CONTRACTILE ACTIVITY IS REQUIRED FOR SARCOMERIC ASSEMBLY IN PHENYLEPHRINE-INDUCED CARDIAC MYOCYTE HYPERTROPHY

Agonist-induced hypertrophy of cultured neonatal rat ventricular myocy tes (NRVM) has been attributed to biochemical signals generated during receptor activation. However, NRVM hypertrophy can also be induced by spontaneous or electrically stimulated contractile activity in the ab sence of exogenous neurohormonal stimuli. Using single-cell imaging of fura 2-loaded myocytes, we found that low-density, noncontracting NRV M begin to generate intracellular Ca2+ concentration ([Ca2+](i)) trans ients and contractile activity within minutes of exposure to the alpha (1)-adrenergic agonist phenylephrine (PE; 50 mu M). However, NRVM pret reated with verapamil and then stimulated with PE failed to elicit [Ca 2+](i) transients and beating. We therefore examined whether PE-induce d [Ca2+](i) transients and contractile activity were required to elici t specific aspects of the hypertrophic phenotype. PE treatment (48-72 h) increased cell size, total protein content, total protein-to-DNA ra tio, and myosin heavy chain (MHC) isoenzyme content. PE also stimulate d sarcomeric protein assembly and prolonged MHC half-life. However, bl ockade of voltage-gated L-type Ca2+ channels with verapamil, diltiazem , or nifedipine (10 mu M) blocked PE-induced total protein and MHC acc umulation and prevented the time-dependent assembly of myofibrillar pr oteins into sarcomeres. Inhibition of actin-myosin cross-bridge cyclin g with 2,3-butanedione monoxime (7.5 mM) also prevented PE-induced tot al protein and MHC accumulation, indicating that mechanical activity, rather than [Ca2+](i) transients per se, was required. In contrast, bl ockade of [Ca2+](i) transients and contractile activity did not preven t the PE-induced increase in cell surface area, activation of the mito gen-activated protein kinases ERK1 and ERK2, or upregulation of atrial natriuretic factor gene expression. Thus contractile activity is requ ired to elicit some but not all aspects of the the hyper-trophic pheno type induced by alpha 1-adrenergic receptor activation.