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.

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.

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.

Kinetics of luminal acidification in cortical tubules of the rat kidney.

1. Some kinetic aspects of renal tubular acidification were studied in proximal and distal tubules of the rat kidney by combining stationary microperfusion methods and continuous measurements of luminal pH changes of phosphate or bicarbonate buffers by means of antimony electrodes. The analysis included the measurement of steady-state pH, steady-state buffer concentrations and acidification half-times. From these data, net rates of tubular bicarbonate reabsorption and of H ion secretion were obtained since it was shown that the rate of phosphate acidification provides a realistic estimate of H ion secretion. 2. Experiments were performed in control rats, in animals undergoing metabolic acidosis or alkalosis and in control and acidotic rats receiving the carbonic anydrase inhibitor Diamox. 3. In all experiments, the rates of tubular bicarbonate reabsorption and of phosphate acidification (H ion secretion) were proportional to luminal buffer levels. The changes of luminal acid concentrations followed first-order kinetics. 4. Steady-state transepithelial pH differences were reduced in metabolic alkalosis and after diamox but augmented during metabolic acidosis. 5. Acidification half-times were prolonged in metabolic acidosis and after Diamox but remained similar to control levels in metabolic alkalosis. 6. From the observation that both bicarbonate reabsorption and phosphate acidification are similarly affected by these experimental manoeuvres, it is concluded that H ion secretion plays a key role in both transport processes.

Conductances, diffusion and streaming potentials in the rat proximal tubule.

1. Transtubular potential differences and specific resistances were measured in rat proximal tubules by means of single and double barrelled glass micro-electrodes. 2. Tip localization was made by observation of effective resistance changes measured with double barrelled micro-electrodes upon passage of oil droplets, and by perfusion with choline C1. 3. Mean early proximal p.d.s. of the order of -1 to -2 mV, and late values of +0-5 to +1mV were found. Mean specific resistances ranged from 12 to 15 omega cm2. 4. Diffusion potentials and single ion relative conductances were evaluated, perfusing the lumen with solutions differing only with respect to one salt concentration. Na and K conductances were similar and greater than those of C1. 5. Luminal and peritubular perfusions with hypotonic solutions showed the occurrence of streaming potentials in this structure suggesting the existence of pores lined with negative charges. The effective diameter of these pores appeared to be reduced by hypotonic perfusion, as evidenced by a significant increase in resistance, indicating that the main ion path across this structure is represented by intercellular spaces.