Effects of cAMP and IBMX on the chick retinal pigment epithelium. Membrane potentials and light-evoked responses.

cAMP is known to alter electrical measures of retinal pigment epithelial (RPE) function (standing potential and DC ERG. To locate the origin of these effects, studies were performed on in vitro preparations of chick retina-RPE-choroid and RPE-choroid, which were separately perfused with cAMP or db-cAMP on the retinal (apical) and choroidal (basal) tissues surfaces. Similar studies were performed using the phosphodiesterase inhibitor IBMX. cAMP and db-cAMP produced essentially identical results. cAMP placed in the retinal or choroidal baths hyperpolarized the RPE basal membrane, increased the apparent basal membrane resistance (Rba), and decreased the ERG c-wave. Experiments in RPE-choroid preparations suggested that these effects of cAMP were not secondary to effects on the neural retina. Effects were observed at 1.0 microM and increased with concentration (1.0 microM-500 microM), with choroidal application producing smaller effects than retinal. The c-wave decrease with retinal cAMP was due principally to the increase in Rba. However, the light-evoked subretinal K+ ([K+]0) decrease, measured with K(+)-specific microelectrodes, showed a small reduction that may have contributed to the c-wave change and that explained an observed decrease in slow PIII. Retinal cAMP also led to a small decrease in ERG a-wave amplitude, which in view of the effects on [K+]0, is consistent with a small reduction in the light response of photoreceptors. The cAMP-induced hyperpolarization of the RPE basal membrane may result from a decrease in basal membrane anion conductance. The results of retinal or choroidal perfusion with IBMX (0.1 mM) in the retina-RPE-choroid preparation were similar and were complicated by effects on photoreceptors. IBMX increased the amplitude of the light response of photoreceptors as indicated by an increase in amplitude of the light-evoked [K+]0 decrease and the a-wave of the ERG. IBMX also produced a transient increase in dark-adapted [K+]0 that altered the membrane potentials of the RPE and obscured any direct effects of IBMX on the RPE. However, IBMX perfusion of the RPE-choroid suggested that the effect on the RPE was similar to that of cAMP perfusion. The IBMX-induced increase in the photoresponse also led to increases in other components of the DC ERG, including the c-wave, fast oscillation and light peak.