PH REGULATION IN FROG CONES STUDIED BY MASS RECEPTOR PHOTORESPONSES FROM THE ISOLATED RETINA

Mass cone photoresponses were recorded across the aspartate-treated fr og retina under treatments chosen to affect putative pH-regulating mec hanisms. The saturated response amplitude (U(max)) was found to be a m onotonically increasing function of perfusion pH in the range 7-8, and thus presumably of intracellular pH (pH(i)). Accepting that U(max) ca n be used as an index of pH(i) changes, two results indicate the impor tance of bicarbonate transport for preventing intracellular acidificat ion: (1) bicarbonate-buffered (6 mM HCO3- + 6 mM HEPES) perfusate incr eased U(max) compared with nominally bicarbonate-free perfusate (12 mM HEPES); (2) the anion transport blocker DIDS (0.1 mM) caused a strong decrease in the amplitude of photoresponses. Substitution of 95 mM ch loride by gluconate in the perfusing fluid boosted photoresponses indi cating that at least part of the bicarbonate transport involves HCO-/C l- exchange. Amiloride (2 mM) also caused a decrease of photoresponse amplitude, which suggests that Na+/H+ exchange contributes to pH(i) re gulation. In all these respects, cones behaved similarly to rods. Cone s differed from rods,(in the intact retina) in that addition of 0.5 mM of the carbonic anhydrase inhibitor acetazolamide reduced (never augm ented) photoresponses. The difference is considered in relation to the presence of carbonic anhydrase in cone, as opposed to rod, outer segm ents.