Fuel Choices by Human Platelets in Human Plasma

Abstract

Despite the fact that homogeneous preparations of isolated cells are now being used very effectively to study a range of important biochemical questions, it is still not known what combination of fuels and energy-producing pathways is used by cells when offered the complex mixture characteristic of plasma or extracellular fluid. We have developed an in vitro system whereby highly purified and functional human platelets are incubated in human plasma that has been minimally modified from its native state. The concentration of platelets and fuels, and the complexity of fuels in the incubation are similar to those in vivo. The preparation thus represents a reasonable approximation of the physiological condition, considering the complex nature of the system being studied. Measurements carried out simultaneously during the incubation are rates of oxygen consumption, lactate production and fuel oxidation. The data allow the calculation of total ATP turnover, and contributions to this turnover by lactate production and the oxidation of individual fuels. Lactate production accounts for 24% of the ATP turnover. The oxidation of glucose and 3-hydroxybutyrate each account for under S%, palmitate for 21 %, oleate for 7% and acetate for 9%, leaving 32% of the ATP turnover as yet unaccounted for. The results confirm some previous measurements in the literature, but show that data collected under non-physiological experimental conditions can be misleading.Keywords: platelet; oxygen consumption ; plasma; glycolysis ; oxidation.The preparations used for the study of cellular metabolism have progressed over the last 40 years from tissue homogenates and slices to homogeneous preparations of isolated cells. The latter systems are now being used very effectively to study a range of important biochemical questions. However, there is still a question that has never really been adequately addressed, which could have important implications for the relevance of these preparations, and which is fundamental to our understanding of cell biology. This question concerns the combination of fuels and energy-producing pathways used by a cell when it is offered the complex mixture characteristic of plasma or extracellular fluid.Where energy budgets have been presented previously, it is often the case that total ATP turnover has been underestimated due to the omission of any measurement of oxygen consumption. This approach results in overestimates of the contributions of particular fuels and pathways to total ATP turnover as the 'total ATP turnover' figure is too low. For example Petch and Butler (1994) and Neermann and Wagner (1996) conclude that the various pathways of glucose and glutamine utilization account for 100% of ATP turnover in various cell lines in the presence of glucose, glutamine and fetal calf serum. Oxygen consumption was not measured and would almost certainly demonstrate the oxidation of other fuels. Similarly it is now obvious that aerobic glycolysis contributes significantly (20 -50%) to ATP production under aerobic conditions...

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