Masaya Sakurada

Pituitary adenylate cyclase-activating polypeptide (PACAP) is an islet substance serving as an intra-islet amplifier of glucose-induced insulin secretion in rats.

1 We examined whether pituitary adenylate cyclase‐activating polypeptide with 38 or 27 residues (PACAP‐38 or PACAP‐27) serves as an intra‐islet regulator of glucose‐induced insulin secretion in rats. PACAP antiserum specific for PACAP‐38 and PACAP‐27 was used to neutralize the effect of endogenous PACAP in islets. PACAP release from islets was bioassayed using the response of cytosolic Ca2+ concentration ([Ca2+]i) in single β‐cells, monitored by dual‐wavelength fura‐2 microfluorometry. Expression of PACAP mRNA was studied by reverse transcription‐polymerase chain reaction (RT‐PCR), while expression of PACAP was studied by metabolic labelling and immunoblotting. Localization of PACAP receptors was studied immunohistochemically. 2 High glucose‐stimulated insulin release from isolated islets was attenuated by PACAP anti‐serum but not by non‐immune sera. 3 The islet incubation medium with high glucose (Med) possessed a capacity, which was neutralized by PACAP antiserum, to increase [Ca2+]i in β‐cells. PACAP antiserum also neutralized the [Ca2+]iincreasing action of synthetic PACAP‐38 and PACAP‐27, but not that of vasoactive intestinal polypeptide (VIP) and glucagon. 4 Both Med and synthetic PACAP increased [Ca2+]i in β‐cells only in the presence of stimulatory, but not basal, glucose concentrations. In contrast, ATP, a substance that is known to be released from β‐cells, increased [Ca2+]i in β‐cells at both basal and stimulatory glucose concentrations. 5 Expression of PACAP mRNA and biosynthesis of PACAP‐38 were detected in islets and a β‐cell line, MIN6. 6 Immunoreactivity for PACAP‐selective type‐I receptor was observed in islets. 7 [Ca2+]i measurements combined with immunocytochemistry with insulin antiserum revealed a substantial population of glucose‐unresponsive β‐cells, many of which were recruited by PACAP‐38 into [Ca2+]i responses. 8 These results indicate that PACAP‐38 is a novel islet substance that is synthesized and released by islet cells and then, in an autocrine and/or paracrine manner, potentiates and arouses β‐cells responses to glucose, thereby amplifying glucose‐induced insulin secretion in islets.

Intraperitoneal PACAP Administration Decreases Blood Glucose in GK Rats, and in Normal and High Fat Diet Mice

PACAP is an islet peptide that serves as an endogenous amplifier of glucose induced insulin secretion. Furthermore, we has recently found that PACAP also potentiates insulin stimulated glucose uptake in adipocytes. Therefore, an antidiabetic action of PACAP is possible. In the present study, we examined the effect of PACAP treatment of the hyperglycemia in GK rats, an animal model of type 2 diabetes, and in high fat fed C47BL/6J mice, an animal model for glucose intolerance. GK rats housed with normal diet exhibited a normal level of blood glucose until three weeks old but significant hyperglycemia at eight weeks. When GK rats were treated with daily PACAP38 (i.p. injection, 6 pmol/kg) from age three weeks, development of hyperglycemia was prevented. In high fat fed mice, i.p. administration of PACAP27 for five (25 nmol/kg twice daily) reduced plasma glucose levels to 6.9 +/- 0.2 mmol/l compared to 8.1 +/- 0.2 mmol/l in saline injected animals (p < 0.001) without altering baseline insulin levels. We conclude that PACAP reduces circulating glucose in animal models of type 2 diabetes and glucose intolerance. The mechanism of this action needs to be established.

First-phase insulin response to glucose in nonobeser or obese subjects with glucose intolerance: Analysis by C-peptide secretion rate

This study was proposed to clarify the impairment of first-phase insulin response to glucose in subjects with glucose intolerance by analysis of C-peptide secretion rate after glucose or glucagon injection. The rate was calculated from kinetic analysis of peripheral C-peptide behavior. The rate reached the peak two minutes after glucose injection and then rapidly declined (first-phase secretion) in control subjects. In nonobese subjects with impaired glucose tolerance (IGT) or non-insulin-dependent diabetes mellitus (NIDDM), the rate promptly increased in response to glucose and was followed by a second phase increase. The time course of the rate in the subjects was slightly different from that in control subjects. There was a progressively greater deficit in the first-phase increase with increasing severity of glucose intolerance. The time course of the rate in the obese subjects with NIDDM was different from that in control subjects. The first-phase increase was reduced in the obese subjects with NIDDM. The glucose disappearance rate was correlated with the first-phase increase. Since the time course of the rate after glucagon injection in all subjects did correspond well with that in the control subjects, variation of metabolic clearance rate of endogenous C-peptide among the subjects may be negligible for this study. This study provides the precise time course of first- and second-phase insulin response to glucose injection in nonobese and obese subjects with IGT or NIDDM as well as convincing evidence of the progressive reduction of first-phase insulin response with increasing severity of glucose intolerance. First-phase insulin response to glucose might be slightly delayed in some obese subjects with NIDDM.

Current Status of PACAP as a Regulator of Insulin Secretion in Pancreatic Isletsa

PACAP-27 and PACAP-38 as low as 10(-13) M stimulate insulin release from rat islets in a glucose-dependent manner. PACAP also glucose dependently increases cAMP and [Ca2+]i in rat islet beta cells. The [Ca2+]i and insulin secretory responses to PACAP exhibit a similar concentration-response relationship, exhibiting a peak at 10(-13) M. When the [Ca2+]i response is abolished by nitrendipine, a blocker of L-type Ca2+ channels, the insulin response is also inhibited. Insulinotropic peptides glucagon, GLP-1, and VIP also increase [Ca2+]i in beta cells, but only in the nanomolar concentration range. PACAP is 4 logs more potent that VIP, a peptide that exhibits 68% amino acid homology and shares the type II PACAP receptor with PACAP. Immunoreactivity for the type I PACAP receptor is demonstrated in rat islets. Furthermore, PACAP immunoreactivity is demonstrated in nerve fibers and islets in rat pancreas. Based on these findings, we can draw the following conclusions: (1) PACAP is localized in pancreatic nerve fibers and islets; (2) PACAP in the subpicomolar range stimulates insulin release from islets; (3) the stimulation of insulin release is mediated by the cAMP-dependent increase in [Ca2+]i in beta cells; (4) all the PACAP effects are glucose-dependent; (5) PACAP is the most potent insulinotropic hormone known, and (6) the type I PACAP receptor appears to mediate the action of PACAP in the subpicomolar range. Finally, we hypothesize that PACAP is a pancreatic peptide of both neural and islet origin and functions as an intrinsic potentiator of glucose-induced insulin secretion in pancreatic islets (FIG 6).

PACAP as low as 10(-13) M raises cytosolic Ca2+ activity in pancreatic B-cells by augmenting Ca2+ influx through L-type Ca2+ channels to trigger insulin release.

Pancreatic insulin secretion is under control by peptides, as well as by nutrients and other substances(1,2). Truncated glucagon like peptide-1 (tGLP-1), gastric inhibitory peptide (GIP) and glucagon, the members of glucagon/VIP/secretin family of peptides, stimulate insulin release in a low concentration range around 10−9 M, and are thought to be involved in the physiological regulation of insulin release(3–5).

Abnormal calcium handling by perifused pancreatic islets from neonatal streptozotocin diabetic model rats

To elucidate the mechanism of impaired insulin release in case of non-insulin-dependent diabetes (NIDDM), we investigated insulin release and 45Ca++ efflux from perifused islets obtained from neonatal streptozotocin diabetic model rats. The model rats were prepared by the intraperitoneal administration of 65 mg/kg streptozotocin (STZ) to neonatal males. Rats treated with STZ did not differ from controls in body weight from 1 week to 16 weeks. The model rats had significant hyperglycemia both in the fasting state and after intraperitoneal administration of 2 g/kg glucose. Although the diameter of the islets from the model rats was not significantly different from that of controls, immunoreactivity to anti-insulin was slightly diminished, and degranulation was slightly observed in B-cells. Insulin content was reduced to 45.6% of the control. Insulin release from the perifused islets of STZ-treated rats responded little to 16.7 mmol/L glucose, but normally to 20 mmol/L arginine in the presence of 5.5 mmol/L glucose. In experiments to test the 45Ca++ efflux from the perifused islets prelabeled with 45Ca++, a rise of 45Ca++ efflux concomitant with the second phase of insulin release from the islets of the model rats was inhibited although a sharp increase of 45Ca++ efflux concomitant with the first phase of insulin release was maintained. 45Ca++ uptake for 30 minutes was reduced in the islets from the model rats in the basal and stimulated state of insulin secretion although the incremental 45Ca++ uptake was similar. It is possible that the abnormal calcium handling in pancreatic B-cells may be one of the causes of defect in insulin release in our model rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of Monensin on Glucose-Induced Insulin Release and 45Ca2+ Outflow

The β-cells in the pancreatic islets respond to glucose with a biphasic insulin release2. Protons generated from glucose metabolism have been proposed to be a coupling factor between metabolic and ionic events in the cells1,7. A rise in cytosol Ca2+ concentration is considered to be essential in stimulus-secretion coupling in the β-cells4. Thus, it seems possible that the biphasic insulin release is a reflection of a biphasic change in the Ca2+ concentration of the cytosol, the change being controlled by the rate of generation of protons. To examine the role of protons and calcium in the insulin response to glucose, we examined the effect of monensin, a carboxylic ionophore8, on the first and second phase of glucose-induced insulin release and on the 45Ca2+ efflux from perifused rat pancreatic islets.