CHARACTERIZATION OF AN L-TYPE CALCIUM-CHANNEL EXPRESSED BY HUMAN RETINAL MULLER (GLIAL) CELLS

The traditional notion that glial cells are permeable only to potassiu m has been revised. For example, glia from various parts of the nervou s system have calcium-permeable ion channels. Since characterization o f the calcium channels in glia is limited, the purpose of this study w as to determine the molecular identity and examine the functional prop erties of a voltage-gated calcium channel expressed by Muller cells, t he predominant glia of the retina. Whole-cell and perforated-patch rec ordings of human Muller cells in culture revealed a high threshold vol tage-activated calcium current that is blocked by dihydropyridines, bu t not by omega-conotoxin GVIA or omega-conotoxin MWC. RT-PCR of cultur ed human Muller cells using primers specific for the calcium channel s ubunits demonstrated the expression of an L-type channel composed of t he alpha(1)D, alpha(2) and beta(3) subunits. The alpha(2) subunit of t he Muller cell calcium channel is a splice variant which is distinct f rom either the skeletal muscle alpha(2)s or the brain alpha(2)b. Our e lectrophysiological experiments indicate that the alpha(1)D/alpha(2)/b eta(3) calcium channel is functionally linked with the activation of a potassium channel that may serve as one of the pathways for the redis tribution by Muller cells of excess retinal potassium.