Two Different Mechanosensitive Calcium Responses in Muller Glial Cells of the Guinea Pig Retina: Differential Dependence on Purinergic Receptor Signaling

Tractional forces or mechanical stimulation are known to induce calcium responses in retinal glial cells. The aim of the study was to determine the characteristics of calcium responses in Muller glial cells of the avascular guinea pig retina induced by focal mechanical stimulation. Freshly isolated retinal wholemounts were loaded with Mitotracker Deep Red (to fill Muller cells) and the calcium-sensitive dye Fluo-4/AM. The inner retinal surface was mechanically stimulated with a micropipette tip for 10 ms. Stimulation induced two different cytosolic calcium responses in Muller cells with different kinetics in dependence on the distance from the stimulation site. Muller cells near the stimulation site displayed an immediate and long-lasting calcium response with high amplitude. This response was mediated by calcium influx from the extracellular space likely triggered by activation of ATP-insensitive P2 receptors. More distant Muller cells displayed, with a delay of 2.4 s, transient calcium responses which propagated laterally in a wave-like fashion. Propagating calcium waves were induced by a calcium-independent release of ATP from Muller cells near the stimulation site, and were mediated by a release of calcium from internal stores triggered by ATP, acting in part at P2Y(1) receptors. The data suggest that mechanically stimulated Muller cells of the guinea pig retina release ATP which induces a propagating calcium wave in surrounding Muller cells. Propagating calcium waves may be implicated in the spatial regulation of the neuronal activity and homeostatic glial functions, and may transmit gliosis-inducing signals across the retina. Mechanical stimulation of guinea pig Muller cells induces two calcium responses: an immediate response around the stimulation site and propagating calcium waves. Both responses are differentially mediated by activation of purinergic receptors.