Magnesium transport in the cortical thick ascending limb of Henle's loop of the rabbit

of Henle's loop were perfused in order to directly evaluate magnesium transport in this segment. Trans-epithelial potential difference was altered by varying the NaCl concentration in perfusate and bath and add-ing 50,uM furosemide to the perfusate. Perfusion un-der standard conditions with isotonic solutions resulted in a mean transepithelial potential difference of +8.8±0.7 mV and net magnesium absorption at a rate of 0.32±0.06 pmol/mm per min. Perfusion with a hy-potonic solution significantly increased potential dif-ference and the net absorptive rate of magnesium, calcium, and potassium. Conversely, reversal of the polarity of the potential difference with low NaCl bath and luminal furosemide produced net secretion of magnesium, calcium, and potassium. Parathyroid hor-mone in a bath concentration of 1.0 U/ml increased magnesium absorption from 0.34±0.06 to 0.63±0.06 pmol/mm per min (P < 0.001) and calcium from 0.52±0.08 to 0.97±0.08 pmol/mm per min (P < 0.001). Dibutyryl cyclic AMP produced similar ef-fects on both calcium and magnesium absorption. In-creasing bath calcium concentration twofold significantly inhibited net calcium absorption from 0.79±0.27 to 0.16±0.02 pmol/mm per min but mag-nesium transport was unaffected. Increasing bath mag-nesium concentration twofold significantly inhibited net magnesium absorption from 0.56±0.14 to-0.09±0.13 pmol/mm per min but had no effect upon net calcium transport. Net absorption of magnesium was significantly increased with increased concentra-Portions of this study were presented at the 12th Annua