Direct action of T₃on phosphorylation potential in the sheep heart in vivo

Abstract

Thyroid acting through ligand binding to nuclear receptors modifies myocardial respiratory kinetics and oxidative phosphorylation in the heart. Direct nongenomic action of thyroid hormone on high-energy phosphate concentrations and respiratory kinetics has never been proven in vivo but might be responsible for observed changes in oxygen utilization efficiency immediately after triiodothyronine (T3) administration. We tested the hypothesis that T3 directly and rapidly modifies myocardial high-energy phosphate concentrations and phosphorylation potential in vivo. Anesthetized sheep (age 28 -40 days) thyroidectomized shortly after birth (Thy) and euthyroid age-matched controls (Con) underwent median sternotomy and received T3 infusion (0.8 g/kg), followed by epinephrine infusion to increase myocardial oxygen consumption (MV O2).31 P magnetic resonance spectra were monitored via a surface coil over the left ventricle. T3 increased phosphocreatine (PCr)/ATP and decreased ADP in Thy animals without causing a change in MV O2. T3 produced no changes in high-energy phosphates in Con animals. T3 did not modify the PCr/ATP or ADP response to epinephrine and elevation in MV O2 in either group. Cardiac mitochondria isolated from Thy and Con animals showed no change in respiratory rate or ADP/ATP exchange efficiency after T3 incubation. T3 infusion in a hypothyroid state decreases ADP concentration, thereby altering the equilibrium between phosphorylation potential and myocardial respiratory rate. These T3-induced effects are not due to changes in ADP/ATP exchange efficiency through action at the adenine nucleotide translocator but may be due to T3 mediation of substrate utilization, confirmed in other models. adenine nucleotide translocator; myocardial energy metabolism; substrate oxidation THYROID HORMONE REGULATES myocardial metabolism at the transcriptional level through binding to nuclear receptors. Ligand-dependent binding of these receptors to thyroid receptor elements controls transcription of various target genes involved in contractile and metabolic processes (5, 16). Although these nuclear receptor-mediated processes have been investigated to some extent, thyroid hormone as its active component, triiodothyronine (T 3 ), also regulates cellular processes through direct binding to membranes or enzymes (2).The direct or nongenomic actions of T 3 on cardiac energy metabolism and high-energy phosphate kinetics remain somewhat obscure. In previous studies, we (20) demonstrated that T 3 directly regulates myocardial substrate oxidation in isolated, perfused hearts. The relationship between substrate oxidation and phosphorylation potential has been established in several cardiac models (23,43). An apparent equilibrium exists between mitochondrial NADH/NAD and cytosolic phosphorylation potential. Accordingly, we considered that T 3 could also elevate phosphorylation potential in vivo. Prior studies in sheep in vivo implicate the adenine nucleotide translocator (ANT), which facilitates ADP/ATP exchange across the mitochondri...

Thyroid hormone interacts with PPARα and PGC-1 during mitochondrial maturation in sheep heart

“…Accordingly, we evaluated content for two respiratory chain components, Cyt aa 3 and Cyt c. Others have previously shown coordination between Cyt aa 3 enzymatic activity and PGC-1 protein expression in several tissues (20). Our data, demonstrating reduced concentration of these respiratory chain cytochromes in the thyroidectomized sheep, reinforce our previous ovine data indicating slowing of mitochondrial maturation in vivo (30,31). Thyroidectomy reduced inherent maximal oxidative phosphorylation capacity while attenuating normal developmental PGC-1 accumulation.…”

“…Using this same ovine model, we previously demonstrated that thyroidectomy delays mitochondrial postnatal maturation in vivo (30,31). Thryoidectomy caused persistence of the newborn-type ADPmediated respiratory control, which occurred coordinately with reduced adenine nucleotide translocator accumulation (ANT) (30).…”

“…Potassium cyanide (2 mm) was added to the cuvette to achieve full reduction of the cytochromes. The cytochrome c oxidase (Cyt aa 3) and cytochrome c (Cyt c) contents were measured using the difference between absorbances measured as previously described (31).…”

“…The principal objective of this study was to test the hypothesis that TH state influences regulation of PPAR␣ and PGC-1 during postnatal maturation in sheep heart. Accordingly, we performed experiments in an established model of ovine neonatal hypothyroidism (30,31). 3 and thyroxine levels were measured before surgery, after 24 h, after 7 days, and after 28 days as reported previously (30 ).…”

The dominant negative thyroid hormone receptor β-mutant Δ337T alters PPARα signaling in heart

“…This transgenic model does not alter circulating thyroid hormone levels, ensuring that experiments were conducted under systemic euthyroid conditions. Therefore, our study design avoids confounding factors such as posttranscriptional triiodothyronine action on cardiac metabolism (13,22) and transcriptional actions on noncardiomyocyte cells and organs that could indirectly alter cardiac response. These findings imply that these nuclear receptor pathways synergistically or cooperatively activate or repress target genes.…”