The study of pars recta function by the perfusion of isolated tubule segments

Regulation of renal proximal and distal tubule transport: sodium, chloride and organic anions

Renal tubular transport and its regulation are reviewed for Na(+) (and Cl(-)), and for fluid and organic anions (including urate). Filtered Na(+) (and Cl(-)) is reabsorbed along the tubules but only in mammals and birds does most reabsorption occur in the proximal tubules. Reabsorption involves active transport of Na(+) and passive reabsorption of Cl(-). The active Na(+) step always involves Na-K-ATPase at the basolateral membrane, but the entry step at luminal membrane varies among tubule segments and among vertebrate classes (except for Na(+)-2Cl(-)-K(+) cotransporter in diluting segment). Regulation can involve intrinsic, neural and endocrine factors. Proximal tubule fluid reabsorption is dependent on Na(+) reabsorption in all vertebrates studied, except ophidian reptiles. Fluid secretion occurs in glomerular and aglomerular fishes, reptiles and even mammals, but its significance is not always clear. A non-specific transport system for net secretion of organic anions (OAs) exists in the proximal renal tubules of almost all vertebrates. Net transepithelial secretion involves: (1) transport into the cells at the basolateral side against an electrochemical gradient by a tertiary active transport process, in which the final step involves OA/alpha-ketoglutarate exchange and (2) movement out of the cells across the luminal membrane down an electrochemical gradient by unknown carrier-mediated process(es). Regulation may involve protein kinase C and mitogen-activated protein kinase. Urate is net secreted in the proximal tubules of birds and reptiles. This process is urate-specific in reptiles but in birds, it may involve both a urate-specific system and the general OA system.

Contribution of Na+-H+ exchange to sodium reabsorption in the loop of henle: a microperfusion study in rats

1. The contribution of apical Na+-H+ exchange to sodium reabsorption in the thick ascending limb of the loop of Henle (TALH) in vivo was examined in anaesthetized rats by perfusing loops of Henle of superficial nephrons with solutions containing the Na+-H+ exchange inhibitor, ethyl isopropyl amiloride (EIPA). 2. Using a standard perfusate, no statistically significant effect of EIPA on net sodium reabsorption (JNa) was detected. However, when sodium reabsorption in the pars recta of the proximal tubule was minimized by using a low-sodium perfusate, EIPA reduced JNa from 828 +/- 41 to 726 +/- 37 pmol min-1 (P < 0.05), indicating that apical Na+-H+ exchange can make a small contribution to net sodium reabsorption in the TALH in vivo. This contribution appears to be dependent on the bicarbonate load, since an increase in the latter led to an enhancement of EIPA-sensitive sodium transport. 3. Addition of the Na+-K+-2Cl- cotransport inhibitor, bumetanide, to the low-sodium perfusate reduced baseline JNa to 86 +/- 27 pmol min-1. In this setting, EIPA reduced JNa further, to -24 +/- 18 pmol min-1 (P < 0.05), an effect similar to that seen in the absence of bumetanide. This finding argues against previous suggestions (based on in vitro evidence) that inhibition of the Na+-K+-2Cl- cotransporter leads to an increase in apical Na+-H+ exchange in the TALH.

Cross-talk and the role of KATP channels in the proximal tubule

Over the last few years it has become evident that an assortment of functionally-related, but diverse, KATP channels provide an important and physiologically-regulated determinant of the K conductive pathways in many, if not all, epithelial cells expressed along the nephron. As such, KATP plays central roles in regulating and maintaining a number of transport processes in concert with physiological demands of the kidney. In the renal proximal tubule, KATP channels and changes in the hydrolytic activity of the Na,K-ATPase permit ATP to act as a coupling modulator of parallel Na,K-ATPase-K recycling. The response insures that cell membrane potential, intracellular K activity and cell volume are protected in the face of physiological variations in transcellular ion transport. In addition to demonstrating the physiological relevance of KATP in renal epithelial, these studies have provided a long awaited answer to the underlying mechanism of pump-leak coupling, a universal and essential homeostatic mechanism observed in nearly all salt translocating epithelia.

Microelectrode characterization of the basolateral membrane of rabbit S3 proximal tubule

The purpose of this study was to characterize the basolateral membrane of the S3 segment of the rabbit proximal tubule using conventional and ion-selective microelectrodes. When compared with results from S1 and S2 segments, S3 cells under control conditions have a more negative basolateral membrane potential (Vbl = -69 mV), a higher relative potassium conductance (tK = 0.6), lower intracellular Na+ activity (ANa = 18.4 mM), and higher intracellular K+ activity (AK = 67.8 mM). No evidence for a conductive sodium-dependent or sodium-independent HCO3- pathway could be demonstrated. The basolateral Na-K pump is inhibited by 10(-4) M ouabain and bath perfusion with a potassium-free (0-K) solution. 0-K perfusion results in ANa = 64.8 mM, AK = 18.5 mM, and Vbl = -28 mV. Basolateral potassium channels are blocked by barium and by acidification of the bathing medium. The relative K+ conductance, as evaluated by increasing bath K+ to 17 mM, is dependent upon the resting Vbl in both S2 and S3 cells. In summary, the basolateral membrane of S3 cells contains a pump-leak system with similar properties to S1 and S2 proximal tubule cells. The absence of conductive bicarbonate pathways results in a hyperpolarized cell and larger Na+ and K+ gradients across the cell borders, which will influence the transport properties and intracellular ion activities in this tubule segment.

Isovolumetric regulation of isolated S2 proximal tubules in anisotonic media

Sudden alteration in medium osmolality causes an osmometric change in proximal tubule cell size followed by restoration of cell volume toward normal in hypotonic but not in hypertonic medium. We determined the capability of isolated nonperfused proximal tubules to prevent a change in cell volume in anisotonic media. The external osmolality was gradually changed over a range from 110 to 480 mosM. At 1.5 mosM/min, cell volume remained constant between 167 +/- 9 and 361 +/- 7 mosM, a phenomenon termed isovolumetric regulation (IVR). Cells lost intracellular solutes in hypotonic and gained intracellular solutes in hypertonic media. Raffinose or choline chloride substitution showed that osmolality, rather than NaCl, signalled cell volume maintenance in hyperosmotic media. Cooling (7-10 degrees C) blocked IVR. IVR was maintained when osmolality was lowered at a rate of 27, but not at 42 mosM/min. IVR was not observed when the rate of osmolality increase exceeded 3 mosM/min. We conclude that proximal tubule cells sensitively regulate intracellular volume in an osmolality range of pathophysiologic interest by mechanisms dependent on the rate of net water movement across basolateral membranes and the absolute intracellular content of critical solutes.

Renal glycosuria in renal homograft recipients

Transient renal glycosuria was observed in eight renal transplant patients during the recovery phase from initial tubular necrosis or acute rejection. In these subjects and three homograft recipients without glycosuria we performed glucose titration experiments. Three patients were found to have type A glycosuria, two had type B and three type C. The titration curve was normal in the three patients without glycosuria. In addition, most subjects presented with hypophosphataemia and hyperphosphaturia. Apart from a direct correlation between the point of splay of the glucose titration curves and the fractional clearance of phosphate, there was no clear-cut relationship between the handling of glucose and phosphorus. Mild hyperchloraemic acidosis was observed in six subjects, but this was unrelated to the type and grade of glycosuria. It is concluded that in homograft recipients the tubular alterations have a patchy and unpredictable distribution and may cause a variety of symptoms which do not necessarily occur in close association.

Renal glycosuria in patients with the nephrotic syndrome

Transient or persistent renal glycosuria may occur in patients with the nephrotic syndrome. In an attempt to elucidate its mechanism, glucose titration experiments were performed in 20 nephrotic patients. The type A titration curve was found in one and type B in four patients with severe organic changes and low glomerular filtration rate. The remaining subjects displayed a particular type of curve (type C) characterized by a low point of splay but an otherwise almost physiological tracing. In type B and C patients the maximal rate of reabsorption per ml glomerular filtrate (TmG/GFR) was significantly increased and correlated inversely with the filtration fraction. In these patients the point of splay correlated with the glomerular filtration rate and the sodium clearance, but not with the plasma albumin concentration or the rate of proteinuria. These observations suggest that type A was due to diffuse tubular atrophy, and type B to increased nephron heterogeneity resulting from chronic organic changes. Type C was presumably caused by a potentially reversible alteration of the late proximal or distal glucose transport related to the nephrotic syndrome itself.