['The underlying mechanism(s) of vulnerability of the diabetic myocardium to ischaemia/reperfusion (I/R)-induced injury is not fully understood. Interleukin-33 (IL-33) has been reported showing the beneficial effect to the myocardium on I/R injury. The aims of this study were to test whether diabetes mellitus (DM) affects myocardial levels of IL-33 and to examine whether reduction in IL-33 is responsible for exaggerated I/R injury in the diabetic myocardium.', 'DM hearts were challenged with I/R in vivo, whereas while isolated cardiomyocytes in vitro were conditioned with high glucose (HG) followed by an anoxia/reoxygenation (A/R) challenge. Myocardial levels of IL-33 were decreased in mice with DM which was associated with increased protein kinase C βII (PKCβII) activation. Exogenous IL-33 prevented the DM-induced PKCβII activation and attenuated I/R injuries (myocardial infarction size and apoptosis). HG-conditioned myocytes incurred exaggerated apoptosis when compared with naïve myocytes after A/R which was attenuated by IL-33. HG activated PKCβII in cardiomyocytes, which was further enhanced by A/R. IL-33 prevented the PKCβII activation in myocytes with HG or HG and A/R. Inhibition of PKCβII prevented the beneficial effect of IL-33. Finally, IL-33 up-regulated diacylglycerol kinase zeta (DGK-zeta) in cardiomyocytes and reversed the down-regulation of myocardial DGK-zeta in mice with DM.'] Does reduction in IL-33 expression exaggerate ischaemia/reperfusion-induced myocardial injury in mice with diabetes mellitus?