Introduction: Heart failure with preserved ejection fraction (HFpEF) represents ~50% of heart failure cases and is characterized by impaired relaxation, ventricular stiffening and fibrosis. Growth hormone releasing hormone agonists (GHRH-A) reduce fibrosis in rat and swine models of ischemic myocardial injury. However, their effect on failing cardiomyocytes (CMs) is unknown.
Hypothesis: The activation of GHRH receptor signaling targets proteins associated with excitation-contraction coupling, reduces affinity of myofilaments for Ca2+ and prevents the development of HFpEF.
Methods: CD1 mice, implanted with mini-osmotic pump (Alzet) to deliver angiotensin-II (Ang-II) for 4 weeks, received daily injections of GHRH-A (MR-356; n=8) or vehicle (n=8). CMs were isolated and Ca2+ and sarcomere length recorded. Ca2+ handling and sarcomeric proteins were assessed. Unmanipulated CD1 mice (n=7) acted as normal controls.
Results: Ang-II-treated CMs resembled features of the HFpEF syndrome. They exhibited reduced sarcomere length consistent with shorter cell length, indicating an inability to relax. These CMs also exhibited impaired contractility that correlated with reduced myosin binding protein C (cMyBPC) expression with no changes in phosphorylation. Response of [Ca2+] transient amplitude to increasing pacing rate was depressed and Ca2+ decay was delayed and associated with lower expression of SERCA2 and NCX1, increased SR Ca2+ leak but enhanced phospholamban phosphorylation (p-PLB) at Ser16. Slower sarcomere re-lengthening and reduced phospho-cTnI (p-cTnI) at Ser 23/24 were observed in HFpEF CMs. MR-356 treatment maintained resting sarcomere length as well as sarcomere shortening at control values, and completely abrogated Ang-II-induced delay in Ca2+ decay and sarcomere relaxation. SR Ca2+ leak was reduced. p-PLB was further enhanced by MR-356, and cMyBPC and p-cTnI were maintained at control levels.
Conclusions: Our findings demonstrate that chronic administration of Ang-II mediates functional changes in CMs consistent with HFpEF and suggest that activation of the GHRH receptor signaling pathways desensitizes myofilaments and prevents HFpEF-associated alterations in Ca2+ handling and dysfunctional CM relaxation.