Arrang J M, Roy J, Morgat J L, Schunack W, Schwartz J C
Unité de Neurobiologie et Pharmacologie (U. 109) de l'INSERM, Centre Paul Broca, Paris, France.
Eur J Pharmacol. 1990 Apr 25;188(4-5):219-27. doi: 10.1016/0922-4106(90)90005-i.
The binding of 3Halpha-methylhistamine, a potent and specific agonist at histamine H3 receptors, was investigated with membranes of rat cerebral cortex. In phosphate buffer the specific binding defined with thioperamide, an H3 receptor antagonist, displayed characters of reversibility and saturability with a Bmax of approximately 30 fmol/mg protein. The KD, derived from either dissociation/association rates or saturation kinetics at equilibrium, was approximately 0.5 nM at 25 degrees C. Competition studies indicated that the binding occurred with a stereoselectivity and pharmacological specificity similar to that of functional H3 autoreceptors regulating histamine release in brain slices. However, whereas the potency of antagonists was closely similar in the two assay systems, that of agonists was approximately 10-fold higher in the binding assay. Among antagonists burimamide was the only one to compete with a pseudo-Hill coefficient significantly different from unity (nH = 0.48 +/- 0.03), indicating a possible heterogeneity among binding sites. The presence of 2.6 mM Ca2+, in a modified Krebs-Ringer medium, promoted the conversion of a larger fraction of sites into a low-affinity component with a KD of 16 nM. The presence of guanylnucleotides in the Krebs-Ringer medium with Ca2+ abolished the binding to this low-affinity component whereas in a phosphate buffer only the KD was slightly increased. It is concluded that the H3 receptor, like many other amine receptors, is coupled to its still unidentified effector system via a G-protein and regulated by Ca2+. However, unlike the latter, the H3 receptor is down-regulated by the divalent cation.
用大鼠大脑皮层膜研究了组胺H3受体强效特异性激动剂3Hα-甲基组胺的结合情况。在磷酸盐缓冲液中，用H3受体拮抗剂硫代酰胺定义的特异性结合表现出可逆性和饱和性，Bmax约为30 fmol/mg蛋白质。在25℃下，由解离/缔合速率或平衡时的饱和动力学得出的KD约为0.5 nM。竞争研究表明，这种结合具有与调节脑片组胺释放的功能性H3自身受体相似的立体选择性和药理特异性。然而，虽然拮抗剂在两种测定系统中的效力非常相似，但激动剂在结合测定中的效力大约高10倍。在拮抗剂中，布立马胺是唯一一种竞争伪希尔系数与1显著不同（nH = 0.48 ± 0.03）的拮抗剂，表明结合位点可能存在异质性。在改良的 Krebs-Ringer 培养基中存在2.6 mM Ca2+会促使更大比例的位点转化为KD为16 nM的低亲和力成分。在含有Ca2+的Krebs-Ringer培养基中存在鸟苷酸会消除与这种低亲和力成分的结合，而在磷酸盐缓冲液中，只有KD略有增加。结论是，H3受体与许多其他胺受体一样，通过G蛋白与其尚未确定的效应系统偶联并受Ca2+调节。然而，与后者不同的是，H3受体受二价阳离子下调。
