Osmotic and aging effects in caviar oocytes throughout water and lipid changes assessed by H-1 NMR T-1 and T-2 relaxation and MRI

By combining NMR relaxation spectroscopy and magnetic resonance imaging techniques, unsalted (us) and salted (s) caviar (Acipenser transmontanus) oocytes were characterized over a storage period of up to 90 days. The aging and the salting effects on the two major cell constituents, water and lipids, were separately assessed. T(1) and T(2) decays were interpreted by assuming a two-site exchange model. At Day 0, two water compartments that were not in fast exchange were identified by the T(1) relaxation measurements on the us oocytes. In the s samples, T(1) decay was monoexponential. During the time of storage, an increment of the free water amount was found for the us oocytes, ascribed to an increased metabolism. T(1) and T(2) of the s oocytes shortened as a consequence of the osmotic stress produced by salting. Selective images showed the presence of water endowed with different regional mobility that severely changed during the storage. Lipid T(1) relaxation decays collected on us and s samples were found to be biexponential, and the T(1) values lengthened during storage. In us and s oocytes, the increased lipid mobility with the storage was ascribed to lipolysis. Selective images of us samples showed lipids that were confined to the cytoplasm for up to 60 days of storage.