Membrane ultrastructure preservation and membrane potentials after isolation of rabbit retinal glial (Müller) cells...

Enzymatically isolated retinal glial (Müller) cells have been the subject of many electrophysiological studies. Local high membrane conductivities for potassium ions have been speculated to correspond with local occurrence of orthogonal arrays of intramembranous particles (OAP) observed in freeze-fracture replicas of retinal Müller cells in situ. We studied whether such OAP are preserved after enzymatic digestion of the retinal tissue which is necessary for isolation of living cells for electrophysiology. We found that strong papain digestion leads not only to disturbances in the cell's ultrastructure as seen in ultrathin sections but evokes both a redistribution of intramembranous particles and a disappearance of OAP as seen in the freeze-fracture replica. Furthermore, such isolated cells have low membrane potentials and lose their topographical specialization in K+ conductance. If, however, the retinae were exposed to papain as short as possible to get just some isolated cells, their cytoplasmic and membranous ultrastructure was preserved very well, and high resting membrane potentials were recorded in cells with marked regional specialization of membrane conductivity. Our results show that indeed sites of high K+ conductance may correspond with the occurrence of OAP, even in isolated cells.