VOLTAGE-DEPENDENT K+ CURRENTS IN GUINEA-PIG MULLER (GLIAL) CELLS SHOW DIFFERENT SENSITIVITIES TO BLOCKADE BY BA2+

The effect of externally applied Ba2+ and Na+ on K+ currents was investigated by means of whole-cell patch-clamp in isolated and in situ Muller cells from guinea pig retina. Muller cells express a typical set of K+ currents, i.e. an ohmic current, an inactivating inward current (I(K(IR))), a delayed rectifier (I(K(DR))) and an inactivating outward current (I(K(A))). Inactivation of the inward current did not occur when extracellular Na+ was replaced by choline. When administered in increasing concentrations, Ba2+ blocked these K+ currents in a typical sequence: the ohmic current and I(K(A)) were most sensitive, followed by I(K(IR)), whereas I(K(DR) was not completely blocked even in 1 mM Ba2+. The differential sensitivity of Muller cell K+ currents to external Ba2+ may be a tool which can be used to improve our understanding of the Muller cell response to physiological stimulation of the retina.