Paracrine hypertrophic factors from cardiac non-myocyte cells downregulate the transient outward current density and Kv4.2 K+ channel expression in cultured rat cardiomyocytes.

OBJECTIVES: Cardiac hypertrophy is characterized by a prolongation of action potential duration (APD) and a reduction of outward K+ currents, primarily the transient outward current (Ito). Since the interaction between cardiac non-myocyte cells (NMCs) and cardiomyocytes (MCs) plays a critical role during the process of myocardial hypertrophy, in the present study, we investigated the effects of NMCs on cell growth and K+ channel expression in cultured newborn rat ventricular cells. METHODS: Single MCs were isolated from day-old Wistar rat ventricles and cultured for a period of five days. The effects of NMCs were examined by MC-NMC co-culture or incubating pure MCs in NMC-conditioned growth medium (NCGM). Whole-cell voltage-clamp recording and Western blot analysis using a polyclonal antibody against rat Kv4.2 channel protein were performed. RESULTS: A marked increase in surface area and total cell protein concentration of MCs was observed in the MC-NMC co-culture. In the pure MC culture, this hypertrophic effect could be mimicked by a 72-h addition of NCGM, with a significant prolongation of APD25 (APD at 25% repolarization) and a 42% decrease in Ito density (at +30 mV). The rates of inactivation and recovery from inactivation of Ito were unchanged. In the NCGM-treated MC culture, Western blots of MC proteins also showed a 36% reduction of the Kv4.2 K+ channel protein level. In addition, the NCGM-induced MC hypertrophy was partially inhibited by anti-insulin-like growth factor-1 (IGF-1) antibody, while it revealed no effects on Ito density and Kv4.2 channel expression. CONCLUSIONS: These findings first demonstrate that some paracrine hypertrophic factors released from cardiac NMCs, although unidentified, downregulate cardiac K+ channel expression.