Cellular pH and Na⁺-H⁺ exchange activity in lens epithelium of Bufo marinus toad

Incubation of lenses with the acetoxymethyl ester of 2,7-biscarboxyethyl-5(6)-carboxyfluorescein (BCECF-AM) led to selective, prolonged entrapment of the deesterified, pH-sensitive, membrane-impermeable form of the dye (BCECF) in the lens epithelial cells allowing, thereby, for the continuous fluorometric monitoring of epithelial intracellular pH (pH(i)) in intact lenses. Under anaerobic conditions in physiological solution, the epithelium maintained a pH(i) of 7.36 ± 0.14 (mean ± SD, n = 6 lenses). In the absence of CO₂ and HCO₃^-, the pH(i) increased to 7.83 ± 0.38 (n = 55). In this condition, either the removal of Na⁺ or the addition of amiloride to the medium resulted in a decrease of pH(i). This acidification was increased in an amiloride-inhibitable fashion when lenses were loaded with Na⁺ before extracellular Na⁺ removal. The extent of artificially generated cellular acidosis (induced by the 'NH₄⁺ loading' technique) was markedly decreased by the presence of extracellular Na⁺ or reverted on its addition when the ion was initially absent; the latter reversal was inhibited by amiloride. This inhibition was by-passed by monensin addition. The existence of a Na⁺-H⁺ antiport in the epithelial cells is concluded.