Ulrich C. Dubach

Distal tubular segments of the rabbit kidney after adaptation to altered Na- and K-intake

SummaryThe baso-lateral cell-membrane area in kidney tubules appears to be associated with the capacity for electrolyte transport; in the rabbit, it decreases from the distal convoluted tubule (DCT-cells) over the connecting tubule (CNT-cells) to the cortical collecting duct (principal cells).Adaptation to low Na-, high K-intake changes this pattern: CNT-cells at the beginning of the connecting tubule have the highest membrane area, which decreases along the segment, but remains two-fold higher than in controls. Principal cells have a four-fold higher membrane area than in controls. Simultaneous treatment with the antimineralocorticoid canrenoate-K inhibits the structural changes in CNT-cells only in end-portions of the connecting tubule and in principal cells.After prolonged high Na-, low K-intake DCT-cells display a two-fold higher membrane area than controls, while CNT-cells and principal cells are not affected. Simultaneous treatment with DOCA does not affect the DCT-cells but provokes a moderate increase in membrane area in CNT-cells, and a 5.5-fold increase in principal cells.The data provide evidence that DCT-, CNT- and principal cells are functionally different cell types. The baso-lateral cell-membrane area, associated with electrolyte-transport capacity, appears to be influenced in DCT-cells mainly by Na-intake, in CNT-cells mainly by K-intake and in part also by mineralocorticoids, and in principal cells mainly by mineralocorticoids.

Sodium gradient-energized concentrative transport of adenosine in renal brush border vesicles

The uptake of adenosine in brush border vesicles of the proximal tubule of the rat kidney has been studied with a filtration technique. The initial rate of uptake was almost 6 times greater in the presence of NaCl than in the presence of KCl. The stimulatory effect of Na+ was strictly dependent on a gradient of Na+ (out>in). The time course of uptake showed an overshoot with a maximum at 20 s with a gradient of NaCl, but not with KCl. Inosine and 5′-AMP were produced from adenosine within the vesicles. In the presence of an inhibitor or adenosine deaminase adenosine was not significantly metabolized during the first 20 s of uptake. Thus, kinetic parameters of transport could be studied in the absence of interferences with metabolism. AKm of 1.1 μM and aVmax of 232 pmol · min−1 · mg protein−1 were calculated for the Na+ gradient-dependent transport. The dependency on a Na+ gradient, the capacity for uphill transport and the high affinity for adenosine situate this transport system apart from the mechanisms of transport of nucleosides described so far. It may be relevant in regard to the role of adenosine in the regulation of glomerular filtration.

NaK stimulated adenosinetriphosphatase: Intracellular localisation within the proximal tubule of the rat nephron

SummaryWith refinements of quantitative histochemistry, i.e. oilwell technique, enzymaticPianalysis and NADP/NADPH cycling an enzymatic polarization of the tubular epithelial cell from the rat nephron to NaKATPase can be established. NaKATPase activity is limited to the basal area of the epithelial cell. Brush border fragments lack NaKATPase activity.

Uphill transport of pyrimidine nucleosides in renal brush border vesicles

The uptake of cytidine, of thymidine and of uridine in brush border vesicles prepared from the cortex of rat kidney has been studied by the technique of rapid filtration. The nucleosides were not metabolized in the vesicles. The time-courses of uptake in the presence of inwardly directed gradients of Na+ and of K+ showed an overshoot, indicating uphill transport. The overshoot was much more pronounced with Na+ than with K+; it was not observed when Na+ was at equilibrium across the membrane. The uptake of the nucleosides was stimulated by an inside negative potential in the presence of Na+. These results provide evidence for a cotransport of pyrimidine nucleosides with Na+. The apparentKms for the uptake of cytidine, of thymidine and of uridine were 3.76 μmol · l−1, 4.18 μmol · l−1 and 7.21 μmol · l−1 respectively. The uptake of the pyrimidine nucleosides was insensitive to 6-nitrobenzylthioinosine. This insensitivity as well as the high affinity for the nucleosides and the capacity for uphill transport indicate that the nucleoside carrier(s) is renal brush border is (are) different from the carriers found in most other cell types.

Transport of Adenosine by Renal Brush Border Membranes

SummaryThe transport of adenosine into brush border membrane vesicles from rat kidney is demonstrated. A first, rapid stage of transport is completed by 20 s. It is stimulated by a gradient of sodium. The effect of the concentration of adenosine in the low micromolar range on the initial rate of uptake indicates a saturability. An apparent Km of 1.48 μM and a Vmax of 215 pmol/mg protein/min have been calculated. In the rapid stage of uptake, adenosine is transported into the intravesicular space with no significant binding to the membrane. The following slow uptake is not stimulated by sodium and reaches steady state after about one hour.

Induction of Na K ATPase in the proximal and distal convolution of the rat nephron after uninephrectomy

SummaryUsing refinements of quantitative histochemistry, i.e. oil-well technique and enzymaticPianalysis combined with an enzymatic cycling system, the activity of Na K ATPase (E.C. 3.6.1.3), an enzyme which is integrated in the transmembrane cation transport, was measured in single dissected segments of the proximal and distal convolution from the superficial nephron of the rat kidney following uninephrectomy. In the distal convolution an acute rise in Na K ATPase activity was apparent already during the first day after uninephrectomy, whereas in the proximal convolution the first significant increase occurred seven days after surgery. These data indicate a rapid adaptation of Na K ATPase which is limited to the distal tubule. The difference between the proximal and distal tubule confirms the functional correlations to uninephrectomy with more reabsorptive work in the distal tubule per unit length. Na K ATPase seems to have a special functional meaning for the distal tubule in regulating cation transport.

Activities of enzymes of the tricarboxylic acid cycle in segments of the rat nephron

The activities of enzymes of the tricarboxylic acid cycle were measured in order to compare the respiratory capacity in different parts of the nephron of the rat. Oxoglutarate dehydrogenase, citrate synthase and isocitrate dehydrogenase were assayed in single nephron segments dissected out of freeze-dried cryostat sections. The activities of the three enzymes per unit weight are higher in the distal segments (thick ascending limb and distal convoluted tubule) than in the proximal tubule. The distal vs. proximal ratios of activities are about 1.5, 2.5 and 2 for oxoglutarate dehydrogenase, citrate synthase and isocitrate dehydrogenase, respectively. Oxoglutarate dehydrogenase shows the lowest activities along the whole nephron and appears to catalyze the rate-limiting step of the tricarboxylic acid cycle. The possibility to estimate the respiratory capacity in the different segments of the nephron on the basis of the activity of oxoglutarate dehydrogenase is discussed. The capacity calculated for the proximal tubule (between 44 and 66 μmol O · min−1 · g−1, depending on the substrate) is in agreement with direct measurements of oxygen consumption as well as with calculations made on the basis of morphometrical data.