Effects of external potassium concentrations on the cell sodium and potassium contents of isolated rat kidney tubules

The effects of 20 mol/l amiloride, 10 mol/l furosemide and 1 mmol/l ouabain on cell Na and K concentrations were investigated by flame microphotometry in isolated rat medullary collecting tubules and medullary thick ascending limbs (MCT and MAL) as a function of the external potassium concentration [K_e]. The results are expressed as Na and K concentrations per liter cell volume ([Na_c] and [K_c], mmol/l) and relative sodium content, [Na_c]/([Na_c]+[K_c]). From the experimental curves, [K_e]_1/2 is defined as the [K_e] value corresponding to half maximal exchange of K against Na in cells. When [K_e] was 5 mmol/l, the relative Na content was less than 15% in control and amiloride-treated MCT as well as in control and furosemide-treated MAL, and about 24% in ouabain-treated MCT and MAL. In MCT, relative cell Na content increased up to 90% or more when [K_e] was reduced from 2.5 to 0.25 mmol/l. [Ke]_1/2 was 0.55, 0.45 and 1.25 mmol/l for control, amiloride-treated and ouabain-treated MCT respectively. In MAL, similar increases in relative Na content were observed when [K_e] was reduced from 0.5 to 0.05 mmol/l. [K_e]_1/2 was 0.25, 0.10 and 1.75 mmol/l for control, furosemide-treated and ouabain-treated MAL respectively. When [K_e] was reduced from 5 to 1 mmol/l, [Na_c] dropped from 16.4 to 8.4 mmol/l (P<0.01) in control MAL. When [K_e] was 5 mmol/l, [Na_c] was lower in furosemide-treated MAL (7.8 mmol/l) than control MAL (P<0.01). At 1 mmol/l [K_e], [Na_c] was similar in both groups.These results are discussed in terms of the balance between the active and passive components of Na and K fluxes across apical and basolateral cell membranes. They indicate that a K-dependent passive Na entry process exists in the membranes of MAL cells but not of MCT cells. This process was proportionally more inhibited than the active Na pump when [K_e] was reduced from 5 to 1 mmol/l. In addition, it was found sensitive to furosemide. It presumably corresponds to the luminal 1 Na-1 K-2 Cl contransport mechanism known to exist in the thick ascending limb.