Establishment of β-Adrenergic Modulation of L-Type Ca 2+ Current in the Early Stages of Cardiomyocyte Development

Abstract

—β-Adrenergic modulation of the L-type Ca²⁺ current (I_CaL) was characterized for different developmental stages in murine embryonic stem cell-derived cardiomyocytes using the whole-cell patch-clamp technique at 37°C. Cardiomyocytes first appeared in embryonic stem cell-derived embryoid bodies grown for 7 days (7d). I_CaL was insensitive to isoproterenol, forskolin, and 8-bromo-cAMP in very early developmental stage (VEDS) cardiomyocytes (from 7+1d to 7+2d) but highly stimulated by these substances in late developmental stage (LDS) cardiomyocytes (from 7+9d to 7+12d), indicating that all signaling cascade components became functionally coupled during development. In early developmental stage (EDS) cells (from 7+3d to 7+5d), the stimulatory response to forskolin and 8-bromo-cAMP was relatively weak. The forskolin effect was strongly augmented by ATP-γ-S. At this stage, basal I_CaL was stimulated by the nonselective phosphodiesterase (PDE) inhibitor isobutylmethylxanthine, by PDE inhibitors selective for the PDE II, III, and IV isoforms, as well as by the phosphatase inhibitor okadaic acid. Stimulation of I_CaL by the catalytic subunit of the cAMP-dependent protein kinase A (PKA) was found to be similar (about 3 times) throughout development and in adult mouse ventricular cardiomyocytes, indicating that no structural changes of the Ca²⁺ channel related to phosphorylation occurred during development. I_CaL was stimulated by isoproterenol in the presence of a PKA inhibitor and GTP-γ-S in LDS but not VEDS cardiomyocytes, suggesting the development of a membrane-delimited stimulatory pathway mediated through the stimulatory GTP binding protein, G_s. We conclude that uncoupling and/or low expression of G_s protein accounted for the I_CaL insensitivity to β-adrenergic stimulation in VEDS cardiomyocytes. Furthermore, in EDS cells at the 7+4d stage, the reduced β-adrenergic response is due, at least in part, to high intrinsic PDE and phosphatase activities.