NA-P-I COTRANSPORT SITES IN PROXIMAL TUBULE AND COLLECTING TUBULE OF WINTER FLOUNDER (PLEURONECTES-AMERICANUS)

Localization of a recently described and cloned Na-P-i cotransport sys tem from flounder was investigated by reverse transcription-polymerase chain reaction (RT-PCR) of microdissected tubules and by immunocytoch emistry of kidney of winter flounder. Histological examination showed a small glomerulus, an extremely short proximal tubule PI with a selec tive affinity to Lens culinaris agglutinin from lentils, and an extens ive second proximal tubule segment PII (>90% of proximal tubules), con sisting of cells with numerous apical clear vesicles and extensive amp lification of basolateral. cell membranes. PII merged with the collect ing tubule/collecting duct (CT/CD) system without a distal segment. By RT-PCR, PII cells revealed high levels of NaPi-II related RNA; low le vels were also observed in CTs. Previously characterized antisera agai nst different epitopes of flounder NaPi-II specifically labeled the ba solateral regions of PII and the apical cell portion of CT/CD cells an d 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 conten t in the subsequent CT/CD system.