Renal transepithelial phosphate secretion: luminal membrane voltage and Ca2+ dependence

The role of apical membrane electrical potential, the possibility of K+ channel involvement, and the role of extracellular Ca2+ in transepithelial P(i) secretion were examined in primary monolayer cultures of flounder renal proximal tubule cells in Ussing chambers. Exposure to 200 nM thapsigargin (TG) significantly increased net P(i) secretion. In TG-stimulated tissues, substitution of 100 mM KCl for 100 mM NaCl in the luminal medium depolarized the apical membrane potential from -64 +/- 2.8 to -26 +/- 3.9 mV and strongly inhibited net P(i) secretion. In 32P(i)-preloaded tissues, cell-to-lumen exit of 32P(i) was significantly decreased to approximately 50% of control by high luminal K+ while cell-to-peritubular bath movement was unchanged. Addition of BaCl2 (2 mM) or charybdotoxin (20 nM) to the luminal surface significantly reduced TG-stimulated net P(i) secretion. The elevation of bath Ca2+ from 2 to 5 mM significantly increased secretory flux and decreased reabsorptive flux. The effect of TG on net P(i) secretion was reduced by the Ca2+ channel blocker verapamil (VE, 100 microM) to 65% of control and by calmodulin antagonist W-7 (20 microM) to 35% of control but it was not blocked by the protein kinase inhibitor H-7 (100 microM). VE also significantly inhibited the P(i) secretion induced by acidification of the peritubular bathing medium. The data indicate that transepithelial P(i) secretion induced by TG is significantly influenced by apical membrane electrical polarity, which may be regulated in part by Ca(2+)-activated K+ channels.