Rosenblatt J, Wolfe R R
Metabolism Unit, Shriners Burns Institute, Galveston, Texas.
Am J Physiol. 1988 Apr;254(4 Pt 1):E526-31. doi: 10.1152/ajpendo.1988.254.4.E526.
The use of stable isotope tracers to calculate substrate kinetics in humans is favored over the use of radioactive isotopes because of their greater safety and versatility. However, potential complications not met when dealing with radioactive tracers are caused by 1) the natural occurrence of the stable isotope used as a tracer and 2) the necessity to administer the tracer in an amount that cannot be treated as "massless." We therefore found it desirable to derive a theoretically valid equation for calculating the rate of appearance, Ra, of a substrate under steady-state conditions using a stable isotope tracer. This theoretically valid equation yields results that differ from those of the equations conventionally used to calculate (endogenous) Ra in steady state. Quantitative determination of the error in one of these equations revealed that for tracers commonly used in metabolic studies the error is negligible, whereas the error made in the other equation is likely to be quite high in commonly encountered situations. Finally, to allow for proper use of different definitions of isotopic enrichment that have arisen from practical considerations, we use the results derived above to determine valid equations for Ra appropriate to the two prevalent definitions.
由于稳定性同位素示踪剂具有更高的安全性和通用性，因此在人体中使用稳定性同位素示踪剂来计算底物动力学比使用放射性同位素更受青睐。然而，在处理放射性示踪剂时未遇到的潜在并发症是由以下原因引起的：1）用作示踪剂的稳定性同位素的天然存在；2）必须以不能被视为“无质量”的量施用示踪剂。因此，我们发现需要推导一个理论上有效的方程，用于使用稳定性同位素示踪剂在稳态条件下计算底物的出现率（Ra）。这个理论上有效的方程得出的结果与传统上用于计算稳态下（内源性）Ra的方程的结果不同。对其中一个方程中的误差进行定量测定表明，对于代谢研究中常用的示踪剂，误差可以忽略不计，而在另一个方程中产生的误差在常见情况下可能相当高。最后，为了能够正确使用出于实际考虑而产生的不同同位素富集定义，我们使用上述结果来确定适用于两种普遍定义的Ra的有效方程。
