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

Abstract

The ability of various neurotransmitters/neuroactive substances to induce fast, transient rises of Ca(2+)-activated K(+) currents (I(BK)) caused by release of Ca(2+) from intracellular stores was investigated in Müller glial cells of the human retina. Müller cells were enzymatically isolated from retinas of healthy donors or of patients with proliferative vitreoretinopathy, and the transmembrane ionic currents were recorded using the whole-cell and the cell-attached patch-clamp techniques. The results of the screening experiments indicate that human Müller 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 I(BK) amplitude with half-maximal effects at 0.33 and 0.50 microM, respectively. Since several different P2 receptor agonists increased the I(BK), it is assumed that human Müller 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 Ca(2+) was buffered by a high-EGTA pipette solution or when IP(3) 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 Müller glial cells express P2Y receptors that, via IP(3) formation, cause intracellular Ca(2+) release. The increased intracellular Ca(2+) 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 Ca(2+)-gated cation channels may be important in respect to proliferative Müller cell gliosis.

Purinergic signaling in retinal degeneration and regeneration

“…In human Müller cells, activation of P2X 7 by ATP or the more selective agonist benzoylbenzoyl-ATP (BzATP) results in membrane depolarization, calcium influx through receptor channels, and sustained activation of calciumactivated, big-conductance potassium (BK) channels ( Fig. 1F) (Pannicke et al, 2000;Bringmann et al, 2001Bringmann et al, , 2002. Similar P2Y and P2X receptor-mediated calcium and current responses were described in RPE cells (Ryan et al, 1999).…”

Group I mGluR-Mediated Inhibition of Kir Channels Contributes to Retinal Müller Cell Gliosis in a Rat Chronic Ocular Hypertension Model

“…However, many studies show no significant glutamate-induced Ca 2ϩ transient in mammalian (rat, guinea pig) retinal Müller cells (Newman and Zahs, 1997;Bringmann et al, 2002b;Newman, 2005;Rillich et al, 2009). Because Müller cells have a relatively hyperpolarized resting membrane potential, the NMDA channels in these cells may be blocked by Mg 2ϩ under physiological conditions.…”

Experimental Dispase-Induced Retinopathy Causes Up-Regulation of P2Y Receptor-Mediated Calcium Responses in Müller Glial Cells

“…However, it is known from previous studies on human [5,18] and rabbit Müller cells [17] that BK channels may activate at the native resting membrane potential. Very probably, the strong positive shift of the BK channel activation observed in whole-cell records may reflect the absence in the pipette solution of essential cytoplasmic components which co-activate BK channels, such as activators of the protein kinase A [12].…”