Phenylephrine hypertrophy, Ca2+-ATPase (SERCA2), and Ca2+ signaling in neonatal rat cardiac myocytes

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Abstract
We endeavored to use a basic and well-controlled experimental system to characterize the extent and time sequence of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) involvement in the development of cardiac hypertrophy, including transcription, protein expression, Ca2+ transport, and cytoplasmic Ca2+ signaling. To this end, hypertrophy of neonatal rat cardiac myocytes in culture was obtained after adrenergic activation with phenylephrine (PE). Micrographic assessment of myocyte size, rise of [14C]phenylalanine incorporation and total protein expression, and increased transcription of atrial natriuretic factor demonstrated unambiguously the occurrence of hypertrophy. An early and prominent feature of hypertrophy was a reduction of the SERCA2 transcript, as determined by RT-PCR with reference to a stable marker such as glyceraldehyde-3-phosphate dehydrogenase. Reduction of Ca2+-ATPase protein levels and Ca2+ transport activity to ∼50% of control values followed with some delay, evidently as a consequence of a primary effect on transcription. Cytosolic Ca2+ signaling kinetics, measured with a Ca2+-sensitive dye after electrical stimuli, were significantly altered in hypertrophic myocytes. However, the effect of PE hypertrophy on cytosolic Ca2+ signaling kinetics was less prominent than observed in myocytes subjected to drastic SERCA2 downregulation with small interfering RNA or inhibition with thapsigargin (10 nM). We conclude that SERCA2 undergoes significant downregulation after hypertrophic stimuli, possibly due to lack of SERCA gene involvement by the hypertrophy transcriptional program. The consequence of SERCA2 downregulation on Ca2+ signaling is partially compensated by alternate Ca2+ transport mechanisms. These alterations may contribute to a gradual onset of functional failure in long-term hypertrophy.