Contractile activity is required for sarcomeric assembly in phenylephrine-induced cardiac myocyte hypertrophy.

Agonist-induced hypertrophy of cultured neonatal rat ventricular myocytes (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 absence of exogenous neurohormonal stimuli. Using single-cell imaging of fura 2-loaded myocytes, we found that low-density, noncontracting NRVM begin to generate intracellular Ca2+ concentration ([Ca2+]i) transients and contractile activity within minutes of exposure to the α1-adrenergic agonist phenylephrine (PE; 50 μM). However, NRVM pretreated with verapamil and then stimulated with PE failed to elicit [Ca2+]itransients and beating. We therefore examined whether PE-induced [Ca2+]itransients and contractile activity were required to elicit specific aspects of the hypertrophic phenotype. PE treatment (48-72 h) increased cell size, total protein content, total protein-to-DNA ratio, and myosin heavy chain (MHC) isoenzyme content. PE also stimulated sarcomeric protein assembly and prolonged MHC half-life. However, blockade of voltage-gated L-type Ca2+ channels with verapamil, diltiazem, or nifedipine (10 μM) blocked PE-induced total protein and MHC accumulation and prevented the time-dependent assembly of myofibrillar proteins into sarcomeres. Inhibition of actin-myosin cross-bridge cycling with 2,3-butanedione monoxime (7.5 mM) also prevented PE-induced total protein and MHC accumulation, indicating that mechanical activity, rather than [Ca2+]itransients per se, was required. In contrast, blockade of [Ca2+]itransients and contractile activity did not prevent the PE-induced increase in cell surface area, activation of the mitogen-activated protein kinases ERK1 and ERK2, or upregulation of atrial natriuretic factor gene expression. Thus contractile activity is required to elicit some but not all aspects of the the hypertrophic phenotype induced by α1-adrenergic receptor activation.