Na-Pi cotransport sites in proximal tubule and collecting tubule of winter flounder (Pleuronectes americanus)

Localization of a recently described and cloned Na-Pi cotransport system from flounder was investigated by reverse transcription-polymerase chain reaction (RT-PCR) of microdissected tubules and by immunocytochemistry of kidney of winter flounder. Histological examination showed a small glomerulus, an extremely short proximal tubule PI with a selective affinity to Lens culinaris agglutinin from lentils, and an extensive second proximal tubule segment PII (> 90% of proximal tubules), consisting of cells with numerous apical clear vesicles and extensive amplification of basolateral cell membranes. PII merged with the collecting tubule/ collecting duct (CT/CD) system without a distal segment. By RT-PCR, PII cells revealed high levels of NaPi-II related RNA; low levels were also observed in CTs. Previously characterized antisera against different epitopes of flounder NaPi-II specifically labeled the basolateral regions of PII and the apical cell portion of CT/CD cells and of some PII cells. These results suggest that tubular secretion of P(i) occurs in PII of teleost fish with modulation of urinary P(i) content in the subsequent CT/CD system.