Regional differences in the in vivo synthesis and degradation of myosin subunits in rabbit ventricular myocardium.

To test for regional differences in the rates of synthesis and degradation of contractile proteins during normal physiological growth of the heart in vivo, fractional rates of protein accumulation and synthesis were assessed for total protein, myosin heavy chain, myosin light chain, phosphorylatable myosin light chain, and actin in the right and left ventricular free walls of growing, New Zealand White rabbits. Fractional rates of protein accumulation were determined from regional protein content growth curves of total and individual myofibrillar proteins measured in 82 animals ranging from 1 day to 9 weeks of age. In vivo right and left ventricular fractional rates of protein synthesis were determined by the [3H]leucine constant infusion method in 9-week-old rabbits. At this age, right and left ventricular fractional accumulation rates for total protein, myosin subunits, and actin were found to be identical, thus allowing for the comparison of the effect of regional hemodynamic load on protein degradation independent of its effect on growth. Total protein and individual myofibrillar protein fractional degradative rates were then derived as the difference between fractional rates of synthesis and accumulation. The results indicate increased fractional synthesis and degradation of myosin heavy chain and light chains, but not actin, in the left compared to the right ventricular free wall. Accelerated left ventricular replacement of myosin subunits may be related to regional differences in hemodynamic load. These results help explain the apparent increase in the in vivo rate of myocardial protein degradation observed in several experimental models of ventricular hypertrophy.