TRANSEPITHELIAL CA2-DEPENDENT PROCESS( AND MG2+ TRANSPORT IN THE CORTICAL THICK ASCENDING LIMB OF HENLES LOOP OF THE MOUSE IS A VOLTAGE)

The mechanisms responsible for transepithelial Ca2+ and Mg2+ transport in the isolated perfused cortical thick ascending limb (cTAL) of Henl e's loop of the mouse nephron were investigated by measuring transepit helial voltages (PD(te)) and transepithelial ion net fluxes (J(Na), J( Cl), J(K), J(Ca), J(Mg)) by electron microprobe analysis. In the prese nce of furosemide (10(-4) mol.l-1, lumen) and diphenylamine-2-carboxyl ate (DPC, 10(-4) mol.l-1, bath), known inhibitors of NaCl reabsorption in the TAL, Ca2+ and Mg2+ reabsorption was completely inhibited. In t he presence of furosemide, JCa fell from 0.75+/-0.07 to -0.08+/-0.09 p mol.min-1.mm-1 (n = 5), and J(Mg) from 0.47+/-0.04 to -0.01+/-0.11 pmo l.min-1.mm-1 (n = 5). In the presence of DPC, J(Ca) fell from 0.57+/-0 .08 to -0.07+/-0.11 pmol.min-1.mm-1 (n = 5), and J(Mg) from 0.16+/-0.0 2 to -0.11+/-0.07 pmol.min-1.mm-1 (n = 5). With furosemide, inhibition of Ca2+ and Mg2+ transport was paralleled by a 93% inhibition of NaCl reabsorption, while in the presence of DPC there was a 60% reduction of NaCl reabsorption. These effects were fully reversed after removal of the inhibitors from the lumen or bath solutions. In the absence of active NaCl transport, a lumen-to-bath directed-NaCl gradient (lumen: 150 mM NaCl + furosemide, bath: 50 mM NaCl + 200 mM mannitol) generate d a negative transepithelial dilution potential of -13.8+/-1.1 mV (n = 8) which induced a significant Ca2+ and Mg2+ secretion into the tubul ar lumen of -0.59+/-0.06 and -0.43+/-0.05 pmol.min-1.mm-1 (n = 8), res pectively. A bath-to-lumen-directed NaCl gradient, on the other hand, (lumen: 50 mM NaCl + furosemide, bath: 150 mM NaCl) generated a positi ve transepithelial dilution potential of +15.9+/-0.6 mV (n = 7), induc ing a significant Ca2+ and Mg2+ reabsorption of 0.62+/-0.08 and 0.38+/ -0.07 pmol.min-1.mm-1 (n = 7), respectively. Linear regression analysi s of individual Ca2+ and Mg2 net flux data versus voltage indicated th at J(Ca) and J(Mg) were highly correlated to PD(te). In conclusion, th ese data indicate that transepithelial Ca2+ and Mg2+ reabsorption in t he mouse cTAL is predominantly a passive process, driven by the lumen- positive PD(te).