Regulation of the Human Brush Border Na<sup>+</sup>/Phosphate Cotransporter (NaPi-3) Expressed in Xenopus Oocytes by Intracellular Calcium and Protein Kinase C

Abstract

Tubular phosphate reabsorption is strongly regulated by parathyroid hormone (PTH) via intracellular activation of protein kinase A (PKA) and C (PKC). In the present study, we expressed the human renal brush border Na⁺-dependent phosphate (Pi) transporter (NaPi-3) in <i>Xenopus </i>oocytes and analyzed its regulation by coexpressed PTH receptors, activation of PKA and PKC and increase of [Ca²⁺]i. Stimulation of coexpressed PTH receptors (with 10 nMPTH 1-34) or activation of PKA (with dibutyryl-cAMP) had no effect on I_p. Increasing [Ca²⁺]i with the calcium ionophore A23187 reduced I_p. Ca²⁺-mediated I_p inhibition was prevented in the presence of staurosporine, but not by the calmodulin-antagonist calmidazo-lium. Activation of PKC by the racemate of phorbol-12, 13-didecanoate (PDD) mimicked the effect of A23187 and decreased Pi-induced currents (I_p). The inactive enantiomere α-phorbol-12, 12-didecanoate (α-PDD) had no effects on I_P. An increase in [Ca²⁺]i accelerated PDD-mediated I_P-inhibition, but [Ca²⁺]i and PDD effects were not additive. It is concluded that [Ca²⁺]i and PDD inhibit I_P via activation of PKC, while activation of PKA has no functional consequence for NaPi-3 expressed <i>Xenopus </i>oocytes.