DiMattio J
Department of Physiology and Biophysics, New York University School of Medicine, New York.
Invest Ophthalmol Vis Sci. 1989 Nov;30(11):2320-31.
The transport rates of radiolabeled ascorbic acid and dehydroascorbic acid, as well as, labeled 3-O-methyl-D-glucose and L-glucose from a central plasma compartment into aqueous and vitreous humors and cerebrospinal fluid were studied in vivo. Normal, male albino Sprague-Dawley rats and English Short Haired guinea pigs were used to explore the mechanism of ascorbic acid entry into ocular humors in a species that can produce ascorbate (the rat) and one that cannot and, like humans, is dependent on dietary sources (the guinea pig). In vivo kinetic studies allowed for the calculation of entry rate constants, Ki (min-1), in double-labeled experiments using L-glucose as an internal passive control. Parallel TLC chromatographic studies were performed to monitor intraocular labeled molecules deriving from the plasma-introduced test molecule. In addition, resting levels of ascorbic acid and D-glucose were determined in order to obtain more reliable data than previously available. Resting levels of D-glucose revealed a consistent pattern of lower levels in aqueous and vitreous humors and CSF than found in plasma for both rat and guinea pig. However, ascorbate levels differed significantly, with the guinea pig demonstrating high ascorbate levels in the aforementioned humors: 58, 77 and 22, respectively, times the circulating plasma value of 0.2 +/- 0.2 mg/dl. In contrast, the rat, like the guinea pig, had low plasma ascorbate levels (3.3 +/- 0.8 mg/dl) compared to glucose (162 +/- 8 mg/dl), with even lower aqueous and vitreous values in a pattern similar to that of D-glucose. In vivo aqueous, vitreous and CSF transport results from the guinea pig indicate active transport mechanisms for ascorbic acid that prefer the ascorbate over the dehydroascorbate moiety and are probably different from the carrier-facilitated diffusion mechanisms for D-glucose, which do not move molecules against a concentration gradient. TLC studies, performed under nitrogen, revealed that only (14C)-ascorbic acid was present in aqueous or vitreous humors regardless of whether the radiolabeled pulse was of ascorbic or dehydroascorbic acid. The rat demonstrated little or no carrier involvement, with ascorbic acid crossing into ocular humors at rates very close to those of L-glucose, which is similar in size and is considered to cross the barriers studied via passive diffusion. Saturation studies with unlabeled glucose and glucose inhibitor drugs phloretin (10(-3) M) and phloridzin (10(-1)) had no apparent effect on ocular entry rates. Dehydroascorbic acid movement was also found to be passive.(ABSTRACT TRUNCATED AT 400 WORDS)
在体内研究了放射性标记的抗坏血酸和脱氢抗坏血酸，以及标记的3 - O - 甲基 - D - 葡萄糖和L - 葡萄糖从中央血浆隔室进入房水、玻璃体液和脑脊液的转运速率。使用正常的雄性白化Sprague - Dawley大鼠和英国短毛豚鼠来探究抗坏血酸进入眼内液的机制，其中大鼠能够产生抗坏血酸盐，而豚鼠不能产生，且像人类一样依赖饮食来源。体内动力学研究通过使用L - 葡萄糖作为内部被动对照，在双标记实验中计算进入速率常数Ki（分钟⁻¹）。进行了平行的薄层层析色谱研究，以监测源自血浆引入的测试分子的眼内标记分子。此外，测定了抗坏血酸和D - 葡萄糖的静息水平，以便获得比以前更可靠的数据。D - 葡萄糖的静息水平显示，大鼠和豚鼠的房水、玻璃体液和脑脊液中的水平始终低于血浆中的水平。然而，抗坏血酸盐水平存在显著差异，豚鼠在上述眼内液中显示出高抗坏血酸盐水平：分别是循环血浆值0.2±0.2mg/dl的58、77和22倍。相比之下，大鼠与豚鼠一样，血浆抗坏血酸盐水平（3.3±0.8mg/dl）低于葡萄糖（162±8mg/dl），房水和玻璃体液中的值甚至更低，其模式与D - 葡萄糖相似。豚鼠体内房水、玻璃体液和脑脊液的转运结果表明，抗坏血酸的主动转运机制更倾向于抗坏血酸盐而非脱氢抗坏血酸部分，并且可能与D - 葡萄糖的载体介导扩散机制不同，后者不会逆浓度梯度转运分子。在氮气下进行的薄层层析研究表明，无论放射性标记脉冲是抗坏血酸还是脱氢抗坏血酸，房水或玻璃体液中仅存在（¹⁴C） - 抗坏血酸。大鼠几乎没有或没有载体参与，抗坏血酸进入眼内液的速率与L - 葡萄糖非常接近，L - 葡萄糖大小相似，被认为通过被动扩散穿过所研究的屏障。用未标记的葡萄糖和葡萄糖抑制剂药物根皮素（10⁻³M）和根皮苷（10⁻¹）进行的饱和研究对眼内进入速率没有明显影响。还发现脱氢抗坏血酸的转运是被动的。（摘要截短于400字）
