Horwitz E M, Gurd R S
Medical Sciences Program, Indiana University, Bloomington 47406.
Arch Biochem Biophys. 1988 Dec;267(2):758-69. doi: 10.1016/0003-9861(88)90085-9.
Biochemical methods have been used to quantitate total, acid-stable and acid-labile association of (mono[125I]iodoTyr10) glucagon with rat hepatocytes in suspension to evaluate internalization of glucagon and its receptors. Internalization is inhibited by low temperature, phenylarsine oxide, and by blocking receptor binding, consistent with receptor-mediated endocytosis. Approximately 30% of the total cell-associated hormone is internalized at 30 min of incubation. The rate declines until 90 min when the internalization of glucagon ceases, although the cells remain competent to internalize asialofetuin. From 90 min to 4 h, 27% of the maximum label internalized at 30 min remains within cells. The number of cell surface receptors decreases but the affinity of those remaining is unchanged. However, 1.7-2.7 surface receptors are lost to binding for each molecule of radiolabeled glucagon internalized. Uptake occurs according to a rate constant of 0.183 min-1 (t1/2 = 3.8 min). We conclude that (i) hepatocytes internalize a finite quantity of glucagon, implying the existence of undefined regulatory mechanisms; (ii) hormone is retained for greater than 2 h within cells and may play a physiological role within cells; and (iii) both occupied and unoccupied receptors become inaccessible to extracellular hormone as internalization proceeds; rapid recycling of receptors does not occur.
已采用生化方法对悬浮培养的大鼠肝细胞中（单[125I]碘酪氨酰10）胰高血糖素的总结合量、酸稳定结合量和酸不稳定结合量进行定量，以评估胰高血糖素及其受体的内化情况。低温、苯胂酸氧化物以及受体结合阻断剂均可抑制内化过程，这与受体介导的内吞作用一致。在孵育30分钟时，约30%与细胞结合的总激素被内化。内化速率持续下降，直至90分钟时胰高血糖素的内化停止，不过此时细胞仍有能力内化去唾液酸胎球蛋白。从90分钟到4小时，在30分钟时内化的最大标记量的27%仍保留在细胞内。细胞表面受体数量减少，但剩余受体的亲和力不变。然而，每内化一分子放射性标记的胰高血糖素，就会有1.7 - 2.7个表面受体失去结合能力。摄取过程的速率常数为0.183分钟-1（半衰期=3.8分钟）。我们得出以下结论：（i）肝细胞内化有限量的胰高血糖素，这意味着存在尚未明确的调节机制；（ii）激素在细胞内保留超过2小时，可能在细胞内发挥生理作用；（iii）随着内化过程的进行，细胞外激素无法再与已被占据和未被占据的受体结合；受体不会快速循环利用。
