Cell density and contraction regulate p38 MAP kinase-dependent responses in neonatal rat cardiac myocytes

In vitro cardiac myocyte hypertrophy is characterized by increased cell siz e, sarcomere organization, and induction of several genes including atrial natriuretic factor (ANF). The hypertrophic growth program has been associat ed with activation of various mitogen-activated protein kinase (MAP) kinase family members, one of which is a stress kinase, p38. In this study, we fo und that the p38-specific inhibitor SE-203580 failed to inhibit phenylephri ne-induced ANF-driven gene expression in low-density myocyte cultures but d id inhibit gene expression in higher density cultures. Dense myocyte cultur es also had a higher metabolic activity and contraction rate than cells pla ted at low density. We found that mimicking this effect by rapid electrical pacing activated ANF-driven gene expression and that this expression was i nhibited by inactivation of p38. However, addition of SE-203580 at time poi nts ranging between 1 and 72 h suggests that the effect of p38 on the ANF p romoter may be both direct and indirect. Electrical pacing induced a small, but consistent, increase in p38 phosphorylation (phospho-p38) at time poin ts ranging from 30 min to 4 h, but at later times phospho-p38 levels were r educed. When myocytes were treated with phenylephrine or electrically paced in the presence of the p38 inhibitor, there was little discernible change in morphology or rates of protein synthesis from DMSO-treated cells at 48 o r 72 h. These data indicate that cell density and myocyte contraction may m odulate p38-dependent pathways for ANF gene expression, but these pathways may not be direct and have limited effects on hypertrophic morphology.