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

Renal calcium and magnesium handling during pregnancy: modeling and analysis

Pregnancy is associated with elevated demand of most nutrients, with many trace elements and minerals critical for the development of fetus. In particular, calcium (Ca2+) and magnesium (Mg2+) are essential for cellular function, and their deficiency can lead to impaired fetal growth. A key contributor to the homeostasis of these ions is the kidney, which in a pregnant rat undergoes major changes in morphology, hemodynamics, and molecular structure. The goal of this study is to unravel the functional implications of these pregnancy-induced changes in renal handling of Ca2+ and Mg2+, two cations that are essential in a healthy pregnancy. To achieve that goal, we developed computational models of electrolyte and water transport along the nephrons of a rat in mid and late pregnancy. Model simulations reveal a substantial increase in the reabsorption of Mg2+ along the proximal tubules and thick ascending limbs. In contrast, the reabsorption of Ca2+ is increased in the proximal tubules but decreased in the thick ascending limbs, due to the lower transepithelial concentration gradient of Ca2+ along the latter. Despite the enhanced transport capacity, the marked increase in glomerular filtration rate results in elevated urinary excretions of Ca2+ and Mg2+ in pregnancy. Furthermore, we conducted simulations of hypocalcemia and hypomagnesemia. We found that hypocalcemia lowers Ca2+ excretion substantially more than Mg2+ excretion, with this effect being more pronounced in virgin rats than in pregnant ones. Conversely, hypomagnesemia reduces the excretion of Mg2+ and Ca2+ to more similar degrees. These differences can be explained by the greater sensitivity of the calcium-sensing receptor (CaSR) to Ca2+ compared with Mg2+.NEW & NOTEWORTHY A growing fetus' demands of minerals, notably calcium and magnesium, necessitate adaptations in pregnancy. In particular, the kidney undergoes major changes in morphology, hemodynamics, and molecular structure. This computational modeling study provides insights into how these pregnancy-induced renal adaptation impact calcium and magnesium transport along different nephron segments. Model simulations indicate that, despite the enhanced transport capacity, the marked increase in glomerular filtration rate results in elevated urinary excretions of calcium and magnesium in pregnancy.

Metabolic Responses of Normal Rat Kidneys to a High Salt Intake

In this study, novel methods were developed, which allowed continuous (24/7) measurement of arterial blood pressure and renal blood flow in freely moving rats and the intermittent collection of arterial and renal venous blood to estimate kidney metabolic fluxes of O2 and metabolites. Specifically, the study determined the effects of a high salt (HS; 4.0% NaCl) diet upon whole kidney O2 consumption and arterial and renal venous plasma metabolomic profiles of normal Sprague-Dawley rats. A separate group of rats was studied to determine changes in the cortex and outer medulla tissue metabolomic and mRNAseq profiles before and following the switch from a 0.4% to 4.0% NaCl diet. In addition, targeted mRNA expression analysis of cortical segments was performed. Significant changes in the metabolomic and transcriptomic profiles occurred with feeding of the HS diet. A progressive increase of kidney O2 consumption was found despite a reduction in expression of most of the mRNA encoding enzymes of TCA cycle. A novel finding was the increased expression of glycolysis-related genes in Cx and isolated proximal tubular segments in response to an HS diet, consistent with increased release of pyruvate and lactate from the kidney to the renal venous blood. Data suggests that aerobic glycolysis (eg, Warburg effect) may contribute to energy production under these circumstances. The study provides evidence that kidney metabolism responds to an HS diet enabling enhanced energy production while protecting from oxidative stress and injury. Metabolomic and transcriptomic analysis of kidneys of Sprague-Dawley rats fed a high salt diet.

Mapping single-nephron filtration in the isolated, perfused rat kidney using magnetic resonance imaging

The kidney has an extraordinary ability to maintain glomerular filtration despite natural fluctuations in blood pressure and nephron loss. This is partly due to local coordination between single-nephron filtration and vascular perfusion. An improved understanding of the three-dimensional (3-D) functional coordination between nephrons and the vasculature may provide a new perspective of the heterogeneity of kidney function and could inform targeted therapies and timed interventions to slow or prevent the progression of kidney disease. Here, we developed magnetic resonance imaging (MRI) tools to visualize single-nephron function in 3-D throughout the isolated perfused rat kidney. We used an intravenous slow perfusion of a glomerulus-targeted imaging tracer [cationized ferritin (CF)] to map macromolecular dynamics and to identify glomeruli in 3-D, followed by a bolus of a freely filtered tracer (gadolinium diethylenetriamine penta-acetic acid) to map filtration kinetics. There was a wide intrakidney distribution of CF binding rates and estimated single-nephron glomerular filtration rate (eSNGFR) between nephrons. eSNGFR and CF uptake rates did not vary significantly by distance from the kidney surface. eSNGFR varied from ∼10 to ∼100 nL/min throughout the kidney. Whole single-kidney GFR was similar across all kidneys, despite differences in the distributions eSNGFR of and glomerular number, indicating a robust adaptive regulation of individual nephrons to maintain constant single-kidney GFR in the presence of a natural variation in nephron number. This work provides a framework for future studies of single-nephron function in the whole isolated perfused kidney and experiments of single-nephron function in vivo using MRI.NEW & NOTEWORTHY We report MRI tools to measure and map single-nephron function in the isolated, perfused rat kidney. We used imaging tracers to identify nephrons throughout the kidney and to measure the delivery and filtration of the tracers at the location of the glomeruli. With this technique, we directly measured physiological parameters including estimated single-nephron glomerular filtration rate throughout the kidney. This work provides a foundation for new studies to simultaneously map the function of large numbers of nephrons.

Sex and species differences in epithelial transport in rat and mouse kidneys: Modeling and analysis

The goal of this study was to investigate the functional implications of sex and species differences in the pattern of transporters along nephrons in the rat and mouse kidney, as reported by Veiras et al. (J Am Soc Nephrol 28: 3504–3517, 2017). To do so, we developed the first sex-specific computational models of epithelial water and solute transport along the nephrons from male and female mouse kidneys, and conducted simulations along with our published rat models. These models account for the sex differences in the abundance of apical and basolateral transporters, glomerular filtration rate, and tubular dimensions. Model simulations predict that 73% and 57% of filtered Na+ is reabsorbed by the proximal tubules of male and female rat kidneys, respectively. Due to their smaller transport area and lower NHE3 activity, the proximal tubules in the mouse kidney reabsorb a significantly smaller fraction of the filtered Na+, at 53% in male and only 34% in female. The lower proximal fractional Na+ reabsorption in female kidneys of both rat and mouse is due primarily to their smaller transport area, lower Na+/H+ exchanger activity, and lower claudin-2 abundance, culminating in significantly larger fractional delivery of water and Na+ to the downstream nephron segments in female kidneys. Conversely, the female distal nephron exhibits a higher abundance of key Na+ transporters, including Na+-Cl− cotransporters in both species, epithelial Na+ channels for the female rat, and Na+-K+-Cl−cotransporters for the female mouse. The higher abundance of transporters accounts for the enhanced water and Na+ transport along the female rat and mouse distal nephrons, relative to the respective male, resulting in similar urine excretion between the sexes. Model simulations indicate that the sex and species differences in renal transporter patterns may partially explain the experimental observation that, in response to a saline load, the diuretic and natriuretic responses were more rapid in female rats than males, but no significant sex difference was found in mice. These computational models can serve as a valuable tool for analyzing findings from experimental studies conducted in rats and mice, especially those involving genetic modifications.

Visualization of the intrarenal distribution of capillary blood flow

This study describes a modified technique to fill the renal vasculature with a silicon rubber (Microfil) compound and obtain morphologic information about the intrarenal distribution of capillary blood flow under a variety of conditions. Kidneys and cremaster muscles of rats were perfused in vivo with Microfil using a perfusion pressure equal to the animal's mean arterial pressure at body temperature. Microfil did not alter arteriolar diameter or the pattern of flow in the microcirculation of the cremaster muscle. The modified protocol reproducibly filled the renal vasculature, including; glomerular, peritubular, and vasa recta capillaries. We compared the filling of the renal circulation in control rats with that seen in animals subjected to maneuvers reported to alter the intrarenal distribution of blood flow. Infusion of angiotensin II, hypotension, volume expansion, and mannitol- or furosemide-induced diuresis redistributed flow between renal cortical and medullary capillaries. The advantage of the current technique is that it provides anatomical information regarding the number, diameter, and branching patterns of capillaries in the postglomerular circulation critical in determining the intrarenal distribution of cortical and medullary blood flow.

Single-Nephron Glomerular Filtration Rate in Healthy Adults

BACKGROUND

The glomerular filtration rate (GFR) assesses the function of all nephrons, and the single-nephron GFR assesses the function of individual nephrons. How the single-nephron GFR relates to demographic and clinical characteristics and kidney-biopsy findings in humans is unknown.

METHODS

We identified 1388 living kidney donors at the Mayo Clinic and the Cleveland Clinic who underwent a computed tomographic (CT) scan of the kidney with the use of contrast material and an iothalamate-based measurement of the GFR during donor evaluation and who underwent a kidney biopsy at donation. The mean single-nephron GFR was calculated as the GFR divided by the number of nephrons (calculated as the cortical volume of both kidneys as assessed on CT times the biopsy-determined glomerular density). Demographic and clinical characteristics and biopsy findings were correlated with the single-nephron GFR.

RESULTS

A total of 58% of the donors were women, and the mean (±SD) age of the donors was 44±12 years. The mean GFR was 115±24 ml per minute, the mean number of nephrons was 860,000±370,000 per kidney, and the mean single-nephron GFR was 80±40 nl per minute. The single-nephron GFR did not vary significantly according to age (among donors <70 years of age), sex, or height (among donors ≤190 cm tall). A higher single-nephron GFR was independently associated with larger nephrons on biopsy and more glomerulosclerosis and arteriosclerosis than would be expected for age. A higher single-nephron GFR was associated with a height of more than 190 cm, obesity, and a family history of end-stage renal disease.

CONCLUSIONS

Among healthy adult kidney donors, the single-nephron GFR was fairly constant with regard to age, sex, and height (if ≤190 cm). A higher single-nephron GFR was associated with certain risk factors for chronic kidney disease and certain kidney-biopsy findings. (Funded by the National Institute of Diabetes and Digestive and Kidney Diseases.).

Physiological roles of claudins in kidney tubule paracellular transport

The paracellular pathways in renal tubular epithelia such as the proximal tubules, which reabsorb the largest fraction of filtered solutes and water and are leaky epithelia, are important routes for transepithelial transport of solutes and water. Movement occurs passively via an extracellular route through the tight junction between cells. The characteristics of paracellular transport vary among different nephron segments with leaky or tighter epithelia. Claudins expressed at tight junctions form pores and barriers for paracellular transport. Claudins are from a multigene family, comprising at least 27 members in mammals. Multiple claudins are expressed at tight junctions of individual nephron segments in a nephron segment-specific manner. Over the last decade, there have been advances in our understanding of the structure and functions of claudins. This paper is a review of our current knowledge of claudins, with special emphasis on their physiological roles in proximal tubule paracellular solute and water transport.

A computational model for simulating solute transport and oxygen consumption along the nephrons

The goal of this study was to investigate water and solute transport, with a focus on sodium transport (T_Na) and metabolism along individual nephron segments under differing physiological and pathophysiological conditions. To accomplish this goal, we developed a computational model of solute transport and oxygen consumption (Q_O2 ) along different nephron populations of a rat kidney. The model represents detailed epithelial and paracellular transport processes along both the superficial and juxtamedullary nephrons, with the loop of Henle of each model nephron extending to differing depths of the inner medulla. We used the model to assess how changes in T_Na may alter Q_O2 in different nephron segments and how shifting the T_Na sites alters overall kidney Q_O2 Under baseline conditions, the model predicted a whole kidney T_Na/Q_O2 , which denotes the number of moles of Na⁺ reabsorbed per moles of O₂ consumed, of ∼15, with T_Na efficiency predicted to be significantly greater in cortical nephron segments than in medullary segments. The T_Na/Q_O2 ratio was generally similar among the superficial and juxtamedullary nephron segments, except for the proximal tubule, where T_Na/Q_O2 was ∼20% higher in superficial nephrons, due to the larger luminal flow along the juxtamedullary proximal tubules and the resulting higher, flow-induced transcellular transport. Moreover, the model predicted that an increase in single-nephron glomerular filtration rate does not significantly affect T_Na/Q_O2 in the proximal tubules but generally increases T_Na/Q_O2 along downstream segments. The latter result can be attributed to the generally higher luminal [Na⁺], which raises paracellular T_Na Consequently, vulnerable medullary segments, such as the S3 segment and medullary thick ascending limb, may be relatively protected from flow-induced increases in Q_O2 under pathophysiological conditions.

Effects of exercise on the nephron of Goto-Kakizaki rats: morphological, and advanced glycation end-products and inducible nitric oxide synthase immunohistochemical analyses

The current study aimed to examine how exercise affects morphology of the nephron, and localization of advanced glycation end-products (AGEs) and inducible nitric oxide synthase (iNOS) immunoreactivity in diabetic Goto-Kakizaki rats. Four groups of male rats were studied. WIS SED (Wistar rats; sedentary) group served as a control. Other groups were WIS EX (Wistar rats; exercise), GK SED (Goto-Kakizaki diabetic rats; sedentary) and GK EX (Goto-Kakizaki diabetic rats; exercise) groups. The rats in EX groups were subjected to 15weeks of treadmill running at a speed of 15m/min for a total of 30minutes, three times a week. Changes in the structure of renal corpuscles and in the distribution of AGEs- and iNOS-immunoreactive cells of the uriniferous tubules were evaluated. Every parameter of GK EX was significantly different from that of GK SED (area of Bowman's capsules: p<0.001, area of glomeruli: p<0.05 and the occupancy of a glomerulus: p<0.05). These findings suggest that exercise may ameliorate glomerular filtration rate (GFR). The localizations of AGEs and iNOS immunostaining in the uriniferous tubules were similar in each group. Immunohistochemical assays revealed that the number of the AGEs and iNOS immunopositive cells of the proximal tubule of cortico-deep layer in EX groups were markedly greater than those in SED groups and that iNOS expression in GK EX was significantly higher than GK SED (p<0.05). Exercise seems to normalize the GFR and glomerular filtrate absorption from the uriniferous tubules in Goto-Kakizaki diabetic rats with the recovered shape of renal corpuscles and may be involved in the absorption and catabolization of AGEs with iNOS-related reactions for reabsorption.

A mathematical model of the rat nephron: glucose transport

Mathematical models of the proximal tubule (PT), loop of Henle (LOH), and distal nephron have been combined to simulate transport by rat renal tubules. The ensemble is composed of 24,000 superficial (SF) nephrons and 12,000 juxtamedullary (JM) nephrons in 5 classes (according to LOH length); all coalesce into 7,200 connecting tubules (CNT). Medullary interstitial solute concentrations are specified. The model equations require that each nephron glomerular filtration rate (GFR) satisfies a tubuloglomerular feedback (TGF) relationship, and each initial hydrostatic pressure yields a common CNT pressure; that common CNT pressure is determined from an overall distal hydraulic resistance to flow. By virtue of the greater GFR for JM nephrons, fluid delivery to SF and JM tubules is comparable. Glucose reabsorption is restricted to the PT, cotransported with one Na in the convoluted tubule (SGLT2), and two Na in the straight tubule (SGLT1). Increasing ambient glucose from 5 to 10 mM increases proximal Na reabsorption and decreases distal delivery. This is mitigated by a TGF-mediated increase in GFR, and may thus be an etiology for TGF-mediated glomerular hyperfiltration. With SGLT2 inhibition by 95%, the model predicts that under normoglycemic conditions about 60% of filtered glucose will still be reabsorbed, so that profound glycosuria is not to be expected. Compared with glucose-driven osmotic diuresis, SGLT2 inhibition provokes greater natriuresis. When hyperglycemia is superimposed on SGLT2 inhibition, the model suggests that natriuresis may be severe, reflecting synergy of a proximal diuretic and osmotic diuresis. In sum, the model captures TGF-mediated diabetic hyperfiltration and predicts glomerular protection with SGLT2 inhibition.

A mathematical model of rat proximal tubule and loop of Henle

Proximal tubule and loop of Henle function are coupled, with proximal transport determining loop fluid composition, and loop transport modulating glomerular filtration via tubuloglomerular feedback (TGF). To examine this interaction, we begin with published models of the superficial rat proximal convoluted tubule (PCT; including flow-dependent transport in a compliant tubule), and the rat thick ascending Henle limb (AHL). Transport parameters for this PCT are scaled down to represent the proximal straight tubule (PST), which is connected to the thick AHL via a short descending limb. Transport parameters for superficial PCT and PST are scaled up for a juxtamedullary nephron, and connected to AHL via outer and inner medullary descending limbs, and inner medullary thin AHL. Medullary interstitial solute concentrations are specified. End-AHL hydrostatic pressure is determined by distal nephron flow resistance, and the TGF signal is represented as a linear function of end-AHL cytosolic Cl concentration. These two distal conditions required iterative solution of the model. Model calculations capture inner medullary countercurrent flux of urea, and also suggest the presence of an outer medullary countercurrent flux of ammonia, with reabsorption in AHL and secretion in PST. For a realistically strong TGF signal, there is the expected homeostatic impact on distal flows, and in addition, a homeostatic effect on proximal tubule pressure. The model glycosuria threshold is compatible with rat data, and predicted glucose excretion with selective 1Na(+):1glucose cotransporter (SGLT2) inhibition comports with observations in the mouse. Model calculations suggest that enhanced proximal tubule Na(+) reabsorption during hyperglycemia is sufficient to activate TGF and contribute to diabetic hyperfiltration.

The glomerulus: the sphere of influence

The glomerulus, the filtering unit of the kidney, is a unique bundle of capillaries lined by delicate fenestrated endothelia, a complex mesh of proteins that serve as the glomerular basement membrane and specialized visceral epithelial cells that form the slit diaphragms between interdigitating foot processes. Taken together, this arrangement allows continuous filtration of the plasma volume. The dynamic physical forces that determine the single nephron glomerular filtration are considered. In addition, new insights into the cellular and molecular components of the glomerular tuft and their contribution to glomerular disorders are explored.

Hormonal regulation of salt and water excretion: a mathematical model of whole kidney function and pressure natriuresis

We present a lumped-nephron model that explicitly represents the main features of the underlying physiology, incorporating the major hormonal regulatory effects on both tubular and vascular function, and that accurately simulates hormonal regulation of renal salt and water excretion. This is the first model to explicitly couple glomerulovascular and medullary dynamics, and it is much more detailed in structure than existing whole organ models and renal portions of multiorgan models. In contrast to previous medullary models, which have only considered the antidiuretic state, our model is able to regulate water and sodium excretion over a variety of experimental conditions in good agreement with data from experimental studies of the rat. Since the properties of the vasculature and epithelia are explicitly represented, they can be altered to simulate pathophysiological conditions and pharmacological interventions. The model serves as an appropriate starting point for simulations of physiological, pathophysiological, and pharmacological renal conditions and for exploring the relationship between the extrarenal environment and renal excretory function in physiological and pathophysiological contexts.

Proximal nephron

The kidney plays a fundamental role in maintaining body salt and fluid balance and blood pressure homeostasis through the actions of its proximal and distal tubular segments of nephrons. However, proximal tubules are well recognized to exert a more prominent role than distal counterparts. Proximal tubules are responsible for reabsorbing approximately 65% of filtered load and most, if not all, of filtered amino acids, glucose, solutes, and low molecular weight proteins. Proximal tubules also play a key role in regulating acid-base balance by reabsorbing approximately 80% of filtered bicarbonate. The purpose of this review article is to provide a comprehensive overview of new insights and perspectives into current understanding of proximal tubules of nephrons, with an emphasis on the ultrastructure, molecular biology, cellular and integrative physiology, and the underlying signaling transduction mechanisms. The review is divided into three closely related sections. The first section focuses on the classification of nephrons and recent perspectives on the potential role of nephron numbers in human health and diseases. The second section reviews recent research on the structural and biochemical basis of proximal tubular function. The final section provides a comprehensive overview of new insights and perspectives in the physiological regulation of proximal tubular transport by vasoactive hormones. In the latter section, attention is particularly paid to new insights and perspectives learnt from recent cloning of transporters, development of transgenic animals with knockout or knockin of a particular gene of interest, and mapping of signaling pathways using microarrays and/or physiological proteomic approaches.

Acute saline expansion increases nephron filtration and distal flow rate but maintains tubuloglomerular feedback responsiveness: role of adenosine A(1) receptors

Temporal adaptation of tubuloglomerular feedback (TGF) permits readjustment of the relationship of nephron filtration rate [single nephron glomerular filtration rate (SNGFR)] and early distal tubular flow rate (V(ED)) while maintaining TGF responsiveness. We used closed-loop assessment of TGF in hydropenia and after acute saline volume expansion (SE; 10% body wt over 1 h) to determine whether 1) temporal adaptation of TGF occurs, 2) adenosine A(1) receptors (A(1)R) mediate TGF responsiveness, and 3) inhibition of TGF affects SNGFR, V(ED), or urinary excretion under these conditions. SNGFR was evaluated in Fromter-Wistar rats by micropuncture in 1) early distal tubules (ambient flow at macula densa), 2) recollected from early distal tubules while 12 nl/min isotonic fluid was added to late proximal tubule (increased flow to macula densa), and 3) from proximal tubules of same nephrons (zero flow to macula densa). SE increased both ambient SNGFR and V(ED) compared with hydropenia, whereas TGF responsiveness (proximal-distal difference in SNGFR, distal SNGFR response to adding fluid to proximal tubule) was maintained, demonstrating TGF adaptation. A(1)R blockade completely inhibited TGF responsiveness during SE and made V(ED) more susceptible to perturbation in proximal tubular flow, but did not alter ambient SNGFR or V(ED). Greater urinary excretion of fluid and Na(+) with A(1)R blockade may reflect additional effects on the distal nephron in hydropenia and SE. In conclusion, A(1)R-independent mechanisms adjust SNGFR and V(ED) to higher values after SE, which facilitates fluid and Na(+) excretion. Concurrently, TGF adapts and stabilizes early distal delivery at the new setpoint in an A(1)R-dependent mechanism.

Aprotinin uptake in the proximal tubules in the rat kidney. II. Uptake site relative to glomerulus

Glomerular filtration rates in whole kidney and in outer, middle and inner cortical zones have previously been estimated by measuring the amount of iodinated Aprotinin, filtered and taken up in the first two thirds of the proximal convoluted tubules, in part positioned more superficial than the parent glomerulus. Thus, an appreciable amount of the absorbed Aprotinin may be located superficial to its filtration site and lead to an underestimate of glomerular filtration in deep cortical layers. Therefore, in this study we have measured the distance from the glomerulus to the center of proximal convoluted tubular ball and the site of Aprotinin uptake. Measurements were made on photos of Microfil-injected tubules and on camera lucida drawings of tubular transections from autoradiographs of nephrons containing both Microfil and iodinated Aprotinin. Both techniques showed that the center of the tubular ball was localized more superficial in all cortical layers. The average distance, in percent of cortical thickness, from all proximal convoluted tubular transections to the parent glomerulus was 9% in deep and 13% in middle and superficial cortex. Corresponding distances for tubular transections containing Aprotinin were 7 and 12%. Grain density in five reconstructed proximal convoluted tubules showed a continuous and exponential fall of Aprotinin along the uptake segment. The results may be used to estimate single nephron filtration rate from Aprotinin uptake and glomerular density in outer, middle, and inner cortex.

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.

Effects of daily sodium intake and ANG II on cortical and medullary renal blood flow in conscious rats

Implanted optical fibers and laser-Doppler flow measurement techniques were used for the sequential measurement of regional renal blood flow in conscious rats to determine the effects of an increase of daily NaCl intake on the renal cortical blood flow and blood flow to the outer and inner medulla. Cortical blood flow was increased significantly (32%) by the second day when NaCl intake was increased from 1 to 7 meq/day and was increased further (50%) on the second day after a further elevation of NaCl intake to 13 meq/day. Blood flow to the outer and inner medulla was not changed as NaCl intake was elevated. The increase in renal cortical flow was closely associated with significant reductions in circulating concentrations of ANG II from 31 to 16 pg/ml. Rats given a continuous infusion of nonpressor does of ANG II (5.0 ng.kg(-1).min-1) to maintain constant plasma concentrations of ANG II as sodium intake was increased exhibited no increase of cortical flow. We conclude that reductions of plasma ANG II associated with incremental increases of daily sodium intake result in a rise of renal cortical flow. The elevated blood flow to the renal cortex may enhance sodium excretion and contribute to long-term sodium homeostasis.

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.

Three-dimensional reconstruction of human juxtaglomerular apparatus (JGA)

A three-dimensional and morphological study of the human JGA was undertaken to establish a background for understanding the changes in this vital apparatus during various physiological and pathological conditions. Three-dimensional reconstruction was carried out using a computer program "GLOM". Serial sections of normal human kidneys were used after staining with specific human renin antiserum. Three-dimensional reconstruction revealed renin-positive cells in the afferent and efferent arterioles and interlobular arteries away from the JGA area. A close contact was demonstrated between renin-positive cells and the macula densa. The frequency of positively stained JGAs was significantly higher in the superficial glomeruli compared to the deep glomeruli. The high renin content of the superficial glomeruli suggests higher generation of angiotensin, which may contribute to the regulation of the GFR as proposed by other workers. This preliminary study on normal human JGA is to be extended to hypertensive and renal failure patients.

The effects of hyperfiltration on serum sickness glomerulonephritis in rats

The effects of hyperfiltration induced due to unilateral nephrectomy on immunologically induced glomerular injuries were studied. Glomerulonephritis was induced in rats by sensitizing them with egg albumin as an antigen. Unilateral nephrectomy did not affect the removal rate of the antigen from the glomeruli in the rats, but accelerated the rate of the glomerular injuries after cessation of the immunologically induced glomerular inflammation. The histopathological features were characterized by sclero-adhesive lesions with aneurysmal dilatation and hyalinosis of the glomerular capillaries. The parietal epithelial cells extended from the Bowman's capsule with matrices to cover the denuded basement membrane and formed adhesions. The neighboring capillaries collapsed, and the sclero-adhesive lesions progressed. These findings indicate that hyperfiltration at the capillary level did not accelerate the recovery from glomerulonephritis, but induced glomerular sclerosis with adhesions and deteriorated the trivial glomerular injuries to produce similar focal segmental lesions.

Effect of salt balance on the renal and hemodynamic actions of benazepril in normal men

Renal and hemodynamic effects of diet alone and of single oral doses of the nonsulphydryl angiotensin converting enzyme (ACE) inhibitor, benazepril (10 mg), were investigated in eight healthy volunteers under stable conditions of high salt intake (300 mmol NaCl/day) and low salt intake (10 mmol NaCl/day), in a double blind, placebo controlled study. There were no changes in blood pressure between the two dietary extremes either during the run-in period or once sodium balance had been achieved. Mean renal plasma flow was higher, by approximately 10% and renal vascular resistance lower by 15%, on high salt diet compared to low salt diet. Glomerular filtration rates were found to be similar irrespective of the state of salt balance. Both plasma urate concentration and plasma renin activity were significantly elevated in the low salt compared to high salt state. Benazepril caused a greater fall in blood pressure in the sodium depleted state. Significant increases in the mean renal plasma flow, in the order of 15-20%, were seen over 6 h postbenazepril when compared with placebo response, regardless of the level of salt intake. Glomerular filtration rate over the same period remained unaltered. Benazepril doubled the urinary excretion of sodium over the first 4 hours after dosing whilst on the low salt diet; the equivalent increase during salt loading was approximately 20%. These results suggest that benazepril may exert direct effects on renal tubular function additional to those achieved through ACE blockade.

Intra- and inter-nephron heterogeneity of ammoniagenesis in rats: effects of chronic metabolic acidosis and potassium depletion

In order to determine intra- and inter-nephron heterogeneity of ammoniagenesis, ammoniagenic activity in microdissected nephron segments of control, acidotic and potassium (K)-depleted rats was examined. Intranephron distribution of ammoniagenic activity in control rats revealed the highest amount at the second segment of the proximal tubule (S2). Chronic metabolic acidosis induced ammoniagenesis markedly at the first segment of the proximal tubule (S1) by 235% and the thick ascending limb of Henle's loop by 198% and moderately at the S2 by 49%. K-depletion increased ammonia production significantly in the S1 by 298% and the S2 by 107%, which is a pattern quite similar to the result of chronic metabolic acidosis. Ammonia production in K-depletion was also increased in the cortical and medullary collecting tubule by 71% and 102%, respectively, probably due to increases in protein amounts (41% and 158%, respectively) there. To evaluate inter-nephron heterogeneity of ammoniagenesis, ammonia formation from glutamine in the S1 of superficial (SF) and juxtamedullary (JM) nephrons was examined. Although there was no difference in ammonia production between SF-S1 and JM-S1 in control rats, ammonia production in SF-S1 was significantly higher than that in JM-S1 in both metabolic acidosis and K-depletion. From these studies, we conclude: The increase of ammonia production in the proximal tubule was quite similar in both acidosis and K-depletion, suggesting that the main trigger of ammoniagenesis in both conditions might be a reduction of intracellular pH. SF-S1 was the nephron most reactive to acidosis and K-depletion. JM nephrons could be considered to be important not for ammonia production but for ammonia secretion.

Redistribution of glomerular filtration and renal plasma flow in CNS-induced natriuresis

Infusion of hypertonic sodium chloride solution into the third cerebral ventricle results in a marked increase in renal sodium output, indicating an important regulator of extracellular volume homeostasis. The intrarenal events governing the enhanced excretion have not been thoroughly studied previously. In 12 anaesthetized male rats a stainless steel cannula was introduced stereotaxically into the right lateral cerebral ventricle. Urine volume and excretion rates, Na, K, and osmotically active particles were measured during control infusion of artificial cerebrospinal fluid and during stimulation of central mechanisms with I M NaCl (520 nl min-1). At the end of the stimulation period, regional renal plasma flow (86RbCl) and glomerular filtration rate (51Cr-EDTA) were measured with single injection techniques. A second group of 12 non-stimulated rats served as controls. During ICV stimulation, the urine flow rate increased from 1.8 +/- 0.19 to 6.4 +/- 1.01 microliter min-1 (P less than 0.001). The urinary concentrations of Na and K increased, leading to a rise in the excretion rates of these ions from 0.12 +/- 0.025 to 0.96 +/- 0.352 mumol min-1 (P less than 0.001) and 0.40 +/- 0.083 to 1.70 +/- 0.196 (P less than 0.001), respectively. The osmolar excretion rate was 2.9 +/- 0.35 mu Osm min-1 before stimulation and 9.6 +/- 1.09 higher (P less than 0.001) during stimulation. Simultaneously the inner medullary plasma flow rose two-fold from 0.7 +/- 0.06 to 1.4 +/- 0.12 microliter min-1 tissue (P less than 0.008).(ABSTRACT TRUNCATED AT 250 WORDS)

Time-dependent heterogeneity of filtration rate in the autoregulating rat kidney

Experiments were performed to study the regulation of the single-nephron glomerular filtration rate (SNGFR) in superficial and juxtamedullary nephrons, as the left kidney of Sprague Dawley rats was submitted to a reduced arterial pressure of 70 mmHg by means of an aortic clamp. The SNGFR at different cortical levels was measured 0.5, 1, 5, 20 or 45 min after the reduction, in order to ascertain whether the effects of the regulatory mechanisms are modified with time. A Hanssen technique was used, which allows one determination of filtration rates per animal. At a renal arterial pressure (RAP) of 100 mmHg (= control animals) the SNGFR amounted to 20 +/- 1.2 and 23 +/- 0.8 nl X min-1 X g-1 kidney weight in the outer and inner cortical (OC, IC) nephrons. When RAP was further reduced to 70 mmHg, the autoregulation of SNGFR, determined after 0.5 min, was highly efficient for both OC and IC nephrons (19 +/- 2.0, 23 +/- 2.6). A prolonged reduction in RAP caused a gradual decline in SNGFR. The filtration rate measured after 5 min was 15 +/- 1.4 for OC and 20 +/- 1.8 for IC nephrons. The decline was most pronounced for OC nephrons, which led to a fractional redistribution in favour of IC nephrons. Thus, SNGFRIC/SNGFROC was 1.16 +/- 0.065 when RAP was 100 mmHg and 1.41 +/- 0.126 after 5 min with an RAP of 70 mmHg. It is well documented that suprarenal aortic occlusion is a powerful stimulus for the release of renin. This was manifested as an increase in the arterial pressure proximal to the aortic clamp.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of pressor doses of Angiotensin II on autoregulation and intrarenal distribution of glomerular filtration rate in the rat

This study was designed to investigate the effect of pressor doses of exogenous Angiotensin II (AII) on autoregulation and intrarenal distribution of single nephron glomerular filtration rate (SNGFR) in anesthetized, normotensive rats. SNGFR at all cortical levels of the left kidney was measured with a modified Hanssen technique at three renal arterial pressures (RAP): Spontaneous, 100 +/- 1 mmHg and 70 +/- 1 mmHg. In control rats, both outer cortical (OC) and inner cortical (IC) nephrons showed complete autoregulation of SNGFR when RAP was reduced to 100 +/- 1 mmHg. Further reduction to 70 +/- 1 mmHg resulted in different responses among the cortical layers, accompanying a decrease in SNGFR. The SNGFRIC/SNGFROC ratio increased from 1.36 +/- 0.053 to 1.52 +/- 0.047 and a fractional redistribution of glomerular filtration rate towards IC nephrons was seen. When the kidney was submitted to a RAP of 70 +/- 1 mmHg, there was a concomitant increase in central arterial pressure (CAP) from 120 +/- 4.3 to 134 +/- 3.2 mmHg. A continuous i.v. infusion of AII (0.5 microgram . min-1 . kg-1 BW) increased mean arterial pressure from 123 +/- 1.4 to 142 +/- 3.8 mmHg, an effect corresponding to that on peripheral vascular resistance during reduction of RAP to 70 +/- 1 mmHg in control rats. This dose reduced SNGFR at all cortical levels, but did not per se lead to redistribution of SNGFR. A reduction in RAP to 100 +/- 1 mmHg during AII administration resulted in impaired autoregulation of SNGFR in both OC and IC nephrons.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of tubulo-glomerular feedback on the autoregulation of filtration rate in superficial and deep glomeruli

Single nephron glomerular filtration rate (SNGFR) of superficial and juxtamedullary nephrons were measured at normal and reduced perfusion pressure in the left kidney of young Sprague Dawley rats. Perfusion pressure was lowered by constricting the aorta proximal to the branching of the left renal artery. The influence of the tubulo-glomerular feedback mechanism on SNGFR was quantitated by measuring SNGFR during intact and interrupted urine flow to the macula densa region. By using a modified Hanssen technique, SNGFR was measured under free-flow conditions. In other experiments, the urine flow to the distal nephron was blocked by a micropuncture technique, which was used for collection of the tubular fluid for measuring the filtration rate. All nephron populations autoregulated SNGFR from 70-80 to 130 mmHg, which was the upper limit of this investigation, when urine flow throughout the nephron was intact. The autoregulation in this pressure range was lost when tubular fluid was prevented from reaching the distal nephron. It was shown that the influence of negative feedback on SNGFR by the macula densa mechanism at normal blood pressure is greater in deep nephrons than in superficial ones.

Kidney function after unilateral nephrectomy

Immediately after unilateral nephrectomy ( uNX ) some different mechanisms of compensatory adaptation begin to act followed by a restoration of sufficient kidney function in a short time period. Beside biochemical changes early compensatory hypertrophy of the remaining kidney occurs. Simultaneously, functional adaptations of renal blood flow, glomerular filtration and exertion of electrolyte and xenobiotics take place. With a suitable pretreatment it is principally possible to accelerate the regeneration phase. Thus the phase of reduced excretion capacity of tubularly secreted xenobiotics after removal of one kidney can be shortened or prevented.

Glucose handling by distal portions of the nephron during pregnancy in the rat

Reabsorption of glucose, salt and water was measured in 7-8 day pregnant and virgin rats by micropuncturing distal nephrons during saline and glucose infusions. Unidirectional fluxes of glucose were measured in loops of Henle and collecting ducts. There were no significant differences in single nephron glomerular filtration rate (S.N.G.F.R.) between virgin and pregnant animals during saline infusion. During glucose infusion S.N.G.F.R. was higher in pregnant animals than in virgins. There is no evidence of a failure of reabsorption of glucose by the proximal tubule in pregnant animals. More glucose is reabsorbed from the loop of Henle in virgin animals than in pregnant animals during saline infusion but during glucose infusion the converse is true. During both saline and glucose infusion there is less reabsorption of glucose from the collecting duct in pregnant animals than in virgin animals. It is concluded that the increased excretion of glucose during pregnancy can be attributed to alteration of glucose handling by distal segments of the nephron.

Efferent renal nerve activity during intracarotid and intracerebroventricular infusions of hypertonic sodium chloride solutions and isotonic volume expansion in the rat

The change in renal nerve activity under conditions known to increase renal sodium excretion was studied. In adult Sprague Dawley rats, anaesthetized with Inactin, normotonic and hypertonic NaCl solutions were infused into 1) a vein, 2) a carotid artery and 3) the third ventricle. The left kidney was freed and placed in a plastic cup. A renal nerve was dissected free and placed on a stainless bipolar electrode. The nerve was cut distal to the electrode. The nerve signals were amplified and recorded on a tape recorder. Simultaneously integrated nerve signals and also atrial and venous pressures were recorded. Intracarotid infusion of a 1 M NaCl solution increased sodium output and temporarily decreased renal nerve activity by some 35%. Corresponding intravenous (i.v.) infusion gave an increase in renal nerve activity and also in sodium output. The latter increase was delayed compared with that caused by the intracarotid infusion. No variations in blood pressure were noted. In control experiments with a slow i.v. infusion of physiological saline, renal nerve activity increased throughout the experiment, while sodium excretion remained constant. During infusion of a 1 M NaCl solution into the third ventricle, renal nerve activity decreased in about half of the cases. This reduction was often accompanied by an increased arterial blood pressure and an increased sodium output. Arterial blood pressure increases were especially pronounced at the highest infusion rats, i.e. 800 ml-min-1. Isotonic volume expansion of 2% of the body weight resulted in a transient decrease in renal nerve activity by about 30%. Venous blood pressure rose and sodium output increased six-fold. The decrease in nerve activity was observed both when the vagal nerves were intact and when they were cut.

Micropuncture study of changes in glomerular filtration and ion and water handling by the rat kidney during pregnancy

1. In rats receiving 200 mul. min(-1) sodium chloride, glomerular filtration rate, single nephron glomerular filtration rate, and fractional reabsorption were measured at various points along the nephron, and ionic concentrations measured in early distal tubular fluid in virgin and 6, 12 and 19 day pregnant rats.2. Glomerular filtration increased progressively until the twelfth day of pregnancy. At 19 days of pregnancy the glomerular filtration rate, while still above virgin levels, was reduced below 12 day pregnant levels.3. Single nephron glomerular filtration rates measured in proximal and distal tubules were different at both 12 and 19 days of pregnancy, indicating an alteration of tubuloglomerular feed-back.4. A change in the ratio of glomerular filtration rate: distal single nephron filtration rate indicated a redistribution of glomerular filtrate to juxtamedullary nephrons by the sixth day of pregnancy.5. Fluid reabsorption is similar up to the early distal tubule but it is not possible to say whether reabsorption is the same in proximal tubules. More fluid is reabsorbed by late distal tubules and collecting ducts in 12 and 19 day pregnant animals than in the virgin and 6 day pregnant animals.6. Changes in ion reabsorption by the loop of Henle occurred during pregnancy; sodium reabsorption was increased by the sixth day of pregnancy and potassium reabsorption by the twelfth day.

Flow resistance of the interlobular artery in the rat kidney

The afferent and efferent arterioles are considered to be the most important resistance vessels within the renal vasculature, but there are indications that a pressure drop occurs along the interlobular artery. This pressure drop was investigated from two aspects: 1) In rat kidneys the "stop-flow pressure" in the efferent arterioles was measured with the micropuncture technique. At arterial pressures between 100 and 130 mmHg the stop-flow pressure did not exceed 85 mmHg, which means that the highest pressure at the end of the interlobular artery was 85 mmHg; 2) A mathematical model was constructed, assuming that the diameter of the interlobular artery decreased stepwise from 60 to 10 micrometers. The artery was divided into 20 segments, each segment containing one afferent arteriole. The flow in the afferent arterioles increased linearly from 100 nl . min-1 in the first segment to 130 nl . min-1 in the last segment. When the pressure in the first segment was 120 mmHg, it was calculated that the pressure in the last segment was 85 mmHg. These findings strengthen the theory that the interlobular artery may participate in the regulation of the intracortical blood flow in the rat kidney. We conclude that the afferent arteriole of the most superficial nephron is nearly maximally dilated and that the juxtamedullary nephron is able to either dilate or constrict its arteriole in normotensive and normohydrated rats.

Adaptive changes of juxtamedullary glomerular filtration in the remnant kidney

The participation of surviving juxtamedullary nephrons in the adaptive changes of glomerular filtration that occur in response to loss of functioning nephron mass was examined by direct micropuncture of the rat renal papilla. The solitary remnant kidney (RK) in rats with an 85% reduction of renal mass demonstrated strikingly elevated values for single nephron glomerular filtration rate (SNGFR) in both superficial (46.1 +/- 3.2 nl/min) and juxtamedullary (73.5 +/- 6.1 nl/min) nephrons in comparison to respective values observed in normal hydrophenic rats (superficial SNGFR = 15.0 +/- 1.9 nl/min, P less than 0.001, and juxtamedullary SNGFR = 30.2 +/- 3.2 nl/min, P less than 0.001). In RK rats, the proximal portions of both superficial and juxtamedullary nephrons exhibited a marked increase in absolute fluid reabsorption as well as a markedly enhanced delivery of fluid to more distal portions of the nephron. These observations indicate that similar, not preferential, functional adaptations in glomerular filtration occur concomitantly in both superficial and juxtamedullary nephrons consequent to reduction of renal mass.

Anatomy of the renal pelvis in the hamster

The hamster renal pelvis has been studied by means of low-power light microscopy, scanning electron microscopy and morphometric analyses. The results of this study are highly suggestive that the contact of pelvic urine with the other medulla as well as with the inner medulla may be an important aspect of final urine formation. The outer medulla constituted nearly 50% of the total pelvic surface area, with the inner stripe of the outer medulla more than twice the pelvic surface area of the outer stripe of the outer medulla. The large outer medullary pelvic surface area was accounted for by the elaboration of the upper pelvic walls into peripelvic columns, opercula ("secondary pyramids"), fornices and secondary pouches. A thin simple-squamous to low cuboidal pelvic epithelium separated pelvic urine from outer medullary parenchyma. The inner medulla which constituted about one quarter of the total pelvic surface area was covered by a cuboidal to columnar pelvic epithelium which appeared morphologically similar to the papillary collecting duct epithelium. Tubules and capillaries of the inner medulla did not appear as closely juxtaposed to the pelvic epithelium as did those of the outer medulla. Cortical tissue comprised only 11.7% of the total pelvic surface area and was covered by transitional epithelium similar to that of ureter and bladder. The previously reported impermeability of this epithelium suggests that pelvic urine contact with the cortex is unimportant in final urine formation. The rich layer of smooth muscle under the transitional epithelium probably functions to move urine into and out of the pelvis during pelvic peristalsis, which has been observed in vivo.

Diuretic action of metolazone in dogs

Metolazone, the sulfonamide diuretic was investigated to determine the sites of action. We used a radioactive microsphere, clearance and stop-flow method in anesthetized dogs. Urine flow and urinary excretion of sodium and potassium were increased at 5--60 min when metolazone was given intravenously at doses of 0.2--5.0 mg/kg, while total renal blood flow, distribution of cortical renal blood flow and GFR did not change. The urinary excretion rate of sodium to potassium (Na/K) increased from 5.69 +/- 0.82 to 8.07 +/- 0.76 in a dose of 1.0 mg/kg, i.v. Osmolar clearance and free water reabsorption increased almost proportionally, indicating that metolazone has little effect on the medullary portion of the ascending limb of Henle and may have a proximal site of action. In stop-flow experiments, a significantly raised U/PNa/U/Pcreatinine was observed at the dip situated distally to the ascending limb of Henle. These findings indicate that the diuretic action of metolazone may be due to the inhibition of sodium reabsorption in the distal nephron segments, in addition to the absence of modification of the cortical regional blood flow.

Histochemical enzyme activity correlated to the structural segmentation of the proximal convoluted tubule in salt-depleted and salt-loaded rat kidneys

In salt-depleted and salt-loaded rat kidneys a study was made of the structural segmentation of the proximal convoluted tubule (PCT) and the histochemical activity of non-specific acid and alkaline phosphatases and succinate dehydrogenase in the same segments. No quantitative structural or segmental alterations were observed, but significant changes in enzyme activity occured. These comprised: 1) A decrease in activity of acid phosphatase in segment 1 and the transitional zone in salt-depleted kidneys, and an increase in enzyme activity in segment 2 in salt-loaded kidneys. 2) a decrease in alkaline phosphatase activity in segment 2 in both salt-depleted and salt-loaded kidneys and 3) a decrease in succinate dehydrogenase activity in segment 2 in salt-depleted kidneys, and an increase in activity in the same segment in salt-loaded kidneys. Thus long-term variation in sodium intake are followed by segment-correlated variations in the activity of acid and alkaline phosphatase and succinate dehydrogenase in the PCT.