Light-evoked disc shedding by rod photoreceptors in vitro: relationship to medium bicarbonate concentration.

Although studies on the control of photoperiod-related shedding by rod photoreceptors in frogs suggested that the mechanisms which initiate shedding are intrinsic features of the eye, attempts to observe light-evoked shedding in opened eye cups in balanced salt solutions have been unsuccessful. We have examined disc shedding in eye cups kept in a complex tissue culture medium. Eye cups were prepared in red light from dark-adapted animals or in white light from constant light-treated animals. Light-evoked shedding did not occur in the standard medium with -20 mM sodium bicarbonate, but a large light-evoked response was consistently seen when medium bicarbonate was raised to 030 mM. In high-bicarbonate medium, light-evoked shedding was comparable in magnitude, size distribution of phagosomes, and time course to similar responses observed in intact animals. Preliminary analysis of culture conditions which support shedding suggests that bicarbonate ion concentration is of critical importance. However, lack of light-evoked shedding in a high-bicarbonate Ringer solution suggests that some additional unidentified factor(s) in the complex medium is also necessary. The data indicate that both light- and dark-dependent processes involved in the control of photoperiod-related shedding are sustained in appropriate culture medium.