G protein-mediated activation of a nonspecific cation current in cultured rat retinal pigment epithelial cells.

We used whole-cell patch-clamp recording techniques to investigate G protein-activated currents in cultured rat retinal pigment epithelial (RPE) cells. Using 140 mM KCl intracellular and 130 mM NaCl extracellular solutions, rat RPE cells possessed both inward and outward K+ currents. Upon addition of the nonhydrolyzable guanine triphosphate analogue, guanosine-5'-O-(3-thiophosphate) (GTPgammaS, 0.1 mM), to the recording electrode, a nonspecific cation (NSC) current was elicited. The NSC current had a mean reversal potential of +5.7 mV in 130 mm extracellular NaCl with Cs+-aspartate in the pipette, and was not affected by alterations in the extracellular Ca2+ or Cl- concentration. The GTPgammaS-activated current was found to be permeable to several monovalent cations (K+, Na+, choline, TRIS, and NMDG). Addition of fluoroaluminate, an activator of large molecular weight heterotrimeric GTP-binding proteins (G proteins), to the intracellular recording solution activated the NSC current. The G protein involved was pertussis toxin (PTX)-sensitive, since GTPgammaS failed to activate the NSC current in cells pretreated with PTX. Further investigation of second messenger molecules suggested that activation of the NSC current was not affected by alterations in intracellular Ca2+ or ATP. From these results, we conclude that a G protein-regulated NSC current is present in rat RPE cells. Activation of the NSC current may sufficiently depolarize RPE cells to activate outward K+ currents. This would provide a mechanism by which these cells could rid themselves of accumulated K+.