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AK Stratis, WH Baldridge; Effects of Aminosulfonates on the Feedback Responses of Goldfish Horizontal Cells . Invest. Ophthalmol. Vis. Sci. 2002;43(13):3776.
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Purpose: Negative feedback from horizontal cells to cones has been proposed to involve an ephaptic mechanism mediated by connexin-26 hemichannels, in part because carbenoxolone, a general gap junction channel blocker, abolished feedback responses in teleost horizontal cells and hyperpolarized the horizontal cell dark resting membrane potential (Kamermans et al., Science, 292:1178, 2001). As the activity of connexin-26 hemichannels, expressed in liposomes, is decreased significantly by protonated aminosulfonates (Bevans & Harris, J. Biol. Chem., 274:3711, 1999), we tested the effects of 2-aminoethanesulfonic acid (taurine) and N-2-hydroxyethyl-piperazine-N'-2-ethanesulfonic acid (HEPES) on the response of goldfish horizontal cells at physiological pH to determine if these aminosulfonates modulate horizontal cell to cone feedback. Methods: The light-evoked responses of horizontal cells were recorded by intracellular microelectrodes using an isolated goldfish (C. auratus) retina preparation. The retina was continuously superfused with a modified Ringer's solution that contained bicarbonate and was bubbled with carbogen to produce a pH of 7.2-7.5. Feedback was assessed by measuring 1) the extent of the rollback of the horizontal cell response during stimulation in monophasic horizontal cells and 2) the depolarizing response to long wavelength stimuli in biphasic horizontal cells. Results: Taurine (5-10 mM) did not abolish the feedback response in either monophasic or biphasic horizontal cells nor did taurine alter the dark resting membrane potential. HEPES (10 mM) reduced rollback in monophasic horizontal cells but did not abolish the depolarizing responses of biphasic horizontal cells. With bicarbonate in the Ringer's solution, HEPES did not alter the dark resting membrane potential of horizontal cells. Conclusion: These results suggest that if connexin-26-like hemichannels mediate feedback from horizontal cells to cones, these hemichannels, unlike those expressed in liposomes, are not sensitive to aminosulfonates. This may indicate that the connexin-26 hemichannels expressed in liposomes are different from horizontal cell connexin-26 hemichannels or that the environment in which the hemichannels are expressed is relevant to their modulation by aminosulfonates. The fact that HEPES did affect rollback, but did not alter dark resting membrane potential or the depolarizing responses of biphasic cells, suggests that the action of HEPES on rollback is unrelated to an effect on connexin-26 hemichannels.
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