Evidence for a Na(+)-Cl(-)-H(+)-HCO3- exchange system in the mammalian lens

36Cl- efflux was studied in the isolated rat lens under two conditions that are known to decrease internal pH. The first follows exposure to a pulse of ammonium chloride (50 mM) and the second accompanies exposure to an acidified propionate (20 mM) solution. Under acidifying conditions, a stimulation in 36Cl- efflux was observed, that was abolished on removing external Na+ and also on removing external Cl- and HCO3-. In the absence of external Cl-, the presence of HCO3- (16 mM) resulted in an increase in 36Cl- efflux during internal acidification. In the absence of internal acidification, the addition of 0.1 mM dibutyrylcAMP or 0.5 mM IBMX to the external medium produced a rapid increase in 36Cl- efflux. This stimulation was reduced by 0.2 mM SITS. Neither cAMP or IBMX had any significant effect on the electrical resistance of the lens membranes. It is suggested that a coupled SITS-sensitive, Na(+)-Cl(-)-H(+)-HCO3- exchange mechanism is activated when the lens internal pH falls and further that cAMP may play a role in regulating this mechanism.