A, changes in cGMP sensitivity after patch excision elicited during a 10 min treatment period when channels were either kept open, by applying saturating cGMP, or kept closed, by using a cGMP-free solution. B, changes in cGMP sensitivity during phosphorylation were estimated by normalizing the change in K1/2 evoked by a 3 min application of 500 μM ATP to the K1/2 before ATP application in the presence or absence of saturating cGMP.