Exercise and Kidney Dopamine Receptor Function in Aging

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DESCRIPTION (provided by applicant): Project Summary: We reported that the ability of dopamine to promote sodium excretion is markedly diminished in old (24-month) compared to adult (6-month) rats. This is due to the hyperphosphorylation of D1 receptors and their uncoupling from G-proteins resulting in the failure of D1 receptor agonist to inhibit the sodium transporters (Na,K-ATPase, Na,H-exchanger) and to produce natriuresis. Age-related increase in oxidative stress in the proximal tubules (PTs) of old rats is responsible for causing D1 receptor phosphorylation and it's uncoupling from G-proteins via PKC/GRK-2 pathway. Regular physical exercise is reported to slow down cell damage and physiological dysfunction that is characteristic of the aging process. In preliminary studies, we found that 6-week treadmill exercise in old rats decreased levels of malondialdehyde (a marker of oxidative stress) and increased expression and activity of superoxide dismutase (an anti-oxidant enzyme) in PTs compared to PTs of sedentary old rats. Further, the plasma levels of IL-10 (an anti-inflammatory marker) and the nuclear levels of Nrf2 (a transcription factor responsible for transcription of anti-oxidant defense genes) in PTs were increased in exercised compared to sedentary old rats. Moreover, D1 receptor numbers increased in the PTs membranes, and D1 receptor agonist SKF38393 increased sodium excretion in exercised compared to sedentary old rats. This application will test the hypothesis that exercise in old rats leads to increased production of anti-inflammatory cytokines, activation of Nrf2 and increased anti-oxidant capacity in renal PTs. The increased anti-oxidant capacity lowers oxidative stress, reduces PKC activity/GRK-2 translocation, and thus restores proximal tubular D1 receptor G-protein coupling and the natriuretic response to dopamine in exercised old rats. In order to test our hypothesis, old rats will be placed on treadmill exercise for 6-week followed by measurement of markers of oxidative stress, anti-oxidant defenses, cytokines, D1 receptor/G- protein coupling and function. Biochemical, immunological, molecular biology and renal functional studies methodologies will be employed to accomplish the proposed studies. These studies will allow us to determine beneficial effects of exercise and form the basis to identify molecular mechanism of anti-inflammatory cytokines-mediated increase in anti-oxidant defenses and in lowering oxidative stress and restoring D1 receptor G-protein coupling and function in old rats. Project Narrative: The number of elderly in the population and therefore age-related disorders is predicted to increase dramatically over the next few decades. Our research will help identify exercise as an intervention to reverse some of the age-associated abnormalities such as increases in oxidative stress, inflammation and loss of drug responsiveness. The results obtained will have far reaching significance in terms of demonstrating that increasing anti-inflammatory cytokines and anti-oxidant defenses with exercise leads to beneficial effects. Perhaps regular exercise can later on be demonstrated as an effective intervention to prevent some of the age- associated cardiovascular and kidney diseases.