Changes in Myosin Isozyme Expression during Cardiac Hypertrophy in Hyperthyroid Rabbits

Abstract

The myosin isozyme distribution in the left ventricle and in the interventricular septum of rabbits was studied after 3 7,11,14 and 21 days of L-thyroxine (500 µg/kg/day) administration. Histochemical procedures were employed to identify V₁ and V₃ by their Ca²⁺ ATPase activity and their proportions were quantified through polyacrylamide gel electrophoresis. In the left ventricle, the subepicardium was the first to show the shift fromV₃ to V₁ followed by the subendocardium. The intermediate region became heterogeneous by 11 days and remained so until 21 days. The right subendocardial and the intermediate regions of the interventricular septum were heterogeneous in the normal rabbit and hyperthyroidism resulted in a shift from V₃ to V₁ in both the right and left subendocardial regions of the septum. Like the left ventricle, the intermediate region of the interventricular septum remained heterogeneous. Localized accumulations of collagen were seen in all regions of the left ventricle and interventricular septum. From these results we conclude that in thyrotoxic myocardial hypertrophy the isozymic shift from V₃ to V₁ is progressive, region-specific and is directly correlated with the period of hyperthyroidism in the first 2 weeks. Prolonged hyperthyroidism results in localized accumulation of collagen which does not exhibit any regional specificity.

Cell Size and Oxidative Enzyme Activity of Type-Identified Fibers in Rat Hindlimb Muscles: a Review

“…The mammalian cardiac muscle has three distinct MHC isoforms: V1, V2, and V3 [71], suggesting that fibers in the muscle can be classified into several types including hybrid types, as shown in a previous study using rabbits [64]. However, it is difficult to classify fibers into several types based on their histochemical ATPase staining intensities alone because all fibers in the muscle showed positive staining by ATPase, irrespective of their preincubation pHs.…”