Adaptive changes in transmembrane transport and metabolism of triiodothyronine in perfused livers of fed and fasted hypothyroid and hyperthyroid rats.
The transport and subsequent metabolism of triiodothyronine (T3) were studied in isolated perfused livers of euthyroid, hypothyroid, and hyperthyroid rats, both fed and 48-hour-fasted. T3 kinetics (transport and metabolism) during perfusion were evaluated by a two-pool model, whereas the metabolism of T3 was also investigated by determination of T3 breakdown products by chromatography of medium and bile. For comparison of groups, metabolism was corrected for differences in transport. Transport parameters in fed hypothyroid livers were not significantly changed as compared with euthyroid livers, whereas metabolism was decreased. In fed hyperthyroid livers, fractional transfer rate constants for influx (k21) and efflux (k12) were decreased and metabolism, corrected for differences in intracellular mass transfer, was increased. Furthermore, for transport in hyperthyroid liver it was shown that only total mass transfer (TMT) into the metabolizing liver compartment (not into the nonmetabolizing liver compartment) was decreased. Transport and metabolic parameters in fasted hypothyroid livers were decreased as compared with euthyroid fed livers. In fasted hyperthyroid livers, transport and metabolism were not significantly different as compared with that in euthyroid fed livers, so transport was increased versus hyperthyroid fed livers. It appeared therefore that fasting normalized the effects of hyperthyroidism on both the transport and metabolic processes of T3 in the liver. The present study demonstrates normal transport and decreased metabolism in livers of hypothyroid fed rats and decreased transport and increased metabolism in livers of hyperthyroid fed rats. In livers of hypothyroid fasted rats transport and metabolism were decreased, whereas in livers of hyperthyroid fasted rats transport and metabolism were not significantly different from that in euthyroid fed livers.(ABSTRACT TRUNCATED AT 250 WORDS)
de Jong
M
M
Department of Internal Medicine III, Medical Faculty, Erasmus University, Rotterdam, The Netherlands.
Docter
R
R
van der Hoek
H
H
Krenning
E P
EP
Hennemann
G
G
D000222:Adaptation, Physiological; D000818:Animals; D001692:Biological Transport; D005215:Fasting; D006980:Hyperthyroidism / Q000378:metabolism*; D007037:Hypothyroidism / Q000378:metabolism*; D008099:Liver / Q000378:metabolism*; D008297:Male; D010477:Perfusion; D051381:Rats; D017208:Rats, Wistar; D013972:Thyrotropin / Q000097:blood; D013974:Thyroxine / Q000097:blood; D014284:Triiodothyronine / Q000378:metabolism*
D014284:Triiodothyronine; D013972:Thyrotropin; D013974:Thyroxine