Liang C T, Barnes J, Sacktor B, Takamoto S
Laboratory of Biological Chemistry, National Institute on Aging, National Institutes of Health, Baltimore, Maryland 21224.
Endocrinology. 1991 Apr;128(4):1780-4. doi: 10.1210/endo-128-4-1780.
Previously we reported that uptake of Ca2+ in cells isolated from rat duodenum declined in senescence. In this paper we examined the possible mechanisms for this age-related defect. Duodenal vitamin D-dependent calcium-binding protein decreased steadily from 3-12 months (mo), followed by a minimal decline at 24 mo. On the contrary, Ca2+ uptake was not different in 3-, 6-, and 12-mo-old rats. A significant decline of Ca2+ uptake was observed at 24 mo. ATP contents in duodenal cells from 6- and 24-mo-old rats were not different. This suggests that the metabolic status of the duodenal cells was not the cause of the change in Ca2+ uptake. Ca2+ uptake activity was significantly lower in brush border membrane vesicles isolated from 24-mo-old rats than in those from 6-mo-old rats. The decrease in Ca2+ uptake activity in old rats was not due to a change in the Ca2(+)-binding capacity of the membranes. Kinetic analysis shows that the Vmax, the apparent maximum uptake capacity of membrane vesicles, decreased in senescent rats, whereas the Km, the apparent affinity to Ca2+, was unchanged. Since duodenal Ca2+ influx at the brush border was regulated by 1,25-dihydroxy-vitamin D3 [1,25-(OH)2D3], we tested the effect of 1,25-(OH)2D3 administration on the uptake activity in isolated membrane vesicles. After 1,25-(OH)2D3 treatment, Ca2+ uptake activity in brush border membranes prepared from senescent rats was only slightly lower than that in membranes from adult rats. We conclude that the decline in the influx of Ca2+ at the brush border membrane was the main cause of the decrease in duodenal Ca2+ uptake activity in aging. This defect was probably due to the low serum 1,25-(OH)2D3 concentration and not the result of impaired response to 1,25-(OH)2D3.
此前我们报道，从大鼠十二指肠分离的细胞中Ca2+摄取在衰老过程中下降。在本文中，我们研究了这种与年龄相关缺陷的可能机制。十二指肠维生素D依赖性钙结合蛋白在3至12个月（mo）时稳步下降，随后在24 mo时轻微下降。相反，3、6和12月龄大鼠的Ca2+摄取没有差异。在24 mo时观察到Ca2+摄取显著下降。6和24月龄大鼠十二指肠细胞中的ATP含量没有差异。这表明十二指肠细胞的代谢状态不是Ca2+摄取变化的原因。从24月龄大鼠分离的刷状缘膜囊泡中的Ca2+摄取活性显著低于6月龄大鼠的。老年大鼠Ca2+摄取活性的降低不是由于膜的Ca2(+)-结合能力的变化。动力学分析表明，膜囊泡的表观最大摄取能力Vmax在衰老大鼠中下降，而对Ca2+的表观亲和力Km不变。由于十二指肠刷状缘的Ca2+内流受1,25-二羟基维生素D3 [1,25-(OH)2D3]调节，我们测试了给予1,25-(OH)2D3对分离的膜囊泡摄取活性的影响。1,25-(OH)2D3处理后，衰老大鼠制备的刷状缘膜中的Ca2+摄取活性仅略低于成年大鼠膜中的。我们得出结论，刷状缘膜处Ca2+内流的下降是衰老过程中十二指肠Ca2+摄取活性降低的主要原因。这种缺陷可能是由于血清1,25-(OH)2D3浓度低，而不是对1,25-(OH)2D3反应受损的结果。
