Occupancy of aldosterone binding sites in rat kidney cytosol

The binding sites for aldosterone and a potent aldosterone antagonist (SC-26304) were studied in kidney cytosol from adrenalectomized rats. Preformed cytosol and kidney slices were incubated with 3H-labeled steroids in a wide range of concentrations. The recovery and characteristics of the binding sites were affected by the incubation and homogenization conditions. High-affinity, Type I mineralocorticoid binding was reduced by more than 95% when cytosol was incubated at 25 degrees C in the presence of calcium. Tissue dilution also affected the binding sites. SC-26304 was bound to high- and low-affinity receptors, similar to the binding of aldosterone. The physiologic response to aldosterone could result from binding to either or both sets of sites. Some of the physiologic responses to spirolactones could represent antagonism of the binding of aldosterone to either or both sites. A convenient method is presented for describing the relative occupancy of several different sites by any particular steroid.

Ion selectivity and proximal salt reabsorption

Electrophysiological techniques were used in isolated perfused superficial (S) and juxtamedullary (JM) rabbit proximal convoluted tubules (PCT) to examine the relative sodium-to-chloride (PNa/PCl) and bicarbonate-to-chloride (PHCO3/PCl) permeability ratios. We found that the great majority of PCT are sodium selective and that PHCO3/PCl depends on the experimental conditions. In the presence of active sodium transport, PHCO3/PCl is high and increases with PNa/PCl. When PHCO3/PCl is determined after inhibition of active sodium transport or at 25 degrees C, PHCO3/PCl approximates the free solution anion mobility ratio of 0.5 and is independent of PNa/PCl. The difference between PHCO3/PCl determined in the presence of and in the absence of active transport suggests that the lowering of bath bicarbonate concentration in the presence of active transport changes both paracellular and transcellular current flow. In addition, we found that during luminal perfusion with high chloride, low bicarbonate, organic solute-free solutions, the transepithelial electrical potential depends on PNa/PCl and PHCO3/PCl. This potential is approximately 4.0 mV in S PCT with low PNa/PCl and falls progressively to zero in JM PCT with high PNa/PCl. From these data we conclude that anion concentration gradients drive an important diffusive flux of sodium chloride through the paracellular pathway only in PCT with low PNa/PCl ratios.

Contribution of leaked load to solute transport by renal tubules

Renal tubules reabsorb solutes from the glomerular filtrate. The relationship between "filtered load" and reabsorption has been previously discussed and analyzed in detail. One aspect which has not been emphasized, however, is that, when reabsorption of a solute causes its concentration (or activity) in the tubule lumen to decrease below the level in the blood, solute may enter the tubule down this concentration gradient adding a "leaked load" to the filtered load. The leaked load should be taken into account when quantifying tubular transport. In the present study we derived equations for estimating the leaked load and its contribution to transport. The importance of the leaked load of glucose in the rabbit proximal convoluted tubules is evaluated with parameters derived from in vitro perfusion and by solving the equations numerically. It is shown that, depending on the conditions, the leaked load of glucose may account for a substantial part of the glucose present in the tubule lumen and reabsorbed from the tubule. Also, the leaked load could conceivably be an important factor in the transport of other solutes such as lactase and bicarbonate in proximal tubules.

Urinary acidification: CO2 transport by the rabbit proximal straight tubule

Proximal straight tubules from rabbit kidneys were perfused in vitro in order to study transport of bicarbonate. Total CO2 content was measured in perfused and collected tubule fluid, using microcalorimetry. When the initial perfusate and bath contained 25 mM bicarbonate, the concentration of total CO2 decreased in the collected tubule fluid, indicating net reabsorption of bicarbonate. When the initial perfusate contained no bicarbonate and the bath contained 25 mM bicarbonate, total CO2 appeared in the collected tubule fluid. The rate at which total CO2 appeared in the tubule fluid was rapid, indicating a high permeability. Proximal straight tubules from superficial and juxtamedullary nephrons were compared and found to differ in permeability to CO2 and in transport rate. This functional heterogeneity may affect urinary acidification when there is redistribution of renal blood flow.