Tubuloglomerular feedback and chloride activity in the juxtaglomerular interstitium in Amphiuma

Fresh-water animals excrete large volumes of dilute urine. The pronounced dilution of the tubular fluid starts in the early distal tubule. In this paper the reabsorptive capacity of the diluting segment is discussed, with special attention to the maximal salt concentration of the reabsorbed fluid. Earlier studies have shown a reabsorption-dependent chloride concentration on the basolateral side of the epithelium (juxtaglomerular interstitium, JGI) at the site of the juxtaglomerular apparatus. In the present study measurements were made of chloride activities in the immediate vicinity of the basolateral side of the tubular epithelial cells at high and low tubular perfusion rates. Extremely high chloride activities (2451 +/- 625 mM (SE), n = 7) were found in the JGI at maximal reabsorption, while more plasma-like values were noted at zero tubular flow. The flow-dependent salt reabsorption may be a powerful signal for tubuloglomerular feedback from the tubular lumen to the effector cells.