Properties and regulation of organic cation transport in freshly isolated human proximal tubules

The kidney, and more specifically the proximal tubule, is the main site of elimination of cationic endogenous metabolites and xenobiotics. Although nu merous studies exist on renal organic cation transport of rat and rabbit, n o information is available from humans. Therefore, we examined organic cati on transport and its regulation across the basolateral membrane of isolated human proximal tubules. mRNA for the cation transporters hOCT1 and hOCT2 a s well as hOCTN1 and hOCTN2 was detected in these tubules. Organic cation t ransport across the basolateral membrane of isolated collapsed proximal tub ules was recorded with the fluorescent dye 4-(4-dimethylamino)styryl-N-meth ylpyridinium (ASP(+)). Depolarization of the cells by rising extracellular K+ concentration to 145 mM reduced ASP(+) uptake by 20 +/- 5% (n = 15), ind icating its electrogeneity. The substrates of organic cation transport tetr aethylammonium (K-i = 63 muM) and cimetidine (K-i = 11 muM) as well as the inhibitor quinine (K-i = 2.9 muM) reduced ASP(+) uptake concentration depen dently. Maximal inhibition reached with these substances was similar to 60% . Stimulation of protein kinase C with 1,2-dioctanoyl-sn-glycerol (DOG, 1 m uM) or ATP (100 muM) inhibited ASP(+) uptake by 30 +/- 3 (n = 16) and 38 +/ - 13% (n = 6), respectively. The effect of DOG could be reduced with calpho stin C (0.1 muM, n = 7). Activation of adenylate cyclase by forskolin (1 mu M) decreased ASP(+) uptake by 29 +/- 3% (n = 10). hANP (10 nM) or 8-bromo-c GMP (100 muM) also decreased ASP(+) uptake by 17 +/- 3 (n = 9) or 32 +/- 5% (n = 10), respectively. We show for the first time that organic cation tra nsport across the basolateral membrane of isolated human proximal tubules, most likely mediated via hOCT2, is electrogenic and regulated by protein ki nase C, the cAMP- and the cGMP-dependent protein kinases.