Kinetics of nitric oxide-cyclic GMP signalling in CNS cells and its possible regulation by cyclic GMP.

Physiologically, nitric oxide (NO) signal transduction occurs through soluble guanylyl cyclase (sGC), which catalyses cyclic GMP (cGMP) formation. Knowledge of the kinetics of NO-evoked cGMP signals is therefore critical for understanding how NO signals are decoded. Studies on cerebellar astrocytes showed that sGC undergoes a desensitizing profile of activity, which, in league with phosphodiesterases (PDEs), was hypothesized to diversify cGMP responses in different cells. The hypothesis was tested by examining the kinetics of cGMP in rat striatal cells, in which cGMP accumulated in neurones in response to NO. Based on the effects of selective PDE inhibitors, cGMP hydrolysis following exposure to NO was attributed to a cGMP-stimulated PDE (PDE 2). Analysis of NO-induced cGMP accumulation in the presence of a PDE inhibitor indicated that sGC underwent marked desensitization. However, the desensitization kinetics determined under these conditions described poorly the cGMP profile observed in the absence of the PDE inhibitor. An explanation shown plausible theoretically was that cGMP determines the level of sGC desensitization. In support, tests in cerebellar astrocytes indicated an inverse relationship between cGMP level and recovery of sGC from its desensitized state. We suggest that the degree of sGC desensitization is related to the cGMP concentration and that this effect is not mediated by (de)phosphorylation.