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

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 mu g/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 [H-3]ryanodine binding was also significantly greater (similar to 40%) in JSR from hyperthyroid rabbits. The Ca2+ sensitivity of [H-3]ryanodine binding and the dissociation constant for [H-3]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.