Local superfusion modifies the inward rectifying potassium conductance of isolated retinal horizontal cells

Horizontal cells were enzymatically and mechanically dissociated from the white perch (Roccus americana) retina and voltage clamped using patch electrodes. Steady-state current-voltage (I-V) relationships of solitary horizontal cells were determined by changing the membrane potential in a rampwise fashion. The I-V curve of cells bathed in normal Ringer solution exhibited a large conductance increase at negative membrane potentials. This conductance activated near the K⁺ equilibrium potential, had not clear reversal potential, was enhanced by raising the extracellular concentration of K⁺, and was suppressed by external Cs⁺. These properties identify the conductance as the inward (anomalous) rectifier. Continuous superfusion of the cells' local environment with drug-free Ringer reduced the magnitude of the inward rectifier current and shifted its activation point to more negative potentials. This effect developed over ~30 s, lasted as long as superfusion continued and was reversible upon cessation of superfusion. Pressure ejection of drug-free Ringer solution onto cells bathed in the identical solution also reduced the magnitude of the inward rectifier current, although the effects were more rapid and more transient than those exerted by superfusion. Pressure ejection had little effect when cells were simultaneously superfused with Ringer, suggesting a common mode of action on the inward rectifier. In the absence of superfusion, pressure ejection of Ringer containing 200 μM L-glutamate had a biphasic effect on membrane conductance. At potentials above -60 mV glutamate caused a conductance increase with a reversal potential near +10 mV. At potentials below -60 mV, glutamate caused a conductance decrease whose reversal potential could not reliably by determined. The latter effect was similar to the suppression of the inward rectifier by application of Ringer alone, suggesting that it may represent an artifact of pressure ejection rather than a direct effect of glutamate. In support of this interpretation, we found that pressure ejection of glutamate in the presence of external Cs⁺ (which blocks the inward rectifier) or during local superfusion with Ringer (which prevents attenuation of the inward rectifier by pressure ejection) did not cause a conductance decrease at negative potentials. Under these conditions, glutamate caused primarily a conductance increase with a reversal potential near +10 mV. The results are consistent with the hypothesis that fluid motion reduces either the concentration or the efficacy of extracellular potassium in the immediate vicinity of the isolated cells and indicate that the microenvironment around cultured neurons must be controlled during pharmacologic studies.