We have altered the spontaneous contractile activity of neonatal cardiac myocytes in culture to investigate the re-lationship between mechanical forces, myofibril assembly, and the localization and translation of (alpha)-myosin heavy chain mRNA. Immunofluorescence and in situ hybridization techniques revealed that contracting myocytes display well aligned myofibrils and a diffuse distribution of (alpha)-myosin heavy chain mRNA. Inhibition of contractile activity with the calcium channel blocker verapamil (10 microM) resulted in myofibril disassembly and a perinuclear mRNA distribution within six hours. There was a significant decrease (P<0. 05) of mRNA levels, 5 to 15 micron away from the nucleus following 6 hours of verapamil treatment compared with control cells. Inhibition of protein synthesis with cycloheximide (10 microM) also resulted in perinuclear mRNA localization despite having little effect on contractile activity or myofibril assembly. To determine if the 3' untranslated region of (alpha)-myosin heavy chain mRNA was sufficient for localizing the entire message, a chimeric construct composed of beta-galactosidase coding region followed by (alpha)-myosin heavy chain 3' untranslated region sequences was made as a reporter plasmid and transfected into cultured myocytes. A perinuclear accumulation of ss-galactosidase was exhibited in many of the contractile arrested cells (48.3+/-2.4%, n=7). In contrast, significantly fewer (P<0.05) contracting control (29.1+/-3.3%, n=7) and strongly contracting, isoproterenol-treated cells (27.2+/-6.1%, n=3) exhibited a perinuclear localization of protein. The distribution of the reporter protein was not affected by the contractile state in cells transfected with a constitutively translated 3'UTR. We propose that mechanical activity of neonatal cardiac myocytes regulates the intracellular localization of alpha-myosin heavy chain mRNA via the 3' untranslated region mediated by an initial block in translation.