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G Malnic

Publications and source records attributed to G Malnic.

At least 19 recordsLinked to original sources
PubMed1992

Effects of parathyroid hormone and calcium and their interrelationship on urinary acidification in the rat.

1. The influence of thyroparathyroidectomy on renal function and specifically on acid excretion was studied in rats with or without oral supplementation of calcium. 2. Thyroparathyroidectomy caused a significant decrease in glomerular filtration rate, in the urinary/plasma inulin ratio and in overall acid excretion. These changes were not corrected by calcium supplementation. 3. Rates of proximal tubular acidification were studied by means of double-barrelled resin/reference microelectrodes. Acidification half-time was significantly increased in both thyroparathyroidectomized and calcium-supplemented thyroparathyroidectomized rats (8.38 s and 7.40 s, respectively) compared with control rats (5.44 s). 4. When 10(-6) mol/l A23187, a calcium ionophore, was added to the luminal bicarbonate solution, the acidification half-time returned to 3.97 s in the thyroparathyroidectomized rats, whereas no significant changes were detected in the properties of acidification in the control rats. 5. These data show that parathyroid hormone and cellular calcium are important factors involved in proximal tubular H+ secretion, which appears to be largely dependent on a well-defined concentration range of these agents.

Animals↗
PubMed1992

Role of corticosteroids in distal acidification of amiloride-treated rats.

The role of amiloride-dependent sodium channels in the action of adrenal cortical steroids on urine-blood PCO2 (U-B PCO2) differences was studied in bicarbonate-infused and amiloride-treated adrenalectomized rats. U-B PCO2 was significantly reduced by amiloride in bicarbonate-infused control rats. Adrenalectomy further reduced U-B PCO2 in amiloride-treated, bicarbonate-infused rats (from 27.9 +/- 1.82 mmHg in sham-operated rats to 21.3 +/- 1.58 mmHg in adrenalectomized (ADX) rats) (1 mmHg = 133.322 Pa). Acute administration of corticosterone and 18-hydroxycorticosterone (18-OH-B), but not of aldosterone, caused recovery of U-B PCO2 to the level of sham-operated animals treated with amiloride. Aldosterone did not affect U-B PCO2 in the presence of amiloride (21.9 mmHg ADX group vs. 20.98 mmHg aldosterone group). Results are compatible with aldosterone affecting distal H ion secretion mostly by a sodium and potential difference dependent mechanism, while corticosterone and 18-OH-B should act by other mechanisms (e.g., increased luminal buffer level).

18-Hydroxycorticosterone↗
PubMed1991

Bicarbonate reabsorption and electrophysiology of proximal tubules in uninephrectomized rats.

1. The kinetics of acidification of luminal fluid in hypertrophied proximal tubules after unilateral nephrectomy was studied by stationary microperfusion and continuous measurement of luminal pH with antimony microelectrodes. 2. Trans-epithelial and basolateral membrane electrical potential differences were measured in order to detect modifications in electrogenic transport mechanisms under these conditions. 3. The values of stationary pH and HCO3- concentration were significantly lower, the mean acidification half-time was not different and net reabsorptive HCO3- fluxes in proximal tubules were significantly increased in uninephrectomized rats. According to an electrical analogue model, these results suggest (a) a reduction in the internal series resistance of the H+ pump, caused perhaps by an increased density of pump sites, and (b) an increase in the protonmotive force of the pump. 4. The trans-epithelial electrical potential difference measured in free flow conditions was significantly more lumen-positive in uninephrectomized rats. The trans-epithelial electrical potential difference measured during intraluminal perfusion with Ringer solution containing 30 mmol/l HCO3- was significantly more negative in all groups studied. In uninephrectomized rats treated with acetazolamide, the trans-epithelial electrical potential difference was more lumen-negative than that in untreated uninephrectomized rats. These results are compatible with a steeper transepithelial Cl- gradient as well as with electrogenic, active H+ secretion. 5. There was no significant difference in the basolateral electrical potential difference between control and uninephrectomized rats. 6. In conclusion, our data show an increase in the transport rates of HCO3- in the proximal tubule of uninephrectomized rats, which may be due to an increase in the density of transporters in the brush-border membrane, and an increased ability of the transport mechanism to create H+ gradients.

Absorption↗
PubMed1991

Effects of parathyroid hormone on urinary acidification in the rat.

To evaluate the effects of parathyroid hormone (PTH) on urinary acidification parameters, thyroparathyroidectomy was performed in normal (TPTX) and in calcium-supplemented rats (TPTX + Ca2+). Both groups were supplemented with thyroxin. Glomerular filtration rate (GFR) fell from 7.79 +/- 0.33 in the control group (C) to 4.88 +/- 0.26 ml min-1 kg-1 in TPTX, while net acid excretion fell from 5.65 +/- 0.22 in C to 3.76 +/- 0.25 mumol min-1 kg-1 in TPTX. Kinetic data of urinary acidification obtained by microperfusion techniques in proximal tubules showed that the half-time of acidification (t/2) rose from 4.75 +/- 0.24 s in C to 8.97 +/- 0.64 s in TPTX and persisted elevated in TPTX + Ca2+ (7.40 +/- 0.43 s); in the latter group, stationary pH was not significantly different from that of the control group. Bicarbonate reabsorption (JHCO3) fell from 2.18 +/- 0.15 in C to 0.823 +/- 0.082 in TPTX and was 1.53 +/- 0.073 nmol s-1 cm-2 in TPTX + CA2+. These data suggest that normal pH gradients depend on normal calcium levels, but acidification half-times are dependent on PTH, which also contributes to keeping glomerular hemodynamics and acidification rates at normal levels.

Acidosis↗
PubMed1990

pH-stat experiments in proximal renal tubules.

The pH-stat technique has been used to measure H+ fluxes in gastric mucosa and urinary bladder "in vitro" while keeping mucosal pH constant. We now report application of this method in renal tubules. We perfused proximal tubules with double-barreled micropipettes, blocked luminal fluid columns with oil and used a double-barreled Sb/reference microelectrode to measure pH, and Sb or 1 N HC1-filled microelectrodes to inject OH- or H+ ions into the tubule lumen. By varying current injection, pH was kept constant at adjustable levels by an electronic clamping circuit. We could thus obtain ratios of current (nA) to pH change (apparent H(+)-ion conductance). These ratios were reduced after luminal 10(-4) M acetazolamide, during injection of OH-, but they increased during injection of H+. The point-like injection source causes pH to fall off with distance from the injecting electrode tip even in oil-blocked segments. Therefore, a method analogous to cable analysis was used to obtain H+ fluxes per cm2 epithelium. The relation between JH+ and pH gradient showed saturation kinetics of H fluxes, both during OH- and H+ injection. This kinetic behavior is compatible with inhibition of JH by luminal H+. It is also compatible with dependence on Na+ and H+ gradients of a saturable Na/H exchanger. H(+)-ion back-flux into the tubule lumen also showed saturation kinetics. This suggests that H+ flow is mediated by a membrane component, most likely the Na(+)-H+ exchanger.

Acetazolamide↗
PubMed1990

Urinary and proximal tubule acidification during reduction of renal blood flow in the rat.

1. The effects of reduction in renal blood flow (RBF) on urinary acidification and proximal tubule H+ ion secretion were studied after partial aortic clamping in rats. 2. Acute reduction of the renal perfusion pressure (from 109 +/- 3.88 to 77.4 +/- 1.05 mmHg) decreased both inulin and PAH (p-aminohippurate) clearances to about one-third of their control values. Absolute levels of urinary sodium excretion also decreased markedly, but fractional sodium excretion did not change significantly. 3. Urine pH and bicarbonate levels were not affected, but titratable acidity increased significantly from 0.12 +/- 0.011 to 0.25 +/- 0.042 muequiv min-1 ml-1 glomerular filtration rate (GFR). During aortic clamping, cortical PCO2 as determined by means of Severinghaus microelectrodes was reduced by a mean value of 7.0 +/- 1.5 mmHg. 4. Proximal tubule acidification kinetics were studied by stationary microperfusion techniques in which the time course of pH changes was monitored by pH microelectrodes. Steady-state pH fell from a mean control value of 6.77 +/- 0.03 to 6.65 +/- 0.02, and stationary bicarbonate concentrations from 4.70 +/- 0.27 to 2.84 +/- 0.18 mM. Acidification half-time decreased from 5.07 +/- 0.30 to 4.39 +/- 0.19 s, and net bicarbonate reabsorption increased from 1.63 +/- 0.14 to 1.99 +/- 0.12 nmol cm-2 s-1, these changes being statistically significant. 5. The experiments demonstrate that both overall acid excretion and proximal acid secretion are not compromised by a large decrease of RBF to about one-third of the control value; titratable acid excretion and proximal net bicarbonate reabsorption were even moderately increased under these conditions.

Animals↗
PubMed1990

Distal perfusion studies: transport stimulation by native tubule fluid.

It is well established that potassium secretion into the distal tubule increases with the rate of flow. In a previous study [G. Malnic, R. W. Berliner, and G. Giebisch. Am. J. Physiol. 256 (Renal Fluid Electrolyte Physiol. 25): F932-F1271, 1989] we found that the increase with the rate of perfusion with a fluid made up to resemble that normally found in the early distal tubule was substantially less than the increase in free-flow conditions [R. N. Khuri, M. Wiederholt, N. Strieder, and G. Giebisch. Am. J. Physiol. 228: 1249-1261, 1975]. Because of the possibility that some important component was missing from the artificial fluid, we have carried out another series of experiments in which distal tubules were perfused with fluid collected from late proximal tubules and compared the results with those obtained when tubules were perfused with an artificial fluid with an electrolyte composition similar to that of late proximal fluid. When proximal tubule fluid was used, the potassium concentrations in the collected distal fluid were higher and better maintained with increasing flow than when the artificial fluid was used, and consequently the rate of potassium secretion was substantially greater with the proximal tubule fluid, approaching the results of previous studies in free flow. The nature of the component missing from the artificial solution is not known.

Absorption↗
PubMed1990

PCO2 in renal cortex.

In a number of recent investigations a renal cortical PCO2 higher than that of systemic blood was reported. We have studied this problem with the use of micro-Severinghaus electrodes based on antimony, H+ liquid ion exchange, and glass pH electrodes with an inner buffer solution containing 0.5 mg/ml carbonic anhydrase (CA). Measurements in renal cortical structures (renal tubules, star vessels, capillaries, and glomeruli in Munich-Wistar rats) were compared with determinations in renal vein or artery performed with the same electrode in sequence. No significant differences in PCO2 were found between cortical structures and renal vein in control rats, in metabolic alkalosis, respiratory acidosis and alkalosis, and after CA inhibition. Nevertheless, absolute PCO2 levels, which followed the PCO2 of systemic blood, were markedly different in these groups. Measurements of pH and PCO2 at the same tubule site were compatible with HCO3- determinations in tubule fluid in vitro (made with use of the Henderson-Hasselbalch equation) in control rats. When proximal tubules were pump-perfused in vivo with a solution containing 30 mM NaHCO3, measured PCO2 approached that of the perfusing solution at high pump rates, and approached the free-flow value as rates were reduced to zero, indicating that the CO2 generated in the lumen equilibrated rapidly across the epithelium. Reducing renal blood flow by aortic clamping reduced renal cortical PCO2. In conclusion, in a large number of experimental conditions renal cortical PCO2 was never higher than that measured in systemic blood.

Animals↗
PubMed1990

Mechanism of potassium transport across proximal tubule epithelium in the rat.

The mechanism of proximal tubule potassium reabsorption was studied by stopped-flow microperfusion and determination of potassium activities by ion-sensitive resin microelectrodes. The proximal tubule was unable to establish transepithelial potassium gradients. Perfusion with 20 mM K+ turned the lumen 3 mV more negative, an effect abolished by Ba2+. The half-time for K+ activities to reach their stationary level after perfusion with 1 mM K+ was significantly increased by Ba2+ from 4.25 +/- 0.14 s to 11.0 +/- 1.71 s, and after perfusion with 20 mM K+, from 5.43 +/- 0.20 to 12.53 +/- 0.90 s. These data indicate that a significant fraction of potassium is transferred across proximal tubule epithelium by a transcellular, K(+)-channel-dependent route.

Animals↗
PubMed1989

Effect of amphotericin B on renal tubular acidification in the rat.

Amphotericin B, a polyene antibiotic known to induce cation-selective pore formation in biological cell membranes, was given to rats by peritoneal injection (10 mg/kg for 21-26 days) or added to luminal perfusates (2 x 10(-5) M). Kinetics of tubular acidification and alkalinization after perfusion with alkaline or acid phosphate Ringer's solution was studied by means of double barrelled antimony/reference microelectrodes in cortical distal tubules. Stationary pH increased both in early and late distal segments. Acidification and alkalinization half-times decreased markedly from 15-18 s to 6-8 s, a value similar to that found in proximal tubule. Net H-ion secretion rates as well as H-ion back-flux approximately doubled after Amphotericin B. Apparent H-ion permeability of distal tubule epithelium measured during perfusion of lumen and peritubular capillaries with phosphate Ringer's solutions doubled both in early and late segments. These data show that amphotericin B produces a distal acidification defect which impairs formation of normal transepithelial pH gradients by increasing H-ion back-flux without reducing rates of net H-ion secretion.

Amphotericin B↗
PubMed1989

Effects of adrenalectomy and acute replacement by corticosteroids on distal acidification.

The role of adrenocortical steroids in distal nephron acidification was studied in rats by measuring urine minus blood PCO2 differences (U-B PCO2) in control, sham-operated, and adrenalectomized (ADX) animals. Operations were performed 48 h before experiments. During the experiments, all rats received an infusion of 0.35-0.60 M NaHCO3, leading to urine bicarbonate concentrations in the order of 100-200 mM. Adrenalectomized rats had significantly decreased U-B PCO2 (11.9 +/- 1.99 mmHg; 1 mmHg = 133.3 Pa) with respect to sham-operated rats (39.9 +/- 1.26 mmHg). In another series, ADX rats received supplements of the adrenal steroids corticosterone, aldosterone, and 18-hydroxycorticosterone 100 min before the experiment. U-B PCO2 increased after hormone administration: corticosterone, 30.0 +/- 2.13 mmHg; aldosterone, 26.6 +/- 1.74 mmHg; 18-hydroxycorticosterone, 29.0 +/- 1.60 mmHg; but none restored these values to normal. Combinations of two hormones were also used; only aldosterone + corticosterone restored U-B PCO2 to normal: 39.0 +/- 1.66 mmHg. Renal phosphate excretion (but not urine phosphate levels) decreased significantly in ADX as compared with sham-operated rats. Extracellular volume was not significantly affected in ADX rats, which received ad libitum 0.9% NaCl for drinking. It is concluded that distal tubular acidification, as evaluated by U-B PCO2, is dependent on cortical steroids.

18-Hydroxycorticosterone↗
PubMed1989

Flow dependence of K+ secretion in cortical distal tubules of the rat.

Superficial distal tubules were pump perfused (range 0-35 nl/min), generally with solutions similar in composition to early distal tubule fluid, in control, K+-depleted, acutely K+-loaded, and K+-adapted rats with the use of double-barreled resin-reference microelectrodes to measure K+ and Na+ activities and transepithelial potential differences (PD). When perfusion rate decreased from 35 to 5 nl/min in control animals, [K+] increased from 2 mM to between 10 and 20 mM, remaining at these levels as perfusion rate was decreased further. In low-K+ rats, the change in K+ activity with flow was greatly attenuated. In K+-loaded and K+-adapted rats, [K+] was higher than in controls at all flow rates. Na+ concentrations and lumen-negative PD increased with high flow rates in control rats. Addition of 10(-3) M amiloride blocked the increase in luminal K+ with low flow rates. In the physiological range of late distal flow rates, luminal [K+] remains constant and similar to the concentration attained in the steady state. At higher flow rates, [K+] declines, and K+ balance, PD, and Na+ reabsorption modulate the relationship between K+ secretion and flow rate.

Amiloride↗
PubMed1989

Renal bicarbonate reabsorption in the rat. III. Distal tubule perfusion study of load dependence and bicarbonate permeability.

Using continuous microperfusion techniques, we studied the load dependence of bicarbonate reabsorption along cortical distal tubules of the rat kidney and their bicarbonate permeability. Net bicarbonate transport was evaluated from changes in tracer inulin concentrations and total CO2 measurements by microcalorimetry. Bicarbonate permeability was estimated from the flux of total CO2 along known electrochemical gradients into bicarbonate-and chloride-free perfusion solution containing 10(-4) M acetazolamide. Transepithelial potential differences were measured with conventional glass microelectrodes. Significant net bicarbonate reabsorption occurred at luminal bicarbonate levels from 5 to 25 mM, and at perfusion rates from 5 to 30 nl/min. Bicarbonate reabsorption increased in a load-dependent manner, both during increments in luminal bicarbonate concentration or perfusion rate, reaching saturation at a load of 250 pmol/min with a maximal reabsorption rate of approximately 75 pmol/min.mm. Rate of bicarbonate reabsorption was flow dependent at luminal concentrations of 10 but not at 25 mM. During chronic metabolic alkalosis, maximal rates of reabsorption were significantly reduced to 33 pmol/min.mm. The bicarbonate permeability was 2.32 +/- 0.13 x 10(-5) cm/s in control rats, and 2.65 +/- 0.26 x 10(-5) cm/s in volume-expanded rats. Our data indicate that at physiological bicarbonate concentrations in the distal tubule passive bicarbonate fluxes account for only 16-21% of net fluxes. At high luminal bicarbonate concentrations, passive bicarbonate reabsorption contributes moderately to net reabsorption of this anion.

Absorption↗
PubMed1988

Solvent drag of sucrose during absorption indicates paracellular water flow in the rat kidney proximal tubule.

Single convoluted proximal tubules of the rat kidney were lumen perfused in situ with isosmotic solutions containing C14-sucrose and H3-inulin as tracers, to evaluate whether the extracellular marker sucrose is entrained by water during proximal tubular reabsorption. Inulin was used as volume marker. The absorptive rate was varied by using as luminal perfusion fluids either a solution made up of (in mmole/l) 120 NaCl, 5 glucose, 25 NaHCO3 and altering the perfusion rate, or a solution containing 110 NaCl and 70 raffinose. Js, the net sucrose efflux is found to be a function of the net volume flow, Jv, such that at Jv = 0, Js is very small and at high rates of Jv, Js is over 60-fold the value observed at low Jv values. In addition, the transported to luminal sucrose concentrations decreased with Jv in a hyperbolic manner. Unstirred layers affect the diffusive component of Js, but only to a small extent. Therefore, the large remaining dependency of Js with Jv must be due to drag of sucrose by water, within the paracellular pathway. This leads to the conclusion that water flows through the paracellular pathway during absorption in the rat proximal tubule, in addition to transcellular water flow. Using equations for molecular sieving and the measured value of sigma s for sucrose of 0.76-0.91, it is calculated that the pathway where entrainment of solute by water occurs must be 1.0-1.1 nm wide. This calculation is only tentative since sigma s depends on the as yet unknown relative contribution of transcellular and paracellular pathways to transepithelial water osmotic permeability.

Absorption↗
PubMed1988

Sodium dependence of early distal H+ secretion in rat kidney.

In order to study the mechanism of H-ion secretion in cortical distal tubules of the rat kidney, the luminal pH and transepithelial potential difference (PD) were measured with double-barrelled, pH-sensitive, resin/reference microelectrodes. Perfusion of peritubular capillaries with low-sodium solutions increased luminal pH by 0.28 +/- 0.024 units. Perfusion of the lumen with 1 mM amiloride increased luminal pH by 0.67 +/- 0.01 units. These changes could not be attributed to modification of transepithelial PD. We conclude that early distal acidification is sodium-dependent, possibly owing to the presence of Na+/H+ exchange.

Animals↗
PubMed1987

Renal bicarbonate reabsorption in the rat. II. Distal tubule load dependence and effect of hypokalemia.

We studied two groups of rats acutely loaded with bicarbonate, control rats on a standard diet and rats kept on a K-free diet for 3 wk. Compared with controls, K-depleted rats had reduced fractional excretion of bicarbonate despite their elevated filtered bicarbonate load. Distal bicarbonate reabsorption increased in K-depleted rats. In the presence of almost identical early distal bicarbonate loads (481 +/- 40 pmol/min in controls and 444 +/- 50 pmol/min in K depletion), distal bicarbonate reabsorption was significantly enhanced in K depletion (247 +/- 17 pmol/min) as compared with controls (179 +/- 18 pmol/min). These values are significantly different from each other, and both are severalfold higher than bicarbonate reabsorption in nonloaded conditions. In conclusion, distal bicarbonate reabsorption is load dependent, and distal bicarbonate reabsorption is stimulated in K depletion.

Animals↗
PubMed1987

Filtered load of buffer and renal H-ion secretion: mechanism of proximal tubule load dependence.

When the filtered load of buffers like bicarbonate or phosphate is increased by elevating GFR or buffer concentration in plasma, the overall renal reabsorption of bicarbonate or the formation of titratable acidity are markedly increased. The same happens when buffer concentration or flow rate are varied during proximal microperfusion. We have recently studied the mechanisms of this functional dependence. We have observed that the rate of bicarbonate reabsorption is always proportional to luminal buffer concentration when a stationary fluid column is injected into the proximal lumen. H-ion secretion is also proportional to luminal levels of non-bicarbonate buffers. Using a pH-stat technique adapted to renal tubules, we have shown that H-ion secretion is dependent on proximal pH independently of the used buffer species. A kinetic analysis of these data shows a non-linear relationship between luminal H+ and H+ secretion, compatible with carrier mediated transport.

Animals↗