Complex rectification of Müller cell Kir currents

Although Kir4.1 channels are the major inwardly rectifying channels in glial cells and are widely accepted to support K⁺- and glutamate-uptake in the nervous system, the properties of Kir4.1 channels during vital changes of K⁺ and polyamines remain poorly understood. Therefore, the present study examined the voltage-dependence of K⁺ conductance with varying physiological and pathophysiological external [K⁺] and intrapipette spermine ([SP]) concentrations in Müller glial cells and in tsA201 cells expressing recombinant Kir4.1 channels. Two different types of [SP] block were characterized: "fast" and "slow." Fast block was steeply voltage-dependent, with only a low sensitivity to spermine and strong dependence on extracellular potassium concentration, [K⁺]_o. Slow block had a strong voltage sensitivity that begins closer to resting membrane potential and was essentially [K⁺]_o-independent, but with a higher spermine- and [K⁺]_i-sensitivity. Using a modified Woodhull model and fitting i/V curves from whole cell recordings, we have calculated free [SP]_in in Müller glial cells as 0.81 ± 0.24 mM. This is much higher than has been estimated previously in neurons. Biphasic block properties underlie a significantly varying extent of rectification with [K⁺] and [SP]. While confirming similar properties of glial Kir and recombinant Kir4.1, the results also suggest mechanisms underlying K⁺ buffering in glial cells: When [K⁺]_o is rapidly increased, as would occur during neuronal excitation, "fast block" would be relieved, promoting potassium influx to glial cells. Increase in [K⁺]_in would then lead to relief of "slow block," further promoting K⁺-influx.