Objectives The circulating, adipocyte-secreted hormone adiponectin (APN) exerts protective effects on the heart under stress conditions. Recent study demonstrated that APN induces a marked Ca2+ influx in skeletal muscle. However, whether APN modulates [Ca2+]i activity, especially modulates [Ca2+]i transients in cardiomyocytes is still unknown. This study was designed to determine whether and how APN modulates [Ca2+]i transients in cardiomyocytes.
Methods Adult male wild-type (WT) and APN knock-out (APN KO) mice were subjected to myocardial ischaemia/reperfusion (I/R, 30 min/30 min) injury.We observed CaMKII-PLB phosphorylation and SR Ca2+-ATPase (SERCA2) activity were downregulated in I/R heart of WT mice (all P <0.01) and further decreased in I/R heart of APN KO mice (P <0.05). Administration of globular adiponectin domain and full-length adiponectin 20 min before reperfusion significantly reversed the decrease in CaMKII-PLB phosphorylation and SERCA2 activity in WT and APN KO mice (all P <0.05). Compared with WT littermates, single myocytes isolated from APN KO mice displayed equal cell lengths, caffeine-induced Ca2+ transient peaks, and number of Ca2+ sparks (all P >0.05), but decreased [Ca2+]i transients (P <0.001), decreased cell shortening (P <0.01), and a prolonged Ca2+ decay rate (P <0.05), all of which were reversed by APN exposure (all P <0.05). Preincubation with KN-93 (a CaMKII inhibitor), but not Rp-cAMP (a PKA inhibitor), completely abrogated the APN-mediated improvement in [Ca2+]i transients (P <0.05). Interestingly, pretreatment with the sphingosine 1-phosphate (S1P) receptor (S1PR1/3) antagonist VPC23019, but not AMPK inhibitor compound C, virtually abolished APN-stimulated CaMKII-PLB-SERCA2 activation during I/R (P <0.05). A short incubation with APN also reversed the decreases in S1P content and CaMKII-PLB signalling in cultured cardiomyocytes from APN KO mice (all P <0.05). Concomitantly, S1P activated CaMKII-PLB signalling in neonatal rat ventricular myocytes dose-dependently and improved [Ca2+]i transients in APN KO myocytes via S1PR1/3 (all P <0.05).
Results Treatment of myocytes after hypoxia/reoxygenation with S1P also increased cell viability and reduced caspase-3 activity and apoptosis (all P <0.05); this effect was partially abolished by siRNAs against CaMKIIδ/PLB/SERCA2 (all P <0.05). More importantly, pharmacological inhibition of S1P/S1PR and siRNA-mediated silencing of SERCA2 could suppress APN-mediated cardioprotection during I/R (all P <0.05).
Conclusions These data demonstrate that S1P is a novel regulator of SERCA2 by activating CaMKII-PLB signalling and mediates APN-induced cardioprotection.