Activation ofα1-Adrenergic Receptor during Ca2+Pre-conditioning Elicits Strong Protection against Ca2+Overload Injury via Protein Kinase C Signaling Pathway

The objective was to test the hypothesis that transient activation of theα1-adrenergic receptor mimics the beneficial effects of Ca2+preconditioning on the Ca2+paradox (Ca2+PD) injury in rat hearts, and that the protection is mediated by protein kinase C (PKC) signaling pathway. Langendorff-perfused rat hearts were subjected to the Ca2+PD (10 min of Ca2+depletion followed by 10 min of Ca2+repletion). The effects ofα1-adrenergic receptor activation and other interventions on functional, biochemical and pathological changes were assessed. In hearts pretreated with 50μmol/l phenylephrine, left ventricular end-diastolic pressure and coronary flow were significantly preserved after Ca2+PD; furthermore, peak loss of lactate dehydrogenase was significantly decreased while ATP was significantly preserved. A remarkable preservation of cell structure was observed in phenylephrine-treated hearts in contrast to non-treated Ca2+PD hearts. However, pre-conditioning elicited by phenylephrine caused only a mild improvement in left ventricular developed pressure (LVDP) as opposed to its impressive recovery of left ventricular end-diastolic pressure (LVEDP), heart rate (HR), or coronary flow (CF). The salutary effects of phenylephrine on the Ca2+PD injury were almost similar to those observed in hearts which underwent Ca2+pre-conditioning (CPC) or were pretreated with 1-stearoyl-2-arachidonoyl-glycerol (SAG), a potent PKC activator. In phenylephrine pretreated hearts, PKC isoform-αwas localized in the sarcolemma and nucleus, while PKC-δand PKC-cwere localized in the cell membrane, and intercalated disk respectively. Prazosin, a specificα1-adrenergic receptor antagonist completely abolished the beneficial effects of phenylephrine on the Ca2+PD and blocked translocation of PKC isoforms. In addition, prazosin (1μmol/l) also reversed salutary effects of CPC. Moreover, theβ-adrenergic antagonist, propranolol, had no effect on the protection provided by phenylephrine against the Ca2+PD injury. This study suggests that the activation of theα1-adrenergic receptor confers protection against the lethal injury of the Ca2+PD via PKC-mediated signaling pathways. The protection is shared by stimuli common with calcium pre-conditioning.