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肽和营养素的控制。两种垂体腺苷酸环化酶激活多肽（PACAP 27和PACAP 38）在低至10 μ <-13>M时以葡萄糖依赖性方式刺激大鼠胰岛释放胰岛素。PACAP还增加胰岛β细胞胞浆Ca^&lt;2+&gt;浓度（[Ca^&lt;2+] _i）。L型Ca^&lt;2+&gt;通道阻断剂可同时阻断[Ca^&lt;2+&gt;] _i和胰岛素反应。PACAP刺激胰岛产生cAMP，cAMP的增加与PACAP增加β细胞[Ca^&lt;2+&gt;] _i相似。血管活性肠肽，一种与PACAP具有高度氨基酸同源性的肽，也增加β细胞中的[Ca^2+] _i，但仅在纳摩尔范围内的浓度下，表明PACAP 27的效力高4个对数。高亲和力的I型PACAP受体在胰岛中被化学定位，我们接下来研究了PACAP在胰岛中的来源和生理作用。PACAP-免疫反应性被证明在胰腺神经纤维和胰岛。在胰岛和β细胞系MIN 6中检测到PACAP mRNA。用特异性PACAP抗血清减弱高糖刺激离体胰岛释放胰岛素的作用。高糖培养液可使β细胞内[Ca^&lt;2+&gt;] i增加，但这种作用可被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，使β细胞内cAMP升高，进而增强L型Ca^&lt;2+&gt;通道活性，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，从而作为自分泌激素。PACAP通过自分泌作用放大胰岛中葡萄糖诱导的胰岛素分泌，这在非胰岛素依赖型糖尿病中是特征性受损的。

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.