Background-Prolonged action potentials (APs) and decreased transient outward K ϩ currents (I to ) are consistent findings in hypertrophic myocardium. However, the connection of these changes with cardiac hypertrophy is unknown. The present study investigated the effects of changes in I to and the associated alterations in AP on myocyte hypertrophy induced by phenylephrine. Methods and Results-Chronic incubation of cultured neonatal ventricular rat myocytes (NVRMs) with phenylephrine (PE) reduced I to density and prolonged AP duration, leading to a 2-fold increase in the net Ca 2ϩ influx per beat and a 1.4-fold increase in Ca 2ϩ -transient amplitude. PE treatment of chronically paced (2-Hz) NVRM also induced increases in cell size, protein/DNA ratio, atrial natriuretic factor mRNA expression, as well as ␤/␣ myosin mRNA ratio. These hypertrophic changes were associated with a 2.4-fold increase in activation of nuclear factor of activated T-cells (NFAT), indicating increased activity of the Ca 2ϩ -dependent phosphatase calcineurin. Overexpression of Kv4.2 channels using adenovirus prevented the AP duration prolongation as well as the increases in Ca 2ϩ influx and Ca 2ϩ -transient amplitude induced by PE. Kv4.2 overexpression also prohibited the PE-induced increases in cell size, protein/DNA ratio, atrial natriuretic factor expression, ␤/␣ myosin mRNA ratio, and NFAT activation.Conclusions-Our results demonstrate that PE-mediated hypertrophy in NRVMs seems to require I to reductions and AP prolongation associated with increased Ca 2ϩ influx and Ca 2ϩ transients as well as calcineurin activation. The clinical implications of these studies and the possible involvement of other signaling pathways are discussed. (Circulation.

1. We examined the mechanisms for rate-dependent changes in twitch force duration by simultaneously measuring force and [Ca2+]i in rat cardiac trabeculae. 2. Peak force decreased when the rate of stimulation was increased from 0.2 to 0.5 Hz, whilst it increased from 1 to 2 Hz. Over the same range of frequencies, peak [Ca2+]i transients increased monotonically, whilst both force and [Ca2+]i transient duration were abbreviated. 3. Changes in peak force or peak [Ca2+]i transients were not responsible for the changes in force or [Ca2+]i transient duration. 4. The changes in twitch force and [Ca2+]i transient duration were completed roughly within one beat following an abrupt change in the rate of stimulation. 5. Rate-dependent changes resembled those observed with isoproterenol (isoprenaline) application. However, kinase inhibitors (i.e. K252-a, H-89, KN-62 and KN-93) had no effect on the rate-dependent changes of twitch force and [Ca2+]i transient kinetics, suggesting that protein kinase A (PKA), protein kinase PKG) and Ca2+-calmodulin-dependent protein kinase II (CaM/kinase II) were not responsible for these kinetic changes. 6. Despite the changes in twitch force and [Ca2+]i transient kinetics, the rate-limiting step for the rate-dependent force relaxation still resides at the level of the contractile proteins. 7. Our results suggest that rate-dependent changes in force and [Ca2+]i transients do not depend on PKA or CaM/kinase II activity but might result from intrinsic features of the contractile and/or Ca2+-handling proteins.