Gαi2- and Gαi3-deficient mice display opposite severity of myocardial ischemia reperfusion injury.

G-protein-coupled receptors (GPCRs) are the most abundant receptors in the heart and therefore are common targets for cardiovascular therapeutics. The activated GPCRs transduce their signals via heterotrimeric G-proteins. The four major families of G-proteins identified so far are specified through their α-subunit: Gαi, Gαs, Gαq and G12/13. Gαi-proteins have been reported to protect hearts from ischemia reperfusion injury. However, determining the individual impact of Gαi2 or Gαi3 on myocardial ischemia injury has not been clarified yet. Here, we first investigated expression of Gαi2 and Gαi3 on transcriptional level by quantitative PCR and on protein level by immunoblot analysis as well as by immunofluorescence in cardiac tissues of wild-type, Gαi2-, and Gαi3-deficient mice. Gαi2 was expressed at higher levels than Gαi3 in murine hearts, and irrespective of the isoform being knocked out we observed an up regulation of the remaining Gαi-protein. Myocardial ischemia promptly regulated cardiac mRNA and with a slight delay protein levels of both Gαi2 and Gαi3, indicating important roles for both Gαi isoforms. Furthermore, ischemia reperfusion injury in Gαi2- and Gαi3-deficient mice exhibited opposite outcomes. Whereas the absence of Gαi2 significantly increased the infarct size in the heart, the absence of Gαi3 or the concomitant upregulation of Gαi2 dramatically reduced cardiac infarction. In conclusion, we demonstrate for the first time that the genetic ablation of Gαi proteins has protective or deleterious effects on cardiac ischemia reperfusion injury depending on the isoform being absent.