Noda C, Shinjyo F, Tomomura A, Kato S, Nakamura T, Ichihara A
J Biol Chem. 1984 Jun 25;259(12):7747-54.
During treatment of primary cultured hepatocytes with either glucagon or isoproterenol for several hours, the stimulations of cAMP formation by these hormones decreased time dependently. Glucagon treatment also reduced the response to isoproterenol, but isoproterenol treatment did not decrease the response to glucagon. Treatment with isoproterenol caused more rapid desensitization than treatment with glucagon. Assays of glucagon and beta-adrenergic receptors showed that the receptor number of only the hormone with which the cells were treated decreased and that dissociation constants of the receptors did not change. Moreover, in glucagon-desensitized cells, the effect of GTP on competition of the bindings of antagonist and agonist for the beta-adrenergic receptor did not change. After treatment with isoproterenol, stimulation of adenylate cyclase activity by the agonist was decreased without any decrease in the stimulations of activity by other effectors. In contrast, glucagon treatment greatly decreased the stimulations of activity by glucagon, isoproterenol, and guanyl-5'-yl imidodiphosphate and slightly decreased that by fluoride. However, after glucagon treatment, the cells showed normal responses to cholera toxin of activation of adenylate cyclase and ADP-ribosylation of guanine nucleotide binding regulatory protein (Ns). The maximal response of glucagon-treated cells to forskolin was about two-thirds that of untreated cells and this treatment also impaired the shift toward a low Kact value for forskolin observed in the presence of either glucagon or isoproterenol. These results indicate that isoproterenol caused homologous desensitization consisting of only a "down regulation" of the beta-adrenergic receptor, whereas glucagon caused heterologous desensitization, mainly by alteration of the Ns component, as well as "down regulation" of glucagon receptor. This altered Ns seems to be coupled normally to the beta-adrenergic receptor, but to have impaired coupling to the catalytic component of adenylate cyclase.
在用胰高血糖素或异丙肾上腺素处理原代培养的肝细胞数小时期间，这些激素对环磷酸腺苷（cAMP）生成的刺激作用呈时间依赖性降低。胰高血糖素处理也降低了细胞对异丙肾上腺素的反应，但异丙肾上腺素处理并未降低细胞对胰高血糖素的反应。与胰高血糖素处理相比，异丙肾上腺素处理导致脱敏更快。对胰高血糖素和β - 肾上腺素能受体的测定表明，仅细胞所处理的那种激素的受体数量减少，而受体的解离常数未改变。此外，在胰高血糖素脱敏的细胞中，鸟苷三磷酸（GTP）对拮抗剂和激动剂与β - 肾上腺素能受体结合竞争的影响未改变。用异丙肾上腺素处理后，激动剂对腺苷酸环化酶活性的刺激作用降低，而其他效应物对活性的刺激作用未降低。相反，胰高血糖素处理大大降低了胰高血糖素、异丙肾上腺素和鸟苷 - 5'- 基亚氨基二磷酸对活性的刺激作用，对氟化物的刺激作用略有降低。然而，胰高血糖素处理后，细胞对霍乱毒素激活腺苷酸环化酶和鸟嘌呤核苷酸结合调节蛋白（Ns）的ADP - 核糖基化表现出正常反应。胰高血糖素处理的细胞对福斯高林的最大反应约为未处理细胞的三分之二，并且这种处理也削弱了在存在胰高血糖素或异丙肾上腺素时观察到的福斯高林向低激活常数（Kact）值的转变。这些结果表明，异丙肾上腺素引起同源脱敏，仅由β - 肾上腺素能受体 的“下调”组成，而胰高血糖素引起异源脱敏，主要通过Ns成分的改变以及胰高血糖素受体的“下调”。这种改变的Ns似乎与β - 肾上腺素能受体正常偶联，但与腺苷酸环化酶的催化成分的偶联受损。
