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Xiao-Hong Wen, Tomoki Isayama, Alexandre Pertzev, Rameshwar K Sharma, Clint L Makino, Teresa Duda; Bicarbonate Regulates Cyclic GMP Synthesis in Retinal Rods and Cones. Invest. Ophthalmol. Vis. Sci. 2014;55(13):3032. doi: https://doi.org/.
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Bicarbonate, which plays a crucial role in physiological pH buffering, also regulates the catalytic activities of some membrane guanylate cyclases. To test for such a role in photoreceptors, we determined the effect of bicarbonate on ROS-GC1 and ROS-GC2 guanylate cyclase activities in biochemical assays and in single cell recordings of salamander rods and cones.
Bovine ROS-GC1, ROS-GC2, or a variety of truncated mutants of ROS-GC1 were transiently expressed in COS cells and catalytic activities were evaluated in the presence of bicarbonate. In some experiments GCAP1 or 2 was added at low calcium. The suction electrode method was used to record flash responses of single salamander rods and cones while exposing them to bicarbonate or to Ringer’s equilibrated with a high concentration of carbon dioxide.
Bicarbonate increased the activities of ROS-GC1 and 2 by three to four-fold with an EC50 of ~30 to 40 mM. Furthermore, bicarbonate and GCAPs stimulated ROS-GC activity synergistically at low calcium. In single cell recordings, the addition of bicarbonate increased circulating current by as much as two-fold in rods and red cones attached to large pieces of retina. Ringer’s with a high concentration of carbon dioxide also increased the circulating current in red cones but the effect was not as readily reversible as that of bicarbonate.
By stimulating the basal rate of cyclic GMP synthesis, bicarbonate increases the circulating current and the size of the photon response. Since bicarbonate and GCAPs regulate guanylate cyclase activity synergistically, bicarbonate could also affect photon response kinetics. These results suggest that bicarbonate provides an additional mechanism of phototransduction modulation.
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