Ca2+, Mg2+ and K+ transport in the cortical and medullary thick ascending limb of the rat nephron: influence of transepithelial voltage

Isolated segments of rat cortical (cTAL) and medullary (mTAL) thick ascending limbs were microperfused and the transepithelial net fluxes (J_x) were determined by measuring the composition of the collected fluid with an electron microprobe. When perfused with symmetrical solutions both segments showed similar J_Na and j_Cl and lumen-positive transepithelial voltage (V_te=7–8 mV). J_Mg, J_Ca and J_K were not significantly different from zero. When perfused with asymmetrical solutions (lumen 50 mM, bath 150 mM NaCl), the mean V_te were 23 mV and 17 mV in the cTAL and mTAL respectively; this rise was accompanied by significant increases in J_Mg and J_Ca in the cTAL, but not in the mTAL, and a marked increase in J_K in both segments. It is concluded that, in the rat, divalent cations can be reabsorbed in the cTAL, and K⁺ can be reabsorbed in the cTAL and mTAL. The transport is voltage-dependent. The mTAL can reabsorb neither Mg²⁺ nor Ca²⁺, whatever V_te.