Objective: To determine whether astrocytes of the human optic nerve head can induce nitric oxide synthase-2 (NOS-2) in response to elevated hydrostatic pressure as a mechanism for directly damaging the axons of the retinal ganglion cells in glaucoma.
Methods: Primary cultures of astrocytes from human optic nerve heads were placed in chambers, either pressurized at elevated hydrostatic pressure (60 mm Hg) or maintained at ambient pressure. The induction of NOS-2 was studied by immunocytochemistry, immunoblot, and semiquantitative reverse transcription polymerase chain reaction.
Results: In astrocyte cultures under ambient pressure, NOS-2 was almost undetectable. In astrocyte cultures under elevated hydrostatic pressure for 24, 48, and 72 hours, intensive labeling of NOS-2 in the Golgi body and the cytoplasm was observed by immunocytochemistry and intense bands of NOS-2 were detected by immunoblotting. As detected by semiquantitative reverse transcription polymerase chain reaction, the messenger RNA level of NOS-2 increased significantly in the astrocytes under elevated hydrostatic pressure within 12 hours, peaking earlier than the protein level of NOS-2.
Conclusion: Elevated hydrostatic pressure induces the astrocytes of the human optic nerve head to express NOS-2.
Clinical relevance: In glaucoma, the appearance of the neurodestructive NOS-2 in astrocytes of the optic nerve head may be a primary response to elevated intraocular pressure, in vivo, and therefore damaging to the axons of the retinal ganglion cells.