IN VITRO EFFECTS OF THYROXINE AND ANALOGUES ON THE RENAL UPTAKE OF PAH AND TEA

¹ Department of Pharmacology, The Bowman Gray School of Medicine of Wake Forest College, Winston-Salem, North Carolina

The in vitro addition of l-thyroxine (7.4 x 10^-9 M), tetraiodothyropropionic acid (7.4 x 10^-9 M), tetraiodothyroacetic acid (7.4 x 10-7 M), l-triiodothyronine (7.4 x 10^-6 M), triiodothyropropionic acid (7.4 x 10^-6 M), and triiodothyroacetic acid (7.4 x 10^-5 M) significantly depressed (P .001) PAR uptake by rat kidney cortex slices. l-Thyronine (7.4 x 10^-5 M) was inactive. The presence of the iodine substituents is essential for inhibitory action. The tetraiodothyronines were more potent inhibitors of PAH uptake than their corresponding triiodo analogues. Modification of the alanine side chain resulted in increased inhibitory potency. Only triiodothyropropionic acid (7.4 x 10^-5 M; P .01) and triiodothyroacetic acid (7.4 x 10^-5 M; P .001) had an inhibitory effect on TEA transport. It is suggested that thyroxine and its analogues depress PAH uptake by competing for the same transport system.