Characterization of an L-type calcium channel expressed by human retinal Muller (glial) cells

The traditional notion that glial cells are permeable only to potassium has been revised. For example, glia from various parts of the nervous system have calcium-permeable ion channels. Since characterization of the calcium channels in glia is limited, the purpose of this study was to determine the molecular identity and examine the functional properties of a voltage-gated calcium channel expressed by Muller cells, the predominant glia of the retina. Whole-cell and perforated-patch recordings of human Muller cells in culture revealed a high threshold voltage-activated calcium current that is blocked by dihydropyridines, but not by omega-conotoxin GVIA or omega-conotoxin MWC. RT-PCR of cultured human Muller cells using primers specific for the calcium channel subunits demonstrated the expression of an L-type channel composed of the alpha(1)D, alpha(2) and beta(3) subunits. The alpha(2) subunit of the Muller cell calcium channel is a splice variant which is distinct from either the skeletal muscle alpha(2)s or the brain alpha(2)b. Our electrophysiological experiments indicate that the alpha(1)D/alpha(2)/beta(3) calcium channel is functionally linked with the activation of a potassium channel that may serve as one of the pathways for the redistribution by Muller cells of excess retinal potassium.