Reactive oxygen species mediate amplitude-dependent hypertrophic and apoptotic responses to mechanical stretch in cardiac myocytes

Oxidative stress stimulates both growth and apoptosis in cardiac myocytes i n vitro. We investigated whether oxidative stress mediates hypertrophy and apoptosis in cyclically stretched ventricular myocytes. Neonatal rat ventri cular myocytes cultured on laminin-coated silastic membranes were stretched cyclically (1 Hz) at low (nominal 5%) and high (nominal 25%) amplitudes fo r 24 hours. Stretch caused a graded increase in superoxide anion production as assessed by superoxide dismutase (SOD)-inhibitable cytochrome c reducti on or electron paramagnetic resonance spectroscopy. The role of reactive ox ygen species (ROS) was assessed using the cell-permeable SOD/catalase mimet ics Mn(II/III)tetrakis(1-methyl-4-peridyl) (MnTMPyP) and EUK-8. Stretch-ind uced increases in protein synthesis (H-1-leucine incorporation) and cellula r protein content were completely inhibited by MnTMPyP (0.05 mmol/L) at bot h low and high amplitudes of stretch. In contrast, while MnTMPyP inhibited basal atrial natriuretic factor (ANF) mRNA expression, the stretch-induced increase in ANF mRNA expression was not inhibited by MnTMPyP. In contrast t o hypertrophy, only high-amplitude stretch increased myocyte apoptosis, as reflected by increased DNA fragmentation on gel electrophoresis and an appr oximate to3-fold increase in the number of TUNEL-positive myocytes. Similar ly, only high-amplitude stretch increased the expression of bax. mRNA. Myoc yte apoptosis and bax expression stimulated by high-amplitude stretch were inhibited by MnTMPyP. Both low- and high-amplitude stretch caused rapid pho sphorylation of ERK1/2, while high-, but not low-, amplitude stretch caused phosphorylation of JNKs. Activation of both ERK1/2 and JNKs was ROS-depend ent. Thus, cyclic strain causes an amplitude-related increase in ROS, assoc iated with differential activation of kinases and induction of hypertrophic and apoptotic phenotypes.