LIMITATIONS IN OXIDATIVE METABOLISM: CENTRAL AND PERIPHERAL FACTORS MANIPULATION

The main metabolic pathway involved during an exercise of duration greater than 1 minute is the oxidative metabolism. The functional evaluation of oxidative metabolism is based on the analysis of two main functional indexes: O2max and O2 kinetics. These indexes are determined by a finite ability to deliver oxygen to the working muscles (central factor) and a limited ability of the muscles to extract oxygen (peripheral factor). The relative contribution of central and peripheral factors to the overall limitation of oxidative metabolism, yet remains controversial. Near Infrared Spectroscopy (NIRS) was recently added to the classical methods of muscle oxidative metabolism functional evaluation. NIRS is a non-invasive technology that continuously monitors changes (relative or absolute) in oxygenated and deoxygenated haemoglobin (HHb). NIRS HHb signal directly depends on the ratio between the muscular O2 utilization rate and the capillary O2 delivery in the region explored by the probe, providing a non-invasive estimate of the changes in O2 extraction occurring inside the muscles. The general aim of the thesis is to elucidate the relative contribution of central and peripheral factors in limiting oxidative metabolism by the application of non invasive techniques. We have proposed to modulate oxidative metabolism with different manipulations: 1) High fat diet; 2) Heavy intensity warm up and 3) two kinds of training: Aerobic and Isotonic. In the study #1 we used a high fat diet (HFdiet) to manipulate the peripheral factor in healthy young moderate trained males. Animal and human studies suggest that fat adaptation induces structural and functional muscle adaptations that may benefit oxidative metabolism. We tested the hypothesis that a long term HFdiet enhances oxidative metabolism by augmenting the muscle’s capacity to extract oxygen. 22 young healthy moderately trained males (28±5 yrs, 53±6 ml*Kg-1*min-1) were randomly assigned to: Hdiet (HFD, 55% of calories from fat, 30% carbohydrate and 15% proteins) or control diet (C, 30, 55 and 15%) for 10 days. Before and after the diet the subjects performed an incremental cycling test to exhaustion and 3 step transitions at moderate intensity. Respiratory variables and heart rate (HR) were measured bbb. The maximal and submaximal response to exercise were evaluated ( O2, respiratory exchange ratio, R) and the kinetics of pulmonary O2 were fitted by a double exponential model. HF diet was associated with a shift in substrate selection towards a higher contribution of fat to the production of energy in the moderate intensity domain of exercise (reduced R). Furthermore, HFD increased the speed of adaptation of pulmonary O2 kinetics by the reduction of the time constant of the primary component. Our findings support the hypothesis that, in healthy young males, a 10-day high fat diet may increase the speed of adaptation of oxidative metabolism at the onset of a moderate intensity exercise by increasing the relative contribution of fats oxidation to ATP production. In the study #2 we applied heavy intensity warm-up (HWu) to manipulate O2 delivery in sedentary healthy older adults. HWu increases VO2 kinetics during successive moderate intensity transitions. We tested the hypothesis that such improvement is due to a better matching of O2 delivery to utilization within the working muscles. We tested the hypothesis that HWu improvement is due to a better matching of O2 delivery to utilization within the working muscles, rather than to an increase in O2 bulk delivery. In 21 healthy older adults (65.7 5 yrs) we measured contemporarily and non-invasively indexes of the overall speed of adaptation of the oxidative metabolism ( i.e. pulmonary O2 kinetic), of the bulk O2 delivery (i.e. ) and of the rate of muscle deoxygenation (i.e. HHb) during moderate intensity step transitions, either with (Wu) or without (nWu) prior Hwu. The local matching of O2 delivery to utilization was evaluated by the HHb/ O2 ratio index. The innovative findings of this study are: i) HWu does not modify the speed of adaptation of bulk O2 delivery (i.e. ) and TPR; ii) HWu reduces the “overshoot” of the HHb/ O2 ratio, suggesting a better matching of O2 delivery to O2 utilization. Our data are compatible with the hypothesis that, in older adults, HWu, may beneficially affect oxidative metabolism thanks to acute improvement of the local matching of O2 delivery to O2 utilization. The evidences of Study # 2 motivated us to apply a manipulation that modifies O2 delivery in an adaptive way. Thus in the study #3 we applied aerobic training (AT) to manipulate O2 delivery in sedentary healthy older adults. We tested the hypothesis that older adults may benefit from 12 weeks of AT mainly thanks to an adaptive enhancement of O2 delivery to the working muscles. 14 healthy elderly (66 ± 6 yrs) were tested before and after a 12-week training consisting of an AT or a control (remained sedentary). Subjects performed: the same test protocol described in study # 2. Cardio-respiratory variables were measured bbb and muscle oxygen extraction (HHb) was measured, at the vastus lateralis, by quantitative NIRS. We calculated the time delay and the time constant of both the primary component of the pulmonary O2 and of HHb. The main findings of this study wereAT: i) increased O2max and pulmonary O2 kinetics, preceded (Wu) or not (nWu) by HWu; ii) did not modify the speed of adaptation of muscle oxygen extraction (HHb kinetics); iii) abolished the HWu effect in pulmonary O2 kinetics; iv) attenuated HWu effect in muscle oxygen extraction kinetics; v) abolished the peak of the HHb/ O2 ratio. Our data are compatible with the hypothesis that, in older adults, AT may beneficially affect oxidative metabolism thanks to an adaptive improvement in the matching of O2 delivery to the local O2 utilization. The attenuation yet not an abolishment of the HWu effect in HHb kinetics suggested us an underlying limitation in muscle O2 extraction, unaffected by AT. Isotonic Training is known to increase strength and muscle mass in older adults. We tested the hypothesis that older adults may benefits from 12 weeks of isotonic + aerobic training (IT) thanks to an adaptive enhancement in O2 delivery and O2 utilization. 14 healthy elderly (66 ± 6 yrs) were tested before and after a 12-week training consisting of an isotonic training added to aerobic training (IT) or a control condition (remained sedentary). Subjects performed the test protocol described in study #2 and were measured the variables and analyzed data as described for study # 4 The main finding of this study was that IT produced at the lungs an attenuation of HWu effect (i.e.TD remained shorter). These results suggested that older adults can be beneficially affected by IT, however it is not enough to produce the necessary variations in muscle O2 delivery to completely avoid HWu effect. Furthermore at muscle level the variations IT supressed the HWu effect. Thus in agreement with our hypothesis isotonic training added to aerobic training was enough to eliminate completely the effect of priming exercise. Regarding nWu exercise, the effect of IT was a reduction in the HHb/ O2 ratio peak, suggestive of a better matching in O2 delivery to the O2 utilization following training. Our data are compatible with the hypothesis that in older adults oxidative metabolism may be beneficially affected by IT thanks to an adaptive improvement of the local O2 delivery and a further adaptations in the working muscle to O2 utilization. In summary, eucaloric high fat diet, priming and training (aerobic and isotonic) beneficially affect the speed of adaptation of oxidative matabolism at the onset of an exercise of moderate intensity. The observed benefits are related to a variable combination of adaptations in O2 delivery and utilization that have been non-invasively evaluated in our studies.The main metabolic pathway involved during an exercise of duration greater than 1 minute is the oxidative metabolism. The functional evaluation of oxidative metabolism is based on the analysis of two main functional indexes: O2max and O2 kinetics. These indexes are determined by a finite ability to deliver oxygen to the working muscles (central factor) and a limited ability of the muscles to extract oxygen (peripheral factor). The relative contribution of central and peripheral factors to the overall limitation of oxidative metabolism, yet remains controversial. Near Infrared Spectroscopy (NIRS) was recently added to the classical methods of muscle oxidative metabolism functional evaluation. NIRS is a non-invasive technology that continuously monitors changes (relative or absolute) in oxygenated and deoxygenated haemoglobin (HHb). NIRS HHb signal directly depends on the ratio between the muscular O2 utilization rate and the capillary O2 delivery in the region explored by the probe, providing a non-invasive estimate of the changes in O2 extraction occurring inside the muscles. The general aim of the thesis is to elucidate the relative contribution of central and peripheral factors in limiting oxidative metabolism by the application of non invasive techniques. We have proposed to modulate oxidative metabolism with different manipulations: 1) High fat diet; 2) Heavy intensity warm up and 3) two kinds of training: Aerobic and Isotonic. In the study #1 we used a high fat diet (HFdiet) to manipulate the peripheral factor in healthy young moderate trained males. Animal and human studies suggest that fat adaptation induces structural and functional muscle adaptations that may benefit oxidative metabolism. We tested the hypothesis that a long term HFdiet enhances oxidative metabolism by augmenting the muscle’s capacity to extract oxygen. 22 young healthy moderately trained males (28±5 yrs, 53±6 ml*Kg-1*min-1) were randomly assigned to: Hdiet (HFD, 55% of calories from fat, 30% carbohydrate and 15% proteins) or control diet (C, 30, 55 and 15%) for 10 days. Before and after the diet the subjects performed an incremental cycling test to exhaustion and 3 step transitions at moderate intensity. Respiratory variables and heart rate (HR) were measured bbb. The maximal and submaximal response to exercise were evaluated ( O2, respiratory exchange ratio, R) and the kinetics of pulmonary O2 were fitted by a double exponential model. HF diet was associated with a shift in substrate selection towards a higher contribution of fat to the production of energy in the moderate intensity domain of exercise (reduced R). Furthermore, HFD increased the speed of adaptation of pulmonary O2 kinetics by the reduction of the time constant of the primary component. Our findings support the hypothesis that, in healthy young males, a 10-day high fat diet may increase the speed of adaptation of oxidative metabolism at the onset of a moderate intensity exercise by increasing the relative contribution of fats oxidation to ATP production. In the study #2 we applied heavy intensity warm-up (HWu) to manipulate O2 delivery in sedentary healthy older adults. HWu increases VO2 kinetics during successive moderate intensity transitions. We tested the hypothesis that such improvement is due to a better matching of O2 delivery to utilization within the working muscles. We tested the hypothesis that HWu improvement is due to a better matching of O2 delivery to utilization within the working muscles, rather than to an increase in O2 bulk delivery. In 21 healthy older adults (65.7 5 yrs) we measured contemporarily and non-invasively indexes of the overall speed of adaptation of the oxidative metabolism ( i.e. pulmonary O2 kinetic), of the bulk O2 delivery (i.e. ) and of the rate of muscle deoxygenation (i.e. HHb) during moderate intensity step transitions, either with (Wu) or without (nWu) prior Hwu. The local matching of O2 delivery to utilization was evaluated by the HHb/ O2 ratio index. The innovative findings of this study are: i) HWu does not modify the speed of adaptation of bulk O2 delivery (i.e. ) and TPR; ii) HWu reduces the “overshoot” of the HHb/ O2 ratio, suggesting a better matching of O2 delivery to O2 utilization. Our data are compatible with the hypothesis that, in older adults, HWu, may beneficially affect oxidative metabolism thanks to acute improvement of the local matching of O2 delivery to O2 utilization. The evidences of Study # 2 motivated us to apply a manipulation that modifies O2 delivery in an adaptive way. Thus in the study #3 we applied aerobic training (AT) to manipulate O2 delivery in sedentary healthy older adults. We tested the hypothesis that older adults may benefit from 12 weeks of AT mainly thanks to an adaptive enhancement of O2 delivery to the working muscles. 14 healthy elderly (66 ± 6 yrs) were tested before and after a 12-week training consisting of an AT or a control (remained sedentary). Subjects performed: the same test protocol described in study # 2. Cardio-respiratory variables were measured bbb and muscle oxygen extraction (HHb) was measured, at the vastus lateralis, by quantitative NIRS. We calculated the time delay and the time constant of both the primary component of the pulmonary O2 and of HHb. The main findings of this study wereAT: i) increased O2max and pulmonary O2 kinetics, preceded (Wu) or not (nWu) by HWu; ii) did not modify the speed of adaptation of muscle oxygen extraction (HHb kinetics); iii) abolished the HWu effect in pulmonary O2 kinetics; iv) attenuated HWu effect in muscle oxygen extraction kinetics; v) abolished the peak of the HHb/ O2 ratio. Our data are compatible with the hypothesis that, in older adults, AT may beneficially affect oxidative metabolism thanks to an adaptive improvement in the matching of O2 delivery to the local O2 utilization. The attenuation yet not an abolishment of the HWu effect in HHb kinetics suggested us an underlying limitation in muscle O2 extraction, unaffected by AT. Isotonic Training is known to increase strength and muscle mass in older adults. We tested the hypothesis that older adults may benefits from 12 weeks of isotonic + aerobic training (IT) thanks to an adaptive enhancement in O2 delivery and O2 utilization. 14 healthy elderly (66 ± 6 yrs) were tested before and after a 12-week training consisting of an isotonic training added to aerobic training (IT) or a control condition (remained sedentary). Subjects performed the test protocol described in study #2 and were measured the variables and analyzed data as described for study # 4 The main finding of this study was that IT produced at the lungs an attenuation of HWu effect (i.e.TD remained shorter). These results suggested that older adults can be beneficially affected by IT, however it is not enough to produce the necessary variations in muscle O2 delivery to completely avoid HWu effect. Furthermore at muscle level the variations IT supressed the HWu effect. Thus in agreement with our hypothesis isotonic training added to aerobic training was enough to eliminate completely the effect of priming exercise. Regarding nWu exercise, the effect of IT was a reduction in the HHb/ O2 ratio peak, suggestive of a better matching in O2 delivery to the O2 utilization following training. Our data are compatible with the hypothesis that in older adults oxidative metabolism may be beneficially affected by IT thanks to an adaptive improvement of the local O2 delivery and a further adaptations in the working muscle to O2 utilization. In summary, eucaloric high fat diet, priming and training (aerobic and isotonic) beneficially affect the speed of adaptation of oxidative matabolism at the onset of an exercise of moderate intensity. The observed benefits are related to a variable combination of adaptations in O2 delivery and utilization that have been non-invasively evaluated in our studies