β-Adrenergic receptors, voltage-operated Ca 2+-channels, nuclear triiodothyronine receptors and triiodothyronine concentration in pig myocardium after long-term low-dose amiodarone treatment

Abstract

Similar features during chronic amiodarone treatment and hypothyroidism suggest that amiodarone induces a state of "triiodothyronine (T3)-resistance" or "cardiac hypothyroidism", which may predispose the heart to pump failure under conditions with severe strain, such as recovery after cardiac surgery. Disagreements exists as to how amiodarone, and possibly its main metabolite desethylamiodarone, act upon the various receptor systems in the heart. The aim of the present study was to elucidate whether chronic amiodarone treatment leads to a functional reduction in the number of myocardial nuclear T3 receptors, the myocardial tissue T3 concentration and the number of beta-receptors and voltage-operated Ca(2+)-channels. Finally, special attention was drawn to any changes that could contribute to explain previous reports on reduced haemodynamic reserve in animals exposed to severe cardiac strain, such as cardiac surgery. Pigs (72 +/- 2 kg) were assigned randomly to amiodarone treatment (20 mg.kg-1.day-1 for 30 +/- 1 days, N = 8); controls received no medical treatment (N = 6). The left ventricle was evaluated for beta-adrenergic receptors, voltage-operated Ca(2+)-channels, T3 nuclear receptors and tissue T3 concentration. Maximum binding capacity for beta-receptors and Ca(2+)-channels was reduced in amiodarone-treated pigs (by 38%, p < 0.05, and by 52%, p < 0.01) and correlated with tissue drug concentrations for both receptor types (p < 0.05). No changes were observed concerning nuclear T3 receptors. In vitro competition studies revealed that amiodarone, but not desethylamiodarone, possessed binding properties to Ca(2+)-channels, whereas neither of the compounds bound to beta-receptors. Desethylamiodarone, but not amiodarone, competitively inhibited T3 binding to its nuclear receptors. Myocardial T3 was undetectable (< 0.05 nmol/kg wet wt) in amiodarone-treated pigs. From our observations we suggest that the active metabolite desethylamiodarone, rather than the parent drug, is mainly responsible for the observed local hypothyroid-like effects during amiodarone treatment. The observed changes after treatment with low-dose amiodarone in pigs are likely to have biological implications. Functionally, the changes may imply reduced cardiac reserve during conditions of extraordinary strain.