Abstract
By generating the second messenger cGMP in retinal rods and cones, ROS-GC plays a central role in visual transduction. Guanylate cyclase-activating proteins (GCAPs) link cGMP synthesis to the light-induced fall in [Ca(2+)]i to help set absolute sensitivity and assure prompt recovery of the response to light. The present report discloses a surprising feature of this system: ROS-GC is a sensor of bicarbonate. Recombinant ROS-GCs synthesized cGMP from GTP at faster rates in the presence of bicarbonate with an ED50 of 27 mM for ROS-GC1 and 39 mM for ROS-GC2. The effect required neither Ca(2+) nor use of the GCAPs domains; however, stimulation of ROS-GC1 was more powerful in the presence of GCAP1 or GCAP2 at low [Ca(2+)]. When applied to retinal photoreceptors, bicarbonate enhanced the circulating current, decreased sensitivity to flashes, and accelerated flash response kinetics. Bicarbonate was effective when applied either to the outer or inner segment of red-sensitive cones. In contrast, bicarbonate exerted an effect when applied to the inner segment of rods but had little efficacy when applied to the outer segment. The findings define a new regulatory mechanism of the ROS-GC system that affects visual transduction and is likely to affect the course of retinal diseases caused by cGMP toxicity.
Highlights
ROS-GCs generate cGMP and control phototransduction in rods and cones
COS cells were induced to transiently express bovine ROS-GC1 or ROS-GC2, and their membranes were assayed for cGMP synthesis
In our initial recordings from salamander photoreceptors attached to pieces of retina with outer segment inside the pipette (OS-in), switching the inner segment perfusion from Ringer’s solution buffered with 30 mM phosphate to one containing 30 mM bicarbonate increased the response to a bright, saturating flash in all five cones tested by 52 Ϯ 14% and in all three rods tested by 18 Ϯ 3%
Summary
Results: Through a unique [Ca2ϩ]i-independent mechanism, bicarbonate stimulates ROS-GC activity to increase circulating current, quicken flash responses, and reduce relative sensitivity. Guanylate cyclase-activating proteins (GCAPs) link cGMP synthesis to the light-induced fall in [Ca2؉]i to help set absolute sensitivity and assure prompt recovery of the response to light. ROS-GCs do, respond to changes in intracellular [Ca2ϩ] with a neuronal calcium sensing subunit, guanylate cyclase activating protein (GCAP). Faster flash responses and lower sensitivity in rods of toad and monkey are associated with bicarbonate (6 – 8). These changes could be produced by direct stimulation of cGMP synthesis, but the effect of bicarbonate on ROS-GC activity is controversial (9 –11). Some of the results have appeared previously in abstract form [70]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
