Lehman S L, Stanley W C
Department of Physical Education, University of California, Berkeley 94720.
Am J Physiol. 1988 Jul;255(1 Pt 1):E94-8. doi: 10.1152/ajpendo.1988.255.1.E94.
When a substrate appears in and disappears from an unmeasured (tissue) compartment, the proper sites for tracer infusion and sampling to measure tracee turnover become controversial. We analyze a three-compartment model representing arterial blood, tissue, and venous blood. The desired quantity, tracee turnover, is the ratio of the steady-state infusion rate to tissue specific activity. However, specific activity in the tissue compartment is unknown. We assume infusion of tracer into the arterial pool at a constant rate and consider sampling of specific activity of either blood compartment in the steady state. We obtain estimates of tissue specific activity from measurement of concentrations of tracer and tracee in blood samples in two extreme cases. In case I, tracee is assumed to appear in the venous compartment but to disappear from the tissue pool. Then tissue specific activity is equal to arterial specific activity. In case II, both appearance and disappearance are from the tissue pool. Tissue specific activity is then less than arterial or venous specific activity. We give formulas for the difference in each case. We discuss the relationship of our models to actual tracer experiments and define physiological locations for our three compartments. Appearance of substrates is probably intermediate between our extreme cases. A numerical estimate of turnover for the substrate lactate in resting humans reveals an error bound of approximately 30%. We discuss sites of infusion and sampling consistent with our model, the effects of relaxing some of our modeling constraints, and experimental necessities for getting beyond the steady state.
当一种底物在一个未测量的（组织）隔室中出现和消失时，用于测量示踪剂周转的示踪剂注入和采样的合适位点就会引发争议。我们分析了一个代表动脉血、组织和静脉血的三室模型。所需的量，即被示踪物周转，是稳态注入速率与组织比活度的比值。然而，组织隔室中的比活度是未知的。我们假设以恒定速率将示踪剂注入动脉池，并考虑在稳态下对任一血隔室的比活度进行采样。我们在两种极端情况下，通过测量血样中示踪剂和被示踪物的浓度来获得组织比活度的估计值。在情况I中，假设被示踪物出现在静脉隔室但从组织池中消失。那么组织比活度就等于动脉比活度。在情况II中，出现和消失都发生在组织池中。此时组织比活度小于动脉或静脉比活度。我们给出了每种情况下差异的公式。我们讨论了我们的模型与实际示踪剂实验的关系，并定义了我们三个隔室的生理位置。底物的出现可能介于我们的极端情况之间。对静息人体中底物乳酸周转的数值估计显示误差范围约为30%。我们讨论了与我们的模型一致的注入和采样位点、放宽我们一些建模约束的影响以及超越稳态的实验必要性。
