THYROID-HORMONE ACTION - EFFECT OF TRIIODOTHYRONINE ON MITOCHONDRIAL ADENINE-NUCLEOTIDE TRANSLOCASE IN-VIVO AND IN-VITRO

Adenine nucleotide translocase (AdNT) levels were measured as the exch ange of extramitochondrial against intramitochondrial adenosine diphos phate (ADP) in liver, spleen, and testes mitochondria isolated from no rmal and hypothyroid rats using the ''back-exchange'' and atractylosid e-stop method of Pfaff and Klingenberg. The results provide confirmati on of previous reports that mitochondria from hypothyroid rats show a markedly diminished AdNT activity, which is restored to normal levels within 72 hours by intraperitoneal injection of 10 to 20 mu g triiodot hyronine (T-3)/100 g bodyweight. The latter dose was found in dose-res ponse studies to result in maximal stimulation of AdNT in liver mitoch ondria. Qualitatively similar results on AdNT activity were obtained i n liver mitochondria within 30 to 60 minutes following intravenous inj ection into hypothyroid rats of a more physiological dose of T-3 (40 n g/100 g body weight). AdNT in mitochondria isolated from spleen and te stes (organs that do not exhibit a calorigenic response after administ ration of thyroid hormone to the whole animal) failed to respond to th yroidectomy and to administration of T-3. More recently, we have obser ved that in vitro replacement of T-3 also stimulates AdNT activity in hypothyroid liver mitochondria. The enzyme adenosine triphosphate (ATP ) synthase was examined as another possible candidate for direct hormo nal stimulation of mitochondria. Simultaneous determinations on the sa me rats after intraperitoneal injection of T-3 (20 mu g/100 g body wei ght) showed little or no effect on ATP synthase until after 37 to 85 h ours, whereas enhanced activity of the translocator was regularly obse rved at 17 hours. These findings support the view that AdNT, which is considered to exert major control over the rate of oxidative phosphory lation, may be a direct target of Tg action on the mitochondria. Incre ased nuclear transcription may be regarded as a sustained delayed effe ct of T-3 administration, in contrast to the early effect an mitochond rial AdNT. A bolus intravenous injection of T-3 into the hypothyroid r at increases the activity of the mitochondrial carrier AdNT within a m atter of minutes as an early direct effect, as also suggested by studi es of addition of Tg in vitro to isolated rat liver mitochondria. In c ontrast, nuclear effects require 12 to 24 hours to show increased tran scription, evidenced by increased specific mRNA directing the formatio n of more AdNT (Luciakova and Nelson). Copyright (C) 1995 by W.B. Saun ders Company