Relaxation Of Retinal Arterioles Induced By Acute Hypoxia Depends On The Perivascular Retina And Is Mediated By NO And A COX Product

Abstract

Purpose: : Retinal hypoxia induces vasorelaxation. The background for this response is unknown but may be a consequence of disturbances in the metabolism of the perivascular retinal tissue. These disturbances can be expected to involve general vasoactive compounds such as NO and prostaglandins. Therefore, the purpose of the present study was to investigate whether retinal vaso-relaxation depends on the perivascular retinal tissue and is mediated by NO and prostaglandins.

Methods: : Porcine retinal arterioles were mounted in a wire myograph for isometric tone measurements. After pre-contraction with 10^-6 M of the thromboxane analogue U46619, the change in vascular tone after changing PO₂ in the gas mixture supplied to the tissue from 0 to 95 % was recorded, and the experiment was repeated after removal of the perivascular retinal tissue. The experiments were repeated during inhibition of cyclo-oxygenase (COX) and nitrogen oxide synthase (NOS).

Results: : Hypoxia caused a dose-dependent vasorelaxation at PO₂ ≤ 10% (p<0.01, repeated measurements ANOVA) in arterioles with perivascular tissue, whereas no effect was observed in isolated arterioles. Inhibition of the COX and NOS enzymes separately each reduced the vasorelaxing effect significantly, and effect that was additive when the two enzymes were blocked simultaneously.

Conclusions: : Hypoxia induces vasorelaxation in porcine retinal arterioles through a retina dependent mechanism. This vasorelaxation is mediated through separate pathways involving both the COX and the NOS enzymes.

Keywords: hypoxia • blood supply • retina