K+ channel density increases selectively in the endfoot of retinal glial cells during development of Rana catesbiana

The radial glial cells that span the retina, described by Muller in 1851, h ave a remarkable distribution of ion channels in adult amphibia that mediat e extracellular K+ spatial buffering. 94% of the total membrane conductance of these cells resides in inward rectifier K+ channels in the endfoot proc esses apposed to the vitreous humour. We now report that this regional spec ialization is found in Muller cells isolated from adult (>120 day old) bull frogs but to a far less extent in those from 10-20 day old tadpoles (stages 34-36). Using the cell attached configuration of the patch-clamp technique , we found, in agreement with previous studies in salamanders, that the end foot of adult cells had 19.2 +/- 2.4 (mean +/- S.E., n = 81) channels/patch , whereas the soma had 1.81 +/- 0.28 (n = 21) channels/patch. In the tadpol e, the respective values were 4.29 +/- 0.26 (n = 79) for the endfoot and 2. 26 +/- 0.24 (n = 27) for the soma. The slope conductance of the inward rect ifier K+ channel in 115 mM: K+, 19.2 +/- 0.25 pS (n = 205), channel kinetic s and the resting membrane potential (-69 +/- 2.7 mV, n = 224) were similar at both the endfoot and soma of both adults and embryos. We conclude that during development, the K+ conductance of the Muller cell endfoot, but not of the soma, increases due to a selective clustering of inwardly rectifyiin g K+ channels in that specific region of the cell membrane. The properties of the channels change little during the transformation from tadpole to adu lt bullfrog. GLIA 25:199-203, 1999. (C) 1999 Wiley-Liss, Inc.