Osmotic properties of the frog retinal pigment epithelium

The water permeabilities of the membranes of the frog retinal pigment epithelial cells (RPE cells) were studied by means of double-barrelled, ion-selective microelectrodes. The cells were loaded with tetramethylammonium ion (TMA+) or with choline ions (Ch+) which served as intracellular volume markers. The intracellular activities of tetramethylammonium ions (TMAi+) and choline ions (Chi+) were measured with K+ selective intracellular microelectrodes. Intracellular activities of K+ and Cl- were also measured, and these ions were investigated as intracellular volume markers. The osmolarity on retinal side of the epithelium was changed abruptly. The rates of change in TMAi+ were linearly related to the changes in extracellular osmolarity in the range between (-46)-(+46) mosmol l-1. The osmotic water permeability of the retinal membrane, Lp,r, estimated from these experiments was 3.8 x 10(-4) cm sec-1 (osmol l-1)-1. For an osmotic gradient of 23 mosmol l-1 there was no difference between Lp,r determined with Ch+, TMA+ and Cl- as intracellular volume markers. This rules out any specific biological artifacts of the volume measurement. Intracellular K+ was not suitable as intracellular volume marker, since the initial rates of change in the intracellular K+ activity were about 30% lower than the rates of change in TMAi+ when osmotic gradients less than +/- 46 mosmol l-1 were applied. Among the exogenous volume markers, TMA+ was found more reliable than Ch+. For osmotic gradients larger than 23 mosmol l-1 the volume change recorded with Ch+ was smaller than that recorded with TMA+ as volume marker. Since the membrane permeability for Ch+ was 23 times larger than for TMA+, this suggests that Ch+ left the cells during higher rates of shrinkage. The cells acted almost as ideal osmometers, since the steady-state changes in intracellular osmolarity closely reflected the retinal extracellular osmolarity changes. It is concluded that the osmotic water permeability of the retinal membrane is larger than that of the choroidal membrane. This suggests that the rate limiting step for osmotic water movements between the eye and the blood is the choroidal membrane of the retinal pigment epithelium.