Potassium currents in endfeet of isolated Müller cells from the frog retina

Voltage dependent potassium currents were recorded using the whole-cell mode of the patch-clamp technique for the first time from endfeet of Müller cells dissociated from the frog retina. Recordings from intact cells and isolated endfeet indicate that the inward rectifier potassium channel is the dominant ion channel in these cells and that the density of these channels is highest in the endfoot as has been previously reported for several other species. The present study uses rapid changes in [K+]o to understand the behavior of these channels in buffering [K+]o in the retina. With rapid changes in [K+]o, it was found that, at a membrane potential of -90mV, which is close to EK, increasing [K+]o from 3 to 10 mM produced an inward K+ current 5.48 +/- 0.89 SD (n = 9) times larger than outward current induced by decreasing [K+]o from 3 to 1 mM. The outward current was maximal at a holding potential of about -80mV and exhibited inactivation at more positive potentials. At -40 mV both the inward and outward currents are markedly reduced. The current voltage curve for the inward current was linear at holding potentials from -50 mV to -140 mV. Using 20 mV voltage steps, it was found that the voltage dependent K+ currents were unaffected by the addition of 2 mM Cd2+, a blocker of Ca(2+)-activated potassium currents, decreasing [Cl-]o from 120 mM to 5 mM or the substitution of 30 mM Na+ by TEA. The addition of 5 mM [Cs+]o blocked only the inward current. Both the outward and the inward currents disappeared in the absence of intracellular and extracellular K+; 0.3 mM [Ba2+]o blocked the inward current completely and strongly inhibited the outward current in a time and voltage dependent manner. We conclude that at physiological [K+]o and membrane potential, the K+ channels in the Müller cell endfoot are well suited to carry K+ both inward and outward across the membrane as required for spatial buffering.