Modulation of outward potassium currents in aligned cultures of neonatal rat ventricular myocytes during phorbol ester-induced hypertrophy

Protein kinase C-stimulating phorbol esters induce a strong hypertrophic re sponse when applied in vitro to cardiac ventricular myocytes. The aim of th is study was to determine if this in vitro model of hypertrophy is associat ed with changes in the expression of voltage-gated K+ channels. Myocytes we re isolated from 3-4-day-old neonatal rats and cultured on aligned collagen thin gels. Membrane currents were measured with the use of the whole-cell arrangement of the patch clamp technique and the expression levels of the K v1.4, Kv4.2 and Kv2.1 alpha subunits quantified using Western blot analysis . Voltage steps positive to -30 mV resulted in the activation of both a tra nsient (I-to) and a sustained (I-sus) component of outward K+ current in th e aligned myocytes. Overnight exposure to phorbol 12-myristate 13-acetate ( PMA) caused a 55% increase in myocyte size and a three-fold reduction in th e peak amplitude of I-to. No differences in the half-maximal voltages requi red for activation and steady-state inactivation were observed between I-to measured in control and PMA-treated myocytes. In contrast, PMA treatment r esulted in a 62% increase in a tetraethylammonium-sensitive component of I- sus (TEA-I-sus) and was associated with the appearance of a slow component of current decay. Expression levels of the Kv1.4 and Kv4.2 alpha subunits w ere strongly depressed in the hypertrophic myocytes, while the density of t he Kv2.1 alpha subunit was enhanced. PMA-induced changes in the Kv alpha su bunits were partially prevented through inhibition of the mitogen-activated protein kinase (MAPK) pathway. Thus, PMA-induced hypertrophy of cultured v entricular myocytes is associated with an altered expression of voltage-gat ed K+ channels. (C) 2001 Academic Press.