Song Y, Ayre E A, Pang S F
Department of Physiology, Faculty of Medicine, University of Hong Kong.
Biol Signals. 1993 Jul-Aug;2(4):207-20.
[125I]Iodomelatonin binding sites have been identified and characterized in kidneys of birds and mammals. These binding sites in the kidneys of guinea pig, duck and chicken were found to be stable, reversible, saturable, specific and of high affinity. The binding densities (Bmax) of [125I]iodomelatonin binding sites in the kidneys of guinea pig, duck and chicken ranged from 1.07 to 6.43 fmol/mg protein and the equilibrium dissociation constants (Kd) from 19.2 to 44.6 pmol/l at the middle of the light period (mid-light). It appears that [125I]iodomelatonin binding in the kidneys of mammalian species may have lower densities compared with birds. In the mammals studied, the guinea pig kidney showed the highest [125I]iodomelatonin binding. Pharmacological data indicated that the [125I]iodomelatonin binding to kidneys of guinea pig, duck and chicken was highly specific to melatonin, 2-iodomelatonin and 6-chloromelatonin. Diurnal variations in the Bmax of [125I]iodomelatonin binding sites were detected in the kidneys of duck and chicken with no difference in affinity. However, there was no diurnal variation in the Kd or Bmax in the guinea pig kidneys. The density of [125I]iodomelatonin binding sites in the cortex of guinea pig kidney was more than 8-fold higher than the binding in the medulla. Guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S; 10 mumol/l) reduced the Bmax and increased the Kd of [125I]iodomelatonin binding sites in the chicken kidney. However, in the membrane preparations of the guinea pig kidney, co-incubation with GTP gamma S (15 mumol/l) increased the Kd with no effect on the Bmax of the [125I]iodomelatonin binding. The effects of GTP gamma S on the kidney [125I]iodomelatonin binding suggest that these binding sites may couple to a G protein. The identification of [125I]iodomelatonin binding sites in the kidneys of mammals and birds supports the possibility of melatonin acting directly on the renal system.
[125I]碘褪黑素结合位点已在鸟类和哺乳动物的肾脏中得到鉴定和表征。在豚鼠、鸭和鸡的肾脏中发现这些结合位点具有稳定性、可逆性、可饱和性、特异性且亲和力高。在光照期中期，豚鼠、鸭和鸡肾脏中[125I]碘褪黑素结合位点的结合密度（Bmax）在1.07至6.43 fmol/mg蛋白质之间，平衡解离常数（Kd）在19.2至44.6 pmol/L之间。与鸟类相比，哺乳动物肾脏中[125I]碘褪黑素的结合密度似乎较低。在所研究的哺乳动物中，豚鼠肾脏显示出最高的[125I]碘褪黑素结合。药理学数据表明，[125I]碘褪黑素与豚鼠、鸭和鸡肾脏的结合对褪黑素、2-碘褪黑素和6-氯褪黑素具有高度特异性。在鸭和鸡的肾脏中检测到[125I]碘褪黑素结合位点Bmax的昼夜变化，而亲和力无差异。然而，豚鼠肾脏中的Kd或Bmax没有昼夜变化。豚鼠肾脏皮质中[125I]碘褪黑素结合位点的密度比髓质中的结合密度高8倍以上。鸟苷5'-O-（3-硫代三磷酸）（GTPγS；10 μmol/L）降低了鸡肾脏中[125I]碘褪黑素结合位点的Bmax并增加了Kd。然而，在豚鼠肾脏的膜制剂中，与GTPγS（15 μmol/L）共同孵育会增加Kd，但对[125I]碘褪黑素结合的Bmax没有影响。GTPγS对肾脏[125I]碘褪黑素结合的影响表明这些结合位点可能与G蛋白偶联。在哺乳动物和鸟类肾脏中鉴定出[125I]碘褪黑素结合位点，支持了褪黑素直接作用于肾脏系统的可能性。
