Extraordinarily Potent Insulinotropin PACAP : Its Characterization as a Pancreatic Peptide and Action Mechanisms in Islet beta-Cells

胰岛的胰岛素分泌受多肽和营养物质的控制。两种低至10^<-13> M的垂体腺苷酸环化酶激活多肽（PACAP27和PACAP38）以葡萄糖依赖的方式刺激胰岛素从大鼠胰岛释放。PACAP还增加了胰岛β细胞胞浆Ca^<2+>浓度（[Ca^<2+>] _i）。l型Ca^<2+>通道阻滞剂可消除[Ca^<2+>] i和胰岛素反应。PACAP刺激胰岛中cAMP的产生，cAMP的升高模仿PACAP增加β细胞中的[Ca^<2+>] _i。血管活性肠肽，一种与PACAP氨基酸高度同源的肽，也增加了β细胞中的[Ca^<2+>] _i，但仅在纳摩尔浓度范围内，表明PACAP27的效力高4倍。高亲和力i型PACAP受体免疫组织化学定位于胰岛。接下来，我们研究了PACAP在胰岛中的来源和生理作用。胰腺神经纤维和胰岛显示pacap免疫反应性。在胰岛和β细胞系MIN6中检测到PACAP mRNA。高葡萄糖对胰岛胰岛素释放的刺激作用被一种特异性的PACAP抗血清减弱。高糖胰岛培养液具有增加β细胞[Ca^<2+>] _i的能力，这种能力被PACAP抗血清中和。结果表明，PACAP与高亲和力的PACAP选择性受体反应，提高β细胞的cAMP水平，从而增强l型Ca^<2+>通道活性，增加[Ca^<2+>] _i，从而增强葡萄糖诱导的胰岛素释放。PACAP是迄今为止已知的最有效的促胰岛素肽。此外，PACAP从胰岛释放并作用于胰岛β细胞，从而作为一种自分泌激素。PACAP通过自分泌作用，增加了葡萄糖诱导的胰岛胰岛素分泌，这在非胰岛素依赖型糖尿病中是典型的受损。

Insulin secretion from pancreatic islets is controlled by peptides as well as by nutrients. Two forms of pituitary adenylate cyclase activating polypeptide (PACAP27 and PACAP38) as low as 10^<-13> M stimulated insulin release from rat islets in a glucosedependent manner. PACAP also increased cytosolic Ca^<2+> concentration ( [Ca^<2+>] _i) in islet beta-cells. A blocker of the L-type Ca^<2+> channel abolished both [Ca^<2+>] _i and insulin responses. PACAP stimulated production of cAMP in islets, and a rise in cAMP mimicked PACAP in increasing [Ca^<2+>] _i in beta-cells. Vasoactive intestinal peptide, a peptide exhibiting high amino acid homology with PACAP,also increased [Ca^<2+>] _i in beta-cells but only at concentrations in the nanomolar range, indicating that PACAP27 is 4 logs more potent. High-affinity type-I PACAP receptor was immunohistochemically localized in islets.We next examined the source and physiological role of PACAP in islets. PACAP-immunoreactivity was demonstrated in pancreatic nerve fibers and islets. PACAP mRNA was detected in islets and in the beta-cell line MIN6. Stimulation of insulin release by high glucose from isolated islets was attenuated by a specific PACAP antiserum. The islet incubation medium with high glucose possessed a capacity, which was neutralized by PACAP antiserum, to increase [Ca^<2+>] _i in beta-cells.The results indicate that PACAP reacts with high affinity PACAP-selective receptor and raises cAMP in beta-cells, which in turn enhances the L-type Ca^<2+> channel activity, increases [Ca^<2+>] _i and consequently potentiates glucose-induced insulin release. PACAP is by far the most potent insulinotropic peptide known. Moreover, PACAP is released from islets and acts on islet beta-cells, thus acting as an autocrine hormone. PACAP,via the autocrine action, amplifies glucose-induced insulin secretion in islets, which is characteristically impaired in non-insulin-dependent diabetes mellitus.