Phorbol Myristate Acetate-induced Hypertrophy of Neonatal Rat Cardiac Myocytes is Associated with Decreased Sarcoplasmic Reticulum CaATPase (SERCA2) Gene Expression and Calcium Reuptake

The decreased expression of the sarcoplasmic reticulum Ca²⁺-ATPase associated with cardiac hypertrophy was investigated in cultured neonatal rat cardiac myocytes. Northern blot analysis indicated a significant 55–60% decrease in Ca²⁺-ATPase mRNA levels and after 12 and 24 h of treatment with the phorbol ester phorbol myristate acetate (PMA). Myocytes treated with the phorbol ester for 80 h showed a significant 34% decrease (relative to vehicle-treated control cells) in the levels of Ca²⁺-ATPase protein, and a significant 38% increase in the levels ofα-sarcomeric actin, as assessed by Western blot analysis using specific antibodies. Immunocytochemistry of myocytes treated for 72 h with the phorbol ester revealed a hypertrophied cell morphology, and showed a marked decrease in Ca²⁺-ATPase staining intensity. Contractile calcium transients were evaluated through the use of indo-1. It was found that thet_1/2for the decline of calcium transient was significantly prolonged by PMA treatment (0.51±0.15) when compared to controls (0.38±0.17,P<0.001). Treatment of myocytes with endothelin-1 also led to a 35% decrease in sarcoplasmic reticulum Ca²⁺-ATPase mRNA levels. It is concluded that phorbol ester treatment of neonatal rat cardiac myocytes induces similar changes in Ca²⁺-ATPase gene expression as observedin vivoin the hypertrophied and failing heart. The observed prolongation int_1/2for [Ca²⁺]_idecline might be due to the observed depressed levels for sarcoplasmic reticulum Ca²⁺-ATPase in PMA treated cells.