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Biomedical subjects

G Malnic

Publications and source records attributed to G Malnic.

At least 73 records · Page 4Linked to original sources
PubMed1983

Peritubular buffering power and luminal acidification in proximal convoluted tubules of the rat.

Proximal tubular acidification was studied varying peritubular buffer concentration as well as the nature of the main peritubular buffer system. Two buffer systems were used: phosphate which varied between 1 and 20 mM, and glycodiazine, at 5 and 20 mM. Luminal perfusate was always 20 mM phosphate Ringer's. Acidification half times increased as peritubular buffer concentration decreased, independently of the nature of the buffer. At 1 mM phosphate, net H-ion flux (JH +) was 0.53 nmol . cm-2 . s-1; at 5 mM it was 0.73 nmol . cm-2 . s-1 and at 20 mM, 0.97 nmol . cm-2 . s-1. When the peritubular buffer was glycodiazine, JH + was 0.77 nmol . cm-2 . s-1 at 5 mM peritubular buffer concentration and 0.99 nmol . cm-2 . s-1 at 20 mM. Acetazolamide (10(-4) M) and DIDS (10(-4) M) both abolished the effect of peritubular buffer concentration changes on acidification half times. It was shown that these effects were related to the capacity of the peritubular buffer to attenuate changes in peritubular pH as consequence of base transfer by the peritubular membrane. Peritubular buffering power could act limiting intracellular pH increments consequent to luminal H-ion secretion.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed1983

Passive driving forces of proximal tubular fluid and bicarbonate transport: gradient dependence of H+ secretion.

The effect of oncotic pressure changes on fluid (Jv) and net bicarbonate transport (JHCO-3) and the transepithelial bicarbonate permeability (PHCO-3) were measured by an improved luminal and capillary microperfusion method that allows paired experiments on the same tubule. Rat proximal tubules were pump-perfused and Jv and [HCO-3] measured with [14C]inulin and a pH glass electrode. Raising peritubular protein (0-8-15 g/100 ml bovine serum albumin) stimulated Jv and HCO-3 reabsorption. The response to oncotic pressure changes was asymmetrical since changes of the luminal protein concentration had no significant effects. Whereas transepithelial solvent drag effects on HCO-3 must be minimal, peritubular protein most likely stimulates translocation of fluid and bicarbonate from intercellular spaces into peritubular capillaries. PHCO-3 was measured from HCO-3 net flux along a lumen-to-capillary-directed electrochemical potential gradient. In these experiments active H+ transport and Jv were minimized by 10(-4) M acetazolamide and luminal raffinose. PHCO-3 was 1.77 X 10(-5) cm X s-1 and was unaffected by increasing luminal flow rate from 10 to 45 nl X min-1. Since bicarbonate backflux is only a small fraction of physiological rates of JHCO-3, net transport alterations at varying [HCO-3] in the lumen must be due to changes in active HCO-3 (H+) transport. Thus, active H+ ion secretion across the luminal membrane of the proximal tubule is gradient dependent.

Acetazolamide↗
PubMed1983

Proximal tubular acidification in metabolic alkalosis.

Metabolic alkalosis was induced in rats by acute bicarbonate loading in the presence and absence of extracellular volume (ECV) expansion. Proximal tubular acidification was studied by stopped-flow microperfusion and the determination of luminal pH by antimony microelectrodes. Stationary pH and bicarbonate were markedly increased in proximal tubules of alkalotic rats, and acidification half-times were increased, leading to a fall in net bicarbonate reabsorption (JHCO-3) to 36% of control values. Net H+ ion secretion (JH+) into phosphate buffer fell to 56% of controls, whereas alkalinization t/2 and H+ fluxes, measured during perfusion with acid phosphate, were unaltered. ECV expansion reduced proximal JHCO-3 36% in control rats, without affecting JH+, and caused a fall in alkalinization t/2, compatible with increased bicarbonate back-flux into the lumen. However, it did not affect JHCO3 and JH+ in alkalotic rats. Carbonic anhydrase inhibition did not significantly reduce acidification in alkalotic rats. An analysis of the components of bicarbonate reabsorption showed that alkalosis reduced catalyzed H+ -ion secretion to 7% of the control values. It is concluded that proximal H+ ion secretion is significantly reduced in metabolic alkalosis leading to lower JHCO-3. No evidence for modification of H+/HCO-3 apparent permeabilities was obtained. The effects of alkalosis were not significantly altered by ECV expansion or by carbonic anhydrase inhibition.

Acid-Base Equilibrium↗
PubMed1982

Voltage transients during ionic substitution in renal cortical tubules.

A voltage transient is described which is found during proximal tubular perfusion with impermeant cation or anion salt solutions in the rat. It was shown that the magnitude of transepithelial diffusion potentials depended on luminal hydrostatic pressure, suggesting that the observed transients might be the consequence of the enlargement of ionic pathways by tubular dilatation. Thus, when reporting PD values, care should be taken to define the pressure levels at which measurements were performed.

Animals↗
PubMed1982

H+ ion secretion in proximal tubule of low-Co2/HCO-3 perfused isolated rat kidney.

Acidification in proximal tubule of the isolated rat kidney, perfused in vitro, was studied by stopped-flow microperfusion techniques, using Sb microelectrodes to measure luminal pH. The kidney was perfused with mammalian Ringer's solution at pH 7.4 buffered by 20 mmol/l phosphate and containing 7.5 g/100 ml bovine albumin, equilibrated with air. Final urine pH was 6.88 +/- 0.5. Steady-state pH in proximal segments was 6.81 +/- 0.03 (n = 80), and acidification half-time (t/2) 7.25 +/- 0.33 (80) s, giving a net secretory H+ ion flux of 0.51 +/- 0.05 nmol . cm-2 . s-1. This flux was about 70% of "in vivo" (blood perfused kidneys). During luminal perfusion with solutions at pH 6.2, back-flux of H+ was 0.82 +/- 0.08 nmol . cm-2 . s-1, with an alkalinization t/2 of 6.33 +/- 0.34 (34) s. The difference between acidification and alkalization t/2 was not significant. This is compatible with a pump-leak system of H+ transport. This is compatible with a pump-leak system of H+ transport. The back flux of H from the lumen was markedly reduced in low Na+ perfused kidneys in the presence of 10(-4) mol/l amiloride in the lumen, indicating that this process is mediated by the luminal Na/H exchanger. Observations in the presence of high K levels suggest that it may have also a charged component. 10(-4) mol/l acetazolamide added to the kidney perfusate reduced acidification to 0.5% of control, and 10(-6) mol/l SITS to 25% of control. Thus, despite the low pCO2 (0.1-0.4 kPa, or 1-3 mm Hg), the CO2/HCO-3 buffer system still plays an important role in tubular acidification in this preparation.

Animals↗
PubMed1982

Effect of temperature on proximal tubular acidification.

The effect of temperature on proximal tubular acidification was studied in isolated rat kidney, perfused with 20 mM phosphate Ringer's containing 7.5 g/100 ml bovine albumin, equilibrated with air. Tubular pH was measured with Sb microelectrodes during stopped-flow microperfusion. The temperature of the kidney was varied between 10 and 46 degrees C. At 10 degrees C the proximal tubule was still able to maintain pH gradients of about 0.7 pH units. However, half-times (t/2) of both acidification and alkalinization were markedly increased, from 6-7 s at 37 degrees C to 27-30 s at 10 degrees C. In consequence, net H+-ion flux into the tubule was reduced to 26% of that at 37 degrees C. In this system, in the absence of exogenous HCO-3 and CO2, t/2 of acidification and alkalinization were very similar at 37 degrees C and below. Above 37 degrees C alkalinization t/2 fell markedly to 1.43 +/- 0.09 (11) s at 46 degrees C, while acidification t/2 stayed at about 7 s. H+-ion back-fluxes increased progressively from 10-46 degrees C, while secretory JH reached a maximal value at 37 degrees C and fell at higher temperatures. Apparent activation energies calculated from rate coefficients were 8.48 kcal . mol-1 for acidification, and 9.30 for alkalinization, and those calculated from JH were 6.30 and 9.55 respectively. These data indicate that both H-ion secretion and back-flux are carrier-mediated, probably flowing through the Na/H exchanger in the luminal membrane, since their activation energies are of the same order of magnitude and markedly higher than those for protons in solution.

Animals↗
PubMed1982

Cell pH and luminal acidification in Necturus proximal tubule.

Cellular potential and pH measurements (pHi) were carried out in the perfused kidney of Necturus on proximal tubules with standard and recessed-tip glass microelectrodes under control conditions and after stimulation of tubular bicarbonate reabsorption. Luminal pH and net bicarbonate reabsorption were measured in parallel experiments with recessed-tip glass or antimony electrodes, both during stationary microperfusions as well as under conditions of isosmotic fluid transport. A mean cell pH of 7.15 was obtained in control conditions. When the luminal bicarbonate concentration was raised to 25 and 50 mM, pH, rose to 7.44 and 7.56, respectively. These changes in pHi were fully reversible. Under all conditions intracellular H+ was below electrochemical equilibrium. Thus the maintenance of intracellular pH requires "active" H+ extrusion across one or both of the cell membranes. The observed rise in pHi and the peritubular depolarization after stimulation of bicarbonate reabsorption are consistent with enhanced luminal hydrogen ion secretion and augmentation of peritubular bicarbonate exit via an anion-conductive transport pathway.

Animals↗
PubMed1981

Role of luminal buffers in renal tubular acidification.

The acidification of kinetics of artificial solutions containing buffers of different permeancy were studied in rat proximal tubules by means of stationary microperfusion techniques. Luminal pH changes were measured by antimony microelectrodes and used to calculate net rates of acidification and the approach to steady-state pH levels. For most buffer species, tracer efflux out of the lumen was compared with changes in buffer concentration as derived from calculations based on the Henderson Hasselbalch equation. Steady-state luminal pH was similar for most buffer systems studied. However, secretory hydrogen ion fluxes into the lumen were significantly higher for permeant than for less permeant buffers. The most likely explanation is that permeant buffers behave as "open" systems maintaining constant low diffusible acid levels in the lumen, whereas impermeant buffers behave as "closed" systems in which non-ionized acid levels are maintained at higher levels. A behavior consistent with this thesis was directly demonstrated for glycodiazine and, to a lesser degree, for DMO. In contrast, phosphate and creatinine behave like buffers in a "closed" system. Characteristics of proximal tubular acidification, of buffer reabsorption, and the effect thereupon of carbonic anhydrase inhibitors are satisfactorily explained by an essential role of (1) hydrogen ion secretion, (2) pK differences, and (3) different permeance of the non-ionized buffer species. However, specific transport mechanisms may, in addition, also contribute to differences in transepithelial buffer movement.

Animals↗
PubMed1980

Effects of changes in electrical potential difference on tubular potassium transport.

To assess directly the role of the transepithelial potential difference (PD) on potassium concentration differences across distal tubular epithelium, continuous and stationary microperfusion experiments were done in tubules voltage-clamped over a wide range of lumen-negative potentials. Potassium was measured either chemically or in situ by potassium-sensitive microelectrodes. Distal cell PD measurements show that most of the potential drop induced by luminal current injection occurred across the luminal cell membrane. Experiments were done in rats either on a control or on a high potassium diet and after amiloride administration. Luminal potassium was highly sensitive to imposed electrical potential changes, attainment of a new steady-state intraluminal potassium concentration was rapid (less than 1 s), and higher luminal potassium concentrations were observed in animals in which potassium secretion had been stimulated. Similar slopes of tubular fluid-to-plasma potassium ratios versus transepithelial potential differences were observed in all experiments. All slopes intersected, at zero PD, at a luminal tubular fluid-to-plasma concentration ratio in excess of unity, indicating the presence of an active component of potassium secretion.

Animals↗
PubMed1980

Hypothalamic stimulation and electrolyte excretion: a micropuncture study.

The anterior part of the lateral hypothalamus was stimulated by injection of carbachol through a stereotaxically implanted cannula in the rat. Re-collection micropuncture experiments showed that this procedure, which leads to diuresis and natriuresis with only transient changes in glomerular filtration rate and renal plasma flow, reduced the TF/P inulin ratio along proximal and distal tubules without significant alteration of single nephron glomerular filtration rate in most experimental groups. Fractional proximal sodium reabsorption was significantly reduced from 0.54 +/- 0.02 to 0.34 +/- 0.05. Treatment with DOCA, vasopressin, and oxytocin caused natriuresis, but additional hypothalamic stimulation (HS) led to further reduction in TF/P inulin ratio and proximal fractional sodium reabsorption from 0.42 +/- 0.03 to 0.33 +/- 0.03. Fluid transport across proximal and distal epithelium was also studied by the split-droplet method. It was markedly reduced in both segments after HS. During hormone treatment only distal segments showed reduction of fluid transport by HS. These experiments indicate that HS caused inhibition of fluid transport in the proximal and distal tubule. This effect was only partly due to the liberation of neurohypophysial hormones, since during their administration an additional effect of HS was still observed.

Animals↗
PubMed1980

CO2 equilibria in renal tissue.

Disequilibrium situations within the bicarbonate/CO2 buffer system have been observed in the kidney. On the one hand, a PCO2 difference between final urine and arterial blood has been detected. On the other, a disequilibrium pH was found in cortical renal tubules and attributed to either increased carbonic acid concentrations or to PCO2 levels above those of arterial blood. Recent methodological developments have yielded better insight into these disequilibrium situations. They include, besides the use of pH microelectrodes, the introduction of microcalorimetric total CO2 determinations and PCO2 microelectrodes. There is agreement concerning the finding of a PCO2 10--40 mmHg above that in arterial blood in renal cortical tubules; however, stellate vessel PCO2 equal to tubular PCO2 was found by only one group of investigators. According to their view, diffusion equilibrium exists between all cortical structures, a finding disputed by others using a similar methodology. CO2 permeability of tubular epithelium is also still controversial, with values ranging from one-half Microcalorimetric method) to one-twentieth (pH-equilibration method) of an equivalent water layer having been reported. Two positions emerge on the basis of recent experimental results: the existence of very high permeability to CO2 and consequent diffusion equilibrium in renal cortex, and the occurrence of CO2 diffusion limitation leading to the establishment of finite PCO2 gradients across renal tubular epithelium.

Animals↗
PubMed1979

Sodium in renal tubular acidification kinetics.

Renal proximal tubules and their peritubular capillaries were perfused with mammalian Ringer solutions containing different sodium concentrations. In stop-flow microperfusion experiments, the pH was measured by means of antimony microelectrodes, permitting calculation of rates of H ion secretion and bicarbonate reabsorption. These rates, as well as transepithelial pH and bicarbonate gradients, were significantly reduced at ambient concentrations of 20 and 4 meq/liter Na+. However, even at the lowest sodium concentrations (4 meq/liter), H ion secretion was still 74%, and bicarbonate reabsorption of 64% of control rates. In similar conditions, sodium reabsorption as measured by the split-droplet technique fell to practically zero. Ouabain, 10(-3) M, in capillaries reduced bicarbonate reabsorption by 31%, and 3 X 10(-4) M furosemide in lumen and capillaries reduced acidification by 29%. At pH 8--9 in capillaries, sodium transport was normal while acidification was markedly reduced. These data show that low sodium levels impair renal tubular acidification, but they do not support a rigid coupling of these transport processes.

Animals↗
PubMed1977

Phosphate transfer and tubular pH during renal stopped flow microperfusion experiments in the rat.

The loss of 32P-phosphate salts by the luminal compartment of cortical tubules was studied in control and in acetazolamide-infused rats, during stopped-flow microperfusion with 100 mM phosphate-raffinose solutions. When the initial pH of the perfusion solution was low (5.5), phosphate was lost more rapidly from proximal tubules than at high initial pH (8.2). The average half-time of phosphate loss was 31.9 s during acid, and 66.0 s during alkaline perfusion in proximal tubules of control rats; in acetazolamide-infused rats half-times were 77.0 and 86.6 s for acid and alkaline perfusions. Thus acetazolamide infusion slows the rate of phosphate loss by proximal tubules, when the perfusion solution is acid, but has no significant effect if its pH is alkaline. These half-times compare to proximal acidification rates of 7.23 s in control and 13.2 s in acetazolamide-infused rats. In distal tubules of control rats no significant loss of phosphate was observed during the period of perfusion. It is concluded that the loss of phosphate, in proximal tubules, is markedly slower than the changes in tubular pH and so its effect on tubular acidification must be of minor importance. In distal tubules changes in pH are not due to transepithelial phosphate movement.

Acetazolamide↗