Hydrostatic pressure-induced release of stored calcium in cultured rat optic nerve head astrocytes.

In the normal eye the aqueous humor (AH) inflow from the ciliary epithelium and its exit through the outflow pathways, mainly through a sieve-like tissue at the corneoscleral junction (the trabecular meshwork), is balanced to create a physiological intraocular pressure (IOP) of ∼15 mm Hg. This normal hydrostatic pressure is essential to maintain the spherical shape of the globe and the refractive surface of the cornea, a requirement for good vision. Increased IOP as a result of abnormally high resistance to AH outflow, gives rise to a condition called primary open angle glaucoma,1,2 the second leading cause of blindness.3 IOP can more than double, and disease progression is marked by gradual deformation of the optic nerve head, where retinal ganglion cell axons exit the eye. Optic nerve head remodeling and deformation associated with increased intraocular pressure are thought to be the probable cause of retinal ganglion cell death and consequent vision loss. Elevated IOP and progressive visual field loss in primary open angle glaucoma appear linked.4–6 Retinal ganglion cell axons in the optic nerve head have a complex relationship with glial cells and the extracellular matrix (ECM). Astrocytes play a key role in the optic nerve head, and ganglion cells are dependent on them for structural support, metabolic support, and regulation of extracellular ion composition and pH.7 Astrocytes synthesize ECM, and it has been reported that abnormal astrocyte function and consequent changes in the extracellular matrix composition contribute to remodeling of the optic nerve head in glaucoma.8–15 Interestingly, it has been found that cultured astrocytes subjected for 24 to 48 hours to a 10- to 20-mm Hg increase in hydrostatic pressure applied in vitro respond by increased expression of MMPs and TIMPs16 and an altered pattern of ECM expression.17 Expression of small heat shock proteins Hsp27 and αB-crystallin18 and several other genes also is changed in cells subjected to hydrostatic pressure.19,20 Recently this laboratory examined earlier responses that occur in optic nerve astrocytes subjected to elevated hydrostatic pressure. After 2 hours, a pressure challenge of 15 mm Hg was found to activate ERK. Subsequently, ERK activation led to the phosphorylation of p90RSK and stimulation of the NHE-1 sodium-hydrogen exchanger.21 Here, we examined astrocytes at the onset of a hydrostatic pressure increase and discovered an important role for calcium. There have been previous reports that an increase of cytoplasmic calcium occurs in bovine articular chondrocytes subjected to elevated hydrostatic pressure.22,23 In one of these studies, the increase in cytoplasmic calcium appeared to be the result of calcium entry from extracellular space since it was abolished in calcium-free solution and inhibited by gadolinium.22,23 In the other study, the increase in intracellular calcium was attributed to release from an IP3-sensitive intracellular calcium store.22,23 We devised a method to measure cytoplasmic calcium in rat optic nerve astrocytes while the cells are subjected to hydrostatic pressure. A pressure challenge of 15 mm Hg caused an immediate increase of cytoplasmic calcium concentration that appeared to originate from a ryanodine-sensitive cytoplasmic store.