Evaluation of age-related changes of physicochemical properties and functional activity of rat adipose plasma membranes and their possible relationship

Aged lymphocyte proliferation following incorporation and retention of dietary omega-3 fatty acids

T cell activation involves events at the plasma membrane; therefore, molecules such as long chain omega-3 fatty acids that alter the structure of the plasma membrane may affect the activation of aged T cells. In this project we investigated whether the incorporation of omega-3 fatty acids (from fish oil), in the presence of vitamin E, improves age-diminished T cell proliferation. Young and old mice were fed diets rich in either fish (menhaden) oil or saturated fat for various lengths of time. Splenocytes were harvested from these mice and stimulated in culture with either mitogen or the antigen keyhole limpet hemocyanin (for a secondary response); proliferation was estimated by [3H]thymidine incorporation. We found no discernible effect of dietary omega-3 fatty acids (with vitamin E supplementation) on lymphocyte proliferation stimulated by the mitogens concanavalin A or phytohemagglutinin. We did, however, find that the saturated fat diet and the menhaden oil diet in young mice lowered protein kinase C activities in the particulate fractions of spleen cells when compared to chow-fed mice. Middle-aged and old mice were less affected by the experimental diets than young mice, but they demonstrated decreased protein kinase C activity as well. These alterations did not affect the ability of splenocytes to respond to mitogenic stimulation. Fatty acid analysis revealed that lymphocytes from mice fed saturated fat for 8.5 months retained significant amounts of the omega-3 fatty acid docosahexaenoic acid, despite the lack of dietary omega-3 fatty acids. However, when aged (but not young) lymphocytes were clonally expanded by antigen in vivo in the presence of dietary omega-3 fatty acids, they produced a greater secondary proliferative response than old lymphocytes expanded during a saturated fat diet. Although our results suggest that omega-3 fatty acids may enhance aged lymphocyte proliferation, the tenacious retention of these fatty acids makes comparison with omega-3-depleted lymphocytes difficult.

Elevated protein tyrosine phosphatase activity and increased membrane viscosity are associated with impaired activation of the insulin receptor kinase in old rats

Insulin resistance is very common in the elderly, and may be associated with glucose intolerance or frank diabetes. In previous studies we demonstrated that insulin resistance in old Wistar rats is associated with decreased autophosphorylation and activation of the hepatic insulin receptor kinase (IRK) in vivo. We now show that this defect can be reproduced in vitro, where the extent of insulin-induced activation of IRK in liver membranes of old rats was decreased by approximately 50% compared with young controls. The defect could be largely abolished after solubilization of the membranes with Triton X-100. We also show that: (a) the viscosity of membranes from the old rats was significantly (P < 0.001, n = 4) higher (by 15%) compared with young controls; (b) incubation of plasma membranes from old animals with lecithin liposomes, which lowered their cholesterol levels, partially abolished the defect in IRK activation; and (c) Triton extracts of liver membranes prepared from old rats did not interfere with the activation of IRK derived from young controls. Additionally, non-membrane components did contribute to the development of this defect. We observed a significant (approximately 30%) (P < 0.001, n = 18) elevation of cytosolic protein tyrosine phosphatase (PTP) activity directed against the beta subunit of the insulin receptor in livers of old rats. No such elevation of PTP activity could be demonstrated with synthetic substrates. Our findings are consistent with a model in which increased membrane viscosity as well as enhancement of a cytosolic PTP activity both markedly inhibit the activation in vivo of the hepatic IRK in old animals.

Age-related variations of lipid peroxidation in cadmium-treated rats

Under specific conditions, Cd can induce a prooxidant state in biological systems resulting in the peroxidation of the polyunsaturated fatty acids. We investigated in vivo this phenomenon in major target organs of Long Evans rats (12- and 36-week-old), 24 hours after being injected i.p. with a range of cadmium chloride doses (0.00, 0.05, 0.25, 1.0 or 2.5 mg Cd/kg). The measurements of the thiobarbituric acid reactive substances demonstrated that the lungs showed the highest Cd-induced LPO in the 12-week-old rats even if the liver and kidneys accumulated the greatest amounts of Cd. Cd-induced LPO as measured by TBARS assay in the 36-week-old groups was, in all organs, less pronounced even if the control levels were higher for these groups. The reasons for this age-dependent differences are not clear. Some components of the antioxidant defense system were differently modulated as a function of age. However, it is not certain whether these changes have a causative role in the induction of LPO following Cd exposure.

Influence of aging on fluidity and coupling between beta-receptors and G-proteins in rat lung membranes

We tested the hypothesis that in rat lung membranes, the age-related decline in the percentage of beta-receptors coupled with high affinity to G-proteins, is due to limitation of the diffusion caused by a decrease in membrane fluidity. We measured both parameters simultaneously in a crude membrane preparation from lungs of rat of different age. In contrast to what is found in crude membrane preparations from rat liver and brain, in rat lung fluidity was increased upon aging. We conclude that the age-related alteration in coupling between receptor and G-protein is difficult to explain by alterations of membrane fluidity.