Contraluminal p-aminohippurate transport in the proximal tubule of the rat kidney

Abstract

Using the stop-flow peritubular capillary microperfusion method contraluminal transport of corticosteroids was investigated (a) by determining the inhibitory potency (apparent K _i values) of these compounds against p-aminohippurate (PAH), dicarboxylate (succinate) and sulphate transport and (b) by measuring the transport rate of radiolabelled corticosteroids and its inhibition by probenecid. Progesterone did not inhibit contraluminal PAH influx but its 17α- and 6β-hydroxy derivatives inhibited with an app. K_i of 0.36 mmol/l. Introduction of an OH group in position 21 of progesterone, to yield 11-deoxycorticosterone, augments the inhibitory potency considerably (app. K _{i, PAH} of 0.07 mmol/l). Acetylation of the OH-group in position 21 of 11deoxycorticosterone, introduction of an additional hydroxy group in position 17 α to yield 11-deoxycortisol or in position 11 to yield corticosterone brings the app. K _{i, PAH} back again into the range of 0.2–0.4 mmol/l. Acetylation of corticosterone or introduction of a third OH group to yield cortisol does not change the inhibitory potency, but, omission of the 21-OH group or addition of an OH group in the 6β position reduces or abolishes it. Cortisol and its derivatives prednisolone, dexamethasone and cortisone exert similar inhibitory potencies (app. K _{i, PAH} 0.12–0.27 mmol/l). But again, omission of the 21-OH group in cortisone or addition of a 6β-OH group reduces or even abolishes the inhibitory potency against PAH transport. The interaction of corticosterone was not changed when 11β, 18-epoxy ring (aldosterone) was formed. On the other hand, the interaction was considerably augmented if the 11-hydroxy group was changed to an oxo group in 11-dehydrocorticosterone (app. K _{i, PAH} 0.02 mmol/l). When the A ring of corticosterone is saturated and reduced to 3α, 11β-tetrahydrocorticosterone the inhibitory potency is not changed very much. But if more than four OH or oxo groups are on the pregnane skeleton or if the OH in position 21 is missing, the inhibitory potency decreases drastically (app. K_{i, PAH} 0.7–1.7 mmol/l). Introduction of a 21-ester sulphate into corticosterone, cortisol and cortisone does not change app. K _{i, PAH} very much. Glucuronidation, however, reduces it (app. K_{i, PAH} ≈ 1.2 mmol/l). None of the tested corticosteroids interacts, in concentrations applicable, with dicarboxylate transport and only the sulphate esters interact with sulphate transport.

Radiolabelled cortisol, d-aldosterone, 11-dehydrocorticosterone, and corticosterone are rapidly transported into proximal tubular cells. With the latter three compounds no sign of saturation and no transport inhibition with probenecid could be seen. Only with cortisol was a shift toward saturation observed. In addition, cortisol transport could be inhibited by probenecid. The data indicate that corticosteroids interact with the contraluminal renal PAH transporter, whereby hydroxylation in position 21 augments, and hydroxylation in the 6β or 3α, 17β position reduces interaction. However, as tested so far, simple diffusion seems to prevail when corticosteroids cross the cell membrane. Sulphation makes corticosteroids also a substrate for the sulphate transporter.