Abstract 122: Pkialpha Protects Hearts From Beta-adrenergic Overstimulation

Abstract

Introduction: Protein kinase inhibitor peptide α (PKIα) isoforms are abundantly expressed endogenous inhibitors of PKA in the heart. The physiological and pathophysiological relevance of PKIα has not been studied. Hypothesis: PKIα inhibits PKA activity in normal and stressed (post-myocardial infarction (MI)) hearts. Methods: PKIα knockout (KO) and c57bl/6 control (CTR) mice were used to test if PKIα affected cardiac β-adrenergic response and if played a role in post-MI hearts. Results: Protein abundance of PKIα was significantly increased in failing human and post-MI mouse ventricular tissue. The knockout of PKIα in mice did not affect the expression of the other two PKI isoforms. Nor did it change the expression of PKA regulatory and catalytic subunits. PKIα KO mice did not show alterations in basal cardiac morphology, function and life span of mice. Intra-LV hemodynamic measurements in response to different doses of isoproterenol showed that the loss of PKIα sensitizes the heart in response to isoproterenol (ISO) with a lower EC50 and the maximum response was increased more in KO mice than in control mice. ISO (10nM) increased myocyte contraction and Ca2+ transient amplitudes more in KO myocytes than control myocytes though 100nM ISO increased myocyte contraction and Ca2+ transients amplitude to the same. At the ISO concentration of 10nM, ISO increased more phosphorylation of the Ser16 site (PKA site) on phospholamban. In post-MI animals, the loss of PKIα enhances cardiac function at 1 week after MI but caused more myocardial infarction and functional depression at 8 weeks after MI. Conclusions: PKIα is an important regulator of cardiac β-adrenergic responses by antagonizing PKA. This mechanism could be protective to avoid β-adrenergic overstimulation, especially in post-MI patients and animal models.