SERCA 2 activity is involved in the CNP ‐mediated functional responses in failing rat myocardium

Background and Purposes Myocardial C‐type natriuretic peptide ( CNP ) levels are increased in heart failure. CNP can induce negative inotropic ( NIR ) and positive lusitropic responses ( LR ) in normal hearts, but its effects in failing hearts are not known. We studied the mechanism of CNP ‐induced NIR and LR in failing hearts and determined whether sarcoplasmatic reticulum Ca 2+ ATP ase2 ( SERCA 2) activity is essential for these responses. Experimental Approach Contractility, c GMP levels, Ca 2+ transient amplitudes and protein phosphorylation were measured in left ventricular muscle strips or ventricular cardiomyocytes from failing hearts of Wistar rats 6 weeks after myocardial infarction. Key Results CNP increased c GMP levels, evoked a NIR and LR in muscle strips, and caused phospholamban ( PLB ) Ser 16 and troponin I ( TnI ) Ser 23/24 phosphorylation in cardiomyocytes. Both the NIR and LR induced by CNP were reduced in the presence of a PKG blocker/c GMP analogue ( Rp ‐8‐ B r‐ P et‐c GMPS ) and the SERCA inhibitor thapsigargin. CNP increased the amplitude of the Ca 2+ transient and increased SERCA 2 activity in cardiomyocytes. The CNP ‐elicited NIR and LR were not affected by the L ‐type Ca 2+ channel activator BAY ‐ K 8644, but were abolished in the presence of isoprenaline (induces maximal activation of c AMP pathway). This suggests that phosphorylation of PLB and TnI by CNP causes both a NIR and LR . The NIR to CNP in mouse heart was abolished 8 weeks after cardiomyocyte‐specific inactivation of the SERCA 2 gene. Conclusions and Implications We conclude that CNP ‐induced PLB and TnI phosphorylation by PKG in concert mediate both a predictable LR as well as the less expected NIR in failing hearts.