The alpha1 isoform of soluble guanylate cyclase regulates cardiac contractility but is not required for ischemic preconditioning

Nitric oxide (NO)-dependent soluble guanylate cyclase (sGC) activation is an important component of cardiac signal transduction pathways, including the cardioprotective signaling cascade induced by ischemic preconditioning (IPC). The sGC alpha subunit, which binds to the common sGC beta 1 subunit, exists in two different isoforms, sGC alpha 1 and sGC alpha 2, but their relative physiological roles remain unknown. In the present study, we studied Langendorff-perfused isolated hearts of genetically engineered mice lacking functional sGC alpha 1 (sGC alpha 1KO mice), which is the predominant isoform in the heart. Our results show that the loss of sGC alpha 1 has a positive inotropic and lusitropic effect on basal cardiac function, indicating an important role for sGC alpha 1 in regulating basal myocardial contractility. Surprisingly, IPC led to a similar 35-40% reduction in infarct size and concomitant protein kinase C epsilon (PKC epsilon) phosphorylation in both wild-type (WT) and sGC alpha 1KO hearts subjected to 40 min of global ischemia and reperfusion. Inhibition of the activation of all sGC isoforms by 1H-[1,2,4] oxadiazolo[4,3-a]quinoxaline-1-one (ODQ, 10 mu mol/L) completely abolished the protection by IPC in WT and sGC alpha 1KO hearts. NO-stimulated cGMP production was severely attenuated in sGC alpha 1KO hearts compared to WT hearts, indicating that the sGC alpha 2 isoform only produces minute amounts of cGMP after NO stimulation. Taken together, our results indicate that although sGC alpha 1 importantly regulates cardiac contractility, it is not required for cardioprotection by IPC. Instead, our results suggest that possibly only minimal sGC activity, which in sGC alpha 1KO hearts is provided by the sGC alpha 2 isoform, is sufficient to transduce the cardioprotective signal induced by IPC via phosphorylation of PKC epsilon.