Lewis K C, Green M H, Green J B, Zech L A
Nutrition Department, Pennsylvania State University, University Park 16802.
J Lipid Res. 1990 Sep;31(9):1535-48.
A compartmental model was developed to describe the metabolism of vitamin A in rats with low vitamin A status maintained by a low dietary intake of vitamin A (approximately 2 micrograms retinol equivalents/day). After the IV bolus injection of [3H]retinol in its physiological transport complex, tracer and trace data were obtained from plasma, organs (liver, kidneys, small intestine, eyes, adrenals, testes, lungs, carcass), and tracer data were obtained from urine and feces. The dietary protocol developed for this study resulted in animals having plasma vitamin A levels less than 10 micrograms retinol/dl and total liver vitamin A levels of approximately 1 microgram retinol equivalent. Four compartments were used to model the plasma: one to describe retinol, one to describe the nonphysiological portion of the dose, and two to simulate polar metabolites derived from retinol. The liver required two compartments and a delay, the carcass (small intestine, eyes, adrenals, testes, and lungs, plus remaining carcass) required three compartments, and the kidneys required two. The model predicted a vitamin A utilization rate of 1.65 micrograms retinol equivalents/day with the urine and feces accounting for most of the output. The plasma retinol turnover rate was approximately 20 micrograms retinol equivalents/day; this was 12 times greater than the utilization rate. This indicated that, of the large amount of retinol moving through the plasma each day, less than 10% of this was actually being irreversibly utilized. Similarly, as compared to the whole-body utilization rate, there was a relatively high turnover rate of retinol in the kidneys, carcass, and liver (9.0, 8.2, and 5.8 micrograms retinol equivalents/day, respectively), coupled with a high degree of recycling of vitamin A through these tissues. Of the total vitamin A that entered the liver from all sources including the diet, approximately 86% was mobilized into the plasma. Similarly, of the vitamin A that entered the carcass, approximately 76% was returned to the plasma. All of the retinol that entered the kidneys was modeled as recycling to the plasma. The present studies provide quantitative and descriptive evidence of an efficient metabolism of vitamin A from absorption through turnover and utilization in rats with very low vitamin A status. Furthermore, although their body stores of vitamin A were extremely low, these rats maintained a high level of recycling of vitamin A throughout the body.
建立了一个房室模型，用于描述低维生素A状态大鼠（通过低维生素A饮食摄入维持，约2微克视黄醇当量/天）体内维生素A的代谢情况。以生理转运复合物形式静脉推注[3H]视黄醇后，从血浆、器官（肝脏、肾脏、小肠、眼睛、肾上腺、睾丸、肺、躯体）获取示踪剂和痕量数据，并从尿液和粪便中获取示踪剂数据。本研究制定的饮食方案使动物的血浆维生素A水平低于10微克视黄醇/分升，肝脏总维生素A水平约为1微克视黄醇当量。用四个房室对血浆进行建模：一个用于描述视黄醇，一个用于描述剂量中的非生理部分，两个用于模拟视黄醇衍生的极性代谢物。肝脏需要两个房室和一个延迟，躯体（小肠、眼睛、肾上腺、睾丸、肺以及剩余躯体）需要三个房室，肾脏需要两个房室。该模型预测维生素A利用率为1.65微克视黄醇当量/天，尿液和粪便占大部分输出量。血浆视黄醇周转率约为20微克视黄醇当量/天；这比利用率高12倍。这表明，每天流经血浆的大量视黄醇中，实际被不可逆利用的不到10%。同样，与全身利用率相比，肾脏、躯体和肝脏中视黄醇的周转率相对较高（分别为9.0、8.2和5.8微克视黄醇当量/天），同时维生素A在这些组织中的循环程度也很高。从包括饮食在内的所有来源进入肝脏的总维生素A中，约86%被转运到血浆中。同样，进入躯体的维生素A中，约76%返回血浆。进入肾脏的所有视黄醇都被建模为循环到血浆中。本研究提供了定量和描述性证据，证明维生素A在维生素A状态极低的大鼠体内从吸收到周转和利用的高效代谢。此外，尽管这些大鼠的维生素A体内储存量极低，但它们在全身维持了高水平的维生素A循环。
