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Keywords:

  • energy-dependent amino acid transporters;
  • glaucoma;
  • glutamate transport;
  • glutamate/aspartate transporter;
  • hypoxia;
  • intraocular pressure;
  • ischemia;
  • retina

Abstract.

Purpose:  To investigate the relationship between intraocular pressure (IOP), retinal glutamate transport and retinal hypoxia during acute IOP elevations of varying magnitude.

Methods:  Female Dark Agouti rats were anaesthetized by ketamine/xylazine/acepromazine (10/5/0.5 mg/kg i.p.). The anterior chamber was cannulated with a 30-gauge needle attached to a saline reservoir. The target IOP (20–120 mmHg, in 10 mmHg increments) was obtained by adjusting the reservoir height. After 10 mins of IOP stabilization, 2 μl of the non-endogenous glutamate transporter substrate, D-aspartate, was injected into the vitreous (final concentration 50 μm), and the elevated IOP maintained for a further 60 mins (total duration of IOP elevation was 70 mins). Glutamate transporter function was assessed by the immunohistochemical localization of D-aspartate. Retinal sections were examined for histological integrity. The experiment was repeated substituting the D-aspartate with the cellular hypoxia marker, Hypoxyprobe-1.

Results:  Under control conditions, D-aspartate was preferentially taken up into the glial Müller cells by glutamate/aspartate transporter (GLAST). This function was maintained at pressures ≤ 70 mmHg, whereafter perturbation of function was evidenced by decreased accumulation of D-aspartate by Müller cells. Failure of GLAST activity was coincident with the appearance of Hypoxyprobe-labelled cells in the inner retina and histological damage.

Conclusions:  Glutamate transport does not appear to change linearly with increased IOP. A pressure threshold exists, above which Müller cell GLAST function is compromised. Moreover, ganglion cell glutamate uptake is only apparent at pressures above those that cause GLAST inhibition. The association between IOP, hypoxia, glutamate transporter dysfunction and subsequent retinal cell death may have important implications for the pathogenesis of IOP/ischaemia-related neuropathy and neuroprotective strategies.