Mechanical activity in heart regulates translation of alpha-myosin heavy chain mRNA but not its localization

Mechanical inactivity depresses protein expression in cardiac muscle tissue and results in atrophy. We explore the mechanical transduction mechanism i n spontaneously beating neonatal rat cardiomyocytes expressing the alpha-my osin heavy chain (alpha-MyHC) isoform by interfering with cross-bridge func tion [2,3-butanedione monoxime (BDM), 7.5 mM] without affecting cell calciu m. The polysome content and alpha-MyHC mRNA levels in fractions from a; suc rose gradient were analyzed. BDM treatment blocked translation at initiatio n (162 +/- 12% in the nonpolysomal RNA fraction and 43 +/- 6% in the polyso mal fraction, relative to control as 100%; P < 0.05). There was an increase in alpha-MyHC mRNA from the nonpolysomal fraction (120.5 +/- 7.7%; P < 0.0 5 compared with control) with no significant change in the heavy polysomes. In situ hybridization of alpha-MyHC mRNA was used to estimate message abun dance as a function of the distance from the nucleus. The mRNA was disperse d through the cytoplasm in spontaneously beating cells as well as in BDM-tr eated cells (no significant difference). We conclude that direct inhibition of contractile machinery, but not calcium, regulates initiation of alpha-M yHC mRNA translation. However, calcium, not pure mechanical signals, appear s to be important for message localization.