G. Rumrich

Phenomenologic description of Na+, Cl− and HCO 3 − absorption from proximal tubules of the rat kidney

SummaryProximal tubules of the rat kidney were perfused in vivo with NaCl-NaHCO3 Ringers solution and the net rates of fluid absorption from Gertz shrinking drops were measured as well as the stationary electro-chemical potential differences for Na+ and Cl− that develop across the tubular wall during constant fluid absorption. By altering the rate of fluid absorption through addition of raffinose to the peritubular perfusate or to the lumen fluid, the relations between the net ion fluxes and the electrochemical potential differences were obtained for Na+, Cl− and HCO3−. From these relations which were reasonably linear for Na+ and Cl− over small deviations from equilibrium, single ion reflection coefficients and active transport rates were calculated. Since the calculations required a knowledge of the permeability coefficients of the tubular wall for Na+ and Cl−, in a separate series of experiments these coefficients were determined from tracer flux experiments. The calculations yield σNa=0.7, and σCl=0.5

Sodium dependence of the amino acid transport in the proximal convolution of the rat kidney

\sigma _{HCO_2 }

Specificity and sodium dependence of the active sugar tansport in the proximal convolution of the rat kidney

can be estimated to be substantially greater than σCl. Comparing the active transport rates to the net fluid absorption under conditions similar to free flow in the normal kidney, the following conclusions can be drawn: approximately one third of the sodium is resorbed by active transport, one third by electrical transference and one third by solvent drag. Chloride transport is entirely passive. One half of the chloride is resorbed by diffusion and one half by solvent drag. Bicarbonate transport appears to be entirely active, and the active transport rate is greater than the net transport pointing to passive bicarbonate back flux.

Coupling between proximal tubular transport processes

SummaryThe fluid reabsorption from the proximal convolution of the rat kidney was measured with the Gertz shrinking droplet technique. Simultaneously, the peritubular capillaries were perfused with artificial solutions. In some experimental series, fluid from the shrinking droplet was withdrawn and analysed for Cl−, Na+, and osmolality so that the transtubular transport of Na+, Cl−, and HCO3− could be calculated. Capillary perfusate in some experiments was also withdrawn and its pH was measured. The following results were obtained: 1. With increasing concentration of HCO3− in the capillary perfusate, the transtubular water, sodium, chloride, and bicarbonate reabsorption increased. 2. The sulfonamide buffers sulfamerazine and glycodiazine (Redul®), which easily penetrate the tubular wall, could, in equimolar concentrations, substitute totally for the bicarbonate buffer in promoting isotonic fluid absorption. 3. Butyrate, propionate, and acetate were also effective; pyruvate, lactate, and paraaminohippurate, however, were not. 4. The effect of HCO3− and glycodiazine on isotonic absorption was shown to depend exclusively on the concentration of the buffer anion and not on the concentration of undissociated acid or pH. From these data it is suggested that for proximal isotonic absorption of water, sodium, and chloride, the reabsorption of buffer anions via H+ secretion and nonionic diffusion may be essential. The H+ secretion or the buffer anion absorption across the luminal cell wall may secondarily influence the active Na+ transporting mechanism located at the basal cell site either by a luminal H+−Na+ exchange mechanism or by a lyotropic effect which would increase the Na+ permeability of the luminal cell site. Thereby more Na+ would be delivered to the Na+ pumping site and the rate of Na+ pumping would be augmented.

Secretion and contraluminal uptake of dicarboxylic acids in the proximal convolution of rat kidney

SummaryWith the technique of stop flow microperfusion with simultaneous capillary microperfusion the zero net flux transtubular concentration differences (Δc) of labelled amino acids which are equivalent to their active transport rates were measured. Alll-amino acids tested (phenylalanine, histidine, aminobicycloheptane-carboxylic acid, aminoisobutyric acid; lysine, ornithine, arginine; aspartic acid; proline and glycine) showed a considerable Δc, i.e. active transport rate. When, however, the ambient sodium was replaced by choline the Δc values dropped to zero. An analysis of the Na+ dependence of the ornithine transport revealed that the sodium-dependence is of the mixed type, i.e. thatKmdecreased andVmax increased with increasing Na+ concentration to the same extent.In contrast to other biological systems no mutual interaction between the Na+-dependentd-glucose andl-histidine transport could be observed.Incidental to these studies it was observed that the active transport rate ofd-histidine was in the range of 40% of that of thel-isomer while ford-phenylalanine it was only in the range of 10% of the active transport of thel-isomer. Furthermore it was found that thel-aspartic acid transport was already saturated at a luminall-aspartic acid concentration of 0.05 mmol/l while that ofl-phenylalanine was not saturated even at a luminal concentration of 9 mmol/l.

Renal phosphate transport: inhomogeneity of local proximal transport rates and sodium dependence.

SummaryUsing the stop flow microperfusion technique with simultaneous capillary perfusion the rate of active Ca2+ reabsorption was evaluated by measuring the static head electrochemical potential difference as well as the permeability of the tubular wall for Ca2+ ions. Under control conditions the active Ca2+ transport was calculated to be 3.35×10−13 mol/cm·s. It declined toward zero if the ambient Na+ was replaced by choline or lithium. Parallel experiments in the golden hamster showed that active Ca2+ transport, vanished completely if active Na+ transport was blocked by ouabain (1 mM). These data indicate that the active Ca2+ reabsorption from the proximal tubule depends on the active reabsorption of Na2+ presumably via a Na+−Ca2+ countertransport at the contraluminal cell membrane. The static head electrochemical potential difference of Ca2+ is the same in late and early proximal tubules. It is also not affected by the presence of acetazolamide (10−4 M) by the absence of bicarbonate or glycodiazine buffer or by the absence or presence of phosphate (2 mM).

Bisubstrates: substances that interact with both, renal contraluminal organic anion and organic cation transport systems

SummaryWith the technique of stop flow microperfusion with simultaneous capillary perfusion, the zero net flux transtubular concentration difference (Δc) of labelled sugars was measured.The following sequence of Δc values, which are a measure for the active transtubular transport rate, were evaluated:d-glucose ≅β methyl-d-glycoside >α-methyl-d-glycoside >d-galactose >3-O-methyl-glucose >d-allose. When 10−4 M phlorrhizin was given in the luminal perfusate the Δcs dropped to zero (±8%). Δc-values in the same range i.e. indicating no active transport, were found for:l-glucose,d-mannose, 2-deoxy-d-glucose,d-fructose,d-glucosamine, 6-deoxy-d-galactose (=d-fucose),d-ribose and the reference polyalcohold-mannitol. Inhibition of thed-galactose δc was achieved by 15 mmol/l of the following sugars: α-methyl-d-glycoside ≅d-glucose ≅ 6-deoxy-d-glucose >3-O-methyl-d-glucose an no significant inhibition byd-xylose andd-mannose. Against Δc of α-methyl-d-glucose the following inhibitory potency was observed:d-glucose >6-deoxy-d-glucose >3-O-methyl-d-glucose ≅d-galactose >d-xylose and no inhibition byd-mannose.When the ambient sodium was replaced by choline, the Δc values of all actively transported sugars dropped toward zero. An analysis of the Na+ dependence of the α-methyl-d-glycoside transport revealed that the sodium dependence is of the affinity type i.e. that onlyKmincreased with increasing Na+ concentration whileVmax remained almost constant.From these data one can conclude: 1. The Crane specificity, i.e. that only the α-position of the OH-group on carbon atom 2 is essential, which was found for the intestinal hexose transport holds for the rat proximal kidney tubule, too. 2. The hexose transport system in the rat works only when Na+-ions are present. The sodium ions augment the affinity of the hexose transport system for the hexoses.

Anion transport through the contraluminal cell membrane of renal proximal tubule. The influence of hydrophobicity and molecular charge distribution on the inhibitory activity of organic anions

SummaryThe rate of active transport by the proximal renal tubule of amino acid (l-histidine), sugar (α-methyl-d-glycoside), H+ ions (glycodiazine), phosphate and para-aminohippurate was evaluated by measuring the zero net flux concentration difference (Δc) of these substances. In the case of calcium the electrochemical potential differenceΔc +zFciΔϕ/RT) was the criterion employed. The rate of isotonic Na+-absorption (JNa) was measured with the shrinking droplet method. The effect of ouabain on the transport of these substances was tested in the golden hamster and the effect of SITS (4-acetamido-4′isothiocyanatostilbene 2,2′-disulfonic acid) was observed in rats.Ouabain (1 mM) applied peritubularly incompletely inhibited JNa (80%), but in combination with acetazolamide (0.2 mM) the inhibition was almost complete (93%). In addition, ouabain inhibited the sodium coupled (secondary active) transport processes ofl-histidine, α-methyl-d-glycoside, calcium and phosphate by more than 75%. It did not affect H+ (glycodiazine) transport and PAH transport was only slightly affected.When SITS (1 mM) was applied from both sides of the cell it inhibited H+ (glycodiazine) transport by 72% and reduced JNa by 38% when given from only the peritubular cell side. SITS (1 mM), however, had no significant affect on H+ secretion and sodium reabsorption if it was applied from only the luminal side. Furthermore it had no affect on the other transport processes tested, regardless of the cell side to which it was applied.When the HCO3− buffer or physically related buffers were omitted from the perfusate the absorption of Na+ was reduced by 66%, phosphate by 44%, andl-histidine by 15%. All the other transport processes tested were not significantly affected.The data are consistent with the hypothesis that the active transport processes of histidine, α-methyl-d-glycoside and phosphate, which are located in the brush border, are driven by a sodium gradient which is abolished by ouabain. This may also apply to the Na+-Ca2+ countertransport located at the contraluminal cell side. The residual Na+ transport remaining in the presence of ouabain is likely to be passively driven by the continuing H+ transport which probably is driven directly by ATP. SITS seems to inhibit the exit step of HCO3− from the cell and secondary to that, the luminal H+-Na+ exchange and consequently the Na+ reabsorption. In the absence of HCO3− buffer in the perfusates the luminal H+-Na+ exchange seems to be affected and the pattern of inhibition of the other transport processes is almost the same as with SITS. The different effects onPi reabsorption observed under these conditions might be explained by possible variations in intracellular pH.