Activation of P2Y receptors stimulates potassium and cation currents in acutely isolated human Muller (glial) cells

The ability of various neurotransmitters/neuroactive substances to induce fast, transient rises of Ca2+-activated K+ currents (I-BK) caused by release of Ca2+ from intracellular stores was investigated in Muller glial cells of the human retina. Muller cells were enzymatically isolated from retinas of healthy donors or of patients with proliferative vitreoretinopathy, and the transmembrane ionic currents were recorded using the wholecell and the cell-attached patch-clamp techniques. The results of the screening experiments indicate that human Muller cells express, in addition to GABA(A) and perhaps glutamatergic and cholinergic receptors, predominantly P2 receptors. ATP and other nucleotides exerted two effects on membrane currents: repetitive transient increases of the I-BK amplitude and, in a subpopulation of cells investigated, the appearance of a transient cation conductance at negative potentials. ATP and UTP increased dose-dependently the IBK amplitude with half-maximal effects at 0.33 and 0.50 muM, respectively. Since several different P2 receptor agonists increased the I-BK, it is assumed that human Muller cells express a mixture of different types of P2Y receptors. In cell-attached patches, extracellular application of ATP or UTP transiently increased the open probability of single putative BK channels. The increase of I-BK and the appearance of the cation conductance in whole-cell records were abolished when intracellular Ca2+ was buffered by a high-EGTA pipette solution or when IP3 was included in the pipette solution. The expression of agonist-evoked transient cation currents was found to be stronger in cells from patients as compared to cells from healthy donors. It is concluded that human Muller glial cells express P2Y receptors that, via IP3 formation, cause intracellular Ca2+ release. The increased intracellular Ca2+ concentration Stimulates the activity of BK channels and may induce opening of cation channels. Both the ATP-induced activity of BK channels and the increased expression of Ca2+-gated cation channels may be important in respect to proliferative Muller cell gliosis. (C) 2002 Wiley-Liss, Inc d.