Adenylate cyclase stimulation alters transport in frog retinal pigment epithelium

The effect of adenylate cyclase inhibitors and activators on enzyme kinetics and cyclic AMP (cAMP) levels was determined in the bullfrog retinal pigment epithelium (RPE). The RPE enzyme has two Km for ATP: 5.2 X 10(-4) and 4.0 X 10(-5) M, with Vmax of 2.5 and 0.25 nmol X mg protein-1 X min-1. Forskolin, the most potent activator, produced a fourfold increase in enzyme activity and, in the presence of isobutylmethylxanthine (IBMX), an inhibitor of phosphodiesterase, caused a 20-fold increase in cAMP levels. Alloxan, a potent inhibitor, blocked the forskolin-induced activation of this enzyme. In the isolated RPE choroid, forskolin (plus IBMX) produced changes in membrane voltage and resistance that were similar in magnitude but slower in time course than those produced by exogenous cAMP. Like exogenous cAMP, forskolin also decreased steady-state fluid and solute transport in isotonic proportions. Therefore, modulation of RPE adenylate cyclase activity plays an important role in the control of RPE transport.