Fluid transport across retinal pigment epithelium is inhibited by cyclic AMP

Fluid transport across the retinal pigment epithelium (bullfrog) has been measured. These experiments were carried out by using a capacitance probe technique and a water-impermeable chamber that allowed the measurements to be made with an accuracy of 0.5-1.0 nl/min. With identical Ringer's solution on both sides of the epithelium, and in the absence of a hydrostatic driving force, the direction of net fluid movement is from the retina to the choroid (absorption). The net transport rate, approximately 10 nl/min (4.8 microliters/cm2 . hr), is comparable to that observed in other amphibian epithelia. It is reduced to zero by the mitochondrial uncoupler 2,4-dinitrophenol but is relatively unaffected by ouabain, which inhibits the Na+/K+-pump located on the apical membrane of this epithelium. A significant decrease in net fluid absorption was produced by dibutyryl cAMP and 3-isobutyl-1-methylxanthine (a potent phosphodiesterase inhibitor). This cAMP-dependent fluid transport may be an important mechanism for controlling the fluid volume in the subretinal space.