MODIFICATION BY CYCLIC ADENOSINE-MONOPHOSPHATE OF BASOLATERAL MEMBRANE CHLORIDE CONDUCTANCE IN CHICK RETINAL-PIGMENT EPITHELIUM

Purpose. We investigated the hypothesis that cyclic adenosine monophos phate (cAMP) modifies the basolateral membrane chloride conductance of chick retinal pigment epithelium (RPE). Methods. Placing freshly diss ected chick retina-RPE-choroid tissues in a per fusion chamber that al lows separate perfusion of its retinal and choroidal sides, the author s examined the effect of choroidal perfusion of forskolin (50 mM), an adenylate cyclase activator, on extracellular and intracellular RPE po tentials, resistances, light-evoked responses, and chloride diffusion potentials.Results. Forskolin hyperpolarized the RPE basolateral membr ane, decreased the apical/basal membrane resistance ratio, increased t he 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 chlori de diffusion potential by 31%. The effects of forskolin were diminishe d by pretreatment of the basolateral membrane with DIDS, a chloride ch annel blocker, or by pretreatment of the apical membrane with bumetani de, a blocker of NaK2Cl cotransport. Transepithelial potential, resist ance, and c-wave amplitude were not changed by 1,9-dideoxyforskolin, w hich does not elevate cAMP levels. Conclusions. Elevation of cAMP resu lts in diminished basolateral membrane chloride conductance in chick R PE. This could be due to a direct effect on the chloride channel or du e to a decrease in intracellular chloride concentration secondary to i nhibition of apical membrane NaK2Cl contransport.