early proximal flow rate - FFP free flow pressure - P_tub intratubular pressure - P mean arterial pressure - SFP stop, flow pressure - SNGFR single nephron glomerular filtration rate - V_u urine flow rate - ø perfusion rate through the loop of Henle Supported by the Deutsche Forschungsgemeinschaft (Hi 97/14)     

Pressure in the glomerular capillaries of the rat kidney and its relation to arterial blood pressure

Summary The pressure in the glomerular capillaries of the rat kidney was determined by micropuncture of individual nephrons. The proximal tubule was blocked by injection of viscous oil. The intratubular hydrostatic pressure increased until it reached a steady state, the intratubular stop-flow pressure. Since the glomerular capillary wall is an ultrafiltration membrane, impermeable to proteins, the sum of intratubular stop-flow pressure plus the plasma colloid osmotic pressure should equal the pressure in the glomerular capillaries. This pressure increased as the arterial pressure was raised from 60 to 90 mm Hg. Thereafter it remained constant at 88±4 mm Hg despite further elevation of the arterial pressure up to 160 mm Hg. Determination of the stop-flow pressure in Bowman's capsule and in the proximal tubules of denervated kidneys gave the same results as above.The hydrostatic pressure drop in the afferent arterioles was determined by subtracting the glomerular capillary pressure from the arterial pressure. This pressure drop was negligible at arterial pressures below 90 mm Hg and increased linearly as the arterial pressure was elevated from 90 to 160 mm Hg.Both the above findings are compatible with and confirm the principle of autoregulation of renal blood flow and glomerular filtration rate.Supported by NIH Grant Nr. AM 06806-03 and by Deutsche Forschungsgemeinschaft.From the Dept. of Pediatrics, University of Wisconsin, Madison, Wisconsin, U.S.A. Supported by the National Cystic Fibrosis Foundation.     

Carbonic anhydrase independent bicarbonate reabsorption

The present study was designed to define the prerequisites of carbonic anhydrase independent bicarbonate reabsorption. In free flow experiments during systemic application of carbonic anhydrase inhibitor benzolamide (50 mg/kg B. W.) bicarbonate recovery in % of filtered load was found to be 74±8% in late proximal convoluted tubules, 39±6% in distal convoluted tubules and 32±4% in urine, indicating that most of carbonic anhydrase independent bicarbonate reabsorption occurs in tubule segments prior to distal convoluted tubules. In vivo continuous microperfusion experiments in proximal convoluted tubules demonstrated that luminal benzolamide (0.5 mmol/l) virtually abolishes net bicarbonate fluxes, when bicarbonate concentration in the luminal perfusate (25 mmol/l) is close to peritubular plasma concentration (24.4 mmol/l). In contrast, a significant downhill reabsorptive flux occurs, when perfusate bicarbonate concentration is 75 mmol/l and a significant downhill secretory flux is observed, when the perfusate is initially free of bicarbonate. The corresponding apparent permeabilities are 1.0±0.1·10^–6 cm²/s for influx and 1.6±0.4·10^–6 cm²/s for efflux of bicarbonate.Clearance studies reveal that carbonic anhydrase dependent and independent bicarbonate reabsorption are not saturable but depend on the rate of volume reabsorption in the kidney. In conclusion, passive movements of bicarbonate do occur in proximal convoluted tubules and most likely contribute to carbonic anhydrase independent bicarbonate reabsorption.This study has been supported by Österr. Fond zur Förderung der wiss. Forschung Nr. 4366. Part of this study has been presented at the 8th Int. Congr. Nephrol. Athens, 1981, and at the Herbsttagung of the Deutsche Physiologische Gesellschaft, Innsbruck 1981     

Inhibitory effect of methylxanthines on feedback control of glomerular filtration rate in the rat kidney

Summary Microperfusion experiments were performed in rats to assess the effect of luminal application of theophylline and the more lipophilic 3-isobutyl-1-methylxanthine (IBMX) on feedback regulation of glomerular filtration rate. Elevation of loop of Henle flow from 0 to 40 nl/min caused a 20.3±4.5% reduction of stop flow pressure (SFP) and a 32.2±3.7% reduction of early proximal flow rate (EPFR) when the perfusate was a 140 mM NaCl solution. When theophylline was added in a concentration of 5 mM SFP fell by only 5.3±1.8% and EPFR by 7.9±3.3%, changes which were significantly smaller than in the control perfusions (P<0.01). An identical change of loop of Henle flow in the presence of IBMX in concentrations of 1 and 5 mM was associated with a 4.5±3.9% decrease and a 12.1±2.7% increase of EPFR. In orthograde perfusion experiments full inhibition of the feedback response was noted at an IBMX concentration of about 1 mM while during retrograde perfusion a concentration of 0.4 mM was sufficient to produce the same effect. This indicates that methylxanthines diffuse out of the loop of Henle to a considerable extent. Methylxanthines reduced absolute and fractional water absorption along the loop of Henle to some extent while Cl absorption rates and early distal Cl concentrations were not significantly altered. Cyclic AMP applied from the luminal side in a concentration of 10 mM did not affect the feedback response of EPFR to flow elevation from 0 to 40 nl/min. Luminal application of dibutyryl cyclic AMP at 10 mM induced a small, but significant reduction of the feedback response when tested by pairedt-test (P<0.05). Our results show that luminal application of methylxanthines strongly interfere with feedback regulation of glomerular filtration rate. It is unclear at present whether this effect is related to inhibition of cyclic nucleotide phosphodiesterase and a rise in tissue cyclic AMP levels or to interference with a mechanism involving the local action of adenosine or 5-AMP.Supported by the Deutsche Forschungsgemeinschaft     

Volume absorption in the pars recta. III. Luminal hypotonicity as a driving force for isotonic volume absorption

This paper examines the possibility that osmotic disequilibrium between luminal and bathing solutions may account for isotonic fluid absorption coupled to active Na+ absorption observed when superficial proximal straight tubules isolated from rabbit kidney are perfused and bathed with NaCl solutions in the absence of CO2, HCO3-, and luminal organic solutes. If luminal hypotonicity provides a driving force for isotonic fluid absorption under these conditions, the luminal fluid must be nearly isotonic; and steady-state luminal hypotonicity should develop sufficiently rapidly that the absolute rate of volume absorption ('JV, nl min-1) coupled to active Na+ transport is relatively independent of perfusion rate, so that the normalized rate of fluid absorption (JV, nl min-1 mm-1) is approximately constant. Our theoretical calculations indicate that these expectations are fulfilled. A 0.42-0.56 mM reduction in luminal NaCl concentration adequately accounts for the JV observed under such conditions, because of the high hydraulic conductivity of these tubules; and within the range of tubule lengths normally employed with isolated proximal straight tubules, JV is relatively indepedent of perfusion rate within the generally observed range of experimental error.     

Impaired potency for feedback regulation of glomerular filtration rate in DOCA escaped rats

The present experiments were performed to study the effect of chronic extracellular volume expansion on the magnitude of tubulo-glomerular feedback responses in the rat kidney. Extracellular volume expansion was achieved by giving isotonic saline as drinking water and by injecting DOCA in a dose of 2.5 mg/kg - day. When Ringer perfusion rate through the loop of Henle was elevated in control rats (receiving only saline as drinking water) stop flow pressure (SFP) fell by an average of 0.47 +/- 0.81 mm Hg (mean +/- S.D.) and 7.93 +/- 2.85 mm Hg at the flow rate steps of 0--15 nl/min and 15--40 nl/min respectively. SN-GFR was reduced by a mean of 1.3 +/- 0.97 nl/min (0--15 nl/min) and 10.3 +/- 2.45 nl/min (15--40 nl/min). In DOCA treated rats the mean reductions of SFP were 0.98 +/- 0.9 mmHg and 2.1 +/- 1.4 mmHg and of SN-GFR 0.06 +/- 1.8 nl/min and 1.94 +/- 2.3 nl/min. Thus, significantly smaller changes of both SFP and SN-GFR were found in DOCA treated animals when flow rate was elevated from 15--40 nl/min. Net loop NaCl absorption rates did not significantly differ between control and DOCA rats. Renin activity of 5 pooled microdissected glomeruli was 15.6 +/- 17.1 ng/hr-0.1 ml in control and 2.94 +/- 2.6 ng/hr-0.1 ml in DOCA treated rats (P less than 0.01). It is possible therefore that the reduced feedback reactivity in DOCA treated rats is related to the diminished juxtaglomerular renin activity.     

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. 100g 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.     

In vitro perfused,non-isolated small intestine: Ontogeny of transmural hydraulic permeability

Summary Net fluid-flux and transmural hydrostatic hydraulic conductance (L_p) were measured in the rat ileum during postnatal ontogeny, utilizing a refined in vitro, capillary-perfused (non-isolated) preparation for free-flow studies. Volume absorption at low transmural pressure gradient (1–2 cmH₂O) was 0.99±0.22 ml/g wet weight·hr in the mature ileum and 1.55±0.42 ml/g wwt·hr in the differentiating intestine. Hydrostatic hydraulic conductance of the mature ileum was 0.76 l/cm²·cm H₂O·hr, indicating that normal hydrostatic pressure gradients are not important for modulation of intestinal absorption. In contrast, L_p was 3.82 l/cm²·cmH₂O·hr in the differentiating ileum, suggesting a significant contribution of pressure-dependent solvent drag to isotonic solute absorption.