Piscine PTHrP regulation of calcium and phosphate transport in winter flounder renal proximal tubule primary cultures

Abstract

Multiple factors control calcium (Ca2+) and inorganic phosphate (Pi) transport in the fish nephron, and the recently discovered members of the piscine parathyroid hormone-like protein family are likely participants in such regulatory mechanisms. The effects of an NH2-terminal peptide (amino acids 1–34) of Takifugu rubripes parathyroid hormone-related protein, (1–34)PTHrP, on Ca2+and Pitransport were investigated in winter flounder ( Pseudopleuronectes americanus ) proximal tubule cells in primary culture (fPTCs). RT-PCR performed on RNA extracted from fPTCs and from intact kidney tissue indicated that expression of PTHrP and types 1 and 3 PTH/PTHrP receptors occurred both in vivo and in vitro and that circulating levels of PTHrP measured by specific radioimmunoassay averaged 2.5 ± 0.13 ng/ml. fPTC monolayers were mounted in Ussing chambers, and under neutral electrochemical conditions, addition of 10 nM (1–34)PTHrP to the basolateral side induced a slight increase in Ca2+transport rate from luminal to peritubular side, significantly stimulating net Ca2+reabsorption. (1–34)PTHrP also significantly increased the Pisecretory flux, and slightly reduced Pireabsorption, evoking a significant increase in Pinet secretion. This stimulatory effect was partially inhibited by bisindolylmaleimide, an inhibitor of protein kinase C. Incubation of ex vivo flounder renal tubules with (1–34)PTHrP resulted in apparent reduction of Na+-Picotransporter type II (NaPi-II) protein in tubule membranes. PTHrP seems therefore to participate in the modulation of Ca2+and Pihomeostasis by fish kidney.