LIGHT-DEPENDENT HYDRATION OF THE SPACE SURROUNDING PHOTORECEPTORS IN THE CAT RETINA

We have studied the effect of retinal illumination on the concentratio n of the extracellular space marker tetramethylammonium (TMA(+)) in th e dark-adapted cat retina using double-barreled ion-selective microele ctrodes. The retina was loaded with TMA(+) by a single intravitreal in jection. Retinal illumination produced a slow decrease in [TMA(+)](o), which was maximal in amplitude in the most distal portion of the spac e surrounding photoreceptors, the subretinal space. The light-evoked d ecrease in [TMA(+)](o) was considerably slower and of a different over all time course than the light-evoked decrease in [K+](o), also record ed in the subretinal space. [TMA(+)](o) decreased to a peak at 38 s af ter the onset of illumination, then slowly recovered towards the basel ine, and transiently increased following the offset of illumination. I t resembled the light-evoked [TMA(+)](o) decreases previously recorded in the in vitro preparations of frog (Huang and Karwoski, 1990, 1992) and chick (Li et al., 1992, 1994) but was considerably larger in ampl itude, 22% compared with 7%. As in frog, where it was first recorded, the light-evoked [TMA(+)](o) decrease is considered to originate from a light-evoked increase in the volume of the subretinal space (or subr etinal hydration). A mathematical model accounting for [TMA(+)](o) dif fusion predicted that the volume increase underlying the response was 63% on average and could be as large as 95% and last for minutes. The estimated volume increase was then used to examine its effect on K+ co ncentration in the subretinal space. We conclude that a light-dependen t hydration of the subretinal space represents a significant physiolog ical event in the intact cat eye, which should affect the organization of the interphotoreceptor matrix, and the concentrations of all ions and metabolites located in the subretinal space.