Omission of luminal potassium reduces cellular chloride in early distal tubule of amphibian kidney.
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Animals↗
Biomedical subjects
H Oberleithner
Publications and source records attributed to H Oberleithner.
Chloride activity in cells of isolated perfused cortical thick ascending limbs of rabbit kidney.
Rabbit cortical thick ascending limb segments were perfused in vitro, and intracellular Cl- activity was estimated in three types of experiments using conventional and chloride selective microelectrodes. In series 1 Ringer like solutions were present on the two epithelial sides. In series 2 limen Cl- was replaced by gluconate, and in series 3 furosemide, 10-20 . 10(-6) mol . 1-1, was added to the lumen perfusate. It was found that under control conditions intracellular Cl- activity, as estimated from the difference of the reading of the conventional (n = 53) and ion selective electrodes (n = 118) was 26 +/- 1 mmol . 1-1. Thi value is approximately three times higher than expected for passive distribution of Cl-. After removal of lumen Cl- (series 2) intracellular Cl- activity fell to 9 mmol . 1(-1) which is only some 4 mmol . 1(-1) above passive distribution. We argue that these 4 mmol . 1(-1) reflect mainly the interference with the Cl- electrode by other anions, such as phosphate. The above estimates for intracellular Cl- activity, have to be diminished by these 4 mmol . 1(-1), and, thus, are close to 22 mmol . 1(-1). In series 3 a rapid and reversible fall in intracellular Cl- from 23 to 7 mmol . 1(-1) was observed. We conclude that the Cl- activity in cTAL cells is clearly above equilibrium under control conditions and that it falls rapidly to values close to equilibrium when Cl- reabsorption is blocked by either removing lumen Cl- or by blocking the Cl- entry via the Na+-2 Cl--K+-carrier with furosemide.(ABSTRACT TRUNCATED AT 250 WORDS)
Animals↗
Evidence for an amiloride sensitive Na+ pathway in the amphibian diluting segment induced by K+ adaptation.
The effect of amiloride on cell membrane potentials and intracellular Na activity (Nai) was tested in early distal tubules of the isolated perfused kidney of control and of K-adapted (high-K diet) Amphiuma. Conventional and Na-sensitive liquid ion-exchanger microelectrodes were employed to measure the peritubular cell membrane potential (PDpt), the transepithelial potential difference (PDte) and the Na electrochemical gradient across the peritubular cell membrane (ENapt), in the absence and the presence of amiloride (1 X 10(-4) mol X 1(-1] in both groups of animals. Amiloride did not affect PDpt and ENapt in control animals but depolarized PDpt and ENapt by about 8 mV in K-adapted animals. Nai (11.0 +/- 0.6 mmol X 1(-1) in early distal cells of control animals) did not change significantly by this maneuver. However, Nai decreased to extremely low values (2.3 +/- 0.2 mmol X 1(-1] when the luminal cotransport system for Na, Cl and K was inhibited by the luminal application of furosemide (5 X 10(-5) mol/l) and when the luminal cell membrane was exposed simultaneously to amiloride. The amiloride-induced effects on PDpt, ENapt and Nai occurred within seconds and were fully reversible. We conclude that high-K diet (K adaptation) induces an amiloride-sensitive pathway in the luminal cell membrane of early distal cells of Amphiuma which exists in parallel with the furosemide-sensitive cotransport system located in this cell barrier. The results suggest a luminal amiloride-sensitive Na/H exchange mechanism which regulates the luminal K permeability.
Adaptation, Physiological↗
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 X 10(-6) cm2/s for influx and 1.6 +/- 0.4 X 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.
Absorption↗
Cellular Mechanism of the furosemide sensitive transport system in the kidney.
Experiments were performed in the distal tubule of the doubly-perfused kidney of Amphiuma to determine active and passive forces, involved in the transport processes of potassium, sodium and chloride. Ion-sensitive microelectrodes and conventional microelectrodes were applied to estimate intracellular ion activities, cell membrane potentials and net flux of potassium and chloride under control conditions and during inhibition of active transport. Sodium chloride cotransport, located in the luminal cell membrane is postulated, based on the following observations: Total omission of sodium from the tubular lumen inhibits furosemide sensitive chloride reabsorption, decreases the lumen positive transepithelial potential difference and leads to a dramatic decrease of intracellular chloride. The experiments further suggest that potassium ions are involved in the sodium chloride transport system because potassium reabsorption is inhibited by furosemide and because intracellular sodium falls significantly when potassium ions are removed from the tubular fluid. Furthermore, there is experimental evidence that the luminal potassium uptake mechanism is suppressed after potassium adaptation. Under these conditions potassium transport is found to be insensitive to furosemide. The data suggest a furosemide sensitive cotransport system for sodium, chloride and potassium, operative in the luminal cell membrane. The energy for this carrier-mediated transport process is provided by the large "downhill" gradient of sodium across the luminal cell membrane which is maintained by the sodium pump located in the peritubular cell membrane.
Animals↗
Effects of inhibition of chloride transport on intracellular sodium activity in distal amphibian nephron.
Previous experiments had demonstrated that cell chloride activities in early distal tubule cells of Amphiuma are above equilibrium distribution. Chloride activities fell sharply towards electrochemical equilibrium following perfusion of the tubular lumen with furosemide or with sodium-free solutions. These results suggested a furosemide-sensitive sodium chloride cotransport system in the luminal cell membrane. The present experiments were carried out to evaluate directly the electrochemical driving forces acting on sodium ions under similar experimental conditions. Intracellular sodium activity measurements were performed in the doubly-perfused kidney of Amphiuma by means of single-barreled liquid ion-exchange microelectrodes. Basolateral cell membrane potential and resistance ratio measurements of tubular cell membranes were also carried out under control conditions and after inhibition of chloride transport by luminal application of furosemide (5 . 10(-5) mol/l) or by omission of chloride. Control conditions were characterized by a steep downhill electrochemical gradient for sodium ions from lumen to cell. Inhibition of chloride transport led to a sharp decrease of intracellular sodium activity and to hyperpolarization of the peritubular membrane potential while the resistance ratio of the tubular cell membranes did not change significantly. These results demonstrate the presence of low cellular sodium activities in early distal tubule cells. The sharp decline of cell sodium after furosemide and after luminal chloride removal is consistent with inhibition of a sodium chloride cotransport system and continued peritubular sodium extrusion. The latter can increase the electrochemical gradient of sodium ions beyond that observed under control conditions.
Animals↗
The effect of respiratory and metabolic acid-base changes on ionized calcium concentration: in vivo and in vitro experiments in man and rat.
Correlation of ionized calcium concentration, [Ca2+] and blood pH has long been recognized. So far no distinction of the acid-base changes, i.e. respiratory changes or metabolic changes seemed necessary. The present study, with the use of a recently developed system for in vivo analysis of [Ca2+], and with in vitro experiments reinvestigates this question. In a first series respiratory and metabolic changes were induced in rats. Changes of [Ca2+] (delta [Ca2+]) and of plasma pH (delta pH) were recorded continuously in vivo, plasma bicarbonate, [HCO-3] was measured in vitro. In a second series respiratory and metabolic changes were induced in sixteen volunteers and, separately, in vitro in plasma and modified Ringer solution, and the same parameters were determined. In all experiments delta [Ca2+] correlates negatively with delta pH. However, the correlation in respiratory changes was significantly less as compared to that in metabolic changes. As expected, delta [HCO-3] correlates positively with pH in metabolic and negatively in respiratory changes. We conclude from these experiments that in metabolic changes the effects of calcium-albumin interaction and calcium complexation with bicarbonate are additive, whereas both effects oppose each other in respiratory changes. This might explain the blunted effect of pH changes on [Ca2+] in respiratory changes.
Acidosis↗
Mechanism of distal tubular chloride transport in Amphiuma kidney.
To characterize the mechanism of chloride transport across individual cell membranes, experiments were carried out on early distal tubules in the doubly perfused Amphiuma kidney and net chloride flux, transepithelial and transmembrane cell potentials, and intracellular chloride activity measured. Net chloride flux was evaluated by a modified stationary microperfusion technique, and intracellular and intraluminal chloride activities by means of double-barreled liquid ion exchange microelectrodes. Control conditions were characterized by significant net volume and chloride reabsorption, a transepithelial potential difference of +9.0 +/- 0.5 mV (lumen positive), and cell chloride activities above electrochemical equilibrium across both luminal and peritubular cell membranes. Following luminal application of furosemide (5 X 10(-5) M) or perfusion with either a sodium- or chloride-free solution, net flux of chloride fell dramatically, the transepithelial potential difference was abolished, and cell chloride activity dropped sharply to approach electrochemical equilibrium. The decrease in transepithelial potential difference was fully accounted for by hyperpolarization of the basolateral cell membrane potential. These results are consistent with a furosemide-sensitive, electrically neutral sodium chloride cotransport mechanism across the luminal cell membrane.
Animals↗
Inefficacy of bicarbonate infusions on the course of postischaemic acute renal failure in the rat.
Since bicarbonate has been reported to elicit fast recovery from acute renal failure in man, clearance studies were performed to compare the effects of sodium bicarbonate and saline infusion on renal function in postischaemic renal failure in the rat. In a first set of experiments the left kidney and in a second both kidneys were clamped for a period of 45 min and renal function monitored up to 210 min after release of the clamp. Glomerular filtration rate (ml/min) decreased following clamping from (mean values +/- SEM) 1.33 +/- 0.09 to 0.12 +/- 0.02 (saline) or 1.43 +/- 0.1 to 0.08 +/- 0.01 (bicarbonate) in the unilaterally clamped kidney and from 2.94 +/- 0.20 to 0.41 +/- 0.10 (saline) or 2.81 +/- 0.17 to 0.22 +/- 0.03 (bicarbonate) when both kidneys were clamped. Fractional excretion of water and sodium increased to a similar extent in saline and bicarbonate treated animals. Plasma potassium decreased (from 3.37 +/- 0.10 to 2.95 +/- 0.07 [unilaterally clamped kidneys] or from 5.2 +/- 0.4 to 4.4 +/- 0.2 [bilaterally clamped kidneys]) in bicarbonate treated but remained constant in saline treated animals, an effect not related to altered renal potassium excretion. In conclusion, no evidence was found that bicarbonate improves renal function in postischaemic renal failure.
Acute Kidney Injury↗
Factors modulating the renal handling of phosphate.
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Animals↗
Ion activity measurements in single renal tubules.
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Ambystoma↗
[Additive phosphaturic action of parathyrin and calcitonin (author's transl)].
Parathyrin and calcitonin exert their effects on phosphate metabolism by influencing the functions of at least three organ systems, i. e. bone, gut and kidneys. To study the renal effects of these hormones under exclusion of systemic effects microinfusion studies were performed in anesthetised rats. After thyroparathyroidectomy radioactively labelled phosphate containing solutions were microinfused into single proximal convoluted tubules. The tracer recovery in the urine allowed calculation of phosphate reabsorption in the nephron segments beyond the micropuncture site. After a control period of 6 minutes the hormones were superfused to the nephron surface and tracer recovery measured during the following 36 minutes. Within few minutes both, parathyrin and calcitonin, clearly reduced phosphate reabsorption. Infusions of supramaximal doses of either hormone abolished the local action of this hormone but did not influence the effect of the other. Thus the phosphaturic actions of parathyrin and calcitonin are additive, indicating that the hormones involve different mechanisms and/or nephron sites.
Animals↗
Additivity of the phosphaturic action of parathyrin and calcitonin in the rat kidney.
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Animals↗
Continuous in vivo measurement of plasma ionized calcium.
A new system for continuous in vivo measurement of ionized calcium in blood is presented. The system consists of a small chamber (400 microliter) which is thermostatized at 37 degrees C and contains a glass electrode for pH measurement and a calcium selective electrode. The chamber is mounted in an arteriovenous shunt between the femoral artery and femoral vein of heparinized (approximately 1IU/min.kg B.W.) rats. The perfusion of approximately 500 microliter/min is driven by blood pressure. The system is tested in rats undergoing acute thyroparathyroidectomy and phosphate infusion. Due to this procedure ionized calcium falls from 0.9 mmol/l (total calcium 2.0 mmol/l) to 0.35 mmol/l (total calcium 1.2 mmol/l) after 5 hours. Tetany then occurs in all animals. The system presented simplifies the measurement of ionized calcium and has the advantages of immediate and continuous recording. It is especially apt for small animals in which repeated measurements with standard techniques involve large blood losses.
Animals↗
Renal handling of urate in healthy man in hyperuricaemia and renal insufficiency: circadian fluctuation, effect of water diuresis and of uricosuric agents.
To differentiate between extrarenal and renal causes of hyperuricaemia and gout, clearances of urate and creatinine were monitored for 3 1/2 days in fifty-two individuals (seven with a history of gout) with no gross impairment of renal function (creatinine clearance 52-137 ml/min). Dietary purine intake was kept constant. Monophasic circadian fluctuations of fractional urate excretion (= urate clearance over creatinine clearance) were observed with peak values in the afternoon, about 50% higher than during the night. Circadian fluctuations of urinary flow rate were almost identical. However, enhancement of urinary flow rate due to water diuresis had no effect on urate clearance. Despite wide variation of plasma urate concentrations among different individuals (+/- 30% SD), daily urate excretion varied little (+/- 4% SD) and did not correlate with plasma urate (r = 0.03). Thus extrarenal factors appear not to account for the occurrence of hyperuricaemia in these patients. In contrast, a clearcut negative correlation was apparent between plasma urate concentration and fractional urate clearance (r = 0.72), which could fully account for the variations of plasma urate concentration. To elucidate further the mechanism responsible for antiuricosuria in hyperuricaemic patients, the effects of the uricosuric agents benzbromarone and probenecid were tested. A clearcut correlation was apparent between control fractional urate excretion and uricosuric effect of both benzbromarone and probenecid (r = 0.83 and 0.88, respectively), suggesting that anti-uricosuria was due to defective secretion. In an additional series, the uricosuric effect of probenecid was tested in ten patients with renal insufficiency. In these patients the uricosuric effect was clearly blunted, indicating that urate reabsorption is reduced in renal insufficiency.
Adult↗
Influence of calcium and ionophore 23187 on tubular phosphate reabsorption.
In previous studies it has been demonstrated that a decline of plasma calcium concentration accounts for the decrease of phosphate reabsorption in thyroparathyroidectomized (TPTX) rats undergoing phosphate loading. Microinfusion studies were performed in TPTX rats in order to discriminate between a systemic effect of calcium an a direct renal effect. Thyroparathyroidectomized animals were infused with a phosphate solution continuously. When plasma calcium concentration fell below 1.30 mmol/l, proximal convoluted tubules were microinfused with a phosphate tracer solution for 42 min. After 18 min a calcium chloride-containing solution was applied superficially (superfused) to the area of the microinfused tubule. This elevation of peritubular calcium concentration led to an immediate increase of phosphate reabsorption up to 12% of the microinfused phosphate load within 24 min. In another series of experiments, the calcium specific ionophore A23187--a substance which is known to increase intracellular calcium--was superfused on the microinfused tubule. This resulted again in an increase of fractional phosphate reabsorption of about 15% after 24 min. In contrast, when calcium chloride-free as well as ionophore-free solutions were superfused fractional phosphate reabsorption decreased (7%). From these data we conclude that 1. calcium has a direct renal effect on phosphate reabsorption in the absence of parathyroid hormone and 2. intracellular calcium appears to be a major parameter in the regulation of renal phosphate transport under these conditions.
Animals↗
Renal handling of urate and oxalate: possible implications for urolithiasis.
Both urate and oxalate are organic acids of considerable clinical interest, owing to their limited solubility. Calcium oxalate is the most frequent constituent of renal calculi and occasionally precipitates in body fluids. Urate precipitations are common in the kidney and in various other tissues. In this paper, a short outline of the present knowledge of renal handling of these substances will be followed by some conclusions as to the possible relevance of this knowledge for the understanding of urolithiasis and intrarenal precipitation. Direct (micropuncture) data are available for urate in the rat (1, 6, 7, 10, 21, 23, 28, 36, 42), rabbit (35), dog (34) and cebus monkey (33) and in the rat only for oxalate (11, 15, 20).
Alkalies↗
Effect of benzolamide on luminal pH in proximal convoluted tubules of the rat kidney.
Luminal pH in early and late proximal tubules was recorded continuously with antimony microelectrodes before and during carbonic anhydrase inhibition. Following i.v. application of benzolamide (25 mumol/kg BW), luminal pH decreased almost immediately in early proximal tubules (deltapH--0.42 +/- 0.06SEM), but increased in late proximal tubules (deltapH +0.27 +/- 0.06). Urinary pH increased (deltapH + 1.6 +/- 0.16) after a delay of some 30 s. Similar results, i.e. decrease of pH in early and increase of pH in late proximal tubules, were obtained, when benzolamide containing solutions were microinfused into early proximal tubules or superfused on the nephron surface. In contrast, luminal pH decreased in late proximal tubules, when benzolamide was microinfused into the same nephron segment. The decrease of luminal pH indicates inhibition of luminally active carbonic anhydrase, leading to delayed buffering of secreted hydrogen ions. The increase of luminal pH in late proximal tubules may be attributed to several factors including increased delivery of bicarbonate, impaired bicarbonate exit at the antiluminal membrane and decreased hydrogen ion formation in the tubular cell due to inhibition of cellular carbonic anhydrase.
Animals↗