Sakamoto C, Matozaki T, Nagao M, Nishisaki H, Baba S
Second Department of Internal Medicine, Kobe University School of Medicine, Japan.
Nihon Naibunpi Gakkai Zasshi. 1987 Aug 20;63(8):978-86. doi: 10.1507/endocrine1927.63.8_978.
To investigate whether somatostatin receptors couple to guanine nucleotide inhibitory protein, Ni, on rat pancreatic acinar membranes, the effects of guanine nucleotide analogs or pretreatment of acini with islet activating protein (IAP), pertussis toxin on labeled somatostatin binding were examined. Guanine nucleotides reduced labeled somatostatin binding to acinar membranes up to 80%, with rank order of potency being guanyl-5'-yl imidodiphosphate (Gpp(NH)p) greater than GTP greater than GDP greater than GMP. Scatchard analysis of the labeled somatostatin binding revealed that the decrease in somatostatin binding caused by Gpp(NH)p was due to the decrease in the maximum binding capacity without a significant change in the binding affinity. The inhibitory effect of Gpp(NH)p was partially abolished in the absence of Mg2+ and Na+ also reduced labeled somatostatin binding. Furthermore, inhibitory effects of 100mM Na+ and Gpp(NH)p were additive in reducing labeled somatostatin binding. A half maximal inhibitory concentration of Gpp(NH)p was decreased to 10(-7)M in the presence of 100mM Na+ and 5mM Mg2+ as compared to 10(-6)M in the presence of 5mM Mg2+ alone. Results therefore suggest that Gpp(NH)p requires Mg2+ for Ni activation and Na+ increases sensitivity of Ni to guanine nucleotide analogs. When pancreatic acini were treated for 4 hours with varying concentrations of IAP, which has been shown to uncouple Ni-mediated communication between inhibitory receptors and adenylate cyclase catalytic unit, subsequent labeled somatostatin binding to the acinar membranes was decreased in a dose dependent manner. These results indicate that somatostatin receptors on pancreatic acinar membranes couple to guanine nucleotide inhibitory protein, Ni and thus somatostatin probably functions in the pancreas to regulate intracellular signal transduction via Ni.
为了研究生长抑素受体是否与大鼠胰腺腺泡细胞膜上的鸟嘌呤核苷酸抑制蛋白Ni偶联，我们检测了鸟嘌呤核苷酸类似物或用胰岛激活蛋白（IAP）、百日咳毒素预处理腺泡对标记生长抑素结合的影响。鸟嘌呤核苷酸使标记生长抑素与腺泡细胞膜的结合减少达80%，其效力顺序为鸟苷-5'-亚基亚氨二磷酸（Gpp(NH)p）＞GTP＞GDP＞GMP。对标记生长抑素结合的Scatchard分析表明，Gpp(NH)p引起的生长抑素结合减少是由于最大结合容量的降低，而结合亲和力无显著变化。在无Mg2+时，Gpp(NH)p的抑制作用部分被消除，Na+也降低了标记生长抑素的结合。此外，100mM Na+和Gpp(NH)p在降低标记生长抑素结合方面具有相加作用。与仅存在5mM Mg2+时的10(-6)M相比，在存在100mM Na+和5mM Mg2+时，Gpp(NH)p的半数最大抑制浓度降至10(-7)M。因此结果表明，Gpp(NH)p激活Ni需要Mg2+，而Na+增加了Ni对鸟嘌呤核苷酸类似物的敏感性。当用不同浓度的IAP处理胰腺腺泡4小时时（IAP已被证明可使抑制性受体与腺苷酸环化酶催化单位之间的Ni介导通讯解偶联），随后标记生长抑素与腺泡细胞膜的结合呈剂量依赖性降低。这些结果表明，胰腺腺泡细胞膜上的生长抑素受体与鸟嘌呤核苷酸抑制蛋白Ni偶联，因此生长抑素可能在胰腺中通过Ni调节细胞内信号转导发挥作用。
