Acute effects of hydrocortisone on the reabsorption of inorganic phosphate in normal,adrenalectomized and parathyroidectomized rats

Clearance studies have been performed in order to examine the acute effects of hydrocortisone on the reabsorption of inorganic phosphate (Pi) in normal, acutely adrenalectomized (Adr. X) and acutely parathyroidectomized (PTX) rats. Hydrocortisone increased the glomerular filtration rates in all groups, normalized the capacity to excrete water in the Adr. X animals, did not change consistently the fractional reabsorption of sodium in all series and diminished the fractional reabsorption of inorganic phosphate in Adr. X rats from 91.4±6.5 (mean ±S.D.) to 78.2±8.5% (P<0.001). It is concluded that the decrease of the fractional reabsorption of Pi in the Adr. X animals was rather the result of the increased filtered load than a direct effect of hydrocortisone on the Pi transport.This work was supported by a grant from the Deutsche Forschungsgemeinschaft (Fr 239/7)     

Stationary microperfusion study of phosphate reabsorption in proximal and distal nephron segments

Summary Micropuncture studies demonstrate phosphate reabsorption in proximal tubules and between the late proximal and early distal convoluted tubule accessible to micropuncture. To further define the sites of phosphate reabsorption, the stationary microperfusion technique was applied to proximal and distal nephron segments. Phosphate reabsorption was evaluated in superficial loops of proximal tubules, descending segments beyond late proximal tubules accessible to micropuncture, ascending segments up to the point of micropuncture in the distal tubule, and superficial loops of distal tubules of thyroparathyroidectomized rats. Microperfusates of 1.3 or 2.6 nl (100 mmol/l mannitol, 100 mmol/l NaCl,³²P-phosphate and³H-inulin) were injected and then withdrawn after contact times of 2–108 s. Phosphate recovery relative to that of inulin was determined. A steep exponential decline of phosphate recovery (R) with increasing contact time (t) was observed in the superficial proximal tubule and descending segments. The slopes of the logarithmic regressions (¹⁰logR)/t, ±SEM) were: –1.68±0.33 and –1.21±0.24 min^–1 in superficial proximal tubules and descending segments respectively. In contrast, no significant decline in phosphate recoveries (–0.02±0.04 and +0.11±0.10 min^–1) was apparent in the ascending segments and distal tubule. It is concluded that phosphate is reabsorbed in the proximal convoluted tubule and adjacent descending segments of the superficial nephron and that there is no significant phosphate reabsorption in distal convoluted tubules and adjacent ascending segments.Study carried out during a one year fellowship at the Mayo Clinic. Present address: Institut für Physiologie, Fritz-Pregl-Strasse 3, A-6020 Innsbruck, Austria     

Chronic thyroparathyroidectomy and tubular handling of phosphate

The fractional reabsorption (FR) of inorganic phosphate (Pi) along the proximal tubule depends upon both the filtered load of Pi (FLPi) and the tubular reabsorptive capacity of the Pi transporting system. To assess the actual effect of parathyroid hormone on the reabsorptive capacity only, the influence of Pi load has to be eliminated. In this study FRPi was determined by free-flow micropuncture along superficial nephrons of chronically (48 h) thyroparathyroidectomized (TPTX) and pair-fed sham-operated (SHAM) rats at identical FLPi [TPTX 3.07±0.14 (n=26) and SHAM 3.07±0.11 (n=26) mol/min±SEM]. The micropuncture results indicate that in the ranges of tubular fluid over plasma inulin concentration [TF/P)_In] 1.00–1.49 and 1.50–1.99, no difference in FRPi between TPTX and SHAM could be detected. It is only between a TF/P_In of 2.0 and 2.49 that chronic TPTX resulted in a significant increase in FRPi. Accordingly the present study indicates that chronic TPTX increases FRPi in late but not in early portions of proximal tubule. Thus in the early proximal tubule the tubular reabsorptive capacity of the Pi transporting system appears to be unaffected by chronic removal of the parathyroid glands. From this result it can be inferred that the increased plasma concentration of Pi which follows the removal of the parathyroid glands, particularly in the chronic stage, will lead to an apparently paradoxical decrease in FRPi in early proximal tubule as a mere consequence of the increased filtered load of Pi.     

Micropuncture study on the effects of lithium on proximal and distal tubule function in the rat kiney

Micropuncture studies were performed in rats infused with LiCl to induce stable plasma lithium concentrations of 2–3 mEq/l, or with an equivalent amount of NaCl. In free flow experiments LiCl reduced proximal tubule fractional reabsorption of sodium and potassium. Reduced reabsorption of bicarbonate, as reflected by a decrease in TF/P_cl, was also observed. Proximal fractional reabsorption of chloride, however, was not affected. The TF/P_In at the end proximal tubule was 2.6±0.2 (mean ±SEM) in controls and 2.1±0.1 in experimental animals (P<0.025). In the distal portions of the nephron lithium treatment caused a fall in fractional reabsorption of water and sodium, while potassium secretion was stimulated in the distal tubule.Previous studies have indicated that lithium influences antidiuretic hormone stimulated water transport in the collecting duct. These experiments demonstrate that lithium also affects the transport of water and electrolytes in multiple nephron segments, including the proximal and distal convolution.     

Postprandial changes of renal blood flow

Summary The standing droplet method has been used in combination with the peritubular perfusion of blood capillaries to determine the build up of transtubular concentration differences of phosphate (P _i ) in the renal proximal convoluted tubule of parathyroidectomized rats. Electron probe analysis was used to estimate P _i . At zero time both the intraluminal and the contraluminal P _i concentration was 2 mM. The time dependent decrease of the intraluminal P _i concentration was approximately 4 times faster in the early than in the late proximal convoluted tubule. After 45 sec an intraluminal steady state concentration of 0.20 mM P _i was achieved in the early part. In the late part the intraluminal P _i concentration approached a steady state value of 0.54 mM at 120 sec. When sodium free solutions were used the intraluminal P _i concentration increased to 2.22 mM in the earlier and to 2.76 mM in the late part. The data indicate that in the proximal convoluted tubule 1. The rate of phosphate reabsorption is greater in the early part than in the later part, and 2. phosphate reabsorption might occur as co-transport with Na⁺ ions.     

Micropuncture study of renal phosphorus transport in hypophosphatemic vitamin D resistant rickets mice

Summary A micropuncture study of inorganic phosphorus (Pi) transport was performed in 6 mice presenting hypophosphatamic vitamin D resistant rickets (Hyp) and results compared to those obtained in 13 normal (N) mice. The mean plasma (P Pi) and fractional excretion of Pi (FE Pi) in N and Hyp mice were 85.10±2.27 mg/l and 15.81±1.90 vs 48.43±3.29 mg/l and 35.34±5.26%, respectively.In N mice, tubular fluid over plasma Pi ratio (TF/P Pi) progressively decreases along the proximal tubule to reach approximately 0.6 in the late accessible part. The fraction of filtered Pi (% E Pi) remaining in this segment of the nephron is therefore 20–25%. In Hyp mice the TF/P Pi in proximal tubule remains relatively high and does not significantly vary with TF/P inulin (mean TF/P Pi=1.19±0.12 S.E., compared to 0.73±0.04 in N mice). Precise conclusions concerning % E Pi at the end of the proximal tubule of Hyp mice are not available because of the scattering of the data. However, all the values of % E Pi (mean % E Pi: 78.68±7.39 compared to 49.10±4.59 in N mice) are far above the total urinary FE Pi (35.34 ±5.26), suggesting a relatively greater distal fractional Pi reabsorption in Hyp mice than in N mice. Since the P Pi, and therefore the amount of filtered Pi is lower in Hyp mice, it is probable that the absolute amount of Pi reabsorbed distally is comparable in the two series of animals.This work was supported by the Medical Research Council of Canada, grant MA 4472 and by special funds from the University of MontrealResearch fellow in renal physiology, supported by the conseil de la recherche en santé du Quebec     

Expression and insights on function of potassium channel TWIK-1 in mouse kidney

Renal distribution and function of TWIK-1, a member of the two-pore-domain potassium channel family, was studied in mouse kidney. TWIK-1 is expressed in apical and subapical localizations of proximal tubule and cytoplasmic sites of thin and thick ascending limbs, distal convoluted tubules and medullary collecting duct. Studies in mice lacking intact TWIK-1 (twik-1 –/–) and wild-type mice (twik-1 +/+) revealed an attenuated ability to increase renal phosphate (Pi) reabsorption and stabilize plasma Pi concentration in response to a low Pi diet in twik-1 –/– mice. Western blot analysis and immunohistochemistry for the electrogenic 3Na⁺-1HPO₄^2–-cotransporter NaPi-2a revealed a reduced reno-cortical expression in twik-1 –/– mice under these conditions. Under normal diet, twik-1 –/– mice presented lower urinary flow rates. Acute pharmacologic blockade of the vasopressin V₂-receptor revealed both an attenuated diuretic response and an attenuated internalization of aquaporin-2 in the inner medullary collecting duct in twik-1 –/– versus +/+ mice. In summary, mice deficient for TWIK-1 presented impaired regulation of (i) Pi transport in proximal tubule and (ii) water transport in medullary collecting duct. TWIK-1 may contribute to membrane trafficking/expression of transport molecules in proximal tubule and medullary collecting duct, and possibly other renal sites of expression.X. Nie and I. Arrighi have contributed equally to this work.     

PTH independent sex difference in renal handling of inorganic phosphate in the rat: effect of oophorectomy

In order to evaluate the sex difference in the renal handling of inorganic phosphate (Pi) in the rat we performed clearance experiments using intact, thyroparathyroidectomized (TPTX), oophorectomized (OophX) and orchiectomized (OrchX) rats. During stepwise elevation of the Pi concentration in plasma (Pi-titration) to about 6 mmol/l the reabsorptive mechanism of Pi was saturated. The ratio Pi-reabs./GFR in intact males was higher than in females. A significant difference in this parameter was also observed in thyroparathyroidectomized rats: in females this value was 3.47±0.13, and in males it was 4.54±0.37 mol/ml (P<0.001). Oophorectomy in the absence of parathyroid hormone (PTH) increased Pi-reabs./GFR from 3.18±0.36 to 4.12±0.24 mol/ml (P<0.001); however, orchiectomy did not significantly change the reabsorption of Pi. In conclusion, the present results demonstrate a PTH independent sex difference in the renal handling of inorganic phosphate and are consistent with the hypothesis that estrogens may play a dominant role in this differentiation.This work was supported by a grant from the Deutsche Forschungsgemeinschaft (Fr 239/9-1)Dedicated to Professor Dr. Karl Julius Ullrich     

Renal handling of calcium and phosphate

Summary In glomerular filtrate, calcium concentration (UF) is some 50–70% of total plasma calcium concentration. About 60% of filtered calcium is reabsorbed in the proximal convoluted tubule, some 15% in pars recta, 15% in the thick ascending limb, and 9% in the distal convoluted tubule including the granular portion of the cortical collecting duct. About 1% is excreted. Approximately 2/3 of proximal tubular calcium transport is passive, driven by a chemical (TF/UF=1.1) and electrical (2 mV, lumen positive) gradient; 1/3 is active, whereby calcium enters the cell at the brush border membrane by passive diffusion and is pumped out at the basolateral membrane in exchange for sodium. Transport in distal convoluted tubules is entirely active and operates both against a chemical (TF/UF 0.3–0.7) and electrical (10–70 mV, lumen negative) gradient. Although saturability of transport is not detectable in microperfusion studies, enhancement of plasma calcium leads to calciuria. Factors stimulating calcium reabsorption are parathyrin, 1.25(OH)₂D₃, thiazides, and alkalosis. Factors inhibiting calcium reabsorption are calcitonin, growth hormone, thyroid hormone, chronic application of mineralo- and glucocorticoids, insulin, glucose, acidosis, sodium infusion, acetazolamide, furosemide, and mannitol.Phosphate concentration in ultrafiltrate is some 90% of total plasma concentration. In intact animals about 2/3 of filtered phosphate is reabsorbed in proximal convoluted tubules, and some 20% is excreted in urine. Shortly after thyroparathreoidectomy (TPTX), 80% of filtered phosphate is reabsorbed in the proximal convoluted tubule, 15% in the pars recta, and 2% is excreted in the urine. Thus, less phosphate is excreted in urine (20% in intact, 2% in TPTX animals) as is recovered in late distal convoluted tubules of superficial nephrons (35% and 5%, respectively). The discrepancy is at least partially due to more avid reabsorption in deep nephrons. Furthermore, evidence exists in favor of phosphate reabsorption in the arcades or collecting duct. Phosphate reabsorption in proximal convoluted tubule and pars recta is active. Uptake of phosphate at the brush border membrane is uphill and is driven by coupling with two sodium ions, whereas exit at the basolateral membrane may be entirely passive. At increasing plasma phosphate concentrations, reabsorption is saturated and excess filtered phosphate excreted. Factors stimulating phosphate transport are phosphate depletion, acute 1,25(OH)₂D₃, thyroxin, growth hormone, magnesium, lithium, and metabolic acidosis. Factors inhibiting phosphate transport are high phosphate or high calcium diets, parathyrin, calcitonin, chronic 1,25(OH)₂D₃, hypocalcemia, magnesium deficiency, respiratory acidosis, metabolic alkalosis, glucose, colchicine, NaCl-infusions, thiazides, furosemide, ethacrynic acid, mannitol, and acetazolamide.     

The relation between the reabsorption of urea and of water in the distal tubule of the rat kidney

Summary Micropuncture studies on rats with hereditary hypophysial diabetes insipidus were undertaken to examine the effects of water and osmotic diuresis and dietary protein on renal tubular urea handling.The results show that in a massive water diuresis the urea present at end of the proximal tubules is practically quantitatively exreted in the final urine. In osmotic diuresis an ever increasing amount of urea reaches the end of the proximal tubule, sinceTF/P _urea of 1.31±0.16 in water diuresis remains constant in osmotic diuresis, whilstTF/P _inulin falls from 2.41±0.21 in water diuresis to values lower than 2.00 in osmotic diuresis. In this case fractional amounts of urea of the same order as in the proximal tubule were measured in the distal convolution and in the final urine. A value of 0.99±0.17×10^–4 mm/sec for the distal tubular urea permeability was obtained by direct measurement at high plasma and high intratubular urea concentrations and did not change significantly after pretreatment on low or high protein diet (1.00±0.20 and 1.05±0.23×10^–4 mm/sec, respectively). From these results we conclude: 1. The magnitude of the urea excretion in a massive diuresis is primarily determined by the water reabsorption in the distal nephron. 2. The urea permeability of the distal convolution is independent of the concentration thus excluding the suggested possibility that there exists a reabsorption mechanism for urea autonomous of water reabsorption. 3. The fall in fractional urea excretion in animals on low protein diet is due entirely to mechanisms located in the collecting duct.Supported by the Deutsche Forschungsgemeinschaft.     

Maleic acid-induced reabsorptive dysfunction in the proximal and distal nephron

Micropuncture and clearance studies were performed to assess reabsorptive function in the proximal and distal nephron of rats with experimental Fanconi's syndrome induced by maleic acid. Anesthetized rats were studied by free-flow micropuncture of the late proximal tubule 90-120 min after continuous intravenous administration of maleic acid, 100 mg X kg-1 X h-1. Compared with control rats, the reabsorption of sodium and phosphate was significantly reduced (P less than 0.001 and less than 0.02, respectively). Tubular fluid-to-ultrafiltrate (TF/UF) chloride concentration ratio was 1.00 +/- 0.02 compared with 1.16 +/- 0.03 (P less than 0.01) in controls, suggesting a nearly total inhibition of proximal bicarbonate reabsorption. Whole kidney fractional excretions of sodium and chloride were increased significantly (P less than 0.02) but could not be explained by enhanced delivery of these solutes out of the late proximal tubule. To assess whether distal nephron reabsorption of sodium and chloride were inhibited by maleic acid, clearance studies were performed during water diuresis in awake rats. During maleic acid administration, 200 mg X kg-1 X h-1, urine flow rate (P less than 0.02) and the fractional excretions of sodium and chloride (P less than 0.001) increased significantly, but fractional free water clearance decreased from 7.16 +/- 0.42 to 4.03 +/- 0.68% (P less than 0.001). In acetazolamide-treated control rats but not in maleic acid-treated rats with similar bicarbonaturia, the magnitude of fractional free water clearance closely approximated the simultaneously measured fractional distal delivery of chloride. These studies suggest that maleic acid inhibits reabsorption at a distal nephron site or sites as well as in the proximal tubule.     

Modulation of the effect of arginine-vasopressin on water and ion transport in the newt early distal tubule and frog urinary bladder by V1-antagonists

In the early distal tubule of the newtTriturus vulgaris L., 1 nM arginine-vasopressin (AVP) increased water reabsorption; the fractional reabsorption of Na⁺ was elevated from 46.2±6.9% to 67.8±3.9% (P<0.001), of Cl^– from 52.7±6.7% to 73.1±3.5% (P<0.001), of Mg²⁺ from 48.0±7.7% to 71.7±6.3% (P<0.001). When V₁-receptors were blocked by 1 nM peptide V₁-antagonist [1-(-mercapto-,-cyclopentamethylene propionic acid), 2-(O-methyl) Tyr]-[Arg⁸]vasopressin, 1 nM AVP increased the fractional reabsorption of fluid by 8.9% of Na⁺ by 10.7% and of Cl^– by 11.2%, as compared with the effect of AVP alone. The fractional reabsorption of Ca²⁺ after addition of AVP did not differ from control; when V₁-receptors were blocked in the presence of AVP, the fractional reabsorption of Ca²⁺ was increased by AVP. The V₁-receptor block in the presence of AVP did not change the fractional reabsorption of Mg²⁺. Experiments on the urinary bladder of the frogRana temporaria L. showed that 1 nM SR 49059, a non-peptide antagonist of V_1a-receptors, like the peptide V₁antagonist, enhanced the AVP effect by 29%. Inhibition of protein kinase C activity by calphostin C (1 nM) mimicked the effect of V₁-antagonists; the AVP hydroosmotic effect was increased by 60%. The results obtained indicate that V₁-receptors modulate the effects of V₂-receptor activation: their block is accompanied by an enhancement of the AVP hydroosmotic effect in the frog urinary bladder and by an increase of Na⁺ and Cl^– reabsorption in the newt early distal tubule. The enhancement of the AVP effect owing to the V₁-receptor activation seems to be mediated by a decrease in protein kinase C activity.     

Reabsorption of inorganic phosphate in the rat kidney

Summary Clearance methods were employed to study reabsorption of inorganic phosphate in the rat kidney.Plasma inorganic phosphate concentration was raised stepwise by phosphate infusions. Reabsorption of inorganic phosphate increased and finally reached a maximal value (Tm_{PO 4}), which, however, decreased during further phosphate infusions to about 60%. This decline of Tm_{PO 4} during phosphate infusions could be identified as the result of the phosphate loadper se rather than of the volume load. No evidence was obtained for net secretion of inorganic phosphate.In experiments at endogenous phosphate levels, fractional reabsorption of inorganic phosphate was decreased by expansion of extracellular fluid volume from 86% in the control to 75% (P<0.001). A positive correlation was found between fractional reabsorption of water and fractional reabsorption of inorganic phosphate, which suggests a common link between sodium and inorganic phosphate reabsorption at different degrees of extracellular fluid volume expansion.This work was supported in part by a grant from the Deutsche Forschungsgemeinschaft.     

Parathormone as a mediator of inorganic phosphate diuresis during saline infusion in the rat

Summary Normal rats were infused with isotonic saline at 0.50 ml/min for 2 hours in order to expand their extracellular fluid volume. Under these conditions fractional excretion of inorganic phosphate was found to be as high as 38.8±3.0% of filtered phosphate, while fractional sodium excretion was 12.9±0.7% of filtered sodium. The combined addition of calcium and magnesium to the infusion solution decreased inorganic phosphate excretion significantly (P<0.001) to 11.2±3.6% (presumably by inhibiting parathyroid gland activity), while sodium excretion was unchanged (13.5±1.1%). Parathyroidectomized rats were infused with isotonic saline at 0.50 ml/min to achieve a similar extent of extracellular fluid volume expansion as in the normal rats. In these animals inorganic phosphate excretion was as low as 0.9±0.9% of filtered phosphate, while sodium excretion was 11.8±2.2% of filtered sodium. Administration of parathormone to volume expanded parathyroidectomized rats resulted in marked increases or inorganic phosphate excretion to 41.5±3.1% of filtered phosphate (P<0.001), while sodium excretion remained unaltered (12.0±2.8% of filtered sodium), thus resembling very closely the results in normal volume expanded rats.From these results it is concluded, that saline induced phosphaturia in normal rats is mediated primarily by parathormone. Furthermore, sodium excretion during volume expansion of extracellular fluid appears to be independent of inorganic phosphate excretion and independent of changes in parathyroid activity.This work was supported in part by a grant from the Deutsche Forschungsgemeinschaft.     

Renal handling ofl-histidine studied by continuous microperfusion and free flow micropuncture in the rat

Renal tubular reabsorption ofl-histidine (His) was measured in vivo et situ by continuous microperfusion and free flow micropuncture of single proximal convoluted tubules of the rat kidney. The reabsorption is shown to be saturable. A permeability coefficient (P) of <29 m² · s^–1, a maximum reabsorption rate (J _max) of 2.75±1.05>J _max>1.97±0.86 (SEM) nmol · m^–1 · s^–1 and an affinity constant (K _m) of 13.8±4.2>K _m>10.9±4.0 (SEM) mol · l^–1 (lower values forP=29 m² · s^–1, higher values forP=0) were calculated from the microperfusion data. Using these constants and taking backflux of His and water reabsorption into account a good fit with the concentration profile of His along the proximal tubule — measured by free flow micropuncture — was obtained.Varying the buffered pH-values of the perfusion fluids (5.0 or 7.4) influenced neither the active reabsorption nor passive permeability of His. This indicates that the charge of the imidazol group of His does not play a significant role in His reabsorption. Further experiments showed that the addition of 20 mmol · l^–1 l-arginine — a strong inhibitor of the reabsorption system for dibasic ammino acids — did not have a significant effect on the reabsorption ofl-histidine. It is concluded, therefore, that His is reabsorbed by a system for neutral amino acids. Non ionic diffusion does not play an important role for His reabsorption.Part of this work was presented at the 51st meeting of the German Physiological Society in Kiel, 1979 [15]     

Carbonic anhydrase independent bicarbonate reabsorption

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

Experssion of chronic thyroparathyroidectomy on phosphate transport in whole kidney and proximal luminal membranes during phosphate deprivation

Both parathyroid hormone (PTH) and the dietary supply of inorganic phosphate (Pi) influence the tubular Pi reabsorptive capacity. In this study we have assessed how removal of parathyroid gland affects the response of Pi transport to Pi restriction in the whole kidney and in brush border membrane vesicles (BBMV) isolated from the renal cortex. Intact and thyroparathyroidectomized (TPTX) rats were switched from normal to low Pi diet. The maximal tubular Pi reabsorption per volume of glomerular filtrate (max. TRPi/ml GF) and the initial (3, 6 and 9 s) Na⁺-dependent Pi uptake by BBMV were determined before and after 1, 2, 3 and 8 days of Pi restriction. The results indicate that before Pi restriction max. TRPi/ml GF was 1.94±0.04 and 4.03±0.14 mol/ml GF in intact and TPTX rats respectively. The corresponding Na⁺-dependent Pi uptake (3 s value) was 137±23 in sham and 272±41 pmol/mg protein in TPTX rats. During Pi restriction the initial difference in max. TRPi/ml GF remained constant throughout the time course study. At 8 days max. TRPi/ml GF was in intact 3.08±0.09 and in TPTX 5.04±0.18 mol/ml GF. In sharp contrast, in BBMV the initial difference between intact and TPTX became completely abolished 8 days after starting the Pi restriction (SHAM=425±69; TPTX=434±pmol/mg protein). In conclusion, the overall tubular Pi transport capacity and the Na⁺-dependent Pi transport system located in the luminal membrane of the proximal tubule appear to be quite differently affected by changes in the PTH status during Pi restriction.     

Regulation of phosphate transport in proximal tubules

Homeostasis of inorganic phosphate (P_i) is primarily an affair of the kidneys. Reabsorption of the bulk of filtered P_i occurs along the renal proximal tubule and is initiated by apically localized Na⁺-dependent P_i cotransporters. Tubular P_i reabsorption and therefore renal excretion of P_i is controlled by a number of hormones, including phosphatonins, and metabolic factors. In most cases, regulation of P_i reabsorption is achieved by changing the apical abundance of Na⁺/Pi cotransporters. The regulatory mechanisms involve various signaling pathways and a number of proteins that interact with Na⁺/P_i cotransporters.     

Glucagon inhibits water and NaCl transports in the proximal convoluted tubule of the rat kidney

The effects of glucagon on water and electrolyte transport in the kidney were investigated on hormone-deprived rats, i.e. thyroparathyroidectomized diabetes insipidus Brattleboro rats infused with somatostatin. Glucagon consistently inhibited the reabsorption of water and Na⁺, Cl^–, K⁺ and Ca²⁺ along the proximal tubule accessible to micropuncture, leaving the reabsorption of inorganic phosphate (P_i) untouched. In the loop, besides its previously described stimulatory effects on Na⁺, Cl^–, K⁺, Ca²⁺ and Mg²⁺ reabsorption, glucagon strongly inhibited P_i reabsorption, very probably in the proximal straight tubule. These effects resulted in a significant phosphaturia and considerable reductions of Mg²⁺ and Ca²⁺ excretions. The effects of glucagon at both the whole kidney and the nephron levels are very similar to those previously described for calcitonin. In the absence of an adenylate cyclase system sensitive to glucagon and calcitonin in the rat proximal tubule, and from the analogy of their physiological effects with those elicited by parathyroid hormone, it is suggested that glucagon and calcitonin exert their inhibitory effects on Na and P_i reabsorption in the proximal tubule through another pathway, which could be the phosphoinositide regulatory cascade.     

Effect of saline infusion on net reabsorption of endogenous sulfate relative to that of phosphate in intact and thyroparathyroidectomized rats

Clearance experiments have been performed to study the effects of saline infusion on the reabsorption of inorganic sulfate (SO₄) at endogenous levels. Adult female Sprague-Dawley rats on a standard diet were used. Both intact and thyroparathyroidectomized (TPTX) animals were infused with a 130 mmol/l sodium chloride solution at a low (0.15 ml/min) and a high (0.375 ml/min) rate. This increase of the infusion rate decreased the reabsorption of SO₄ in both groups of animals significantly. The fractional excretion of SO₄ in theintact rats increased from 9.9±5.6 to 18.4±3.6% (mean values±SD,p<0.001) and in theTPTX rats from 5.3±2.5 to 22.4±6.3% (p<0.001). It is concluded that endogenous parathyroid hormone has no major effect on the saline-induced inhibition of reabsorption of SO₄.This work was supported by a grant from the Deutsche Forschungsgemeinschaft (Fr 239/9-1)