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 thyro id state-dependent alterations in heart rate, myocardial contractility, and metabolism. We investigated the influence of hyperthyroid state on the exp ression 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 ve sicles derived from ventricular myocardium of euthyroid and hyperthyroid ra bbits. Hyperthyroidism, with attendant cardiac hypertrophy: was induced by the injection of L-thyroxine (200 mu g/kg body wt) daily for 7 days. Wester n blotting analysis using cardiac RyR-specific antibody revealed a signific ant increase (>50%) in the relative amount of RyR in the hyperthyroid compa red with euthyroid rabbits. Ca2+-dependent, high-affinity [H-3]ryanodine bi nding was also significantly greater (similar to 40%) in JSR from hyperthyr oid rabbits. The Ca2+ sensitivity of [H-3]ryanodine binding and the dissoci ation constant for [H-3]ryanodine did not differ significantly between euth yroid and hyperthyroid hearts. Measurement of Ca2+-release rates from passi vely Ca2+-preloaded JSR vesicles and assessment of the effect of RyR-Ca2+-r elease channel (CRC) blockade on active Ca2+-uptake rates revealed signific antly enhanced (>2-fold) CRC activity in the hyperthyroid, compared with eu thyroid, 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.