PH CHANGES IN FROG RODS UPON MANIPULATION OF PUTATIVE PH-REGULATING TRANSPORT MECHANISMS

Rod intracellular pH (pH(i)) in the intact frog retina was measured fl uorometrically with the dye 7'-bis(2-carboxyethyl)-5(and-6)-carboxyflu orescein under treatments chosen to affect putative pH-regulating tran sport mechanisms in the plasma membrane. The purpose was to relate pos sible pH(i) changes to previously reported effects on photoresponses. In nominally bicarbonate-free Ringer, application of amiloride (1 mM) or substitution of 95 mM external Na+ by K+ or choline triggered monot onic but reversible acidifications, consistent with inhibition of Na+/ H+ exchange. Bicarbonate-dependent mechanisms were characterized as fo llows: (1) Replacing half of a 12 mM phosphate buffer by bicarbonate c aused a sustained rise of pH(i). (2) Subsequent application of the ani on transport inhibitor 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS, 0.2 mM) set off a slow acidification. (3) Substitution of external Cl- by gluconate (95 mM) caused a rapid pH(i) rise both in no rmal Na+ and low-Na+ perfusion. (4) This effect was inhibited by DIDS. The results support a consistent explanation of parallel electrophysi ological experiments on the assumption that intracellular acidificatio ns reduce and alkalinizations (in a certain range) augment photorespon ses. It is concluded that both Na+/H+ exchange and bicarbonate transpo rt control rod pH(i), modulating the light-sensitive current. Part of the bicarbonate transport is by Na+-independent HCO3-/Cl- exchange, bu t a further Na+-coupled bicarbonate import mechanism is implicated. Co pyright (C) 1996 Published by Elsevier Science Ltd.