Norwich K H, Wasserman D H
Am J Physiol. 1986 Dec;251(6 Pt 1):E715-20. doi: 10.1152/ajpendo.1986.251.6.E715.
Traditional calculations of metabolic fluxes using isotope dilution are based on the assumption that tracer and tracee enter the distribution space through effectively identical ports. If the tracer infusion site is not identical with the site of endogenous release of the tracee, the traditional equations for calculating rate of appearance (Ra) of a metabolite may give rise to appreciable errors due to the presence of gradients in specific activity. When tracer and tracee enter by means of anatomically disparate sites, such as may be encountered in the study of metabolite (e.g., lactate, alanine, and glycerol) or free fatty acid turnover, one must employ a modification of the traditional specific activity. This modified specific activity is obtained as the ratio of tracer concentration "near" (in the same compartment as) the source of tracee to the concentration of tracee near the source of tracer infusion. This concept is employed to derive equations for calculating metabolic turnover in both steady- and non-steady-state conditions when entry sites of tracer and tracee are dissimilar.
示踪剂和被追踪物通过实际上相同的端口进入分布空间。如果示踪剂注入部位与被追踪物内源性释放部位不同，由于比活度存在梯度，用于计算代谢物出现率（Ra）的传统方程可能会产生明显误差。当示踪剂和被追踪物通过解剖学上不同的部位进入时，比如在代谢物（如乳酸、丙氨酸和甘油）或游离脂肪酸周转研究中可能遇到的情况，就必须对传统比活度进行修正。这种修正后的比活度是示踪剂注入源“附近”（与被追踪物在同一隔室）的示踪剂浓度与被追踪物注入源附近的被追踪物浓度之比。当示踪剂和被追踪物的进入部位不同时，这一概念可用于推导稳态和非稳态条件下计算代谢周转的方程。
