Endothelium-derived hyperpolarizing factor and protein kinase G Iα activation: H2O2 versus S-nitrosothiols

Protein kinase G (PKG) I alpha mediates the cyclic guanosine monophosphate-mediated vasodilatory effects induced by NO. Endothelium-derived hyperpolarizing factors (EDHFs), like H2O2 can activate PKGI alpha in a cyclic guanosine monophosphate-independent manner, but whether this is true for all EDHFs (e. g., S-nitrosothiols) is unknown. Here, we investigated the contribution of PKGI alpha to bradykinin-, H2O2-, L-S-nitrosocysteine-, and lightinduced relaxation in porcine coronary arteries, making use of the fact that thioredoxin reductase inhibition with auranofin or 1-chloro-2,4-dinitrobenzene potentiates PKGI alpha. Thioredoxin reductase inhibition potentiated bradykinin and H2O2, but not L-S-nitrosocysteine or light. The relaxations by the latter 2 and bradykinin, but not those by H2O2, were prevented by the soluble guanylyl cyclase (sGC) inhibitor 1H-[1,2,4] oxadiazolo[4,3-a] quinoxalin-1-one. Yet, after S-nitrosothiol depletion with ethacrynic acid, thioredoxin reductase inhibition also potentiated light-induced relaxation, and this was prevented by the Na+-K+ ATPase inhibitor ouabain. This indicates that photorelaxation depends on sGC activation by S-nitrosothiols, while only after S-nitrosothiol depletion oxidized PKGI alpha comes into play, and acts via Na+-K+ ATPase. In conclusion, both bradykinin-and light-induced relaxation of porcine coronary arteries depend, at least partially, on oxidized PKGI alpha, and this does not involve sGC. H2O2 also acts via oxidized PKGI alpha in an sGC-independent manner. Yet, S-nitrosothiol-induced relaxation is PKGI alpha-independent. Clearly, PKG activation does not contribute universally to all EDHF responses, and targeting PKGI alpha may only mimick EDHF under certain conditions. It is therefore unlikely that PKGI alpha activators will be universal vasodilators.