Activators of soluble guanylate cyclase (sGC) mimic nitric oxide (NO) and have therapeutic potential as vasodilators. Several novel sGC activators, which fall into two distinct classes defined by their interactions with NO and sensitivity to the sGC inhibitor ODQ, have recently been described (Stasch et al 2002a,b). We investigated the properties of representative compounds from each class, BAY 41-8543 and BAY 58-2667.

Acitivity of purified human 1ß1 sGC (Kosarikov et at 2001), was determined by measuring cGMP production using liquid chromatography mass spectroscopy. Stimulatory effects of BAY 41-8543 and BAY 58-2667 on sGC activity, their interactions with the NO-donor (SIN-1, 1mM) and their inhibition by ODQ were investigated. Endothelium denuded aortic rings from male Sprague-Dawley rats (250 - 350 g) were mounted in tissue baths under a resting tension of 2g at 37°C. Isometric preconstriction of tissues with noradrenaline (30nM) was followed by cumulative concentration-response curves either to BAY 41-8543 and BAY 58-2667 in the presence or absence of the NO-donor DETA-NO (1-3 µM) and ODQ (3 µM), n = 6 for each group.

In the presence of SIN-1, BAY 41-8543 caused a concentration- dependent 10-fold increase in sGC activity (logEC₅₀ = -6.38) that was attenuated by ODQ. In the absence of SIN-1, BAY 41-8543 was less potent (EC₅₀ could not be generated) and failed to attain the same maximum response observed in the presence of SIN-1. BAY 58-2667 caused only a 2-fold increase in sGC activity, but was more potent than BAY 41-8543 (logEC₅₀-9). SIN-1 failed to alter the ability of BAY 58-2667 to activate sGC. However, in the presence of SIN-1, ODQ slightly enhanced the potency of BAY 58-2667.

In the absence of DETA-NO, BAY 41-8543 produced a concentration dependent relaxation of aortic tissue (logEC₅₀ = -7.25 ± 0.03) However in the presence of DETA-NO the potency of BAY 41-8543 was significantly enhanced (logEC₅₀ = -7.98 ± 0.05, P < 0.05). ODQ completely inhibited BAY 41-8543 evoked relaxation. BAY 58-2667 also caused a concentration-dependent relaxation of aortic rings that was not altered by the presence of DETA-NO (logEC₅₀ in the presence and absence of DETA-NO = -9.68 ± 0.07 nM and -9.54 ± 0.09 respectively, P = NS). ODQ significantly enhanced the potency of BAY 58-2667 (logEC₅₀ = -10.07 ± 0.04, P < 0.05).

BAY 41-8543 activates sGC by a mechanism that is synergistic with NO and inhibited by ODQ (Stasch et al 2002a). BAY 58-2667 activates sGC in a manner that is completely independent of NO but enhanced by ODQ (Stasch et al, 2002b). Concentrations of BAY 41-8543 required to cause relaxation are lower than might be predicted from enzyme data. Despite appearing less efficacious than BAY 41-8543 in the enzyme assay, BAY 58-2667 is able to potently elicit full relaxation of aortic rings. For the first time we demonstrate that ODQ enhances the functional potency of BAY 58-2667. It is apparent that low levels of sGC activation are required for maximum vasodilator actions, demonstrating functional overcapacity in the NO-sGC signalling cascade.

Kosarikov et al. (2001) Arch. Biochem. Biophys., 388, 185-197.
Stasch, J-P et al. (2002a) Br. J. Pharmacol., 135, 333-343.
Stasch, J-P et al. (2002b) Br. J. Pharmacol., 136, 773-783.