K transport in upper portion of descending limbs of long-loop nephron from hamster

By use of the in vitro microperfusion technique, we investigated potassium transport in the upper portion of the descending limb of the long-loop nephron isolated from hamster kidney. The net potassium flux determined by ultramicro-flame photometry was -0.63 +/- 1.84 pmol X mm-1 X min-1, a value that was not significantly different from zero. The salt permeability for KCl was calculated from the amount of potassium entering the tubular lumen when the concentration of potassium in the bath was increased by approximately 5 mM. The value was 38.9 +/- 1.9 X 10(-5) cm X s-1. The bidirectional fluxes of 86Rb were measured as indices of potassium fluxes. Flux coefficients from lumen-to-bath and bath-to-lumen were 51.2 +/- 9.2 and 48.8 +/- 13.5 X 10(-5) cm X s-1, respectively. These values were not significantly different, confirming that there was no net flux for 86Rb. In another series of five experiments, the lumen-to-bath 86Rb flux coefficients were 69.4 +/- 13.2 X 10(-5) cm X s-1 in the absence of unlabeled Rb, and 70.2 +/- 13.9 X 10(-5) cm X s-1 in the presence of 5 mM unlabeled Rb. The lumen-to-bath 42K flux coefficient measured in the same series of animals was 85.3 +/- 10.5 X 10(-5) cm X s-1 (n = 10), a value that is slightly higher than, but not significantly different from, that of 86Rb. These data show that active potassium transport may exist in this segment and that the passive permeability for potassium is very high.(ABSTRACT TRUNCATED AT 250 WORDS)