β-adrenergic agonists increase phosphorylation of elongation factor 2 in cardiomyocytes without eliciting calcium-independent eEF2 kinase activity

The β-adrenergic agonist isoproterenol increased the phosphorylation of elongation factor eEF2 in ventricular cardiomyocytes from adult rats (ARVC). Phosphorylation of eEF2 inhibits its activity, and protein synthesis was inhibited in ARVC concomitantly with increased eEF2 phosphorylation. eEF2 kinase activity in ARVC extracts was completely dependent upon Ca²⁺/calmodulin. In contrast to other cell types, however, treatments designed to raise intracellular cAMP failed to induce Ca²⁺/calmodulin-independent activity. Instead, they increased maximal eEF2 kinase activity. Similar data were obtained when partially purified ARVC eEF2 kinase was treated with cAMP-dependent protein kinase in vitro. These data suggest that ARVC possess a distinct isoform of eEF2 kinase.