Pharmacology of the nitric oxide receptor, soluble guanylyl cyclase, in cerebellar cells.

The nitric oxide (NO) receptor, soluble guanylyl cyclase (sGC), is commonly manipulated pharmacologically in two ways. Inhibition of activity is achieved using 1-H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-l-one (ODQ) which oxidizes the haem prosthetic group to which NO binds, while the compound 3-(5-hydroxymethyl-2-furyl)-1-benzylindazole (YC-1) is considered an 'allosteric' activator. Knowledge of how these agents function and interact in a normal cellular environment is limited. These issues were addressed using rat cerebellar cells. Inhibition by ODQ was not simply competitive with NO. The rate of onset was ODQ concentration-dependent and developed in two kinetic phases. Recovery from inhibition occurred with a half-time of approximately 5 min. YC-1 slowed the rate at which sGC deactivated on removal of NO by 45 fold, consistent with YC-1 increasing the potency of NO for sGC. YC-1 also enhanced the maximal response to NO by 2 fold. Furthermore, when added to cells in which sGC was 90% desensitized, YC-1 abruptly enhanced sGC activity to a degree that indicated partial reversal of desensitization. After pre-exposure to YC-1, sGC became resistant to inhibition by ODQ. In addition, YC-1 rapidly reversed inhibition by ODQ in cells and for purified sGC, suggesting that YC-1 either increases the NO affinity of the oxidized sGC haem or reverses haem oxidation. It is concluded that the actions of ODQ and YC-1 on sGC are broadly similar in cells and purified preparations. Additionally, YC-1 transiently reverses sGC desensitization in cells. It is hypothesized that YC-1 has multiple actions on sGC, and thereby both modifies the NO binding site and enhances agonist efficacy.