We have identified and partially sequenced a soluble factor, myotrophin, from spontaneously hypertensive rat hearts and hypertrophic human hearts that enhances myocyte protein synthesis and stimulates myocardial cell growth. Our studies suggest that myotrophin may be a biochemical link between hemodynamic stress and myocardial cellular hypertrophy. When rat neonatal cardiac myocytes maintained in culture were incubated with myotrophin for 30 minutes, they showed a marked increase in c-myc, c-fos, and c-jun messenger RNA levels. Cardiac myocytes treated for 24 hours with myotrophin showed a fourfold increase in connexin 43 (gap junction protein), a sixfold increase in atrial natriuretic factor, a threefold increase in skeletal alpha-actin, and a threefold increase in total myosin transcript levels. Studies on myosin isoforms showed a selective increase in the beta-myosin heavy chain transcript levels but no reciprocal decrease in alpha-myosin heavy chain transcript levels. Our data suggested that myotrophin appears to be a primary modulator for myocardial cell growth and differentiation and may play an important role in the pathogenesis of cardiac hypertrophy. Myotrophin may be involved in the upregulation of myofibrillar protein and the activation of cardiac gene transcription during growth and hypertrophy of the myocardium, and the induction of early response gene expression may be linked to this response.