ALPHA-ADRENERGIC STIMULATION INDUCES PHOSPHORYLATION OF RETINOBLASTOMA PROTEIN IN NEONATAL RAT VENTRICULAR MYOCYTES

Mammalian cardiac myocytes become postmitotic shortly after birth, and the subsequent myocardial growth in adaptation to increasing workload s becomes primarily dependent on hypertrophy of existing myocytes. Alt hough hypertrophic growth of cardiac myocytes has been extensively stu died by using both in vitro and in vivo models, the molecular mechanis m controlling the switch from hyperplastic to hypertrophic growth of c ardiac myocytes is largely unknown. Since the majority of terminally d ifferentiated cardiac myocytes are growth-arrested in G(1)/G(0) phase, it has been hypothesized that the retinoblastoma protein (Rb) or its related pocket proteins which block G(1)/S transition becomes constitu tively active during myocardial terminal differentiation. To test this hypothesis, we studied the regulation of Rb activity by alpha-adrener gic stimulation in neonatal rat ventricular myocytes which are mostly postmitotic in culture. Our results demonstrate that Rb is predominant ly in the active hypophosphorylated state in control neonatal ventricu lar myocytes. alpha-Adrenergic stimulation activates G(1)/S transition in foetal but not neonatal rat ventricular myocytes. Although alpha-a drenergic stimulation does not activate G(1)/S transition in neonatal myocytes, it induces hyperphosphorylation of Rb to the same extent as in proliferating skeletal-muscle myoblasts or foetal ventricles. Hyper -but not hypo-phosphorylated Rb in stimulated neonatal myocytes or pro liferating skeletal-muscle myoblasts fails to bind to the transcriptio n factor, E2F, suggesting that hyperphosphorylated Rb is inactive. The refore G(1)/S transition could also be blocked at steps in addition to Rb inactivation during terminal differentiation and these blockades a re refractory to alpha-adrenergic stimulation.