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 cGMP in response to ligand binding. The production of cGMP is essential for the ability of retinal photoreceptor cells to restore the dark state after photoexcitation. GC activity is enhanced in rod outer segments (ROS) by a decrease in the cytosolic free Ca2+ concentration. We recently developed a new real-time assay to measure initial rates of ROS GC activity with much improved precision [Wolbring, G. & P. P. M. Schnetkamp (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 IC50 for Ca2+ of the particulate guanylyl cyclase from bovine retinal rod outer segments. The IC50 for Ca2+ ranged between 30 and 270 nM Ca2+ dependent on the presence of Na+, choline, cAMP, cGMP, 8-bromo-cAMP, 8-bromo-cGMP, or the catalytic subunit of protein kinase A.