bluuebunny/crossref_metadata_2025 huggingface dataset:
Is urea formation regulated primarily by acid-base balance in vivo?
Authors: Halperin M. L. , Chen C. B. , Cheema-Dhadli S. , West M. L. , Jungas R. L.
Published: December 2017

Large quantities of ammonium and bicarbonate are produced each day from the metabolism of dietary protein. It has recently been proposed that urea synthesis is regulated by the need to remove this large load of bicarbonate. The purpose of these experiments was to test whether the primary function of ureagenesis in vivo is to remove ammonium or bicarbonate. The first series of rats were given a constant acid load as hydrochloric acid or ammonium chloride; individual rats received a constant nitrogen load at a time when their plasma acid-base status ranged from normal (pH 7.4, 28 mM HCO3) to severe metabolic acidosis (pH 6.9, 6 mM HCO3). Urea plus ammonium excretions and the blood urea, glutamine, and ammonium concentrations were monitored with time. Within the constraints of non-steady-state conditions, the rate of urea synthesis was constant and the plasma glutamine and ammonium concentrations also remained constant; thus it appears that the rate of urea synthesis was not primarily regulated by the acid-base status of the animal in vivo over a wide range of plasma ammonium concentrations. In quantitative terms, the vast bulk of the ammonium load was converted to urea over 80 min; only a small quantity of ammonium appeared as circulating glutamine or urinary ammonium. Urea synthesis was proportional to the nitrogen load. A second series of rats received sodium bicarbonate; urea synthesis was not augmented by a bicarbonate load. We conclude from these studies that the need to dispose of excess bicarbonate does not primarily determine the rate of ureagenesis in vivo. The data support the classical view that ureagenesis is controlled by the quantity of ammonium to be removed.
Reversal of renal and smooth muscle actions of the thromboxane mimetic U-44069 by diltiazem
Authors: Loutzenhiser R. , Epstein M. , Horton C. , Sonke P.
Published: December 2017

U-44069 is a stable prostaglandin (PG) H2 analogue and a potent vasoconstrictor. Its in vivo and in vitro actions mimic those of thromboxane A2. We have studied the effects of the calcium antagonist diltiazem upon the vasoconstriction induced by U-44069 using isolated rat aortic smooth muscle and isolated perfused rat kidney (IPRK). The administration of 10(-6)M U-44069 elicited maximally effective contractions in isolated aortic rings and increased 45Ca uptake from a control value of 285 +/- 6 mumol/kg to 344 +/- 8 mumol/kg. Diltiazem reduced U-44069-induced tension development and 45Ca uptake of isolated aortic smooth muscle 73 +/- 2 and 91 +/- 3%, respectively. The dose dependency of each of these effects of diltiazem was similar (EC50 = 369 nM and 334 nM for tension and 45Ca flux, respectively). When administered to the IPRK, 10(-6) M U-44069 caused a 82 +/- 3% decrease in glomerular filtration rate (GFR) and a 80 +/- 4% decrease in filtration fraction but reduced renal perfusate flow (RPF) only 13 +/- 8% (P less than 0.005). Diltiazem completely reversed the actions of U-44069 on the IPRK (EC50 = 288 nM and 323 nM for GFR and RPF, respectively). Diltiazem thus inhibited U-44069-induced tension development and 45Ca uptake by vascular smooth muscle and increased GFR within identical dose ranges. The contractile response of isolated rat glomeruli was also assessed. U-44069 reduced the volume of isolated glomeruli, but this action was neither prevented nor reversed by diltiazem. These results are consistent with the hypothesis that diltiazem increased GFR by inhibiting U-44069-induced Ca influx at preglomerular vessels.
Water permeability and fluidity of renal basolateral membranes
Authors: Verkman A. S. , Ives H. E.
Published: December 2017

Water and nonelectrolyte permeability in basolateral membrane vesicles (BLMV) isolated from rat and rabbit renal cortex were characterized. Osmotic water (Pf) and solute (Ps) permeabilities were determined from the time course of scattered light intensity in response to osmotic gradients. In rabbit BLMV, Pf = 2.5 X 10(-2) cm/s, Purea = 1.2 X 10(-6) cm/s, and sigma urea = 0.95 (23 degrees C). Relative solute permeabilities were urea, 1; thiourea, 0.83; ethylene glycol, 9.7; glycerol, 4.6; formamide, 20; and acetamide, 12. Pf and Purea were not altered by organic mercurials, phloretin, urea, and high-affinity urea analogues. delta H (12-50 degrees C) was 10.7 and 2.5 kcal/mol for Purea and Pf, respectively; in contrast, reported delta H for Pf in rabbit brush-border membrane vesicles (BBMV) was 2 kcal/mol (T less than 33 degrees C) and 14 kcal/mol (T greater than 33 degrees C). To examine whether membrane fluidity changes were associated with this difference, fluorescence anisotropy decay was measured using 1,6-diphenyl-1,3,5-hexatriene (DPH). In both BLMV and BBMV, DPH rotation was hindered at all temperatures (5-53 degrees C). Changes in steady-state anisotropy were attributable to changes in DPH rotational freedom rather than to changes in DPH rotational rate without evidence for a definitive membrane thermotropic phase transition. These results suggest that BLMV urea transport occurs by lipid diffusion and that osmotic water transport is rapid and may be facilitated. A comparison of transepithelial Pf with BLMV and BBMV Pf is consistent with transcellular osmotic water flow in the proximal tubule.
Serine synthesis in rat kidney: studies with perfused kidney and cortical tubules
Authors: Lowry M. , Hall D. E. , Brosnan J. T.
Published: December 2017

Renal serine synthesis was studied in the isolated perfused kidney and in isolated cortical tubules. Serine was produced by the perfused kidney from both glycine and aspartate, indicating flux through at least two separate pathways: serine hydroxymethyltransferase and either the nonphosphorylated or phosphorylated intermediate pathways. The precise nephron site of serine production was determined by measuring serine synthesis from various precursors and the activities of enzymes of both pathways in isolated tubules fractionated on a Percoll gradient into proximal tubule and distal tubule fractions. Both pathways of serine synthesis were located in proximal tubules. Detailed studies of serine synthesis from glycine demonstrated extremely tight coupling between the glycine cleavage enzyme and serine hydroxymethyltransferase, since the rate of 14CO2 production from [2-14C]glycine was less than 5% of that of [1-14C]glycine, whereas the rate of incorporation of 14C into serine from [2-14C]glycine was double that from [1-14C]glycine. These studies demonstrate that the kidney can synthesize serine by two separate pathways, both located in the cells of the proximal tubule.
Chronic metabolic acidosis augments acidification along the inner medullary collecting duct
Authors: Bengele H. H. , Schwartz J. H. , McNamara E. R. , Alexander E. A.
Published: December 2017

The inner medullary collecting duct (IMCD) of the rat is a major site of acidification. However, previous micropuncture studies have failed to demonstrate acidification along the terminal IMCD during chronic acid feeding. To more completely evaluate this question we used the microcatheterization method in rats fed ammonium chloride for 3-7 days. Arterial pH was 7.30 +/- 0.015, and PCO2 was set at 40 +/- 0.6 mmHg. The IMCD data were analyzed as a function of IMCD length between 40% and the tip. Equilibrium pH decreased from 6.21 +/- 0.11 to 5.47 +/- 0.03, whereas PCO2 was unchanged (28 +/- 1 mmHg between the deep samples and tip). Bicarbonate delivery decreased from 92 +/- 14 to 10 +/- 1 nmol/min, titratable acid increased from 462 +/- 33 to 762 +/- 40 nmol/min, and ammonium delivery increased from 2,235 +/- 121 to 3,528 +/- 140 nmol/min. Thus estimated net acid increased from 2,638 +/- 134 to 4,303 +/- 161 nmol/min. To determine whether increasing delivery of buffer to the IMCD would stimulate acid secretion in acute acidosis, rats were studied during the infusion of HCl and creatinine. Arterial pH was 7.18 +/- 0.02. IMCD acidification was not increased compared with our previously published studies during HCl infusion [Am. J. Physiol. 241 (Renal Fluid Electrolyte Physiol. 10): F669-F676, 1981]. We conclude that chronic ammonium chloride ingestion stimulates IMCD acidification and that this increase may be an intrinsic modification of the acidification mechanism of the IMCD.
Amelioration of hypoxia-induced lactic acidosis by superimposed hypercapnea or hydrochloric acid infusion
Authors: Abu Romeh S. , Tannen R. L.
Published: December 2017

Recent studies have shown that ketoacid production is exquisitely sensitive to changes in systemic pH, with a decrease inhibiting and an increase stimulating the production rate. To determine whether inhibition of net endogenous acid production is a widely applicable mechanism for the defense of acid-base homeostasis, we examined the effect of superimposed acidosis on lactic acid production by hypoxic rats. Anesthetized paralyzed mechanically ventilated rats with normocapnia increased blood lactate progressively in response to a fractional inspired O2 (FIO2) of 8% (PaO2, 35-38 mmHg) and achieved a level of 7.0 +/- 1.2 mM at 3 h. Superimposition of either mild respiratory acidosis (PCO2, 59 mmHg) or exogenous inorganic metabolic acidosis (intra-arterial HCl sufficient to decrease pH from 7.33 to 7.23) after 1 h of hypoxia dramatically diminished the rise in blood lactate. At the end of the third hour, blood lactate levels averaged 1.7 +/- 0.6 mM with superimposed respiratory acidosis and 2.7 +/- 0.4 mM with superimposed metabolic acidosis, both values being significantly less than the hypoxic controls. Termination of the superimposed respiratory acidosis resulted in a rapid increase in blood lactate levels, demonstrating the reversibility of the pH modulation of lactic acid production. Thus systemic acidosis appears to feed back in a protective fashion to inhibit net lactic acid production in rats with hypoxia-induced lactic acidosis. These findings suggest that finely tuned feedback control mechanisms that keep systemic pH within a narrow range operate under both major conditions of enhanced endogenous acid production (i.e., keto- and lactic acidosis).
Atrial natriuretic factor: reduced cardiac content in cirrhotic rats with ascites
Authors: Jimenez W. , Martinez-Pardo A. , Arroyo V. , Gaya J. , Rivera F. , Rodes J.
Published: December 2017

The present study was designed to investigate whether cirrhosis with ascites is associated with altered tissue content of atrial natriuretic factor. Atrial extracts from 14 cirrhotic rats with ascites and increased plasma renin activity (PRA) (14.4 +/- 4.6 ng X ml-1 X h-1) and aldosterone concentration (148.3 +/- 17.3 ng/dl) and from 10 control rats (PRA, 3 +/- 0.5 ng X ml-1 X h-1; aldosterone, 34.7 +/- 3.7 ng/dl) were intravenously injected into anesthetized normovolemic rats. Only one extract was assayed in each bioassay rat. Atrial extracts from control rats increased diuresis and natriuresis 513 +/- 91 and 3,029 +/- 752%, respectively (means +/- SE). In contrast, atrial extracts from cirrhotic rats increased urine volume 199 +/- 49% (P less than 0.001) and sodium excretion 546 +/- 132% (P less than 0.001). These results strongly suggest that atrial content of atrial natriuretic factor is reduced in cirrhotic rats as compared with control animals.
Postischemic ATP-MgCl2 provides precursors for resynthesis of cellular ATP in rats
Authors: Stromski M. E. , Cooper K. , Thulin G. , Avison M. J. , Gaudio K. M. , Shulman R. G. , Siegel N. J.
Published: December 2017

Postischemic administration of ATP-MgCl2 is known to enhance recovery of renal function and accelerate the restitution of cellular ATP levels. To differentiate between a direct and indirect effect of the exogenous nucleotide, rats were subjected to 45 min of bilateral renal ischemia and were infused with either ATP-MgCl2, AMP-MgCl2, or normal saline. The immediate recovery of the cellular ATP was similar in all three groups of animals, whereas the subsequent recovery was accelerated by the infusion of either nucleotide. Since ATP-MgCl2 and AMP-MgCl2 produced similar results, this study provides evidence that exogenous ATP may act by providing precursors for the resynthesis of the cellular adenine nucleotide pool rather than being a direct source of energy.
Osmotic diuresis in a mathematical model of the rat proximal tubule
Authors: Weinstein A. M.
Published: December 2017

Solute reabsorption in the presence of an osmotic load has been examined in a model of the rat proximal convoluted tubule. The model is a computer simulation of a 0.5-cm segment of tubule comprised of compliant cellular and paracellular compartments, which tracks the luminal profiles of Na, K, Cl, HCO3, phosphate, glucose, and urea. In one series of calculations, the peritubular and initial luminal glucose concentrations are varied from 1.0 to 50 mmol/liter. The resulting proximal reabsorption of glucose increases monotonically to 1.5 nmol X s-1 X cm-2. Sodium reabsorption increases with glucose perfusion concentrations between 1.0 and 10 mmol/liter and then declines with greater glucose loads. Above 10 mmol/liter glucose, there is progressive decline in mean luminal Na concentration so that diffusive paracellular backflux, as well as decreased convective reabsorption, are responsible for the natriuresis. Diuresis per se blunts reabsorption of species requiring the development of lumen-to-bath concentration gradients (i.e., K, Cl, and urea). Diminished bicarbonate reabsorption is also predicted with large glucose loads due to intraepithelial alkalinization. This derives both from cellular depolarization and bicarbonate trapping (interspace closure). It is also observed that when interspace closure occurs, a region of intraepithelial K depletion may be formed, promoting diffusive reabsorption of potassium across the tight junction. Thus a 'middle compartment model' for potassium may provide a means of achieving tubule fluid-to-plasma K ratios less than 1.0, in the absence of specific cellular uptake mechanisms.
Tubuloglomerular feedback control of distal fluid delivery: effect of extracellular volume
Authors: Moore L. C. , Mason J.
Published: December 2017

A closed-feedback loop micropuncture method was used to examine tubuloglomerular feedback (TGF) and the regulation of distal fluid delivery in hydropenic rats (CON), moderately hemorrhaged rats (HEM), and rats given desoxycorticosterone (DOC) and 0.6% saline to drink. Distal delivery and TGF response curves were measured with four samples per nephron: a spontaneous early distal collection, two distal collections during moderate (7.5 nl/min) and saturating (30 nl/min) perturbations in nephron fluid load, and a proximal collection to measure single-nephron glomerular filtration rate (SNGFR) during TGF inhibition. Arterial pressure, predistal volume reabsorption, SNGFR, and early distal flow were significantly higher in DOC than in HEM; the CON group exhibited intermediate values. Except for a greater maximum TGF response in HEM, the normalized TGF responses were similar in all three groups, as was the regulation of distal fluid delivery. However, the TGF onset threshold and the TGF operating point, defined by the spontaneous rates of early distal flow and SNGFR, were reset such that distal fluid delivery and SNGFR were higher in DOC than in HEM, as was renal sodium excretion. The results show that the level around which TGF stabilizes distal fluid delivery is reset when extracellular fluid volume is altered.
Effect of respiratory acidosis on intracellular pH of the proximal tubule
Authors: Trivedi B. , Tannen R. L.
Published: December 2017

In contrast to chronic metabolic acidosis, chronic respiratory acidosis does not result in an adaptation in either renal ammonia or glucose production. To examine the possibility that this might be explained by a difference in proximal tubule intracellular pH, the response of two pH-sensitive metabolites, citrate and alpha-ketoglutarate, were assessed. Metabolic acidosis of 3 days duration, induced by drinking 1.5% NH4Cl, significantly reduced urinary citrate excretion (172 to 15 mumol/day) and renal cortical citrate (1.33 to 0.88 mumol/g) and alpha-ketoglutarate (0.90 to 0.46 mumol/g) concentrations in comparison with normal rats. Chronic respiratory acidosis, produced by 3 days in a 10% CO2 environment, lowered systemic pH similar to metabolic acidosis but had no effect on either urinary citrate excretion or renal cortical citrate and alpha-ketoglutarate concentrations. By contrast, acute respiratory acidosis (3, 6, or 24 h duration) reduced urinary citrate excretion and renal cortical citrate and alpha-ketoglutarate concentrations in a fashion similar to acute metabolic acidosis. These data suggest that acute acidosis of either respiratory or metabolic origin lowers the intracellular pH of the proximal tubule. However, when the acid-base abnormality enters the chronic phase, proximal tubular intracellular pH remains low with metabolic acidosis but returns to normal values with respiratory acidosis.(ABSTRACT TRUNCATED AT 250 WORDS)
Ion microprobe determination of bone surface elements: effects of reduced medium pH
Authors: Bushinsky D. A. , Levi-Setti R. , Coe F. L.
Published: December 2017

When neonatal mouse calvariae are cultured in a medium having a low pH they release calcium (Ca) while buffering protons (H). However, for 1 neq Ca released, 16-21 neq H enter the calvariae, arguing strongly against simple dissolution of bone mineral as the mechanism of H buffering. To determine if H for sodium (Na) exchange could explain the lack of stoichiometry between H and Ca, we cultured calvariae for 3 h in control (pH = 7.40 +/- 0.01) or reduced-pH (pH = 7.21 +/- 0.01) medium and then examined the surface Na, K, and Ca using a high-resolution scanning ion microprobe. The calvarial surface was rich in Na and K relative to Ca (Na/Ca, 52 +/- 17; K/Ca, 61 +/- 17; all values are the ratios of counts per second of detected secondary ions, means +/- SE). Compared with the surface the calvarial cross section contained far less Na and K relative to Ca (Na/Ca, 2 +/- 1; K/Ca, 1 +/- 1; both P less than 0.01 vs. surface). Compared with the control surface, culture in a reduced-pH medium reduced the surface Na and K relative to Ca (Na/Ca, 5 +/- 1; K/Ca, 7 +/- 1; both P less than 0.025 vs. surface) to values still greater than the cross section (P less than 0.05 for both). Neonatal mouse calvariae have a surface that is rich in Na and K relative to Ca. Reduced medium pH depletes surface Na and K of cultured calvariae with respect to Ca.
Endotoxemic acute renal failure in awake rats
Authors: Kikeri D. , Pennell J. P. , Hwang K. H. , Jacob A. I. , Richman A. V. , Bourgoignie J. J.
Published: December 2017

The sequence of changes in renal function in endotoxemic acute renal failure (ARF) and the role of hypotension and systemic hemodynamics were evaluated in awake female Sprague-Dawley rats given an intravenous bolus of Escherichia coli endotoxin (20-40 mg/kg). After endotoxin ARF was abrupt in onset as glomerular filtration rate (GFR) fell promptly and progressively by 53% within 3.5 h, whereas renal blood flow decreased by 42% and renal vascular resistance nearly doubled. Systemic hemodynamics remained stable, including mean arterial blood pressure, cardiac output, and total peripheral resistance. Early endotoxemic ARF was associated with oliguria and sodium retention, a finding consistent with intact tubular function. Three and one-half hours after endotoxin, however, fractional water and sodium excretion were significantly increased. Ultrastructural studies then demonstrated sequestration of phagocytic leukocytes and intracellular edema in the peritubular capillaries with normal glomeruli. The decrease in GFR was spontaneously reversible within 7-9 days. Extracellular fluid volume expansion with saline either before or 24 h after administration of endotoxin failed to prevent the decrease in GFR or to normalize renal function. The data suggest that endotoxin has direct renal effects. The endothelial cells may be the primary target of endotoxin in the kidney.
Increased tubuloglomerular feedback activity in Milan hypertensive rats
Authors: Boberg U. , Persson A. E.
Published: December 2017

Studies of whole-kidney function and micropuncture measurements in superficial nephrons were performed to investigate the role of the tubuloglomerular feedback (TGF) in the excretion of salt and water in hydropenic and volume-expanded rats of the spontaneously hypertensive Milan strain (MHS). The rats were 3.5-5 and 5-7 wk old, and age-matched animals from the Milan normotensive strain (MNS) served as controls. There was no difference in mean arterial blood pressure (Pa) between the 3.5- to 5-wk-old prehypertensive MHS (MHSp) and MNS rats, but the glomerular filtration rate (GFR) was higher in MHSp than in MNS [1.35 vs. 0.80 ml X min-1 X g kidney wt (KW)-1, P less than 0.01]. The distal single-nephron glomerular filtration rate (SNGFR) was also higher in MHSp than in MNS (28.6 vs. 20.2 nl X min-1 X g KW-1, P less than 0.05). TGF was determined from both stop-flow pressure response and proximal and distal SNGFR. It was found that MHSp exhibited essentially no TGF response. During development of hypertension 5- to 7-wk-old MHS (MHSd) had a higher Pa than MNS (120 vs. 98 mmHg, P less than 0.01). Normally GFR and SNGFR increase with age, and such was the case with MNS (0.8 to 1.02 ml X min-1 X g KW-1 and 20.2 to 23.4 nl X min-1 X g KW-1), but in MHSd there was a decrease in both GFR and SNGFR with age (1.35 to 1.10 ml X min-1 X g KW-1 and 28.3 to 18.3 nl X min-1 X g KW-1).(ABSTRACT TRUNCATED AT 250 WORDS)
Interaction between epinephrine and renal nerves in control of renin secretion rate
Authors: Kopp U. C. , DiBona G. F.
Published: December 2017

To determine whether the increase in renin secretion rate (RSR) produced by the beta 2-adrenoceptor agonist epinephrine was dependent on intact renal innervation, epinephrine (10 ng X kg-1 X min-1) was infused bilaterally into an innervated and a denervated kidney (ira) of the same anesthetized dog at spontaneous and reduced renal arterial pressure (decreases RAP, 100 mmHg). Epinephrine ira did not affect mean arterial pressure, renal hemodynamics, or urinary sodium excretion of either kidney. At spontaneous RAP epinephrine ira increased RSR from 633 +/- 134 to 926 +/- 137 ng/min in innervated kidneys but did not change RSR in denervated kidneys. decreases RAP in the presence of epinephrine ira resulted in an increase in RSR from 969 +/- 248 to 2,564 +/- 630 ng/min in innervated kidneys, which was greater than that produced in the absence of epinephrine, from 741 +/- 244 to 1,606 +/- 431 ng/min. In denervated kidneys decreases RAP resulted in similar increases in RSR in the absence and presence of epinephrine ira from 41 +/- 15 to 166 +/- 60 ng/min and from 59 +/- 210 to 235 +/- 78 ng/min, respectively. These results demonstrate that the increase in RSR produced by epinephrine is dependent on intact renal innervation at spontaneous and decreases RAP and suggest that epinephrine increases RSR by a prejunctional mechanism. The beta 1-adrenoceptor antagonist metoprolol (0.3-0.5 microgram X kg-1 X min-1 ira) abolished the enhanced RSR response to decreases RAP produced by epinephrine ira. Similarly, the beta 2-adrenoceptor antagonist ICI 118551 (0.005-0.25 microgram X kg-1 X min-1 ira) abolished the enhanced RSR response to decreases RAP produced by epinephrine.(ABSTRACT TRUNCATED AT 250 WORDS)
Regulation of chloride self exchange by cAMP in cortical collecting tubule
Authors: Tago K. , Schuster V. L. , Stokes J. B.
Published: December 2017

The hormonal control of Cl transport was examined in rabbit cortical collecting tubules using the lumen-to-bath 36Cl tracer rate coefficient (KCl, nm/s). Tracer movement via Cl-HCO3 exchange was minimized by using HCO3-CO2-free solutions. The electrical driving force was minimized by treating with amiloride. Under these conditions, net Cl transport was zero, yet there was a large KCl that fell 88% on removing bath (trans) Cl. These results are consistent with the mechanism of tracer flux being predominantly Cl self exchange. KCl fell spontaneously with time in vitro; after this decline KCl could be stimulated with 8-bromo-cAMP. cAMP present from the onset of perfusion prevented the time-dependent fall in KCl. When tracer movement was restricted to diffusion by eliminating Cl self exchange (0 Cl bath), cAMP had no effect on KCl. Although both isoproterenol and vasopressin are known to stimulate adenylate cyclase in this epithelium, only isoproterenol mimicked the cAMP effect on KCl. The isoproterenol effect was blocked by either propranolol or prostaglandin E2. Lumen addition of the disulfonic stilbene DIDS had no effect on KCl. Lumen addition of furosemide or trichloromethiazide had minimal or no effect. Taken together, these results indicate that Cl self exchange is regulated by beta-adrenergic agents acting via cAMP. The lack of an effect of vasopressin suggests cellular heterogeneity in this response to cAMP.
Renal reabsorption and utilization of hydroxybutyrate and acetoacetate in starved rats
Authors: Barac-Nieto M.
Published: December 2017

Are there increases in the renal reabsorption (R) and utilization (Q) of ketoacids during starvation? R and Q of D-3-hydroxybutyrate and acetoacetate were measured in anesthetized normally fed (C) and 3-day starved (S) rats, using clearance and arteriovenous extraction methods. R was nearly complete in both groups at low filtered ketoacids loads (less than 2mumol/g X min). At higher filtered loads, R of the two ketoacids increased in proportion to their filtered loads (r2 greater than 0.95) but with significantly steeper slopes in the S than in the C group; fractional R of the two ketoacids were significantly higher in the S than in the C animals. Saturable and nonsaturable components for ketoacid reabsorption were evident, but only the nonsaturable component was significantly increased with starvation. Increased R of ketoacids during starvation was in the absence of changes in the initial rates of hydroxybutyrate influx into isolated rat renal brush-border membrane vesicles. Thus the rate-limiting step for ketoacid reabsorption through the nonsaturable pathway may not be located at the luminal membrane of the proximal cells. The increases in R of ketoacids in the S group were associated with simultaneous decreases in the net rates of renal utilization of the two ketoacids. Thus increased renal conservation of ketoacids is a transport and not a metabolic event. Overall, during complete starvation, both the increased R and the decreased renal Q of the ketoacids contribute to increase the availability of these metabolites to other organs.
Automatic derivative evaluation in solving boundary value problems: the renal medulla
Authors: Wexler A. S. , Kalaba R. E. , Marsh D. J.
Published: December 2017

Automatic evaluation of derivatives becomes essential when large systems of equations of many variables are to be solved. This paper presents a set of easy-to-use FORTRAN subroutines that perform automatic derivative evaluation. They were used in conjunction with the method of quasilinearization to solve a 13th-order boundary-value problem. This problem has been proposed as a test of numerical methods used to solve models of the renal concentrating mechanism. Quasilinearization gives the same result as has been reported by others with finite difference or multiple shooting methods. The approach described here offers the important potential advantage of being easier to apply to larger problems, which can be anticipated when attempts are made to simulate the three-dimensional structure of the renal medulla.
Phosphate transport across renal proximal tubular cell membranes
Authors: Hammerman M. R.
Published: December 2017

The transport of phosphate across the plasma membrane of the renal proximal tubular epithelial cell is thought to take place through the activities of specific transporters located in the membrane. The activities of these transporters are essential to effect the reabsorption of phosphate present in glomerular ultrafiltrate. In addition, their activities are thought to be important for the maintenance of metabolic functions in the proximal tubular cell. Studies utilizing proximal tubular brush-border and basolateral membranes isolated from mammalian kidney have provided significant insights into the mechanisms by which phosphate transport across the brush-border and basolateral membranes of the intact proximal tubular cell occurs and is modulated. In this editorial review, the results of many of these studies are summarized. Particular emphasis is placed on studies that utilized isolated membranes to determine the mechanism by which the phosphaturic action of parathyroid hormone is mediated in the renal proximal tubule. On the basis of studies conducted in isolated membranes and in more physiologically intact preparations, models are constructed to integrate the role of the brush-border and basolateral membranes in the transport of phosphate into and out of the renal proximal tubular cell.
Protection by thyroxine in nephrotoxic acute renal failure
Authors: Cronin R. E. , Brown D. M. , Simonsen R.
Published: December 2017

Thyroxine (T4) protects against ischemic and nephrotoxic experimental acute renal failure (ARF). This study examined functional, biochemical, and morphological markers of uranyl nitrate (UN)-induced renal injury in the rat to determine the cellular site at which T4 exerts its protective effect. In experimental group UNT4, 1-thyroxine (10 micrograms/100 g body wt) was given for 10 days prior to and for 4 days following a single subcutaneous injection of UN (0.5 mg/kg body wt). Group UN received only UN, and group CT4 received only T4 for 14 days. Five days after UN administration, plasma creatinine rose from base line in group UNT4 (0.52 +/- 0.30 to 0.84 +/- 0.08 mg/dl, P less than 0.025) and group UN (0.52 +/- 0.03 to 1.64 +/- 0.13 mg/dl, P less than 0.001) but not in group CT4 (0.47 +/- 0.02 to 0.48 +/- 0.04 mg/dl, NS). However, plasma creatinine in group UNT4 was significantly lower than group UN (0.84 +/- 0.08 vs. 1.64 +/- 0.13 mg/dl, P less than 0.001). T4 administration stimulated the basolateral membrane-bound enzyme Na-K-ATPase in the renal cortex homogenate in group UNT4 (13.9 +/- 0.5 micron X mg protein-1 X h-1, P less than 0.005) and group CT4 (16.3 +/- 0.6 micron X mg protein-1 X h-1, P less than 0.001) when compared with controls (11.7 +/- 0.5 micron X mg protein-1 X h-1). Na-K-ATPase activity fell in group UN to 10.0 +/- 0.6 micron X mg protein-1 X h-1 (P less than 0.025).(ABSTRACT TRUNCATED AT 250 WORDS)
Na-K-Cl cotransport in apical membrane of rabbit renal papillary surface epithelium
Authors: Sands J. M. , Knepper M. A. , Spring K. R.
Published: December 2017

The renal papillary surface epithelium is exposed to pelvic urine on its apical surface and to inner medullary interstitium on its basolateral surface. To investigate transport in this epithelium, we dissected it free from the renal papilla of rabbits and mounted it in a chamber that allowed both sides to be bathed independently. Cell volume was measured at 25 degrees C utilizing computerized quantitative microscopy. Addition of ouabain (10(-4) M) to the basolateral solution induced a 20% volume increase. This volume increase was completely inhibited by the removal of apical bath NaCl, Na+, K+, or Cl- but not by the removal of urea. Bumetanide, down to 10(-9) M in the apical bath, completely inhibited the ouabain-induced swelling. Changes in apical bath osmolality, resulting from addition or removal of NaCl, caused cell volume changes that were greater than could be accounted for by osmotic water flow alone. This hyperresponse was blocked by bumetanide and was stimulated by vasopressin (10(-8) M). These observations are consistent with the presence of Na-K-ATPase in the basolateral membrane and a bumetanide-sensitive, vasopressin-responsive Na-K-Cl co-transporter in the apical membrane.
Functional significance of exaggerated renal thromboxane A2 synthesis induced by cyclosporin A
Authors: Perico N. , Benigni A. , Zoja C. , Delaini F. , Remuzzi G.
Published: December 2017

Animals and humans undergoing a chronic treatment with cyclosporin A (CyA) show a reduction in glomerular filtration rate (GFR). The cause of this abnormality has not been established. Since CyA interferes with arachidonic acid (AA) metabolism in various cells, we wished to determine whether alterations in renal AA metabolites contribute to deteriorating renal function in rats on CyA. We show that chronic CyA treatment induces a progressive increase in the renal synthesis of thromboxane (TX) A2. This is a selective abnormality in that CyA does not influence the renal synthesis of prostaglandin E2 (PGE2) and prostacyclin (PGI2). A significant negative correlation has been found between TXB2 urinary excretion rate and inulin clearance. No correlation has been observed between TXB2 excretion and p-aminohippuric acid clearance. The withdrawal of CyA is followed by a normalization of both TXB2 urinary excretion rate and GFR. The administration of a selective TXA2 inhibitor, UK-38,485, resulted in a significant reduction in urinary excretion of TXB2 accompanied by a significant increase in GFR. We conclude that chronic treatment with CyA in rats is associated with a selective increase in renal TXA2 synthesis and suggest that this abnormality may play a role in the reduction of GFR.
Citrate transport in rabbit nephron
Authors: Brennan T. S. , Klahr S. , Hamm L. L.
Published: December 2017

Citrate is an important renal metabolic substrate and urinary inhibitor of calcium stone formation. Our purpose was to characterize citrate reabsorption in the rabbit nephron using isolated perfused tubules. Citrate reabsorption, measured by luminal disappearance of [14C]citrate, was found only in the proximal tubule, not in the cortical thick ascending limb or the cortical collecting tubule. In the proximal convoluted tubule, the collected fluid was also analyzed by thin-layer chromatography and by measurements of chemical citrate concentration using an ultramicroassay. Luminal disappearance of [14C]citrate was determined to accurately represent citrate reabsorption; no significant citrate secretion was found. The absolute magnitude of citrate reabsorption was approximately 3.4 pmol X mm-1 X min-1 using either 1 or 3 mM citrate in the perfusate. This rate of citrate reabsorption in the rabbit proximal tubule could account for all of renal citrate reabsorption but was severalfold lower than glucose reabsorption, which was studied for comparative purposes. In contrast, the magnitude of succinate transport (which probably occurs via the same transport system as citrate) was comparable with that of citrate. Citrate reabsorption was inhibited approximately 80% by 10(-5) M ouabain. This characterization of citrate transport in the intact proximal tubule should provide a useful model to study regulation of urinary citrate excretion.
Substrates induce hypoxic injury to medullary thick limbs of isolated rat kidneys
Authors: Brezis M. , Rosen S. , Spokes K. , Silva P. , Epstein F. H.
Published: December 2017

Under certain conditions, excess of substrates may be detrimental to the kidney. In isolated rat kidneys perfused with cell-free medium, oxidative metabolism to support reabsorptive transport in the presence of a limited oxygen supply results in hypoxic injury to medullary thick ascending limbs (mTAL). Since inhibitors of mitochondrial respiration markedly reduced this injury, we evaluated the effects of altering the availability of substrate for oxidative metabolism in the mTAL. Inhibition of glucose utilization with 2-deoxyglucose (50 mM) and simultaneous inhibition of long-chain fatty acid metabolism with 2-tetradecylglycidic acid (10(-4) M) in the absence of exogenous substrates consistently reduced hypoxic cell injury to mTAL. Similarly, the direct inhibition of substrate oxidation by the citric acid cycle with monofluoroacetate (5 mM) also reduced the extent of damage to this nephron segment. Bypassing these metabolic blockades with L-lactate, pyruvate, or alpha-ketoglutarate stimulated renal oxidative metabolism and increased hypoxic damage to mTAL. Enhanced renal metabolism and function (higher renal oxygen consumption, tubular reabsorption of sodium, and glomerular filtration rate) were paradoxically associated with greater damage to mTAL. Thus, when oxygen supply is limited, substrate-supported aerobic metabolic activity for tubular transport may induce hypoxic injury in the renal medulla.
In vitro stimulation of Na-K-ATPase in rat thick ascending limb by dexamethasone
Authors: Doucet A. , Hus-Citharel A. , Morel F.
Published: December 2017

Dexamethasone has been reported to stimulate Na-K-ATPase activity in the medullary thick ascending limb of adrenalectomized animals within a few hours. The present study was aimed at characterizing the mechanism of this action by investigating the stimulatory effect of the hormone in vitro. Dexamethasone (10(-8) M) added in vitro to segments of the medullary thick ascending limb of Henle's loop, which were microdissected from adrenalectomized rats, restored in a dose-dependent manner the depressed Na-K-ATPase activity within one h of incubation. This stimulation of Na-K-ATPase was inhibited by cycloheximide and actinomycin D. Dexamethasone also stimulated the component of oxidative metabolism coupled to sodium transport. These results, which confirm previous in vivo observations, demonstrate that dexamethasone-induced stimulation of Na-K-ATPase is a direct tubular action of the hormone mediated by protein synthesis. They suggest that this short-term effect of dexamethasone corresponds to the stimulation of sodium reabsorption by the dilution segment.
Ca and P regulation of 1,25(OH)2D3 synthesis by vitamin D-replete rat tubules during acidosis
Authors: Langman C. B. , Bushinsky D. A. , Favus M. J. , Coe F. L.
Published: December 2017

Elevated blood ionized Ca concentration appears to be necessary for suppression by chronic metabolic acidosis (CMA) of serum 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] levels in vitamin D-replete rats eating a low-Ca diet (LCD). The present study asks whether in vitro 1,25(OH)2D3 production by proximal tubule cells from such rats is suppressed and whether suppression can be reversed in vitro by an altered ionic milieu. Young rats were fed LCD and given 1.5% NH4Cl in their drinking water for 10 days to produce CMA. Compared with controls, CMA rats had low serum 1,25(OH)2D3 levels. Tubules prepared from CMA rats produced 1,25(OH)2D3 at a low rate compared with control; variation of medium Ca and pH from 7.2 to 7.4 did not increase 1,25(OH)2D3 production. Reducing medium phosphorus concentration ([P]) increased 1,25(OH)2D3 production by tubules from control but not CMA rats. Increasing medium [P] increased 1,25(OH)2D3 production by tubules from CMA but not control rats. CMA appears to alter proximal tubule cell response to medium [P] so that 1,25(OH)2D3 production rises when medium [P] is increased but not when it is reduced. Medium pH and Ca concentration do not appear to be important regulators of renal 1,25(OH)2D3 production in short-term incubations.
ATP turnover and renal response of dog tubules to pH changes in vitro
Authors: Manillier C. , Vinay P. , Lalonde L. , Gougoux A.
Published: December 2017

In vivo the dog kidney responds to metabolic or respiratory acidosis by a marked increment of its ammonia production (expressed per 100 milliliters glomerular filtration rate). This phenomenon is related to a switch from metabolic utilization of nonammoniagenic (lactate) to ammoniagenic (glutamine) substrates to support ATP turnover in the proximal tubules. We have proposed that in vivo the maximum activity of the ammoniagenic process is fixed by the ATP turnover in this segment of the nephron. The maximal glutamine metabolism is reached when 100% of this turnover is supported by glutamine metabolism. We have studied how these concepts apply to the adaptation of glutamine metabolism and ammonia production to a low pH in vitro using proximal tubules of dogs incubated when one (lactate or glutamine) or several (glutamine plus lactate or plus palmitate) substrates are provided. At pH 7.4 glutamine alone supports already 71-76% of the tissue ATP turnover (minimal and maximal values). With acidification this fraction rises to nearly 87-94%, but this increases only modestly the ammonia production. Reducing the ATP turnover with ouabain at pH 7.4 decreases the absolute glutamine utilization, which now supports only 45-50% of the ATP turnover. Again acidification increases this fraction to 90-99%. Addition of lactate with glutamine displaces part of the glutamine metabolized, but greatly stimulates the synthesis of alanine. Fatty acids depress ammonia production and blunt the tissue response to acidification. Gluconeogenesis from lactate is minimally influenced by incubation pH. It is concluded that the ATP turnover limits the metabolism of glutamine by proximal tubules in vitro as in vivo in the dog, and that the response to acidification is small in vitro because of the absence of alternative substrates.
A sodium-hydrogen exchange system in isolated apical membrane from LLC-PK1 epithelia
Authors: Moran A. , Biber J. , Murer H.
Published: December 2017

We have monitored transmembrane pH gradients using acridine orange fluorescence quenching and traced Na+ flux to study the properties of Na+-H+ exchange in apical membrane vesicles isolated from LLC-PK1 epithelia. The membranes have low conductance for Na+, H+, and K+ ions. An outwardly directed K+ gradient in the presence of valinomycin and carbonyl cyanide p-trifluoromethoxyphenyl hydrazone produced intravesicular acidification. This pH gradient was collapsed by addition of extravesicular Na+ or Li+ ions but not by tetramethylammonium. Amiloride (10(-4) M) inhibited the effect of both Na+ and Li+ . An outwardly directed Na+ gradient stimulated H+ influx, which was also inhibited by 10(-4) M amiloride. Membrane short-circuit conditions affected neither Na+ nor H+ flux, consistent with transport mediated by an electroneutral process. The interaction of amiloride and sodium is consistent with noncompetitive inhibition with Ki = 100+/- 10 microM for amiloride and an apparent Km for Na+ of approximately 20 mM. This finding is in agreement with previous studies of intact LLC-PK1 epithelia but differs from observations in brush-border membrane vesicles isolated from kidney proximal tubule in which competitive and mixed inhibition have been reported. These observed differences can be reconciled if two types of Na+-H+ exchange systems exist along the nephron, one with competitive and the other with noncompetitive inhibition, and if only the latter is expressed in the homogeneous cultured cells.
Vasopressin-induced increases of cytosolic calcium in LLC-PK1 cells
Authors: Tang M. J. , Weinberg J. M.
Published: December 2017

LLC-PK1 cells, a permanent cell line of renal tubule origin, which has vasopressin (VP) receptors and an adenosine 3',5'-cyclic monophosphate (cAMP) response to VP were grown to confluence on glass cover slips and loaded for 30-45 min with fura-2. Exposure to fura-2 did not affect cell viability (greater than 99%), K+, or ATP levels. Free cytosolic Ca2+ (Caf) was estimated spectrofluorometrically on washed cover slips. Basal levels averaged 73 +/- 3 nM. Peak Caf levels induced were: 10(-8) M VP - 128 +/- 24 nM, 10(-7) M VP - 301 +/- 51 nM, 10(-6) M VP - 537 +/- 117 nM. Peak Caf after 10(-6) M VP was reached in 42 +/- 5 s, followed by a return toward basal levels. Addition of a second dose of 10(-6) M VP following an initial dose of 10(-6) M VP did not raise Caf. Chelation of medium Ca2+ with ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid just prior to 10(-6) M VP did not reduce the response of Caf (peak of 359 +/- 53). In contrast to VP, 10(-6) M calcitonin and PTH did not significantly increase Caf. The response to 10(-6) M VP was not significantly modified by prior prostaglandin E2 (3 microM) or dibutyryl-cAMP (100 microM). The response to 1-desamino-8-D-arginine vasopressin was less than that to VP. However, studies with VP-receptor antagonists did not allow definitive delineation of receptor type. These data provide evidence for VP-induced increases of Caf via release from intracellular stores in a renal epithelial cell.
Escherichia coli endotoxin injections potentiate experimental ischemic renal injury
Authors: Zager R. A.
Published: December 2017

The purpose of this study was to assess the influence of Escherichia coli endotoxin on the evolution of ischemic acute renal failure (ARF). Rats were subjected to 25 min of bilateral renal artery occlusion (RAO) plus subcutaneous/intraperitoneal injections of either boiled E. coli or purified E. coli endotoxin (1.5 mg). Boiling the E. coli renders them nonviable but leaves endotoxin intact. Ischemic controls were injected with saline. The E. coli injections exacerbated ischemic ARF, doubling the degree of azotemia and increasing the extent of tubular necrosis and cast formation. This occurred without an adverse effect on renal blood flow or blood pressure during the initiation phase or maintenance phase of the ARF. Endotoxin tolerant rats were totally protected against the E. coli-ARF potentiating effect. Neither Staphylococcus aureus nor E. coli with denatured endotoxin adversely affected ischemic injury. Purified E. coli endotoxin reproduced the E. coli-ARF potentiating action. Neither E. coli nor purified endotoxin induced azotemia in the absence of renal ischemia. Conclusion: E. coli endotoxin can exacerbate ischemic renal injury without compromising renal hemodynamics. This action appears to be mediated by an adverse effect of endotoxin on critical subcellular determinants of ischemic tissue injury.
ADH increases apical Na+, K+, 2Cl- entry in mouse medullary thick ascending limbs of Henle
Authors: Molony D. A. , Reeves W. B. , Hebert S. C. , Andreoli T. E.
Published: December 2017

These studies were designed to evaluate the mechanism for the ADH-dependent increase in transcellular conductance (Gc, mS X cm-2), which accompanies hormone-dependent increases in the spontaneous transepithelial voltage (Ve, mV) and in the net rate of Cl- absorption in single medullary thick ascending limbs of Henle (mTALH) isolated from mouse kidney. The total transepithelial conductance (Ge, mS X cm-2) was measured with perfusing solutions containing 5 mM K+, zero Ba2+; Gc was that component of Ge blocked by luminal 20 mM Ba2+, zero K+. In paired experiments, antidiuretic hormone (ADH) increased Gc from 44.5 +/- 5.6 to 58.9 +/- 8.9 mS X cm-2 (delta = 14.3 +/- 5.5; P less than 0.02); however, in the presence of 10(-4) M luminal furosemide, ADH had no significant effect on Gc (delta = 5.0 +/- 4.3; NS). A set of similarly paired measurements together with paired observations on the effects of bath Cl- deletion, permitted an assessment of the effect of ADH on the magnitude of the fall in Gc on bath Cl- removal (delta GClc, mS X cm-2). delta GClc was clearly larger with ADH, 29.6 +/- 4.3, than without ADH, 19.2 +/- 1.0 (delta = 10.4 +/- 4.9; P less than 0.05). However, with luminal furosemide, ADH had no significant effect on delta GClc (delta = 1.7 +/- 4.5; NS). These results indicate that the ADH-dependent increase in Gc is secondary to increased salt entry across the apical membrane. We computed apical (ga, mS X cm-2) and basolateral (gb, mS X cm-2) membrane conductances from the Gc measurements and apical-to-basolateral membrane resistance ratios (Ra/Rb) obtained from cell impalement: the ADH-dependent Gc increase was due to an increase in gb, which was blocked entirely by luminal furosemide. We propose that ADH increases the number of functioning apical membrane Na+,K+,2Cl- transport units, and that gb increases because cell Cl- activity rises and depolarizes the basolateral membrane. Thus the calculated cellular Cl- activity was 16.3 mM without ADH, and 25 mM with ADH.