Evidence for differential regulation of renal proximal tubular p-aminohippurate and sodium-dependent dicarboxylate transport

In renal proximal tubules, the basolateral organic anion [p-aminohippurate (PAH)] transporter is functionally coupled to the sodium-dependent dicarbox ylate transporter. This study was undertaken to elucidate whether protein k inases differentially modulate the activities of these transporters. In iso lated S-2 segments of proximal tubules microdissected from rabbit kidneys, we investigated whether the transporters are regulated by tyrosine kinases, phosphatidylinositol 3-kinase (PI3K), and mitogen-activated protein kinase (MAPK). The tubules were collapsed; hence, tubular uptake of the marker su bstances [H-3]PAH and [C-14]glutarate reflects transport across the basolat eral cell membrane. Genistein, a selective inhibitor of tyrosine kinase, di minished PAH uptake at 10(-7) M by 15.6 +/- 11.7% and at 10(-6) M by 25.6 /- 9.1%. An inactive analog of genistein, diadzein, was without effect even at a concentration 100-fold higher than the lowest concentration of genist ein, which produced significant reduction of PAH uptake. At 10(-7) M, wortm annin, a selective inhibitor of PI3K, reduced PAH uptake by 24.1 +/- 11.3% and, at 10(-6) M, it reduced it by 32.9 +/- 31.8%. The selective inhibitor of MAPK, PD98059, diminished PAH uptake at 5 x 10(-5) M by 23.2 +/- 6.8% an d at 10(-4) M by 18.3 +/- 5.2%. Glutarate uptake was not reduced by any of these protein kinase inhibitors. Insulin had no effect on PAH uptake. These findings indicate that, in addition to protein kinase A, protein kinase C and calcium/calmodulin-dependent protein kinase II (former studies from thi s laboratory), as well as tyrosine kinases, PI3K, and MAPK, modulate renal basolateral PAH transport, whereas none of these protein kinases affects ba solateral glutarate transport. Thus, the results provide evidence for diffe rential regulation of basolateral transporters for PAH and dicarboxylates.