Myosin isozyme synthesis and mRNA levels in pressure-overloaded rabbit hearts.

The in vivo synthesis rates of myosin isozyme heavy chains beta and alpha were measured in right ventricular (RV) muscle at 2 and 4 days following pulmonary artery constriction in rabbits, together with measurements of their relative mRNA levels. The synthesis rate of beta-myosin heavy chains was elevated in 2-day (0.27 +/- 0.06 day-1 or 2.5 +/- 0.7 mg/g RV/day, mean +/- SD) and in 4-day (0.25 +/- 0.08 day-1 or 2.8 +/- 1.0 mg/g RV/day) pressure overload, when compared to untreated rabbits (0.15 +/- 0.04 day-1 or 1.5 +/- 0.4 mg/g RV/day). However, the synthesis rates of alpha-myosin heavy chains in the same hearts were not altered significantly. There was a differential increase in the fractional synthesis rate of beta vs. alpha heavy chains in 2-day and 4-day pressure overload and in 2-day shams, suggesting switching toward beta heavy chain synthesis had occurred at these time points. beta heavy chain synthesis, as a proportion of total (alpha + beta) heavy chain synthesis, was significantly higher in 4-day pressure overload (78 +/- 9%) than in 4-day sham rabbits (63 +/- 6%). This increase in relative beta-synthesis was associated with a significant increase in the relative proportion of beta heavy chain mRNA level (76 +/- 13% vs. 56 +/- 7%). Furthermore, relative beta-synthesis and the beta-mRNA levels correlated linearly with each other in all experimental groups.(ABSTRACT TRUNCATED AT 250 WORDS)