Comparative studies of atrial and ventricular myosin from normal, thyrotoxic, and thyroidectomized rabbits

Changes in enzymatic and structural properties of ventricular myosin in thyrotoxic rabbit hearts have been investigated extensively. However, there is little information regarding the effect of thyroid hormone on the atrial myosin. In this study, we have compared enzymatic and structural changes of ventricular and atrial myosin from euthyroid, hypothyroid, and hyperthyroid rabbits. In euthyroid rabbits, Ca++- and actin-activated ATPase activities of atrial myosin were 2-fold greater than those of the ventricular myosin. The Ca++- and actin-activated ATPase activities of atrial myosin from hypothyroid and hyperthyroid rabbits were identical with the values for atrial myosin from euthyroid rabbits. The same ATPase activities of ventricular myosin decreased in hypothyroid hearts but increased in hyperthyroid rabbits. The K+ (EDTA)-ATPase activities of all myosins were the same, irrespective of the thyroid status of the animal. Pyrophosphate-polyacrylamide gel electrophoresis patterns showed two isoenzymes (designated as A1 and A3) of atrial myosin in euthyroid hearts. The same electrophoretic patterns also showed in atrial myosin from hypothyroid and hyperthyroid hearts. The ventricular myosin from euthyroid hearts also exhibited two isoenzymes (designated as V1 and V3) but each with slower electrophoretic mobilities than the corresponding atrial myosin. In hypothyroid hearts, only V3 isoenzyme was seen, whereas, in hyperthyroid hearts, only V1 isoenzyme was seen. These results suggest that thyroid hormone controls ventricular myosin ATPase activity by controlling synthesis of a specific ventricular isoenzyme, whereas thyroid hormone does not affect atrial myosin ATPase, possibly due to its inability to control atrial myosin synthesis.