Studies have suggested that chronic amiodarone treatment (a class III antiarrhythmic) may have effects on the myocardium that resemble those of hypothyroidism. Since hypothyroidism is associated with decreased expression of Na,K-ATPase alpha 1, alpha 2 and beta 1 subunits in heart and alpha 2 subunit in skeletal muscle, we aimed to test the hypothesis that chronic in vivo administration of amiodarone would result in changes in the expression of Na,K-ATPase isoforms that resembled those seen in hypothyroid rat. Rats were treated with 40 micrograms/g body weight amiodarone for 3 and 6 weeks. Na,K-ATPase alpha 1, alpha 2, beta 1 mRNA and protein levels and beta 2 mRNA were measured in cardiac ventricle and skeletal muscle. After 3 weeks of amiodarone cardiac alpha 1, alpha 2 and beta 1 protein levels were decreased to 0.65, 0.42, and 0.54 of control, respectively. After 6 weeks of treatment, cardiac alpha 1 and beta 1 levels returned to control and alpha 2 remained depressed. At the mRNA level alpha 2 and beta 2 decreased to 0.6-fold of control after 6 weeks treatment. There was no effect of amiodarone on skeletal muscle Na,K-ATPase expression at either time point. We conclude that the effects of amiodarone on myocardial Na,K-ATPase expression are similar to those of hypothyroidism after 3 weeks of amiodarone treatment, but by 6 weeks the similarities are limited to the decrease in alpha 2 and beta 2 expression.(ABSTRACT TRUNCATED AT 250 WORDS)