Abstract 19461: Endogenous Protein Kinase Inhibitor Peptide (pki) Regulates β-Adrenergic Responses of the Heart

Abstract

Introduction: Protein kinase inhibitor peptide (PKI) isoforms are endogenous inhibitors of PKA and existing in the heart. The physiological relevance of PKI has not been studied. Aims: We will determine the role of PKIα (one of the major PKI isoforms in the heart) in β-adrenergic regulation of the heart with PKIα knockout (KO) versus c57bl/6 control (CTR) mice. Methods: In vivo cardiac function was evaluated by conventional echocardiography (ECHO) in PKIα KO mice. In vivo cardiac β-adrenergic response was determined by intra-LV hemodynamic measurements in response to different doses of isoproterenol (10-16g/g BW∼10-5g/g BW). Isolated ventricular myocytes were used to determine cellular responses to β-adrenergic agonist, isoproterenol (ISO, 1nM, 10nM and 100nM). Western blotting was used to determine phosphorylation levels of PKA-targets in the heart. Results: (1) The loss of PKI did not change basal cardiac morphology, function and life span of mice; (2) ISO (2μ g/g BW) enhanced cardiac function more in KO mice than in CTR mice when evaluated by ECHO and intra-LV hemodynamic measurements; (3) The loss of PKIα sensitizes the heart in response to ISO (Log[EC50] shifted from -9.2 to -10.8) and the maximum dp/dt increases more (by 25%) in KO mice than in CTR mcie; (4) PKIa KO myocytes were more sensitive to ISO in that 10nM ISO increased myocyte contraction and Ca2+ transient amplitudes more in KO myocytes; (5). At the ISO concentration of 10-9M, ISO increased more phosphorylation of the Ser16 site (PKA site) on phospholamban. Conclusion: PKI plays an important role in regulating cardiac β-adrenergic responses by antagonizing PKA.