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1 Department of Pediatrics, University of Washington, Division of Cardiology, Seattle, WA, USA; Department of Pediatrics, Childrens Hospital and regional Medical Center, Seattle, WA, USA
2 Department of Pediatrics, University of Washington, Division of Cardiology, Seattle, WA, USA
* To whom correspondence should be addressed. E-mail: Michael.Portman{at}seattlechildrens.org.
Thyroid acting through ligand binding to nuclear receptors modifies myocardial respiratory kinetics and oxidative phosphorylation in heart. Direct non-genomic 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 T3 administration. We tested the hypothesis that that triiodothyronine (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) were open chested and received T3 infusion (0.8µg/kg), followed by epinephrine infusion to increase myocardial oxygen consumption (MVO2). 31P magnetic resonance spectra were monitored via a surface coil over the left ventricle. Results: T3 increased PCr/ATP and decreased ADP in THY without causing a change in MVO2. T3 produced no changes in high energy phosphates in CON. T3 did not modify the Phosphocreatine/ATP or ADP response to epinephrine and elevation in MVO2 in either group. Cardiac mitochondria isolated from THY and CON showed no change in respiratory rate or ADP/ATP exchange efficiency after T3 incubation. Conclusions: T3 infusion in a hypothyroid state decreases [ADP], 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.
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