Erich E. Windhager

RENAL TUBULAR TRANSFER OF SODIUM, CHLORIDE AND POTASSIUM.

T HIS review is concerned with some of the new developments in the area of renal tubular transport of sodium, chloride and potassium, particularly those dealing with the tubular transfer of these ions at the level of the single nephron. This latter approach has attracted considerable attention recently since it can provide information on many physiologic parameters which are not otherwise open to investigation. Most importantly, studies on single nephrons have provided a direct method to deal with many controversial problems in the area of electrolyte transport. Thus, such an approach has yielded unique insight into the cellular mechanisms of transtubular electrolyte and water movement, particularly with regard to the electrochemical driving forces involved in ion transfer across the tubular cell membranes. Furthermore, studies on single nephrons have firmly established the existence of the countercurrent nature of the urinary concentrating mechanism and have excluded the transtubular colloid-osmotic pressure gradient as being importantly involved in normal salt and water reabsorption. Finally, although recent investigations have provided a considerable body of data concerned with the contribution of various tubular segments to the urinary excretion pattern of sodium, chloride and potassium ions, it becomes clear from the present discussion that many important problems await clarification.

Selectivity of the Renal Collecting Duct Water Channel Aquaporin-3

Aquaporin-3 (AQP3) is a water channel found in the basolateral cell membrane of principal cells of the renal collecting tubule as well as in other epithelia. To examine the selectivity of AQP3, the permeability to water (Pf), urea (Pur), and glycerol (Pgly) of Xenopus oocytes injected with cRNA encoding AQP3 was measured. Oocytes injected with cRNA encoding either human or rat aquaporin-1 (AQP1) were used as controls. Although both aquaporins permit water flow across the cell membrane, only AQP3 was permeable to glycerol and urea (Pgly > Pur). The uptake of glycerol into oocytes expressing AQP3 was linear up to 165 mM. For AQP3 the Arrhenius energy of activation for Pf was 3 kcal/mol, whereas for Pgly and Pur it was >12 kcal/mol. The sulfhydryl reagent p-chloromercuriphenylsulfonate (1 mM) abolished Pf of AQP3, whereas it did not affect Pgly. In addition, phloretin (0.1 mM) inhibited Pf of AQP3 by 35%, whereas it did not alter Pgly or Pur. We conclude that water does not share the same pathway with glycerol or urea in AQP3 and that this aquaporin, therefore, forms a water-selective channel.

Intracellular calcium ions as regulators of renal tubular sodium transport

SummaryThis review addresses the putative role of intracellular calcium ions in the regulation of sodium transport by renal tubules. Cytoplasmic calcium-ion activities in proximal tubules of Necturus are less than 10−7 M and can be increased by lowering the electrochemical potential gradient for sodium ions across the peritubular cell membrane, or by addition of quinidine or ionomycin to peritubular fluid. Whereas lowering of the peritubular Na concentration increases cytosolic [Ca++] and [H+], ionomycin, a calcium ionophore, raises intracellular [Ca++] without decreasing pHi. The intracellular calcium-ion level is maintained by transport processes in the plasma membrane and membranes of intracellular organelles, as well as by calcium-binding proteins. Calcium ions inhibit net transport of sodium by reducing the rate of sodium entry across the luminal cell membrane. In the collecting tubule this inhibition is caused, at least in part, by an indirect reduction in the activity of the amiloride-sensitive sodium channel.ZusammenfassungÜbersicht über die Rolle intrazellulärer Calcium-Ionen in der Regulation des Natriumtransportes der Zellen von Nierentubuli.Die Aktivität zytoplasmatischer Calcium-Ionen in den proximalen Tubuli des Necturus liegen unter 10−7 M und können durch eine Absenkung des elektrochemischen Potentialgradienten für Natrium-Ionen über die peritubuläre Zellmembran erhöht werden. Zusatz von Chinidin oder Ionomycin zur peritubulären Flüssigkeit haben den gleichen Effekt. Die Verminderung der peritubulären Natriumkonzentration erhöht (Ca++) und (H+) im Zytosol, Ionomycin, ein Calcium-Ionophor erhöht (Ca++) intrazellulär ohne Verminderung von pHi. Der intrazelluläre Calcium-Ionengehalt ist durch Transportprozesse aufrecht erhalten, die in der Plasmamembran ablaufen sowie in Membranen der intrazellulären Organellen, jedoch sind auch calciumbindende Proteine von Bedeutung. Calcium-Ionen hemmen den Nettotransport von Natrium durch Verminderung der Natriumeintrittsrate über die luminale Zellmembran. Im Sammelrohr wird diese Hemmung zumindesten zum Teil durch eine indirekte Reduktion der Aktivität des amiloridsensitiven Natrium-Kanals bewirkt.

Transport Functions of the Distal Convoluted Tubule

The distal convoluted tubule (DCT) has traditionally been described as the nephron segment extending from the macula densa to the first confluence with another DCT to form a collecting tubule. Virtually all data on DCT function derive from in vivo studies in the rat using micropuncture or microperfusion techniques. Rat DCT are 2.4–2.5 mm in length(1,2) They can be identified on the kidney surface with light microscopy by their contrast to proximal tubules: the lumina are narrower and the contour more irregular than in proximal tubules. Distal tubular epithelium lacks a brush border and therefore fails to exhibit the light reflex seen in proximal tubules.(3) Eighty percent of the rat DCT is accessible to micropuncture, with only the initial 20% below the kidney surface. At present, the rat provides the most convenient model for studies of DCT function. While the in vitro perfused rabbit nephron technique has been applied extensively to study the function of other tubular segments, the technique is not readily applied to the DCT because of its short length in the rabbit.

URINE CONCENTRATION AND DILUTION

For the cells in the body to function properly they must be working in the optimal environment. This includes a correct and optimal fluid osmolarity, which is the concentration of electrolytes and other solutes in the plasma. This is sometimes referred to as the water balance, since, to a large extent, extracellular fluid sodium concentration and osmolarity are regulated by the amount of extracellular water.

CYTOSOLIC Ca ION ACTIVITY IN PROXIMAL TUBULAR CELLS OF NECTURUS KIDNEY

Publisher Summary This chapter describes that the level of ionized calcium (Ca) is one of the critical factors in the regulation of cellular functions. Models of ion and water transport in epithelia have assumed that the cytosolic Ca-ion level of epithelial cells is similar to that of excitable tissues and is much lower than the Ca-ion level in extracellular fluid. It has been proposed that a process of Na–Ca exchange across the basolateral cell membrane operates to maintain low cytosolic Ca-ion activity and is involved in the regulation of Na and water transport in epithelial cells. To test this hypothesis and its inherent assumptions, Ca-ion activity is measured in proximal tubular cells of Necturus kidneys perfused with Ringers solution containing normal or low Na concentrations. It is suggested that an increase in cytosolic Ca 2+ activity, secondary to a change in Na–Ca exchange, leads to an inhibition of net Na transport by Necturus proximal tubule.

INHIBITION OF FLUID REABSORPTION AND SODIUM EFFLUX BY LOW PERITUBULAR Na AND BY A23187 OR QUINIDINE IN ISOLATED PERFUSED PROXIMAL TUBULES OF THE RABBIT

Publisher Summary Evidence suggests the level of cytosolic calcium ion activity ([Ca 2+ ] cyt ) modulates trans epithelial Na transport in several epithelia. In tight epithelia, maneuvers which raise [Ca 2+ ] cyt , such as removal of serosal Na or addition of either quinidine or the calcium ionophore A21387 to the serosal side, lead to a calcium-dependent inhibition of short-circuit current and hence of the transepithelial Na transport rate. The chapter reviews whether these experimental maneuvers affect water and Na reabsorption by the proximal tubule, a leaky epithelium which transports large quantities of iso-osmotic fluid. Results are consistent with the view that low peritubular [Na] decreases the Na-Ca exchange at peritubular cell membranes and consequently increases [Ca 2+ ] cyt . This chapter highlights that maneuvers thought to raise cytosolic calcium ion activity decrease trans epithelial sodium efflux and net fluid reabsorption in isolated perfused proximal convoluted tubules of the rabbit kidney. The results are consistent with the view that changes in [Ca 2+ ] cyt regulate the transport of sodium and water across the proximal tubular epithelium.