RT Journal Article SR Electronic T1 Modes of accessing bicarbonate for the regulation of membrane guanylate cyclase (ROS-GC) in retinal rods and cones JF eneuro JO eNeuro FD Society for Neuroscience SP ENEURO.0393-18.2019 DO 10.1523/ENEURO.0393-18.2019 A1 Clint L. Makino A1 Teresa Duda A1 Alexandre Pertzev A1 Tomoki Isayama A1 Polina Geva A1 Michael A. Sandberg A1 Rameshwar K. Sharma YR 2019 UL http://www.eneuro.org/content/early/2019/01/29/ENEURO.0393-18.2019.abstract AB The membrane guanylate cyclase, ROS-GC, that synthesizes cyclic GMP for use as a second messenger for visual transduction in retinal rods and cones, is stimulated by bicarbonate. Bicarbonate acts directly on ROS-GC1, because it enhanced the enzymatic activity of a purified, recombinant fragment of bovine ROS-GC1 consisting solely of the core catalytic domain. Moreover, recombinant ROS-GC1 proved to be a true sensor of bicarbonate, rather than a sensor for CO2. Access to bicarbonate differed in rods and cones of larval salamander, Ambystoma tigrinum, of unknown sex. In rods, bicarbonate entered at the synapse and diffused to the outer segment, where it was removed by Cl- dependent exchange. In contrast, cones generated bicarbonate internally from endogenous CO2 or from exogenous CO2 that was present in extracellular solutions of bicarbonate. Bicarbonate production from both sources of CO2 was blocked by the carbonic anhydrase inhibitor, acetazolamide. Carbonic anhydrase II expression was verified immunohistochemically in cones but not in rods. In addition, cones acquired bicarbonate at their outer segments as well as at their inner segments. The multiple pathways for access in cones may support greater uptake of bicarbonate than in rods and buffer changes in its intracellular concentration.SIGNIFICANCE STATEMENT Bicarbonate accentuates the role of the membrane guanylate cyclase, ROS-GC, in phototransduction. In current research, bicarbonate rather than gaseous CO2 is proven to be the ligand for ROS-GCs. Bicarbonate may be preferred because its movements are subject to tighter control. In rods, membrane impermeant bicarbonate gains entry at the synaptic zone and is cleared by Cl--dependent exchange at the outer segment. A novel insight is that while cones are similar, they also obtain bicarbonate at their outer segments and express an enzyme for making their own bicarbonate from endogenous together with exogenous CO2. Multiple pathways for access in cones indicate a need for greater uptake of bicarbonate and better buffering of its intracellular concentration than in rods.