Potassium transfer in distal tubule of normal and remnant kidneys

As renal mass is lost, adaptive changes occur within the residual tissue which aid in the maintenance of external potassium balance. Recently, it has been suggested that the collecting system is the nephron site wherein these adaptive changes occur. These conclusions, however, were based on observations made in rats in which the superficial distal tubule was not a source of potassium addition to the urine. The present studies examined the effect of a reduction in renal mass on potassium transport in the superficial distal tubule of rats in which this site is a potential source of urinary potassium. At 10 to 14 days after sham operation or partial nephrectomy, the glomerular filtration rate (GFR) (ml/min) in sham-operated control rats (stage I), 2.40±0.26 (SE), was comparable to be value of 2.37±0.23 after 25% nephrectomy (stage II). Both values were significantly greater than the GFR of rats with 75% nephrectomy, 1.25±0.12 (stage III). Absolute rates of potassium excretion (μeq/min) were similar: stage I, 1.52±0.22; stage II, 1.71±0.02; stage III, 1.69±0.20. Absolute delivery of potassium to the late distal tubule of stage III rats, 127±15.2 peq/min, was greater than that of stage I rats, 39±4.4 (P<0.005) as a result of both an increase in potassium delivery to the early distal tubule, 13±2.5 vs. 46±10.7 (P<0.02) and potassium secretion along the distal tubule, 26±3.9 vs. 82±14.6 (P<0.01). The rats with 25% nephrectomy (stage II) were not a homogeneous group and the quantity of potassium secreted in distal tubule, 42±11.8 peq/min, was not different from stage I or III rats. The mean fractional excretion of potassium in the urine (U) was not different from the mean fractional delivery of potassium to the late distal (LD) tubule: stage I LD, 20.7±1.4 vs. (U) 16.1±1.8%; stage II LD, 23.5±4.2 vs. (U) 26.1±4.3%; stage III LD, 30.8±3.5 vs. (U) 31.7±2.7%. However, the relationship between the parameters was such that both net addition and net reabsorption of potassium was observed beyond the distal tubule in individual experiments. The results indicate that the distal tubule may contribute to the maintenance of potassium balance in the presence of renal insufficiency, and that other factors, e.g., an increase in delivery to the distal tubule, and, on occasion, addition of potassium beyond this site, may also be of importance.