The long-term objectives of this research project are to characterize the membrane properties of muller cells of the vertebrate retina and to relate these properties to possible Muller cell functions, including the regulation of extracellular K+ and pH within the retina and the generation of the electroretinogram. Studies conducted during previous grant periods have demonstrated that the membrane conductance of Muller cells is distributed in a highly non-uniform manner over the cell surface. Conductance is localized to cell endfeet and, perhaps, to cell processes in contact with retinal blood vessels.
Specific aims for the coming grant period are as follows: 1) Describe the types and spatial distribution of ion channels present in muller cells. This research will test the hypothesis that the high K+ conductance of the Muller cell endfoot is mediated by the same K+ channel(s) that are present in other cell regions.. 2) Describe changes in the ion channels of Muller cells during development, during retinal degeneration, and following retinal injury. This research will test two hypotheses: (i) That blood vessels induce """"""""endfoot-like"""""""" properties in Muller cell processes contacting them. (ii) That significant changes occur in Muller cell ion channels and transport processes during retinal degeneration and following retinal injury produced by a penetrating wound. 3) Characterize the electrogenic Na+-HCO3- cotransport system of Muller cells. The stoichiometry of this transport system and its distribution over the cell surface will be studied. 4) Test the hypothesis that K+ and HCO3- are released preferentially from Muller cell endfeet and from processes in contact with blood vessels. Such ion fluxes may help regulate K+ and pH levels in the retina. 5) Test the hypothesis that glial cells regulate blood flow in the retina. 6) Test hypotheses of Muller cell function using computer simulations of Muller cells and retinal K+ dynamics. Electrophysiological investigations on mouse (normal and retinal degeneration mutant), rabbit, salamander and dogfish Muller cells will be conducted using the following preparations: (i) enzymatically dissociated Muller cells, (ii) retinal slices, and (iii) isolated retinas. Whole-cell voltage-and current-clamp, patch-clamp, and ion-selective microelectrode recording techniques will be employed. Elucidating Muller cell membrane properties is critical for understanding the role that these cells play in retinal function. Knowledge of the changes that occur in Muller cells during retinal degeneration and injury will help us understand such clinical conditions as retinitis pigmentosa and retinal trauma.
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