Müller (glial) cell development in vivo and in retinal explant cultures: morphology and electrophysiology, and the effects of elevated ammonia.

Retinal explant cultures have been established as a useful tool to study both the normal development of the mammalian retina and the effects of pathogenic agents. We used such cultures as a model for the (ammonia-induced) hepatic retinopathy, earlier observed in humans with chronical liver failure, and ascribed to a breakdown of Müller (glial) cell function. In the explant cultures, one day exposure to elevated (7 mM) ammonia was sufficient to cause Müller cell reactivity as indicated by increasing immunopositivity for glial fibrillary acidic protein. After 4 days in elevated ammonia, the Müller cells were severely deformed, the layered structure of the retinae became disorganized, and significant neuronal cell death occurred. Using whole-cell voltage-clamp recordings, the expression of K+ channels was compared in Müller cells isolated from retinae of rabbits at postnatal days 9 to 12 and from neonatal explants cultured for 9 to 12 days, respectively. Müller glial cells grown both in vivo and in vitro express the same set of K+ channels in their membranes: (i) inwardly rectifying K+ (K(IR)) channels which were selectively blocked by Ba2+ ions; (ii) large-conductance, Ca2+-activated K+ (BK(Ca)) channels which were blocked by iberiotoxin and were activated by phloretin; and (iii) delayed rectifying voltage-gated K+ channels. The presence of K(IR) channels indicates successful differentiation of the Müller cells grown in vitro, as these channels are not expressed in cells from neonatal animals. Four days of elevated ammonia in the culture medium caused a complete loss of K(IR) channels in Müller cell membranes, and a significant decrease of the membrane potential. The results indicate that in hepatic retinopathy, the well-known morphological and enzymatical alterations of Müller glial cells may be accompanied by changes in their membrane permeability for K+.