The α1-Adrenoceptor Subtype- and Protein Kinase C Isoform-dependence of Norepinephrineʹs Actions in Cardiomyocytes

Catecholamines modulate cardiac function at least in part through α1-adrenergic receptors linked to the activation of protein kinase C (PKC). This study examines the molecular forms of theα1 -receptor and PKC that mediate norepinephrineʹs actions in cardiomyocytes; distinct approaches (activation-dependent down-regulation of PKC isoforms) and novel reagents (A61603, an α1A/c-receptor agonist) are used to resolve this issue which has been the focus of dispute in previous studies. Norepinephrine (NE) induces a rise in diacylglycerol levels which is sustained for 24 h and is associated with the translocation (at 5 min) and down-regulation (at 24 h) of PKC δ and PKC ξ (but not PKC α). The selective targeting of theα1 -adrenergic receptor to activate novel PKC isoforms is remarkable, given an 8-fold greater abundance of PKC α relative to PKC ξ in this preparation. NE activates the extracellular signal-regulated protein kinase (ERK) subfamily of mitogen-activated protein kinases through a PKC δ/PKC ξ -dependent pathway. WB-4101 (α1A/c- and α1D-receptor antagonist) and 5-methylurapidil (α1A/c-receptor antagonist) inhibit norepinephrine-dependent accumulation of inositol phosphate and diacylglycerol, down-regulation of PKC δ and PKC ξ, and activation of ERK. Each of these responses is stimulated by A61603, but not attenuated by high concentrations of chloroethylclonidine (which irreversibly inactivates theα1B -, and to a lesser extent, the α1D-receptor) or BMY 7378 (selectiveα1D -receptor antagonist). A61603 also activates p38-MAPK and induces hypertrophy. These studies establish that NEʹs actions in cardiomyocytes can be attributed to the α1A/c-adrenergic receptor subtype and nPKC isoforms, thereby identifying specific targets for the development of pharmaceuticals to influence cardiac contractile function and/or growth responses.