Regional differences in in vivo myocardial protein synthesis in the neonatal rabbit heart.

Left ventricular free wall (LV) myofibrillar proteins accumulate at a rate approximately 2.5 times faster than the rate of right ventricular free wall (RV) proteins during postnatal cardiac development of the rabbit heart. In this study, the contribution of regional differences in in vivo protein synthesis to the differential rates of growth of the RV and LV myocardium was assessed in 4-d, 3-wk, and 9-wk old rabbits. In vivo total protein fractional synthetic rates were measured by a modification of the flooding infusion method, with calculations based upon the rate of equilibration of plasma leucine and cardiac leucyl-tRNA specific radioactivities following intravenous administration of a large dose of labeled amino acid. This method was also applied to the analysis of the fractional synthetic rate of a contractile protein subunit (myosin heavy chain) in the RV and LV of 4-d and 9-wk old rabbits. Accelerated growth of the LV during the first week of postnatal cardiac development was associated with increased total protein fractional synthetic rates (35.3 +/- 3.6 vs. 45.5 +/- 4.7%/day for RV and LV total protein of 4-d old rabbits, respectively). In addition, RV and LV fractional synthetic rates for myosin heavy chain in the same 4-d old rabbits were considerably greater than those values observed for total protein (84.0 +/- 13.1 and 98.4 +/- 16.9%/day for RV and LV myosin heavy chain, respectively. However, the fractional synthetic rates of both total protein and myosin heavy chain in both the RV and LV rapidly declined during the transition from fetal to adult hemodynamics. Thus, regional and developmental differences in myocyte-specific protein synthesis both contribute to the extensive restructuring of ventricular muscle during this period of extremely rapid growth. These differences may be in response to regional and developmental changes in hemodynamic load.