Imaging of 13C-labeled glucose and sorbitol in bovine lens by 1H-detected 13C nuclear magnetic resonance spectroscopy

Mapping Glucose Uptake, Transport and Metabolism in the Bovine Lens Cortex

Purpose: To spatially correlate the pattern of glucose uptake to glucose transporter distributions in cultured lenses and map glucose metabolism in different lens regions. Methods: Ex vivo bovine lenses were incubated in artificial aqueous humour containing normoglycaemic stable isotopically-labelled (SIL) glucose (5 mM) for 5 min-20 h. Following incubations, lenses were frozen for subsequent matrix-assisted laser desorption/ionisation (MALDI) imaging mass spectrometry (IMS) analysis using high resolution mass spectrometry. Manually dissected, SIL-incubated lenses were subjected to gas chromatography-mass spectrometry (GC-MS) to verify the identity of metabolites detected by MALDI-IMS. Normal, unincubated lenses were manually dissected into epithelium flat mounts and fibre cell fractions and then subjected to either gel-based proteomic analysis (Gel-LC/MS) to detect facilitative glucose transporters (GLUTs) by liquid chromatography tandem mass spectrometry (LC-MS/MS). Indirect immunofluorescence and confocal microscopy of axial lens sections from unincubated fixed lenses labelled with primary antibodies specific for GLUT 1 or GLUT 3 were utilised for protein localisation. Results: SIL glucose uptake at 5 min was concentrated in the equatorial region of the lens. At later timepoints, glucose gradually distributed throughout the epithelium and the cortical lens fibres, and eventually the deeper lens nucleus. SIL glucose metabolites found in glycolysis, the sorbitol pathway, the pentose phosphate pathway, and UDP-glucose formation were mapped to specific lens regions, with distinct regional signal changes up to 20 h of incubation. Spatial proteomic analysis of the lens epithelium detected GLUT1 and GLUT3. GLUT3 was in higher abundance than GLUT1 throughout the epithelium, while GLUT1 was more abundant in lens fibre cells. Immunohistochemical mapping localised GLUT1 to epithelial and cortical fibre cell membranes. Conclusion: The major uptake site of glucose in the bovine lens has been mapped to the lens equator. SIL glucose is rapidly metabolised in epithelial and fibre cells to many metabolites, which are most abundant in the metabolically more active cortical fibre cells in comparison to central fibres, with low levels of metabolic activity observed in the nucleus.

Carbohydrate nutrition, glycemic index, and the 10-y incidence of cataract

BACKGROUND

Although dietary carbohydrates are thought to play a role in cataractogenesis, few epidemiologic studies have examined links between carbohydrate nutrition and cataract.

OBJECTIVES

We investigated the associations between dietary glycemic index (GI), glycemic load (GL), total carbohydrate intake, and 10-y incident nuclear, cortical, and posterior subcapsular cataract.

DESIGN

Of 3654 baseline participants in an Australian population aged >/=49 y (1992-1994), 933 were seen after 5 and/or 10 y, had completed a detailed semiquantitative food-frequency questionnaire, had no previous cataract surgery or baseline cataract, and had photographs taken to assess incident cataract with the Wisconsin Cataract Grading System. Dietary information was collected with a validated food questionnaire. GI was calculated from a customized database of Australian foods. GI, GL, and all other nutrients were energy adjusted. Hazard ratios (HRs) and 95% CIs were calculated with the use of discrete logistic models.

RESULTS

After age, sex, diabetes, and other factors were controlled for, each SD increase in GI significantly predicted incident cortical cataract (HR: 1.19; 95% CI: 1.01, 1.39). Participants within the highest compared with the lowest quartile of GI were more likely to develop incident cortical cataract (HR: 1.77; 95% CI: 1.13, 2.78; P for trend = 0.035). These findings were similar after excluding participants with diabetes, although they were slightly attenuated and marginally nonsignificant (HR: 1.16; 95% CI: 0.98, 1.37, per SD increase in GI). No association was found between GI and nuclear or posterior subcapsular cataract and between GL or carbohydrate quantity and any cataract subtype.

CONCLUSION

In an Australian cohort, poorer dietary carbohydrate quality, reflected by high GI, predicted incident cortical cataract.

Carbohydrate intake and glycemic index in relation to the odds of early cortical and nuclear lens opacities

BACKGROUND

Animal studies suggest a role for dietary carbohydrate in cataractogenesis. However, few published human studies have evaluated associations between carbohydrate nutrition and lens opacification.

OBJECTIVE

Our objective was to test the hypothesis that long-term carbohydrate intake and dietary glycemic index are associated with the odds of early cortical and nuclear opacities.

DESIGN

Subjects were 417 Boston-area members of the Nurses' Health Study cohort aged 53-73 y. Dietary information was based on an average from 5 semiquantitative food-frequency questionnaires collected over a 14-y period. Opacities were assessed by using the Lens Opacity Classification System III (LOCS III). We used eyes (n = 711) as the unit of analysis and generated odds ratios by using a generalized estimating approach to logistic regression to account for the lack of independence between the 2 eyes of each subject.

RESULTS

After multivariate adjustment, the odds of cortical opacities (LOCS III >/=1.0) among women in the highest tertile of carbohydrate intake (>/=200 g/d) was 2.46 times (95% CI: 1.30, 4.64; P for trend = 0.005) that among women in the lowest tertile (<185 g/d). This association was not affected by adjustment for dietary glycemic index, which was not associated with early cortical opacities. Carbohydrate nutrition was not associated with the odds of nuclear opacities (LOCS III >/=2.5).

CONCLUSIONS

These data suggest that carbohydrate quantity, but not carbohydrate quality, is associated with early cortical opacities, and that neither the quantity nor the quality of dietary carbohydrate affects the risk of nuclear opacities in middle-aged women.