Regulation of superficial nephron filtration rate by tubulo-glomerular feedback

Summary Possible regulation of glomerular filtration rate by tubulo-glomerular feedback from a late tubular site was studied in microperfusion experiments on rats. During perfusion of loops of Henle with varying flow rates and different perfusion solutions, filtration rate of the perfused nephrons was measured by total proximal fluid collection and inulin determination. During perfusion with isotonic Ringer's solution nephron filtration rate decreased significantly with increasing perfusion rates. Since proximal intratubular pressure was experimentally kept constant, this response must reflect decreased glomerular capillary pressure. Increasing the flow rate during perfusion with isotonic sodium sulfate or mannitol solutions was not associated with significant changes of filtration rate. Thus some correlate of the flow rate of normal loop of Henle fluid can affect filtrate formation. Such an effect may be mediated by the juxtaglomerular apparatus. Our results are consistent with the concept that the triggering signal is a function of the amount of distal sodium that is able to permeate the cell membrane at the receptor site rather than of distal intratubular sodium concentration.On leave from the Department of Physiology, University of Melbourne as recipient of a scholarship from the Deutscher Akademischer Austauschdienst.     

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.     

Effect of barbiturates on GFR and fluid reabsorption along proximal convoluted tubules and loops of henle in rats

Summary In the present study we investigated the possibility that GFR and fluid reabsorption are systematically altered by dose and/or structure of different barbiturate compounds. The results show that filtration and absorption rates are identical in rats anaesthetized with 100 or 150 mg/kg BW. Elevating the administered amount by intravenous injections of inactin did not alter renal functions compared to values measured immediately before the additional administration. No differences of GFR and absorption rates were noted when the effect of inactin, amytal, and nembutal in doses necessary to induce and maintain anaesthesia was compared. Absolute and fractional absorption was observed to decrease by 20 to 25% in the first 3 h after induction of anaesthesia independent of the anaesthetic used. Our results suggest that filtration and absorption rates are not systematically modified by specific anaesthetic effects, at least not when doses are kept close to the necessary minimum.Work performed during tenure of an Alexander von Humboldt-Dozentenstipendium.     

Micropuncture studies on the filtration rate of single superficial and juxtamedullary glomeruli in the rat kidney

Summary Single nephron filtration rates of superficial and juxtamedullary nephrons were determined in high and low sodium rats. Single nephron GFR was calculated from TF/P inulin and tubular flow rate in superficial nephrons and single juxtamedullary GFR from corresponding data in long loops of Henle. In low sodium rats superficial nephron GFR was 23.5±6.4 (SD)×10^–6 ml/min×g KW, juxtamedullary nephron GFR was 58.2±13.6 and total kidney GFR (C _In) was 0.94±0.16 ml/min×g KW. Using these single nephron values, total kidney GFR and a total number of 30,000 glomeruli per kidney, the number of superficial and juxtamedullary glomeruli was calculated to be 23,267 and 6,733, respectively. During high sodium diet superficial nephron GFR increased to 38.1±11.3 and single juxtamedullary GFR decreased to 16.5±6.6, total kidney GFR increasing to 1.01±0.24. Calculation again revealed the same distribution of the two nephron types. End-proximal TF/P inulin in superficial nephrons was 2.36±0.36 in low sodium and 2.31±0.28 in high sodium rats. Loops of Henle TF/P inulin and intratubular flow rate were inversely related: the highest TF/P inulin values and lowest intratubular flow rates were found in the descending limb. These data quantify the distribution of superficial and juxtamedullary nephrons on a functional basis and suggest a mechanism by which the kidney adjusts sodium excretion by altering the contribution of each nephron type to total kidney GFR.Supported by the Deutsche Forschungsgemeinschaft and the U.S. Department of the Army, through its European Research Office.     

The effect of saline infusion and hemorrhage on glomerular filtration pressure and single nephron filtration rate

Summary Single nephron filtration rate (GFRs) and effective glomerular filtration pressure (EFP) measured as the difference between intratubular stop-flow (SFP) and free-flow pressures (FFP), were determined in control rats and following saline infusion or hemorrhage. Infusion of isotonic or 4% NaCl increased EFP and GFRs without significantly affecting TF/P inulin. These findings could not be related to changes in arterial blood pressure. Controlled bleeding produced a marked decrease in EFP and in GFRs, again without significant change in TF/P-inulin. In both infusion and hemorrhage the change in GFRs was disproportionately greater than the change in EFP. Analysis of the components of the filtration process suggests that elevation of EFP is attended by an increase in permeability of the filtering membrane. This sensitive dependence of GFRs upon EFP, combined with a demonstrated constancy of total kidney GFR (GFR_T) over a wide range of urine concentrations and flow rates, connotes a close regulation of EFP in this experimental animal.Supported by Deutsche Forschungsgemeinschaft.     

Activation of tubulo-glomerular feedback by chloride transport

Summary To define the luminal agent(s) responsible for the reduction of nephron filtration rate following increases of loop of Henle flow rate early proximal flow rate (EPFR) during loop perfusion with 17 different salt solutions were compared to the non-perfused fubules. During orthograde microperfusions a reduction of EPFR as indication of a feedback response was noted with a number of nonovalent Cl^– and Br^– salts (LiCl, KCl, NaCl, RbCl, CsCl, NH₄Cl, choline Cl, NaBr, KBr), with Na⁺ salts except Na acetate (NaHCO₃, NaNO₃, NaF, NaI, NaSCN), and with CaCl₂ and MgCl₂. These latter 2 solutions where used in a concentration of 70 mM while all other solutions had a concentration of 140 mM. During retrograde perfusion from the distal to the proximal end of the loop of Henle EPFR fell significantly with Cl^– and Br^– salts with percentage changes of EPFR ranging from –8.0 to –44.3%. In contrast, Cl^– free salts and Cl^– salts of divalent cations were associated with percentage changes of EPFR ranging from +7.1 to –6.2%, significance being reached only during perfusion with NaSCN.When furosemide (5×10^–4M) was added to NaBr or KBr a feedback response was not observed. During orthograde perfusion with NaNO₃ distal Cl^– concentrations were 44.2±5.08 mM (mean±S.E.) at a perfusion rate of 10nl/min and 59.1±3.93 mM at a rate of 40 nl/min. CaCl₂ perfusion induced a marked elevation of distal Cl^– concentrations to levels higher than 140 mM. Loop chloride handling was normal during RbCl perfusion.The magnitude of the feedback response during retrograde perfusion was not changed by lowering NaCl concentration from 140 to 60 mM, but fell when NaCl concentration was further reduced. In contrast to orthograde perfusions it was insensitive to changes in flow rate.Our results are compatible with the thesis that feedback responses depend critically upon the rate of Cl^– transport probably across the macula densa cells. Br^– ions can replace Cl^– because they appear to share a common transport pathway which can be inhibited with furosemide. Unspecificity of feedback responses during orthograde microperfusions is due to presence of Cl^– ions in the macula densa region even when solutions are initially Cl^– free. Cl^– salts of divalent cations do not elicit a feedback response because Cl^– transport is severely curtailed.Supported by the Deutsche Forschungsgemeinschaft.     

The effect of experimental alterations in urine concentration on nephron filtration rate

Summary Experiments were performed to clarify the cause of the dependency of juxtamedullary nephron filtration rate (JN-GFR) on the diuretic state of the animal. Using the ferrocyanide technique of de Rouffignacet. al. [5] the distribution of nephron filtration rates was determined during selective modification of final urine osmolarity by papillary superfusion with concentrated or isotonic solutions. Papillary superfusion with a 2000 mosmolar solution led to a mean urine osmolarity of 1848±78 mOsm/l. JN-GFR displayed a mean increase of 66.6% over the superficial nephron filtration rate (SN-GFR). When the superfusion fluid was isotonic urine osmolarity averaged 754±53 mOsm/l and JN-GFR increased by a mean of only 26.3% over the SN-GFR. Using the micropuncture technique it was shown that SN-GFR in a given animal was not altered when the superfusion fluid was interchanged. We conclude therefore that the change in the percentage increase in GFR from superficial to juxtamedullary nephrons is caused by a change of juxtamedullary nephron filtration rate. Since plasma ADH concentration was not altered this effect appears to be elicited by the changed medullary solute concentration rather than by a vasomotor action of the antidiuretic hormone.This work was supported by the Deutsche Forschungsgemeinschaft.     

Effects of prostaglandin E2 on excretion and reabsorption of sodium and fluid in rat kidneys (micropuncture studies)

Summary The effects of i.v. infusions of Prostaglandin E₂ on renal excretion of sodium and fluid were investigated in rats. Increasing diuresis was observed at infusion rates from 10 to 100 ng/min·100 g BW. Higher rates depressed the arterial blood pressure and urinary excretion rates. At 100 ng PGE₂/min·100 g BW urinary flow rate increased approximately 3 times and sodium excretion approximately 8–10 times above the control level. The urine/plasma concentration ratio of Na averaged 1.7 compared with 0.5 in controls. Potassium excretion, however, was only slightly enhanced.The effects on sodium and fluid excretion were not correlated with changes of total kidney GFR. Single nephron GFR of superficial nephrons remained unchanged during the PGE₂ infusion.Fractional proximal reabsorption of sodium and fluid, calculated from endproximal tubular fluid/plasma concentration ratios of Na and Inulin, were not inflenced by PGE₂. Transtubular net movements, calculated from split drop half time, were not significantly affected, too. Tubular transit time, however, was shortened, and fractional proximal reabsorption was decreased, when calculated fromt1/2 andT. This, too, indicates thatt1/2 andT do not always sufficiently define the fractional reabsorption.The normal decrease of the TF/P ratio of Na along the distal convolution is reduced under the influence of PGE₂. Approximately 2% of the filtered Na remain unreabsorbed in the distal convolution in addition to the amount found in controls. The high Na concentration in the final urine is established in the collecting ducts by an ADH like action.Preliminary reports of the results were presented in part at the 33rd meeting of the Deutsche Pharmakologische Gesellschaft in Heidelberg 1970.     

Glomerular filtration changes during renal artery infusion of various hypertonic solutions in the dog

Summary Effects of renal artery infusion of hypertonic equiosmolar solutions of NaCl, Na₂SO₄, urea, glucose and mannitol on the glomerular filtration rate (GFR) were studied in anesthetized dogs. GFR was determined as a product of the renal plasma flow—calculated from directly measured renal blood flow and hematocrit—and the extraction ratio of exogenous creatinine (RPF·E _cr).Hypertonic solutions of urea, glucose and mannitol depressed GFR while NaCl and Na₂SO₄ were without significant effect. The change in filtration was a linear function of the inhibition of the net absolute tubular reabsorption of water (T _{H 2}O) caused by hypertonic infusions. This relation was observed with all the infusates except Na₂SO₄ which depressed totalT _{H 2}O but did not affect GFR. It is suggested that a definite inhibition of proximal reabsorption, probably absent with Na₂SO₄ infusion, determines the fall in GFR in osmotic diuresis.     

饮食钾缓解盐诱导的冠状动脉损伤*

 目的：探讨饮食钾对盐诱导的冠状动脉损伤的保护机制。方法: 将4周龄SD大鼠随机分为3组,每组10只,分别是对照组（NS,蒸馏水）、高盐组（HS,含1.5% NaCl蒸馏水）和高盐补钾组（HS+HP,含1.5% NaCl和0.5% KCl蒸馏水）干预16周。每2周监测各组大鼠尾动脉血压。干预16周后,硝酸还原酶法检测大鼠血清中NO的含量;硫代巴比妥酸法检测各组大鼠血清中丙二醛（MDA）的含量; HE染色观察各组大鼠左冠状动脉的大体形态;免疫荧光染色观察各组大鼠冠状动脉内皮型一氧化氮合酶（eNOS）的表达;二氢乙啶荧光探针染色/Western blotting法观察各组大鼠冠状动脉氧化应激的程度。结果：高盐干预16周后,高盐组大鼠根据尾动脉血压的变化分为盐敏感性大鼠和盐抵抗性大鼠。本实验只研究HS组中盐敏感性大鼠。在HS组中,大鼠血压较NS组显著升高,给予补钾后可以缓解血压的升高。HS组较NS组大鼠血清中NO降低,MDA升高,冠状动脉eNOS表达降低,还原型烟酰胺腺嘌呤二核苷酸磷酸（NADPH）氧化酶的gp91亚基表达升高,冠状动脉管壁厚度显著增加,且 DHE荧光探针染色发现其超氧阴离子增加。高盐摄入的同时给予补钾可以缓解高盐摄入引起的有害效应。结论：高盐摄入通过氧化应激引起冠状动脉结构和功能的改变,补钾可以缓解这一效应的发生。     

Filtration in surface glomeruli as regulated by flow rate through the loop of henle

Summary A new pressure transducer microperfusion system has been used to measure quantitatively SN GFR, early proximal free flow pressure and stop flow pressure of varying flow rates through the loop of Henle in the range of 0 to 50 nl/min. Perfusing the loop with an isoosmolal artificial tubular fluid at physiological flow rates SN GFR was 19.9±1.1 and 27.7±1.0 nl/min in two different strains of antidiuretic rats. SN GFR increased when loop perfusion was interrupted. The question whether a feedback control mechanism of SN GFR is operative in the rat kidney was evaluated in further experiments in which early proximal tubular pressure was measured in functionally isolated proximal convolutions in vivo under conditions of constant flow as well as stopped flow. Experiments in which perfusion rate through the loop of Henle was varied demonstrated the existence of a feedback signal which originates downstream of the late proximal convolution and which affects filtration into individual early proximal segments. This feedback mechanism exhibited an asymmetrical behaviour: Elevation of loop perfusion above the control value resulted in an early proximal pressure drop, under simulated free flow conditions as well as under stop flow conditions. In contrast lowering of perfusion rate below the predetermined physiological value had no significant effect on early proximal pressures.Index of Abbreviations FFP free flow pressure - P _a blood pressure - P _G mean glomerular capillary pressure - PCT proximal convoluted tubule - P _tub intratubular hydrostatic pressure - SFP stop flow pressure - SN GFR single nephron glomerular filtration rate - TF tubular fluid - TF/P concentration ratio between tubular fluid and plasma - V_(TF) collected volume of tubular fluid, flow rate Supported by Deutsche ForschungsgemeinschaftParts of the present work have been presented at the following meetings: Int. Symp. on Renal Handling of Sodium, Brestenberg1971; Workshop of Renal Micropuncture Techniques, Yale University 1971; Int. Congress of Nephrology, Mexico City 1972.     

Modification of feedback influence on glomerular filtration rate by acute isotonic extracellular volume expansion

The behavior of the feedback mechanism, that causes glomerular capillary pressure and filtration rate to decrease when tubule fluid flow rate through the loop of Henle of the same nephron is increased, was examined in rats before and during isotonic extracellular fluid volume expansion. The loop of Henle was perfused from the late proximal tubule at either 10 or 40 nl/min while proximal fluid was collected to measure single nephron filtration rate (SNGFR), while proximal stop-flow pressure (P_SF) was measured, or while fluid was collected from the early distal tubule to assess reabsorption of fluid and electrolytes by the loop of Henle. During control periods increasing loop perfusion caused SNGFR to decrease 37%, P_SF to decrease 19%, and absorption of fluid, sodium and chloride by the loop of Henle to increase. After 1 h of infusion of isotonic NaCl solution the same change in loop flow causes a 19% decrease in SNGFR and an 8% decrease in P_SF. Fluid absorption by the loop of Henle did not increase with increased loop perfusion. Increases in Na and Cl absorption were similar to the increases in control periods. The smaller decreases in SNGFR and P_SF indicate that acute volume expansion decreases the sensitivity of the feedback response. The mechanism of this decrease in gain could involve interference with local generation or action of angiotensin, or a change in the composition or pressure of interstitial fluid tending to dilate the pre-glomerular resistance vessels.     

Changes in “Active” and “Inactive” renin in the juxtaglomerular apparatuses of rat nephrons and plasma induced by different salt intake

The juxtaglomerular apparatuses (JGA) of deep and superficial nephrons were isolated by microbiopsy or by microdissection. Inactive renin content was determined by acidification of JGA or plasma to pH 3.0.In rats with low salt intake the renin content of superficial JGA was 13.4±3.0 ng AI/JGA/h before and 20.4±3.4 ng AI/JGA/h (n=9,P<0.05) after acidification. The corresponding values for deep JGA were 9.1±1.2 ng AI/JGA/h and 12.7±2.7 ng AI/JGA/h (n=9,P<0.01). The plasma renin concentration was 54.1±15.0 ng AI/ml/h before and 56.0±10.6 ng AI/JGA/h (n=7, N.S.) after acidification.In rats with a normal salt intake the superficial renin JGA renin content was 11.6±2.3 ng AI/JGA/h before and 11.0 ±2.7 ng AI/JGA/h (n=9, N.S.) after acidification. The renin content of deep JGA was 4.6±0.6 ng AI/JGA/h before and 8.6±3.1 ng AI/JGA/h (n=9,P<0.005) after acidification. Plasma renin concentration was 34.5±4.7 ng AI/ml/h and did not change after acidification.In rats with a high salt intake superficial JGA content was 6.8±1.7 ng AI/JGA/h before and 8.4±2.1 ng AI/JGA/h (n=9, N.S.) after acidification. The corresponding values for deep JGA were 5.7±1.6 ng AI/JGA/h and 6.9±1.6 ng AI/JGA/h (n=9, N.S.) respectively. Plasma renin concentration was 13.1±1.1 ng AI/ml/h and this to 21.8±2.9 ng AI/ml/h (n=8,P<0.01) after acidification.These results suggest that although the synthesis of active and inactive renin is linked, the secretion of the two forms may be independent.     

Compatibility of postural behavior induced by two aspects of visual feedback: time delay and scale display

Fourteen healthy adults were tested to assess the potential influence on stance maintenance of two parts of the visual feedback technique (display scale and time delay). The task consisted in their keeping a spot on the screen representing their center of pressure, CoP (i.e. successive points of application of the ground reaction forces detected by the force platform on which they were standing) to a minimum size. The analysis focused on elementary motions computed from the complex CoP trajectories, that is the horizontal motion of the center of gravity (CoG_h) and the difference between the CoP and the vertical projection of the center of gravity (CoP–CoG_v). The former is recognized as the main variable in postural control, and several interesting features can be extracted from the latter. The results indicate that setting a delay and increasing the display scale induce substantial reductions in CoP–CoG_v and CoG_h displacements, respectively. Interestingly, when the two effects are combined, these single effects cohabit quite happily. Fractional Brownian motion modeling of these trajectories revealed clearly that, in each case, these effects originate principally from poor or improved control, respectively. This feature confirms that these elementary motions are involved differently in the postural system and that study of the complex CoP might not be of great interest. By generating opposing but complementary trends, the visual feedback technique should thus be perceived as a promising tool for inducing particular postural behavior in healthy and disabled individuals.     

Effect of water and salt intake on pituitary catecholamines in the rat and domestic pig

The dopamine content of the combined neural and intermediate lobes of the rat pituitary gland was increased when the secretion of posterior lobe hormones was maximally stimulated by a restricted water intake followed by the administration of NaCl in the drinking water (dehydration). This was accompanied by vasodilatation in the posterior lobe and by an increase in the intensity of catecholamine fluorescence in the perivascular nerve fibres. The quantity of noradrenaline in the neural and intermediate lobes, which amounts to about 10% of that of dopamine, was also increased after dehydration. Bilateral removal of the sympathetic superior cervical ganglia did not prevent the rise in pituitary dopamine levels after dehydration but the perivascular catecholamine-containing nerve fibres were no longer detectable. The operation caused a decrease in the noradrenaline content of the neural and intermediate lobes by about 50% in rats on normal water intake and in dehydrated rats. Thus, one half of the noradrenaline present in the neural and intermediate lobes is not contained in vasomotor fibres or other nerve fibres with cell bodies in the superior cervical ganglion. The increase in pituitary dopamine levels after dehydration was also observed in domestic pigs.It is concluded that the increase in the concentration of dopamine in the combined neural and intermediate lobes that follows dehydration and salt-loading does not occur in the perivascular sympathetic nerve fibres and that it may be related to the fall in the pituitary content of oxytocin and vasopressin.     

Paradoxical stimulation of food intake by larger loads of glucose,fructose and mannose: Evidence for a positive feedback effect

The phenomenon of paradoxical stimulation of food intake by larger alimentary loads of isotonic glucose was studied with regard to a variety of experimental and nutritional conditions. Thus, paradoxical feeding response was induced not only by the infusions of glucose but also of the other insulinogenic sugars—fructose and mannose. Stimulation of food intake was further observed following the administration of larger volumes of isotonic glucose solutions via the duodenal and intraperitoneal infusion routes in free feeding as well as in 12 hr food-deprived animals. This paradoxical alimentary response was not eliminated or reduced by repeating these infusions daily over a longer period of time; in fact, food in the first postinfusions hour and, unexpectedly, also the total daily food intakes, showed a gradually increasing trend with daily repetitions. Drinking of isotonic glucose for 1 hr in 12 hr water-deprived animals did not suppress the subsequent intake of regular food despite the substantial amount of glucose drunk (94.1 ml vs 76.5 ml of water in controls during the same time period). The conbination of glucose drinking with subsequent food intake resulted in a significant caloric imbalance during the first hour which was not fully compensated within the 24 hr. These findings indicate that larger loads of insulinogenic sugar loads are capable of triggering an antiregulatory positive feedback alimentary response which can induce caloric imbalance and thus adversely affects the short-term maintenance of energy balance.