Abstract P275: Time-of-Day--Dependent Variation in Myocardial Ischemia/Reperfusion Injury Is Lost in Mice Lacking the Regulator of Calcineurin 1 Gene

Many important components of the cardiovascular system display circadian rhythmicity. In humans, the incidence of myocardial infarction peaks in the morning. Furthermore, studies in mouse models have demonstrated that damage from ischemia/reperfusion (I/R) occurring during the transition to waking is greater than at the end of the active period. In the hearts of wild type mice we have observed large circadian oscillations in the transcript and protein levels of the exon 4 isoform of the calcineurin regulatory protein ( Rcan1.4 ). This 15 to 20-fold oscillation peaks around circadian time 0 (CT:0) when mice enter a period of rest. Using an in vivo murine model of myocardial I/R injury we subjected wild type and Rcan1 KO mice to 45 minutes of myocardial ischemia followed by reperfusion for 24 hrs. Mice were housed in hermetic light boxes with opposing light cycles such that the surgeon was blinded as to the light entrainment of individual mice. I/R was performed at the beginning (CT:0 to CT:2) and the end (CT:10 to CT:12) of the mouse9s light entrainment cycle. Twenty-four hours after reperfusion Evan9s Blue and 2,3,5-Triphenyltetrazolium chloride staining were used to quantify the area-at-risk per left ventricle (AAR/LV) and the extent of myocardial infarction per area-at-risk (INF/AAR). Serum troponin-I levels were quantified as an indication of muscle damage. AAR/LV was the same in all animals, however, I/R injury in wild-type mice at CT:12 was 30% higher than in mice subjected to I/R at CT:0. Plasma troponin-I levels were also higher. Remarkably, there was no circadian difference in the extent of I/R injury or troponin-I levels in the Rcan1 KO mice. Damage in the Rcan1 KO mice at both time points was comparable to the level of damage observed in wild type mice at CT:12, when Rcan1.4 expression is low. Importantly, expression of all central clock genes remained circadian in the hearts of Rcan1 KO mice suggesting that circadian changes in the vulnerability of the myocardium to I/R damage is directly dependent upon Rcan1 function.