Endothelial Progenitor Cells (EPCs) are circulating cells released from bone marrow which are important for maintaining cardiovascular health. The prevalence of cardiovascular disease in older adults is associated with reduced circulating EPC numbers. Studies have shown reduced EPC number and function in old vs. young individuals, and endurance exercise training increases EPC number and function in young adults. Oxidative stress adversely affects endothelial cells and preliminary evidence indicates that oxidative stress negatively affects EPC function. Conversely, regular exercise reduces markers of oxidative stress and may enhance EPC function in older adults. The investigators hypothesize that older endurance-trained athletes and matched sedentary individuals will have markedly divergent EPC function and that altering the physical activity levels of both groups will move them to intermediate points between these two extremes. The investigators also propose that the investigators can "mimic" the effect of exercise training on EPC function in cell culture by altering intracellular levels of a key enzyme and a signaling molecule which the investigators have shown to regulate EPC function with respect to exercise training in young individuals.

Inclusion Criteria:

• 50-80 years of age, BMI = 18-35 kg/m2, non-smoking, women must be postmenopausal

Exclusion Criteria:

• history of cardiovascular/cerebrovascular disease, diabetes, cancer, renal, liver disease, HIV; uncontrolled hyperlipidemia/hypertension