C0_2 METABOLISM IN HEALTH AND DISEASE

摘要

The oxidation of carbohydrates (CHO) and non-esterified fatty acids (NEFA) provide most energy by which ATP is generated, and are, therefore, the main source of C0_2 in respiring organisms. Oxidation of CHO produces 1 mol of C0_2 per 6 mol of ATP, whereas the oxidation of NEFA yields 1 mol of C0_2 for 8 mol of ATP1.The total amount of C0_2 (TC0_2) in blood is the sum of bicarbonate (HC0_3), the dissolved C0_2, and C0_2 bound to hemoglobin. The majority of C0_2 in all compartments is HC0_3" (> 90%). The contribution of HC0_3" to C0_2 transport requires rapid conversion of C0_2 into HC0_3" and H+. Carbonic anhydrases catalyzes the reversible reaction involving the hydration/dehydration of C0_2 (H_20 + C0_2 <-CA-> H2C0_3 <-> H~+ + HC0_3~- <-> H~+ + C0_3~(2-) and increases the rate of the Jacobs Stewart cycle and chloride exchange (Hamburger shift) several folds to optimize C0_2 elimination by facilitating the transformation of C0_2 to HC0_3" in peripheral tissues and HC0_3" back to molecular C0_2 across the lung. The half time for completion of these reactions is ~1sec. Carbonic anhydrase inhibition slows the hydration/dehydration of CO_2 with consequent hypercapnia (increased TC0__2) and acidosis, which has important implications for muscle metabolism and the development of fatigue.Carbon dioxide metabolism is closely linked to pulmonary transvascular fluid dynamics in horses and possibly other athletic mammalian species. Any alteration in the Jacobs-Stewart cycle may affect transvascular fluid shifts in the pulmonary vasculature and the diffusion distance between the pulmonary alveoli and pulmonary capillaries; a phenomena that can be independent of Starling forces.