The effects of acetazolamide in albino rabbits, pigmented rabbits, and humans

Modes of Accessing Bicarbonate for the Regulation of Membrane Guanylate Cyclase (ROS-GC) in Retinal Rods and Cones

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.

Comparison between albino and pigmented rabbit ERGs

BACKGROUND

Pigmented and albino rabbits are commonly used in visual research; however, the lack of pigment in the eyes may affect retinal responses. Here, we compare and describe the differences of retinal function between pigmented (English Butterfly) and albino (New Zealand) rabbits.

METHODS

Electroretinograms were recorded in pigmented and albino rabbits in the dark-adapted eye, in the light-adapted eye and for four temporal frequencies in the light-adapted eye. The implicit time and amplitude of the a- and b-waves were analyzed, as well as the amplitude and phase of the first harmonic component of the photopic flicker response.

RESULTS

Albino rabbits presented significantly larger amplitudes for both a- and b-waves at all intensities and frequencies. The intensity-response function of the scotopic b-wave also showed that the albino retina is more sensitive than the pigmented retina and the larger flicker amplitudes found in the albino group also revealed post-receptoral changes specifically related to cone pathways.

CONCLUSIONS

The larger amplitude of albino receptoral and post-receptoral activities might be attributed to greater availability of light due to scatter and reflection at the retinal layer, and as the differences in response amplitudes between the groups increase with flicker frequency, we suggest that ON bipolar cells recover faster in the albino group, suggesting that this might be a mechanism to explain the higher temporal resolution for albinos compared to the pigmented group.

Bicarbonate and Ca(2+) Sensing Modulators Activate Photoreceptor ROS-GC1 Synergistically

Photoreceptor ROS-GC1, a prototype subfamily member of the membrane guanylate cyclase family, is a central component of phototransduction. It is a single transmembrane-spanning protein, composed of modular blocks. In rods, guanylate cyclase activating proteins (GCAPs) 1 and 2 bind to its juxtamembrane domain (JMD) and the C-terminal extension, respectively, to accelerate cyclic GMP synthesis when Ca(2+) levels are low. In cones, the additional expression of the Ca(2+)-dependent guanylate cyclase activating protein (CD-GCAP) S100B which binds to its C-terminal extension, supports acceleration of cyclic GMP synthesis at high Ca(2+) levels. Independent of Ca(2+), ROS-GC1 activity is also stimulated directly by bicarbonate binding to the core catalytic domain (CCD). Several enticing molecular features of this transduction system are revealed in the present study. In combination, bicarbonate and Ca(2+)-dependent modulators raised maximal ROS-GC activity to levels that exceeded the sum of their individual effects. The F(514)S mutation in ROS-GC1 that causes blindness in type 1 Leber's congenital amaurosis (LCA) severely reduced basal ROS-GC1 activity. GCAP2 and S100B Ca(2+) signaling modes remained functional, while the GCAP1-modulated mode was diminished. Bicarbonate nearly restored basal activity as well as GCAP2- and S100B-stimulated activities of the F(514)S mutant to normal levels but could not resurrect GCAP1 stimulation. We conclude that GCAP1 and GCAP2 forge distinct pathways through domain-specific modules of ROS-GC1 whereas the S100B and GCAP2 pathways may overlap. The synergistic interlinking of bicarbonate to GCAPs- and S100B-modulated pathways intensifies and tunes the dependence of cyclic GMP synthesis on intracellular Ca(2+). Our study challenges the recently proposed GCAP1 and GCAP2 "overlapping" phototransduction model (Peshenko et al., 2015b).

Bicarbonate Modulates Photoreceptor Guanylate Cyclase (ROS-GC) Catalytic Activity

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.

Enhancing effects of acetazolamide on neuronal activity correlate with enhanced visual processing ability in humans

Acetazolamide is a potent inhibitor of the reversible hydration of CO(2) catalyzed by the enzyme carbonic anhydrase and is commonly used to increase cerebral blood flow e.g. in order to estimate cerebrovascular reserve. However it is not known whether acetazolamide may positively affect the excitability of neurons in the brain in vivo or cortical processing abilities. To test these possibilities we intravenously administered a low dose (7 mg/kg) acetazolamide to volunteers who performed a demanding visual signal detection task while undergoing whole brain electroencephalographic examinations. Two groups were tested twice on the same task, while receiving acetazolamide or a saline treatment in between the two sessions. Our data indicate that, while the control group showed a decrease in global gamma (30-49 Hz) power across sessions, with no correlation to performance, the acetazolamide group showed increased global gamma power that strongly related to their performance in the signal detection task. This was accompanied by a decrease in the early part of the event related potential in the control group, a decrease not seen in the acetazolamide group. There were no significant differences in blood pressure, ventilation rate, or heart rate between the two groups. It is possible that the differences between the groups, observed in this study, are related to the enhancing effect of acetazolamide on the nitric oxide generation catalyzed by carbonic anhydrase, or to other actions of acetazolamide, e.g. opening of Ca(2+) activated K(+) channels and inhibition of Ca(2+) channels.

Mutant carbonic anhydrase 4 impairs pH regulation and causes retinal photoreceptor degeneration

Retina and retinal pigment epithelium (RPE) belong to the metabolically most active tissues in the human body. Efficient removal of acid load from retina and RPE is a critical function mediated by the choriocapillaris. However, the mechanism by which pH homeostasis is maintained is largely unknown. Here, we show that a functional complex of carbonic anhydrase 4 (CA4) and Na+/bicarbonate co-transporter 1 (NBC1) is specifically expressed in the choriocapillaris and that missense mutations in CA4 linked to autosomal dominant rod-cone dystrophy disrupt NBC1-mediated HCO3- transport. Our results identify a novel pathogenic pathway in which a defect in a functional complex involved in maintaining pH balances, but not expressed in retina or RPE, leads to photoreceptor degeneration. The importance of a functional CA4 for survival of photoreceptors implies that CA inhibitors, which are widely used as medications, particularly in the treatment of glaucoma, may have long-term adverse effects on vision.

Acetazolamide affects performance on the Nagel II anomaloscope

BACKGROUND

Recent reports have indicated that acetazolamide alters human electroretinograms. We wished to determine the effects of administering acetazolamide on performance on the Nagel II anomaloscope.

METHODS

We tested 15 subjects matches of blue-green light to a mixture of blue and green lights (luminance match) on a Nagel type II anomaloscope 2.5 h after ingesting 500 mg of acetazolamide or a placebo.

RESULTS

The mean of the luminance settings for the subjects was 54.4 for the placebo condition and 58.5 for the acetazolamide condition. The mean difference of 4.1 was statistically significant, indicating that following ingestion of acetazolamide subjects were less sensitive to a blue-green light. In two supplementary experiments we tested (1) a second group of four normal subjects using the Nagel type II anomaloscope and (2) the previously untreated eyes of four patients with primary open-angle glaucoma before and after placing them on acetazolamide therapy. In both groups, more blue-green light was needed to make the match after ingestion of acetazolamide.

CONCLUSIONS

Acetazolamide alters the sensitivity of one or more cone populations, probably the carbonic anhydrase-containing cones. The sensitivity loss is reversible and does not appear to be clinically significant. However, the results suggest that patients administered acetazolamide should be excluded from studies which compare the color vision of glaucomatous patients to that of normals.