EXPRESSION OF PROXIMAL TUBULAR NA-P-I AND NA-SO4 COTRANSPORTERS IN MDCK AND LLC-PK1 CELLS BY TRANSFECTION

Two cDNAs coding for proximal tubular Na-P-i cotransport (NaPi-2) and Na-SO4 cotransport (NaSi-1) have been transfected by the use of a dexa methasone-inducible vector (pLK-neo) into MDCK and LLC-PK1 cells. By r everse transcription-polymerase chain reaction, expression of correspo nding mRNAs was observed after stimulation with dexamethasone only. Si milarly, expression of the NaPi-2 protein was detected only after indu ction with dexamethasone. In transfected Madin-Darby canine kidney (MD CK) cells, dexamethasone induced a large increase of Na-Pi or Na-SO4 c otransport, whereas, in transfected LLC-PK1, cell transport was only m inimally expressed. In MDCK cells grown on filter supports, transfecte d Na-P-i-cotransport activity was equally expressed at both cell surfa ces; dual location of expressed NaPi-2 protein was also observed by im munohistochemistry. In contrast, transfected Na-SO4 cotransport activi ty was predominantly expressed at the apical cell surface of MDCK cell s. The results demonstrate that 1), in MDCK cells, the sorting behavio r of two proximal tubular cotransport systems seems to be different: a pical for Na-SO4 cotransport (NaSi-1) and dual location for Na-P-i cot ransport (NaPi-2); and 2) LLC-PK1 cells seem not to be a suitable syst em to functionally express sodium-dependent cotransport systems for ph osphate and sulfate.