Thyroid hormone-induced overexpression of functional ryanodine receptors in the rabbit heart

Abstract

Modifications in the Ca2+-uptake and -release functions of the sarcoplasmic reticulum (SR) may be a major component of the mechanisms underlying thyroid state-dependent alterations in heart rate, myocardial contractility, and metabolism. We investigated the influence of hyperthyroid state on the expression and functional properties of the ryanodine receptor (RyR), a major protein in the junctional SR (JSR), which mediates Ca2+ release to trigger muscle contraction. Experiments were performed using homogenates and JSR vesicles derived from ventricular myocardium of euthyroid and hyperthyroid rabbits. Hyperthyroidism, with attendant cardiac hypertrophy, was induced by the injection of l-thyroxine (200 μg/kg body wt) daily for 7 days. Western blotting analysis using cardiac RyR-specific antibody revealed a significant increase (>50%) in the relative amount of RyR in the hyperthyroid compared with euthyroid rabbits. Ca2+-dependent, high-affinity [3H]ryanodine binding was also significantly greater (∼40%) in JSR from hyperthyroid rabbits. The Ca2+sensitivity of [3H]ryanodine binding and the dissociation constant for [3H]ryanodine did not differ significantly between euthyroid and hyperthyroid hearts. Measurement of Ca2+-release rates from passively Ca2+-preloaded JSR vesicles and assessment of the effect of RyR-Ca2+-release channel (CRC) blockade on active Ca2+-uptake rates revealed significantly enhanced (>2-fold) CRC activity in the hyperthyroid, compared with euthyroid, JSR. These results demonstrate overexpression of functional RyR in thyroid hormone-induced cardiac hypertrophy. Relative abundance of RyR may be responsible, in part, for the changes in SR Ca2+ release, cytosolic Ca2+ transient, and cardiac systolic function associated with thyroid hormone-induced cardiac hypertrophy.