Elevated pressure triggers a physiological release of ATP from the retina: Possible role for pannexin hemichannels.

Increased hydrostatic pressure can damage neurons, although the mechanisms linking pressure to neurochemical imbalance or cell injury are not fully established. Throughout the body, mechanical perturbations such as shear stress, cell stretching, or changes in pressure can lead to excessive release of ATP. It is thus possible that increased pressure across neural tissues triggers an elevated release of ATP into extracellular space. As stimulation of the P2X(7) receptor for ATP on retinal ganglion cells leads to elevation of intracellular calcium and excitotoxic death, we asked whether increased levels of extracellular ATP accompanied an elevation in pressure across the retina. The hydrostatic pressure surrounding bovine retinal eyecups was increased and the ATP content of the vitreal compartment adjacent to the retina was determined. A step increase of only 20 mm Hg induced a threefold increase in the vitreal ATP concentration. The ATP levels correlated closely with the degree of pressure increase over 20-100 mm Hg. The increase was transient at lower pressures but sustained at higher pressures. The rise in vitreal ATP was the same regardless of whether nitrogen or air was used to increase pressure, implying changes in oxygen partial pressure did not contribute. Lactate dehydrogenase activity was not affected by pressure, ruling out a substantial contribution from cell lysis. The ATP increase was largely inhibited by either 30 muM 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB) or 10 muM carbenoxolone (CBX). While this pharmacological profile is consistent with physiological release of ATP through pannexins hemichannels, a contribution from anion channels, vesicular release or other mechanisms cannot be ruled out. In conclusion, a step elevation in pressure leads to a physiologic increase in the levels of extracellular ATP bathing retinal neurons. This excess extracellular ATP may link increased pressure to the death of ganglion cells in acute glaucoma, and suggests a possible role for ATP in the neuronal damage accompanying increased intracranial pressure.