Modification by cyclic adenosine monophosphate of basolateral membrane chloride conductance in chick retinal pigment epithelium.

PURPOSE We investigated the hypothesis that cyclic adenosine monophosphate (cAMP) modifies the basolateral membrane chloride conductance of chick retinal pigment epithelium (RPE). METHODS Placing freshly dissected chick retina-RPE-choroid tissues in a perfusion chamber that allows separate perfusion of its retinal and choroidal sides, the authors examined the effect of choroidal perfusion of forskolin (50 mM), an adenylate cyclase activator, on extracellular and intracellular RPE potentials, resistances, light-evoked responses, and chloride diffusion potentials. RESULTS Forskolin hyperpolarized the RPE basolateral membrane, decreased the apical/basal membrane resistance ratio, increased the amplitude of the RPE membrane c-wave hyperpolarizations, decreased the amplitude of the transepithelial c-wave, and suppressed the light peak. In addition, forskolin decreased the basolateral membrane chloride diffusion potential by 31%. The effects of forskolin were diminished by pretreatment of the basolateral membrane with DIDS, a chloride channel blocker, or by pretreatment of the apical membrane with bumetanide, a blocker of NaK2Cl cotransport. Transepithelial potential, resistance, and c-wave amplitude were not changed by 1,9-dideoxyforskolin, which does not elevate cAMP levels. CONCLUSIONS Elevation of cAMP results in diminished basolateral membrane chloride conductance in chick RPE. This could be due to a direct effect on the chloride channel or due to a decrease in intracellular chloride concentration secondary to inhibition of apical membrane NaK2Cl cotransport.