PRIMARY STRUCTURE AND FUNCTIONAL EXPRESSION OF THE APICAL ORGANIC CATION TRANSPORTER FROM KIDNEY EPITHELIAL LLC-PK1 CELLS

Renal secretion of organic cations involves at least two distinct tran sporters, located in the basolateral and apical membranes of proximal tubule cells. Whereas the basolateral transporter has recently been cl oned, sequence information about the apical type was not yet available . An organic cation transporter, OCT2p, was cloned from LLC-PK1 cells, a porcine cell line with properties of proximal tubular epithelial ce lls. OCT2p was heterologously expressed and characterized in human emb ryonic kidney 293 cells, OCT2p-mediated uptake of the prototypical org anic cation [C-14]tetraethylammonium ([C-14]TEA) into 293 cells was sa turable. There was a highly significant correlation between the K-i va lues for the inhibition of apical [C-14]TEA uptake into LLC-PK1 cells and 293 cells transfected with OCT2p (r = 0.995; p < 0.001; n = 6). Al though OCT2p is structurally related to OCT1r, the basolateral organic cation transporter from rat kidney, the transporters could be clearly discriminated pharmacologically with corticosterone, decynium22, and O-methylisoprenaline. The findings at hand suggest that OCT2 correspon ds to the apical type of organic cation transporter. Reverse transcrip tase-polymerase chain reaction indicates that mRNA of OCT1r is limited to non-neuronal tissue, whereas OCT2r, the OCT2p homologue from rat, was found in both the kidney and central nervous regions known to be r ich in the monoamine transmitter dopamine.