Saturable uptake of [125I]L-triiodothyronine at the basolateral (blood) and apical (cerebrospinal fluid) sides of the isolated perfused sheep choroid plexus.
The uptake of 125I-labelled L-triiodothyronine (T3) was measured on the blood side of the isolated perfused choroid plexus of the sheep using steady-state and single-circulation paired tracer techniques. The steady-state uptake of T3 was 33.5% (perfusion fluid protein content was 0.05 g.dl-1) which could be reduced to 9.4% in the presence of 500 microM unlabelled T3 showing partial saturation. The CSF to blood steady-state [125I]T3 measurements gave plasma/CSF ratio, R%, of 24.6 +/- 4.8% which was reduced to 9.8 +/- 2.1% in the presence of 500 microM unlabelled T3 in the mock CSF. The transport of T3 across the blood face of the choroid plexus and the CSF to blood transport, failed to show sodium dependence. Using the single circulation paired tracer technique, the initial uptake in less than 60 s, Umax of [125I]T3 was 50.4 +/- 3.9% relative to the extracellular marker [3H]D-mannitol. However, when 250 microM unlabelled T3 was present, Umax was reduced by 66%, although further significant inhibition at higher concentrations was not observed. Uptake of T3 at the blood side of the choroid plexus was partially saturated in the presence of unlabelled reverse T3 and DT3, suggesting little uptake stereospecificity. Unlabelled thyroxine (T4) and the amino acid analogues cycloleucine (aminocyclopentane-1-carboxylic acid) and BCH (beta-2-aminobicyclo-[2,2.1]-heptane-2-carboxylic acid) each reduced [125I]T3 uptake significantly, but not to the same degree as T3 stereoisomers. The neutral amino acids alanine and phenylalanine, had no effect on uptake.(ABSTRACT TRUNCATED AT 250 WORDS)
Preston
J E
JE
Sherrington School of Physiology, United Medical School, Guy's Hospital, London, UK.
Segal
M B
MB
D000596:Amino Acids / Q000494:pharmacology; D000818:Animals; D001667:Binding, Competitive; D001692:Biological Transport; D002831:Choroid Plexus / Q000378:metabolism*; D006706:Homeostasis; D066298:In Vitro Techniques; D007454:Iodides / Q000493:pharmacokinetics; D007457:Iodine Radioisotopes; D012756:Sheep; D012964:Sodium / Q000494:pharmacology; D014284:Triiodothyronine / Q000097:blood / Q000134:cerebrospinal fluid / Q000493:pharmacokinetics*
D000596:Amino Acids; D007454:Iodides; D007457:Iodine Radioisotopes; D014284:Triiodothyronine; D012964:Sodium