The
small G protein Ras-mediated signaling pathway has been implicated in the development of
hypertrophy and diastolic dysfunction in the heart. Earlier cellular studies have suggested that the Ras pathway is responsible for reduced
L-type calcium channel current and sarcoplasmic reticulum (SR)
calcium uptake associated with sarcomere disorganization in neonatal cardiomyocytes. In the present study, we investigated the in vivo effects of Ras activation on cellular
calcium handling and sarcomere organization in adult ventricular myocytes using a newly established transgenic mouse model with targeted expression of the H-Ras-v12 mutant. The transgenic hearts expressing activated Ras developed significant
hypertrophy and postnatal lethal
heart failure. In adult ventricular myocytes isolated from the transgenic hearts, the
calcium transient was significantly depressed but membrane L-type
calcium current was unchanged compared with control littermates. The expressions of sarco(endo)plasmic reticulum Ca(2+)-
ATPase (SERCA)2a and
phospholamban (PLB) were significantly reduced at
mRNA levels. The amount of SERCA2a
protein was also modestly reduced. However, the expression of PLB
protein and gross sarcomere organization remained unchanged in the hypertrophic Ras hearts, whereas Ser(16) phosphorylation of PLB was dramatically inhibited in the Ras transgenic hearts compared with controls. Hypophosphorylation of PLB was also associated with a significant induction of
protein phosphatase 1 expression. Therefore, our results from this in vivo model system suggest that Ras-induced contractile defects do not involve decreased
L-type calcium channel activities or disruption of sarcomere structure. Rather, suppressed SR
calcium uptake due to reduced SERCA2a expression and hypophosphorylation of PLB due to changes in
protein phosphatase expression may play important roles in the diastolic dysfunction of Ras-mediated
hypertrophic cardiomyopathy.