CA2- INFLUENCE OF TRANSEPITHELIAL VOLTAGE(, MG2+ AND K+ TRANSPORT IN THE CORTICAL AND MEDULLARY THICK ASCENDING LIMB OF THE RAT NEPHRON )

Isolated segments of rat cortical (cTAL) and medullary (mTAL) thick as cending limbs were microperfused and the transepithelial net fluxes (J (x)) were determined by measuring the composition of the collected flu id with an electron microprobe. When perfused with symmetrical solutio ns both segments showed similar J(Na) and J(Cl) and lumen-positive tra nsepithelial voltage (V(te)=7 - 8 mV). J(Mg), J(Ca) and J(K) were not significantly different from zero. When perfused with asymmetrical sol utions (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 m TAL, and a marked increase in J(K) in both segments. It is concluded t hat, 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-dep endent. The mTAL can reabsorb neither Mg2+ nor Ca2+, whatever V(te).