Sodium chloride and water transport in the renal distal tubule of the rainbow trout

Abstract

Isolated segments of the renal tubules from the freshwater trout, Salmo gairdneri, were perfused in vitro to characterize ion and water transport. The distal tubule showed a transepithelial voltage (Vt) positive in the lumen (+17.8 +/- 1.4 mV). Furosemide added to the lumen and Na cyanide and ouabain added to the bath reduced the lumen-positive Vt of the distal tubule. Removal of either Cl- or Na+ from both perfusate and bathing medium abolished the lumen-positive Vt. When the distal tubule was perfused and bathed with isosmotic solution, net water flux (Jv) was nearly zero. Jv and hydraulic conductivity remained low when the osmolality of the bathing fluid was increased with raffinose. Neurohypophysial hormones added to the bath showed no effect. Chloride efflux (lumen to bath, 171.1 +/- 17.1 peq x mm-1. min-1) was significantly higher than chloride influx (bath to lumen, 105.6 +/- 12.3 peq x mm-1 x min-1), suggesting that net chloride reabsorption exists. These results suggest that in the freshwater trout, which lack the loop of Henle, the distal tubule acts as a diluting segment. The presence of sodium, in addition to chloride, is required to generate the lumen-positive Vt in the distal tubule.

Kidney-specific chloride channel, OmClC-K, predominantly expressed in the diluting segment of freshwater-adapted tilapia kidney

“…Nishimura et al (2) found by a perfusion technique that the distal tubule of the freshwater-adapted rainbow trout was nearly impermeable to water, whereas net chloride absorption was demonstrated. This finding suggests that the distal tubule acts as a diluting segment that helps to excrete excess water as dilute urine by extracting only NaCl from the forming urine, as shown by the mammalian TALH.…”

“…It is most probable that Cl Ϫ enters the cell across the apical membrane by way of Na-K-2Cl cotransport as in other Cl Ϫ transporting epithelia, although the apical Na-K-2Cl cotransporter in the distal tubule has not yet been identified in the teleost. According to Nishimura et al (2), furosemide, an inhibitor of Na-K-2Cl cotransport, added to the lumen of the distal tubule of freshwater trout markedly decreased transepithelial ion transport, whereas no change was noted when it was added to the basolateral side. These results suggest that the Cl Ϫ reabsorption mechanism of the distal tubule in freshwater tilapia is the same as that of the mammalian TALH.…”

“…Nishimura et al (2) measured water and ion transport in nephron tubules isolated from freshwater-adapted rainbow trout, Oncorhynchus mykiss. They perfused nephron tubules and measured water and ion transport in the distal tubule.…”

Differential expression of Na+-Cl−cotransporter and Na+-K+-Cl−cotransporter 2 in the distal nephrons of euryhaline and seawater pufferfishes

“…The distal tubule of rainbow trout actively reabsorbs Na ϩ and Cl Ϫ and has been characterized as a diluting segment (37). Although the role of the collecting duct in Na ϩ and Cl Ϫ reabsorption has not been well analyzed in teleosts, the collecting duct is a major site of Na ϩ and Cl Ϫ reabsorption in freshwater-acclimated river lamprey (35) (reviewed in Refs.…”

“…The proximal tubule also secretes Mg 2ϩ and SO 4 2Ϫ that will be excreted in seawater (4,26). The distal tubule, which is nearly impermeable to water and reabsorbs Cl Ϫ , functions as a diluting segment (37). The collecting duct has not been well characterized in either freshwater or seawater fishes.…”