Bicarbonate enters a rod thru its synapse to stimulate ROS-GC in its outer segment, whereas cones generate bicarbonate intracellularly from CO₂

Abstract

Purpose : Bicarbonate stimulates the membrane guanylate cyclase (ROS-GC) in rods and cones, increasing the circulating current and accelerating flash response kinetics. Experimentally, this effect is elicited by bicarbonate perfusion of rods and red cones attached to a piece of retina and of isolated red cones. Isolated rods do not appear to respond to bicarbonate. Red cones are thought to express carbonic anhydrase, so the apparent difference in bicarbonate uptake between rods and cones may be due to the ability of cones to convert CO₂ to bicarbonate intracellularly.

Methods : Single rods and cones were recorded with a suction electrode while being perfused with bicarbonate with or without acetazolamide, a carbonic anhydrase inhibitor. In addition, recombinant ROS-GC activity was assayed with or without carbonic anhydrase in the presence of CO₂, and with or without carbonic anhydrase and acetazolamide in the presence of CO₂.

Results : In vitro ROS-GC activity was increased slightly by CO₂ in the absence of carbonic anhydrase, but increased several fold when carbonic anhydrase was present; the latter stimulation was inhibited by acetazolamide. Isolated rods typically failed to respond to bicarbonate, but were able to do so in electrical recordings from the outer segment, if they retained their synaptic zone. Acetazolamide did not block the effect. But acetazolamide did block stimulation of ROS-GC by bicarbonate in isolated red cones and in red cones attached to a piece of retina.

Conclusions : CO₂ affects phototransduction by two modes. A rod takes in bicarbonate at its synapse. In contrast, a red cone expresses carbonic anhydrase that converts intracellular CO₂ to bicarbonate.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.