VASCULAR ENDOTHELIAL REACTIVITY MEDIATED BY [HbO2] THE EFFECTS OF EXERCISE TRAINING ON SEDENTARY SUBJECTS

Mounting evidence is linking decreased endothelial reactivity as mediated by the concentration of oxygen bound to hemoglobin ([HbO2]) to a sedentary lifestyle. Previously, a cross sectional study revealed that subjects with a sedentary lifestyle showed an attenuated increase in skeletal muscle blood flow (MBF) resulting from a decrease in [HbO2] when compared with recreationally physically active subjects. PURPOSE: To perform a longitudinal study to determine the effects of exercise on endothelial reactivity as mediated by [HbO2]. METHODS: As a surrogate measure of endothelial function, the thermal dilution technique was used to measure quadriceps MBF and thereby assess changes in endothelial reactivity. Submaximal MBF was measured in 5 sedentary males (18.4 ± 0.3 yrs) prior to and after completion of an 8 week knee- extensor ergometer exercise training protocol in which subjects exercised for 1 hour 3 times a week. Subjects were studied during two graded exercise tests, the first under normoxic (N) conditions followed by conditions of decreased [HbO2] which was attained by the administration carbon monoxide (CO) + hyperoxia. RESULTS: At the onset of the investigation, the subjects attained an average maximal normoxic work rate of 25 ± 2 Watts, which increased to an average normoxic work rate of 37 ± 1 Watts post training. In the pre-training study, MBF at 50% of maximal effort increased from 2737 ± 167 ml/min with an average [HbO2] of 19 ± 0.3 to 2954 ± 148 ml/min with an average [HbO2] of 15.4 ± 0.2, an increase of ∼216 ml/min resulting from a drop in [HbO2] of ∼3.6 mL/dL. After completion of the eight week training protocol there was a much larger increase MBF from 2317 ml/min at 50% of maximum work rate with an average [HbO2] of 19.1 ± 0.2 mL/dL to 2973.0 ml/min at an average of 15.2 ± 0.1 mL/dL, an increase of ∼650 ml/min for a similar drop in [HbO2] of ∼3.9 mL/dL. CONCLUSION: The enhanced MBF response to changes in [HbO2] with regular exercise is suggestive of an endothelial dependent vasodilatory mechanism and may ultimately assist in the understanding of MBF control.