MODULATION OF THE CALCIUM SENSITIVITY OF BOVINE RETINAL ROD OUTER SEGMENT GUANYLYL CYCLASE BY SODIUM-IONS AND PROTEIN-KINASE-A

Guanylyl cyclases (GC, EC 4.6.1.2) serve as receptors that produce cGM P in response to ligand binding, The production of cGMP is essential f or the ability of retinal photoreceptor cells to restore the dark stat e after photoexcitation, GC activity is enhanced in rod outer segments (ROS) by a decrease in the cytosolic free Ca2+ concentration. We rece ntly developed a new real-time assay to measure initial rates of ROS G C activity with much improved precision [Wolbring, G. & P. P. M. Schne tkamp (1995) Biochemistry 34, 4689-4695]. With this assay we examined the Ca2+ sensitivity of ROS GC, and we report here that protein kinase A-mediated phosphorylation and Na+ cause significant shifts in the IC 50 for Ca2+ of the particulate guanylyl cyclase from bovine retinal ro d outer segments. The IC50 for Ca2+ ranged between 30 and 270 nM Ca2dependent on the presence of Na+, choline, cAMP, cGMP, 8-bromo-cAMP, 8 -bromo-cGMP, or the catalytic subunit of protein kinase A.