[Reason for the discrepancy between blood plasma osmolality and its Na+ concentration during cold stress in rats]

The body temperature decrease to 24 degrees C or intraperitoneal injection of adrenaline (0.3 mg per 100 g body mass) to rats entailed the increased plasma osmolality and reduction in it of Na+ concentration. All plasma samples were ultracentrifuged. Osmolality was reduced and sodium concentration was increased in the internatant of the same samples where the maximal alterations were recorded. Experiments with the blood plasma of the lamprey which is rich under natural conditions in nonesterified fatty acids demonstrated analogous changes in osmolality and Na+ concentration in the plasma and plasma internatant. No significant changes were discovered in breast milk in which the percentage of high density lipoproteins is minimal. The authors assume that high density lipids are involved in the regulation of blood osmolality, with these lipids changing the solvent water activity and thereby lowering or raising blood plasma osmolality.