Background: 

The soluble guanylyl cyclase (sGC) is activated by NO and catalyses the formation of cGMP which mediates smooth muscle relaxation and inhibition of thrombocyte aggregation. sGC is a heterodimer of two subunits (a and b) of which the a subunit exists in two isoforms (a1 and a2). The a1 subunit mediates the NO-induced inhibiton of platelet aggregation in vitro and is the only subunit expressed in thrombocytes. We studied the contribution of the a subunit in platelet aggregation in vivo at levels of NO released by the non-stimulated endothelium in arterioles.

Method: 

Thrombus formation was studied in the cremaster muscle microcirculation in vivo in wildtype (wt) and a1 subunit deficient (a1^-/-) mice. Thrombus formation was induced by excitation (488 nm) of FITC-dextran which was injected intravenously ten minutes prior to excitation. Excitation was interrupted upon blood flow stoppage in the arteriole.

Results: 

In the presence of indomethacin to inhibit prostaglandin formation time to thrombus visibility (onset) and time to complete occlusion were significantly shorter in arterioles of a1^-/- mice (25.4 ± 2.8 and 64.7 ± 7.2 s; wt: 64.5 ± 8.6 and 124.3 ± 11.3 s). Additional superfusion of N-nitro-L-arginine to block NO-synthase accelerated thrombus onset and occlusion only in wildtype mice. Consequently, differences between genotypes were not detectable in the absence of endogenous NO (a1^-/-vs. wt; onset: 28.9 ± 3.7 vs. 29.6 ± 2.9 s, time to occlusion: 90.7 ± 13.0 vs. 79.0 ± 5.7 s).

Conclusion: 

NO released by the endothelium in arterioles in vivo inhibits platelet aggregation. This inhibition is completely excerted through the action on the a1 subunit of the sGC.