Retinal Layer–Specific Calcium Demand and Retinovascular Oxygenation in Experimental Diabetes

Abstract

Purpose: : It has not been possible to analytically and specifically measure diabetes–induced changes in retinal cellular calcium ion demand in vivo. Because Mn²⁺ ion is both a Ca²⁺ ion surrogate and a strong MRI contrast agent, we used high resolution (23.4 µm intraretinal resolution) manganese–enhanced MRI (MEMRI) to non–invasively test the hypothesis that diabetes induces changes in retinal cellular demand for calcium that are unrelated to oxidative stress and retinovascular oxygenation dysfunction.

Methods: : Four groups of Lewis rats were studied: age–matched controls and 3 mo experimentally–induced diabetic rats fed either normal chow or chow admixed with the antioxidant α–lipoic acid. α–lipoic acid is reported to prevent retinal biochemical abnormalities and vascular histopathology in long–term diabetic rats (Diabetes (2004) 53, 3233). After systemic administration of non–toxic levels of MnCl₂ concentrations to awake rats, retinal cellular calcium demand–dependent accumulation of Mn²⁺ ion in the inner and outer retina was assessed during either light or dark adaptation using non–invasive MEMRI. Retinal layer thickness was also measured in vivo from the MRI data as well as ex vivo from thin slice histology. In separate non–MnCl₂ injected groups, ΔPO₂ measurements were obtained during a 4 min carbogen challenge.

Results: : In vivo and ex vivo retinal thicknesses were in agreement and not different (P > 0.05) between groups. In diabetic rats, a superior, but not inferior, hemiretinal retinal oxygenation abnormality (P < 0.05) was found and this abnormality was corrected (P < 0.05) with α–lipoic acid treatment. In light and dark adapted diabetic rats, outer retinal uptake of Mn²⁺ was subnormal (P < 0.05) and this was not corrected with α–lipoic acid treatment.

Conclusions: : The oxygenation data provide further evidence that correcting an early abnormal retinal ΔPO₂ is a useful prognostic metric of subsequent positive treatment efficacy. In addition, the data support our overlying hypothesis, at least at the 3 mo time point, that there is dissociation between diabetes–induced changes in retinal cellular calcium demand, and oxidative stress and vascular dysfunction.

Keywords: calcium • oxidation/oxidative or free radical damage • imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound)