Synthesis of myosin heavy and light chains in the afterloaded guinea pig right ventricle.

Increased afterload causes increased cardiac myosin synthesis and ultimately leads to hypertrophy. Since the latter is associated with altered myosin ATPase activity, it was of interest to study the synthesis of myosin subunits in the acute response to this stress. An in vitro guinea pig heart preparation was used which allowed application of afterload to the right ventricle with unaltered coronary flow, and also permitted measurement of synthesis of myosin heavy chains (HC) and combined light chains (LC) by continuous perfusion with labelled amino acids (3H-lysine and/or 3H-phenylalanine) of constant specific activity. Isolation of 3H-labelled HC and LC with heterologous unlabelled carrier was possible because of identical mobilities of HC's and LC's from unlabelled lamb carrier myosin and 3H-labelled guinea pig myosin. This permitted study of comparative synthesis of the HC and LC in small samples as the single guinea pig right ventricle (100--150 mg) and avoided errors inherent in pooling hearts or in measurement of turnover in the nonsteady state. After 3 h or perfusion, the ratio of synthesis of HC/LC was 2 : 1 in controls. This ratio increased significantly to 3 : 1 in after load. It is possible that the disproportionate increase in HC synthesis may lead to stoichiometric problems in myosin assembly which ultimately effect altered myosin ATPase activity.