Myocardial adenosine A1-receptor-mediated adenoprotection involves phospholipase C, PKC-ε, and p38 MAPK, but not HSP27

Abstract

Adenosine via an adenosine A1 receptor (A1R) is a negative feedback inhibitor of adrenergic stimulation in the heart, protecting it from toxic effects of overstimulation. Stimulation of the A1R results in the activation of Gi protein, release of free Gβγ-subunits, and activation/translocation of PKC-ε to the receptor for activated C kinase 2 protein at the Z-line of the cardiomyocyte sarcomere. Using an anti-Gβγ peptide, we investigated the role of these subunits in the A1R stimulation of phospholipase C (PLC), with the premise that the resulting diacylglycerol provides for the activation of PKC-ε. Inositol 1,4,5-triphosphate release was an index of PLC activity. Chlorocyclopentyl adenosine (CCPA), an A1R agonist, increased inositol 1,4,5-triphosphate production by 273% in mouse heart homogenates, an effect absent in A1R knockout hearts and inhibited by anti-Gβγ peptide. In a second study, p38 MAPK and heat shock protein 27 (HSP27), found by others to be associated with the loss of myocardial contractile function, were postulated to play a role in the actions of A1R. Isoproterenol, a β-adrenergic receptor agonist, increased the Ca2+ transient and sarcomere shortening magnitudes by 36 and 49%, respectively. In the rat cardiomyocyte, CCPA significantly reduced these increases, an action blocked by the p38 MAPK inhibitor SB-203580. While CCPA significantly increased the phosphorylation of HSP27, this action was inhibited by isoproterenol. These data indicate that the activation of PKC-ε by A1R results from the activation of PLC via free Gβγ-subunits released upon A1R-induced dissociation of Giαβγ. Attenuation of β-adrenergic-induced contractile function by A1R may involve the activation of p38 MAPK, but not HSP27.