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

胰岛胰岛素的分泌既受肽的控制，又受营养物质的控制。两种形式的垂体腺苷酸环化酶激活多肽(PACAP27和PACAP38)以糖依赖的方式刺激大鼠胰岛胰岛素的释放。PACAP还可增加胰岛β细胞内钙离子浓度([Ca^&lt；2&gt；]i)。L型钙通道阻断剂可阻断[Ca^&lt；2；]_i和胰岛素反应。PACAP刺激胰岛cAMP的生成，cAMP的升高与PACAP类似，使胰岛β细胞[Ca~(2+)]_i升高。血管活性肠肽是一种与PACAP氨基酸同源性很高的多肽，也能增加β细胞内的[Ca^&lt；2&gt；]_i，但仅在纳摩尔范围内，表明PACAP27的效力比PACAP强4倍。高亲和力的I型PACAP受体被免疫组织化学定位于胰岛。接下来，我们研究了胰岛中PACAP的来源和生理作用。胰腺神经纤维和胰岛呈PACAP免疫反应阳性。在胰岛和胰岛β细胞系MIN6中检测到PACAP mRNA。高糖对胰岛胰岛素释放的刺激作用可被PACAP抗血清减弱。结果表明，PACAP能与高亲和力的PACAP选择性受体结合，使胰岛细胞内cAMP含量升高，进而增强L钙通道的活性，增加[Ca^&lt；2&gt；]i，从而增强葡萄糖诱导的胰岛素释放。PACAP是目前已知的最有效的促胰岛素多肽。此外，PACAP从胰岛释放出来，作用于胰岛β细胞，从而起到自分泌激素的作用。PACAP通过自分泌作用，放大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.