Renewal versus Retention: Isotopic Composition of Intestinal Epithelium and Eye Lens

Aims: To compare isotopic signatures of contrasting (due to the structure and metabolism) organs in mice of two contrasting ages.
Study Design: Cross-sectional study.
Place and Duration of Study: Biology Department of Moscow State University; Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences; 2007–2011.
Methodology: Mass spectrometric measurements of carbon and nitrogen isotopic ratios of jejunal epithelium and lens in 1- and 22-mo mice fed a monotonous diet.
Results: The lenses are enriched in carbon and nitrogen as compared with intestinal epithelium (by 5.5% and 4.5% in 1-mo mice and 8.3% and 6% in 22-mo mice, respectively). The 15N content is also higher in lenses than in intestinal epithelium (8.97‰ vs. 7.62‰ in 1-mo mice, and 7.40‰ vs. 6.58‰ in 22-mo mice). The 13C content of lenses exceeds that of intestinal epithelium in 1-mo mice (-20.27‰ vs. -21.69‰), although 13C content is equal in 22-mo mice (-22.56‰ vs. -22.67‰). 15N content is depleted in the intestinal epithelium of 22-mo mice (-1.04‰), whereas 13C depletion (-0.98‰) is non-significant. 13C and 15N content in lenses is also significantly decreased in 22-mo mice (-2.29‰ and -1.57‰).
Conclusion: The intestinal epithelium represents a structure with short-term isotopic memory lasting a few days, whereas the events of the organism’s entire lifetime are retained in lens isotopic memory. The difference of the parameters measured is evidently determined by structural contrast, metabolic rate, and rejuvenation modes of the tissues. The 15N depletion in both the intestinal epithelium and lenses, as well as 13C depletion in lenses of 22-mo mice might be considered as a sign of ageing. In contrast, the depletion of 15N in lenses of 22-mo mice should be considered primarily as a result of dilution of breastmilk isotopic signature that probably obscures age-related alterations of the organ. Comparison of isotopic compositions of these contrasting organs may be useful for physiological and ecological determinations.