Mechanisms and physiological significance of the cholinergic control of pancreatic beta-cell function

Acetylcholine (ACh), the major parasympathetic neurotransmitter, is released by intrapancreatic nerve endings during the preabsorptive and absorptive phases of feeding. In beta-cells, ACh binds to muscarinic M(3) receptors and exerts complex effects, which culminate in an increase of glucose (nutrient)-induced insulin secretion. Activation of PLC generates diacylglycerol. Activation of PLA(2) produces arachidonic acid and lysophosphatidylcholine. These phospholipid-derived messengers, particularly diacylglycerol, activate PKC, thereby increasing the efficiency of free cytosolic Ca(2+) concentration ([Ca(2+)](c)) on exocytosis of insulin granules. IP3, also produced by PLC, causes a rapid elevation of [Ca(2+)](c) by mobilizing Ca(2+) from the endoplasmic reticulum; the resulting fall in Ca(2+) in the organelle produces a small capacitative Ca(2+) entry. ACh also depolarizes the plasma membrane of beta-cells by a Na(+)- dependent mechanism. When the plasma membrane is already depolarized by secretagogues such as glucose, this additional depolarization induces a sustained increase in [Ca(2+)](c). Surprisingly, ACh can also inhibit voltage-dependent Ca(2+) channels and stimulate Ca(2+) efflux when [Ca(2+)](c) is elevated. However, under physiological conditions, the net effect of ACh on [Ca(2+)](c) is always positive. The insulinotropic effect of ACh results from two mechanisms: one involves a rise in [Ca(2+)](c) and the other involves a marked, PKC-mediated increase in the efficiency of Ca(2+) on exocytosis. The paper also discusses the mechanisms explaining the glucose dependence of the effects of ACh on insulin release.

Diacylglycerol hydrolysis to arachidonic acid is necessary for insulin secretion from isolated pancreatic islets: sequential actions of diacylglycerol and monoacylglycerol lipases

Arachidonic acid has been implicated as a second messenger in insulin secretion on the basis of (1) mobilization of intracellular Ca2+ from the endoplasmic reticulum of islets and (2) amplification of voltage-dependent Ca2+ entry. The insulin secretagogues D-glucose and the muscarinic agonist carbachol both increase unesterified arachidonic acid accumulation in isolated islets. We now show that diacylglycerol, a product of phospholipase C action, is a major source of free arachidonic acid in islets. Diacylglycerol hydrolysis in islets occurs through a two-step process. In the first step, the sn-1 bond of 1-stearoyl-2-arachidonyl-sn-glycerol is hydrolyzed by a diacylglycerol lipase, giving rise to 2-arachidonyl-sn-glycerol. Next, the sn-2 bond of 2-arachidonyl-sn-glycerol is hydrolyzed by a monoacylglycerol lipase, which is the rate-limiting step, releasing unesterified arachidonic acid. Both diacylglycerol lipase and monoacylglycerol lipase are highly enriched in the plasma membrane of beta-cells. Diacylglycerol lipase activity in islet homogenates is selectively inhibited in a dose-dependent manner by the compound RHC-80267, a specific diacylglycerol lipase inhibitor. RHC-80267 inhibits glucose- and carbachol-induced insulin release from intact islets in a dose-dependent manner that parallels its inhibition of diacylglycerol lipase activity. Importantly, RHC-80267, at concentrations that almost completely inhibit diacylglycerol lipase activity and glucose- and carbachol-induced insulin secretion by islets, markedly inhibits glucose- and carbachol-induced increases in islet arachidonic acid levels, as measured by gas chromatography with electron-capture detection of its pentafluorobenzyl esters. RHC-80267 did not significantly affect islet glucose oxidation, phospholipase C, monoacylglycerol lipase, or phospholipase A2. Since glucose and carbachol are known to stimulate phospholipase C, our observations indicate that diacylglycerol is an important source of arachidonic acid and other free fatty acids in islets. Furthermore, production of arachidonic acid from the hydrolysis of diacylglycerol is essential for glucose- and carbachol-induced insulin secretion.

Fuel secretagogue stimulation of arachidonic acid accumulation in fresh and cultured pancreatic islets

It has been previously demonstrated that glucose stimulation of islets of Langerhans causes an accumulation of unesterified arachidonic acid that correlates with insulin secretion. In addition, it is well established that glucose metabolism is essential for insulin secretion. We show that non-metabolizable analogs of glucose which do not stimulate insulin secretion fail to cause significant accumulation of unesterified arachidonic acid. In addition, mannoheptulose, an inhibitor of glucose metabolism, completely blocks the glucose-induced increase in arachidonic acid accumulation. Among the nutrient secretagogues tested, only alpha-ketoisocaproic acid causes a significant increase in unesterified arachidonic acid accumulation. Mannose, fructose, and glyceraldehyde, in particular, failed to elicit a significant increase in unesterified arachidonic acid accumulation. Our data, taken together with previous studies, suggests that glucose must be metabolized to induce accumulation of unesterified arachidonic acid in pancreatic islets.

Inhibition of phospholipase A2 and insulin secretion in pancreatic islets

Arachidonic acid may be an important mediator of insulin secretion since (1) glucose activates phospholipase A2 thus increasing endogenous unesterified levels of arachidonic acid, (2) arachidonic acid mobilizes Ca2+ from the islet endoplasmic reticulum and (3) arachidonic acid has been proposed to regulate voltage-dependent Ca2+ channels in the beta-cell. We have used the phospholipase A2 inhibitor, (p-amylcinnamoyl)anthranilic acid (ACA), to determine whether phospholipase A2 activation is required for glucose-induced insulin secretion. ACA inhibited in a dose-dependent manner glucose-induced insulin secretion, as well as glyceraldehyde and alpha-ketoisocaproic acid-induced insulin secretion. ACA also totally abolished glucose-induced arachidonate accumulation but did not affect phospholipase C suggesting that it was specific for phospholipase A2. Furthermore, ACA did not inhibit glucose oxidation. These observations suggest that glucose-induced arachidonate increase is essential for insulin secretion.

Epidermal growth factor modulates intracellular arachidonic acid levels in MA-10 cultured Leydig tumor cells

Epidermal growth factor (EGF) acts on various cell types, including the mouse Leydig tumor cell line MA-10, where it has been shown to stimulate steroidogenesis, apparently in a cAMP-independent manner. In the process of examining other possible signaling pathways for EGF in these cells, we found rapid changes in the intracellular concentration of arachidonic acid (AA) following addition of EGF. For example, a significant increase in AA was detected 1 min after incubating the cells with EGF, with the maximal effect observed at an EGF concentration of 10 ng/ml. In addition, exogenous AA increased steroidogenesis, and the steroidogenesis enhanced by AA and EGF was reduced by lipoxygenase inhibitors, suggesting a possible role of an AA metabolite(s) in promoting steroidogenesis. Consistent with this hypothesis is our observation that several exogenous lipoxygenase metabolites were capable of enhancing progesterone production. The EGF-stimulated steroidogenesis was also inhibited by two phospholipase A2 inhibitors, again confirming a probable role of AA or a metabolite in this process. Therefore, AA appears to be an important intracellular mediator responsible, at least in part, for some of the acute metabolic effects mediated by EGF in MA-10 cells.

Biosynthesis of peptidyl leukotrienes and other lipoxygenase products by rat pancreatic islets. Comparison with macrophages and neutrophils

Biochemical evidence in support of a role for arachidonic acid 5-lipoxygenase activity in pancreatic islet insulin secretion has been obtained. Peptidyl leukotriene metabolism was studied in rat islets using a dual-labeling technique in extended culture, with analysis of arachidonic acid metabolites by reverse-phase high-performance liquid chromatography. The production of [3H]arachidonoyl/[35S]cysteinyl leukotrienes C4 and E4 by islets was compared with that by mouse resident peritoneal macrophages and with the lipoxygenase metabolism of rabbit polymorphonuclear leukocytes. The stimulus-specific nature of leukotriene biosynthesis was characterized by low basal biosynthesis in unstimulated islet cells with a calcium-mediated activation of 5-lipoxygenase product formation.