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Summary: Repaglinide (RPG) is a short-acting insulin secretagogue widely prescribed for the treatment of type 2 diabetes. It boosts insulin secretion by inhibiting the pancreatic ATP-sensitive potassium channel (KATP). However, the mechanisms by which RPG binds to the KATP channel are poorly understood. Here, we describe two cryo-EM structures: the pancreatic KATP channel in complex with inhibitory RPG and adenosine-5’-(γ-thio)-triphosphate (ATPγS) at 3.3 Å and a medium-resolution structure of a RPG-bound mini SUR1 protein in which the N terminus of the inward-rectifying potassium channel 6.1 (Kir6.1) is fused to the ABC transporter module of the sulfonylurea receptor 1 (SUR1). These structures reveal the binding site of RPG in the SUR1 subunit. Furthermore, the high-resolution structure reveals the complex architecture of the ATP binding site, which is formed by both Kir6.2 and SUR1 subunits, and the domain-domain interaction interfaces. : Ding et al. report the detailed binding site and the inhibitory mechanism of the insulin secretagogue repaglinide on the pancreatic KATP channel, revealed by a 3.3 Å cryo-EM structure and electrophysiology experiments. Keywords: diabetes, KATP channel, repaglinide, glibenclamide, sulfonylurea, glinides, Kir, SUR, ABC transporter, KNtp

Objective Binding of ghrelin to its receptor, growth hormone secretagogue receptor (GHSR), stimulates GH release, induces eating, and increases blood glucose. These processes may also be influenced by constitutive (ghrelin-independent) GHSR activity, as suggested by findings in short people with naturally occurring GHSR-A204E mutations and reduced food intake and blood glucose in rodents administered GHSR inverse agonists, both of which impair constitutive GHSR activity. In this study, we aimed to more fully determine the physiologic relevance of constitutive GHSR activity. Methods We generated mice with a GHSR mutation that replaces alanine at position 203 with glutamate (GHSR-A203E), which corresponds to the previously described human GHSR-A204E mutation, and used them to conduct ex vivo neuronal electrophysiology and in vivo metabolic assessments. We also measured signaling within COS-7 and HEK293T cells transfected with wild-type GHSR (GHSR-WT) or GHSR-A203E constructs. Results In COS-7 cells, GHSR-A203E resulted in lower baseline IP3 accumulation than GHSR-WT; ghrelin-induced IP3 accumulation was observed in both constructs. In HEK293T cells co-transfected with voltage-gated CaV2.2 calcium channel complex, GHSR-A203E had no effect on basal CaV2.2 current density while GHSR-WT did; both GHSR-A203E and GHSR-WT inhibited CaV2.2 current in the presence of ghrelin. In cultured hypothalamic neurons from GHSR-A203E and GHSR-deficient mice, native calcium currents were greater than those in neurons from wild-type mice; ghrelin inhibited calcium currents in cultured hypothalamic neurons from both GHSR-A203E and wild-type mice. In brain slices, resting membrane potentials of arcuate NPY neurons from GHSR-A203E mice were hyperpolarized compared to those from wild-type mice; the same percentage of arcuate NPY neurons from GHSR-A203E and wild-type mice depolarized upon ghrelin exposure. The GHSR-A203E mutation did not significantly affect body weight, body length, or femur length in the first ∼6 months of life, yet these parameters were lower in GHSR-A203E mice after 1 year of age. During a 7-d 60% caloric restriction regimen, GHSR-A203E mice lacked the usual marked rise in plasma GH and demonstrated an exaggerated drop in blood glucose. Administered ghrelin also exhibited reduced orexigenic and GH secretagogue efficacies in GHSR-A203E mice. Conclusions Our data suggest that the A203E mutation ablates constitutive GHSR activity and that constitutive GHSR activity contributes to the native depolarizing conductance of GHSR-expressing arcuate NPY neurons. Although the A203E mutation does not block ghrelin-evoked signaling as assessed using in vitro and ex vivo models, GHSR-A203E mice lack the usual acute food intake response to administered ghrelin in vivo. The GHSR-A203E mutation also blunts GH release, and in aged mice leads to reduced body length and femur length, which are consistent with the short stature of human carriers of the GHSR-A204E mutation., Graphical abstract Image 1, Highlights • We generated mice with a GHSR mutation replacing Ala at position 203 with Glu. • The A203E mutation ablates constitutive GHSR activity & hyperpolarizes NPY neurons. • GHSR-A203E mice lack the usual orexigenic response to administered ghrelin. • The GHSR-A203E mutation blunts GH release and causes reduced body length. • This finding is consistent with short stature in human carriers of the GHSR-A204E mutation.

Previously we identified a sodium caseinate (NaCas) hydrolysate, LFC25, which significantly increased secretion of satiety hormone glucagon-like peptide-1 (GLP-1) in vitro and reduced food intake in mice when administered intraperitoneally. This study investigates whether LFC25, administered orally, promotes GLP-1 secretion and/or reduces food intake in vivo. Over an 8-hour period, mice received LFC25 by oral gavage had similar food intake to mice received NaCas. Postprandial blood glucose, plasma active GLP-1, amino acids, insulin and food consumed at the next meal were not significantly different in pigs that consumed LFC25 compared to NaCas in a dairy beverage, at a dosage relevant for human consumption. Simulated in vitro gastro-duodenal digestion of LFC25 revealed a significant reduction in bioactivity. In contrast, the harsh conditions of the upper gut appear to functionalize intact NaCas as a GLP-1 secretagogue. In conclusion, LFC25 will need enteric protection to be used as a food ingredient for satiety.

Extract of pine nodules (matsufushi) formed by bark proliferation on the surface of trees of Pinus tabulaeformis or Pinus massoniana has been used as an analgesic for joint pain, rheumatism, neuralgia, dysmenorrhea and other complaints in Chinese traditional medicine. Here we report the effects of matsufushi extract and its components on catecholamine secretion and synthesis in cultured bovine adrenal medullary cells. We found that matsufushi extract (0.0003–0.005%) and its component, SJ-2 (5-hydroxy-3-methoxy-trans-stilbene) (0.3–100 μM), but not the other three, concentration-dependently inhibited catecholamine secretion induced by acetylcholine, a physiological secretagogue. Matsufushi extract (0.0003–0.005%) and SJ-2 (0.3–100 μM) also inhibited ^Ca^ influx induced by acetylcholine in a concentration-dependent manner, similar to its effect on catecholamine secretion. They also suppressed 14C-catecholamine synthesis and tyrosine hydroxylase activity induced by acetylcholine. In Xenopus oocytes expressing α3β4 nicotinic acetylcholine receptors, matsufushi extract (0.00003–0.001%) and SJ-2 (1–100 μM) directly inhibited the current evoked by acetylcholine. The present findings suggest that SJ-2, as well as matsufushi extract, inhibits acetylcholine-induced catecholamine secretion and synthesis by suppression of nicotinic acetylcholine receptor-ion channels in bovine adrenal medullary cells., 論文

Stimulation of insulin production by insulin secretagogue use may impact T helper cells' cytokine production. This dataset presents the relationship between baseline insulin secretagogues use in women diagnosed with breast cancer and type 2 diabetes mellitus, the T-helper 1 and 2 produced cytokine profiles at the time of breast cancer diagnosis, and subsequent cancer outcomes. A Pearson correlation analysis evaluating the relationship between T-helper cytokines stratified by of insulin secretagogues use and controls is also provided.

Follicle-stimulating hormone (FSH) is a glycoprotein that is secreted by the pituitary gland and, in concert with luteinizing hormone, plays a central role in mammalian reproduction. FSH acts directly on the gonads in both sexes, binding to its receptor on the surface of target cells, initiating a cascade of molecular events beginning with the activation of specific signaling pathways and ending with the expression of gene products. The gene products, in turn, set in motion processes specific to each gonad. In the ovary, FSH stimulates folliculogenesis and steroidogenesis, which, in turn, prepare the female reproductive tract for fertilization, implantation, and pregnancy. In the testis, this gonadotropin plays a central role in testicular development and spermatogenesis. In both sexes, the products of FSH stimulation regulate the secretion of FSH either by modifying the release of the hypothalamic secretagogue, gonadotropin-releasing hormone (GnRH), or by acting directly at the pituitary gland (Fig. 1).

A 55-year-old man visits his physician with complaints of fatigue, frequent urination, peripheral neuropathy (PN) and a swollen big toe. Energy utilization and insulin secretion in type II diabetes mellitus (DM) are discussed, including the role of incretins in stimulating insulin secretion and the efficacy of AMP-activated protein kinase inhibitors in facilitating insulin action. The role of hydrogen peroxide signaling and recent evidence that excess proinsulin can increase beta cell stress and decrease insulin production are explored. Roles of nutrients in insulin secretion and action including leucine, a known insulin secretagogue, food-derived polyphenols, carnitine, thiamin, zinc, and other nutrients are examined, including interactions of vitamin A and iron in obesity and their impact on insulin secretion. The roles of nutrients such as vitamin B12, protein, magnesium, and fructose in the pathogenesis of comorbidities such as sarcopenia, PN, and gout are explored. Current recommendations for prevention and control of type II DM and its complications are presented.

Ghrelin, a 28 amino-acid octanoylated peptide predominantly produced by the stomach, was discovered to be the natural ligand of the type 1a GH secretagogue receptor (GHS-R1a). It was thus considered as a natural GHS additional to GHRH, although later on ghrelin has mostly been considered a major orexigenic factor. The GH-releasing action of ghrelin takes place both directly on pituitary cells and through modulation of GHRH from the hypothalamus; some functional antisomatostatin action has also been shown. However, ghrelin is much more than a natural GH secretagogue. In fact, it also modulates lactotroph and corticotroph secretion in humans as well as in animals and plays a relevant role in the modulation of the hypothalamic-pituitary-gonadal function. Several studies have indicated that ghrelin plays an inhibitory effect on gonadotropin pulsatility, is involved in the regulation of puberty onset in animals, and may regulate spermatogenesis, follicular development and ovarian cell functions in humans. In this chapter ghrelin actions on the GH/IGF-I and the gonadal axes will be revised. The potential therapeutic role of ghrelin as a treatment of catabolic conditions will also be discussed.

Ghrelin has emerged as an important player in the regulation of glucose homeostasis. During caloric restriction, the presence of acyl ghrelin (the active isoform) is necessary for the defense against hypoglycemia and survival [1]. In fasted humans, ghrelin administration enhances growth hormone (GH) secretion, suppresses insulin release and promotes insulin resistance to maintain normoglycemia [2]. The exact mechanism for the insulinostatic effect of ghrelin is not known. Ghrelin is secreted primarily by gastric endocrine cells, although a novel endocrine cell type (ɛ cells) in the pancreatic islet [3] also produces ghrelin raising the possibility of intra-islet regulation. Ghrelin has been shown to activate its receptor, the GH secretagogue receptor (GHSR) 1a, on β cells to inhibit glucose-stimulated insulin secretion (GSIS) through a Gαi2 signaling pathway [4] and acts directly on α cells to stimulate glucagon secretion by elevating intracellular calcium and promoting ERK phosphorylation [5]. Another major inhibitor of insulin secretion from β cells is somatostatin (SST) through the SST receptor subtype-5 (SSTR5). Park and colleagues reported collaborative actions between ghrelin and SST via their receptors in INS-1 β cells to modulate GSIS during low- and high-energy balance states [6]. The close proximity of islet cells makes it difficult to distinguish between direct and indirect effects of hormones on β cells. However, the recent advances in molecular genetic techniques and next-generation sequencing allows for unbiased evaluation of gene expression in individual cells and provides a unique opportunity to intercept cross talk between pancreatic islet cells. In this issue of Molecular Metabolism, DiBruccio et al. [7] report a novel insulinostatic mechanism of ghrelin by acting directly on δ cells to promote SST release.

Regulation of metabolic fuel homeostasis is a critical function of β-cells, which are located in the islets of Langerhans of the animal pancreas. Impairment of this β-cell function is a hallmark of pancreatic β-cell failure and may lead to development of type 2 diabetes mellitus. β-Cells are essentially "fuel sensors" that monitor and react to elevated nutrient load by releasing insulin. This response involves metabolic activation and generation of metabolic coupling factors (MCFs) that relay the nutrient signal throughout the cell and induce insulin biosynthesis and secretion. Glucose is the most important insulin secretagogue as it is the primary fuel source in food. Glucose metabolism is central to generation of MCFs that lead to insulin release, most notably ATP. In addition, other classes of nutrients are able to augment insulin secretion and these include members of the lipid and amino acid family of nutrients. Therefore, it is important to investigate the interplay between glucose, lipid, and amino acid metabolism, as it is this mixed nutrient sensing that generate the MCFs required for insulin exocytosis. The mechanisms by which these nutrients are metabolized to generate MCFs, and how they impact on β-cell insulin release and function, are discussed in detail in this article.

The ghrelin receptor, also known as growth hormone secretagogue receptor (GHS-R), was identified in porcine and rat anterior pituitary membranes, where the synthetic secretagogue MK-0677 causes amplified pulsatile growth hormone (GH) release. In addition to its function in the stimulation of GH secretion, ghrelin, the natural ligand of ghrelin receptor is now recognized as a peptide hormone with fundamental influence on energy homeostasis. Despite the potential existence of multiple subtypes of ghrelin receptor, the effects of ghrelin on energy metabolism, obesity, and diabetes are mediated by its classical receptor GHS-R1a, whose activation requires the n-octanoylation of ghrelin. Here we review the current understanding of the role of the ghrelin receptor in the regulation of energy homeostasis. An overview of the ghrelin receptor is presented first, followed by the discussion on its effects on food intake, glucose homeostasis, and lipid metabolism. Finally, potential strategies for treating obesity and diabetes via manipulation of the ghrelin/ghrelin receptor axis are explored.

The regulation of gastric HCl secretion by the parietal cell occurs primarily through the regulated recycling of H,K-ATPase. Secretagogue stimulation of the parietal cell initiates HCl secretion by effecting an intracellular signaling and biochemical pathway that culminates with the membrane fusion of intracellular H,K-ATPase-rich tubulovesicles with, and thus, the delivery of the H,K-ATPase to the secretory canaliculus (apical membrane). On withdrawal of the secretagogue, cessation of secretion occurs concomitantly with the retrieval of the H,K-ATPase from the secretory canaliculus, and the reformation of the tubulovesicular membrane compartment. The H,K-ATPase resides quiescently in this compartment until another round of secretion is initiated. Intracellular signaling pathways, cytoskeletal elements, and vesicular trafficking machinery must be coordinately regulated to effect this membrane-recycling pathway. This chapter summarizes key findings from experimental approaches taken toward the elucidation of the mechanism of gastric HCl secretion, through the use of electron microscopy, electrophysiology, membrane biochemistry, molecular biology, cell biology, and atomic structure, in the characterization of the regulation of activity of the H,K-ATPase itself, to that of associated ion channels and membrane transporters, as well as of the regulation of H,K-ATPase trafficking.

A minimal-model framework is that growth hormone (GH) secretion is controlled by an ensemble of interlinked peptides, namely, GH-releasing hormone (GHRH), somatostatin (SS), and ghrelin. Clinical studies, laboratory experiments, rare sporadic mutations, targeted gene silencing, and biomathematical models establish that at least three signals regulate GH secretion. A clarion implication of the concept of integrative control is that no one peptidic effector operates alone or can be adequately studied alone. A major unanswered question is how pathophysiology disrupts the core regulatory ensemble, thereby forcing relative GH and IGF-1 deficiency or excess. However, salient technical hurdles exist, namely, the lack of reliable experimental strategies and the paucity of validated analytical tools to distinguish the interlinked roles of GHRH, SS, and ghrelin. To address these significant obstacles requires administering peptide secretagogues in distinct combinations akin to the classical insulin/glucose clamp and implementing an analytical formalism to parse the interactive roles of GHRH, SS, and ghrelin objectively.

Publisher Summary This chapter reviews new and emerging information about important changes in the regulation of insulin secretion that occur during aging. Findings from animal models and from humans are presented and reviewed. The glucose level is known to be a key regulator of insulin secretion The effects of glucose appear to be multiple and complex. For example, exposure of pancreatic islets in vitro or in vivo to a constant glucose stimulus leads to a complex multiphasic insulin secretory response. This response includes an acute phase occurring within minutes, thought to be largely due to an immediate release of preformed insulin granules that are available near the cell surface of pancreatic beta cells, and a more prolonged second phase of insulin secretion that gradually increases in magnitude over time and is dependent on new protein synthesis. In addition to being a direct insulin secretagogue, glucose can potentiate many other factors that can contribute to insulin secretion. Thus, nutrients such as some amino acids and fatty acids can stimulate insulin secretion, but only in the presence of adequate amounts of glucose. In addition, this chapter links the agerelated changes in regulation of insulin secretion to the high risk for development of diabetes mellitus observed in populations of older adults.

Ghrelin is a 28-amino-acid peptide predominantly produced by the stomach and displays strong growth hormone (GH)-releasing action mediated by the activation of the GH secretagogue (GHS) receptor (GHS-R) type 1a. GHS-R are concentrated in the hypothalamus–pituitary unit but are also distributed in other central and peripheral tissues. In addition to its potent GH-releasing action, ghrelin has other actions, including stimulation of prolactin (PRL) and adrenocorticotropic hormone (ACTH) secretion, effects on the pituitary–gonadal axis, stimulation of appetite, control of energy balance, effects on sleep and behavior, control of gastric motility and acid secretion, effects on exocrine and endocrine pancreatic function and glucose metabolism, effects on the cardiovascular system, and modulation of proliferation of neoplastic cells. The discovery of ghrelin opened many new avenues of research in neuroendocrinology and metabolism and other fields of internal medicine, such as gastroenterology, immunology, oncology, and cardiology. The possibility that ghrelin and/or GHS analogues, acting as either agonists or antagonists on different activities, might have clinical impact has been suggested and is receiving great attention.

This chapter coveres stimulus-secretion coupling as it applies to secretagogue stimulation of calcium influx and cyclic AMP elevation leading to calcium sensor activation, protein phosphorylation, and ultimately secretory vesicle fusion with the plasma membrane. The modes of secretagogue signaling have been discussed. These include depolarization via sodium channel activation leading to VSCC opening and calcium influx; modulation of outwardly directed potassium channels leading to altered calcium influx; directfirst-messenger stimulation of VSCC opening; cyclic AMP stimulation of secretory granule exocytosis via interactions with components of the calcium pathway; and cooperative roles of intracellular calcium mobilization and calcium influx in exocytosis. Stimulus-secretion-synthesis coupUng was discussed to illustrate that secretogogue-based regulation involves control of the export economy of the secretory cell, including both the quantity and quality of the neurosecretory quantum of the SSV, the endocrine hormone cocktail of the LDCV, and the zymogen mixture of the exocrine secretory granule. In the future, secretion is likely to be viewed within a more unitary framework for neuronal, endocrine, and exocrine secretion, but with a better molecular understanding of the diversity among cell types and various types of exocytosed cargo.

Publisher Summary The late phase response (LPR) is considered to be an appropriate model for allergic inflammation for many reasons; however, a major similarity between the LPR and chronic allergic inflammation is the cellular influx. In both, an influx of basophils, eosinophils and monocytes is observed. The initial allergen exposure causes the release of cell mediators that may in turn cause the upregulation of adhesion molecules and the influx of inflammatory cells. The IgE-dependent histamine-releasing factors (HRFs) is both a marker for disease severity and is found in the LPR. Because of its presence in the LPR and its activation of basophils, eosinophils and monocytes, human recombinant HRF (HrHRF) is likely to be a pivotal secretagogue in the LPR. Moreover, these findings suggest that HrHRF plays an important role in chronic allergic inflammation. Consequently, receptor identification and further characterization of HrHRF will increase the understanding of chronic allergic disease and may help to explain the varying severity of allergies.

Publisher Summary In recent times, highly active growth hormone releasing peptides (GHRPs) and GHRP peptidomimetics, with short-acting and long-acting pharmacodynamic profiles, has received considerable attention as potential alternatives to injectable growth hormone (GH) replacement therapy. These compounds can restore and enhance pulsatile GH release in humans. It is, therefore, anticipated that inducing physiologically relevant, pulsatile release of GH, with GH-secretagogues (GHSs), may provide the beneficial effects of GH replacement therapy without the associated side effects that result from bolus GH administration. Furthermore, the GH-secretagogues may offer the potential advantages of oral dosing. The mechanisms, by which pulsatile GH secretory pattern result from GH secretagogue treatment, are being elucidated. GHRP-6 acts directly on somatotrophs to cause GH release, to potentiate the actions of GH-stimulatory hormone, growth hormone-releasing hormone (GHRH; a hypothalamic peptide), and by being a functional antagonist of somatostatin, a hypothalamic peptide that inhibits GH release by several mechanisms. The relevance of this synergy with GHRH is exemplified by the loss of GHS sensitivity in hypothalamic/pituitary-stalk sectioned animals that lack portal blood derived growth hormone-releasing hormone (GHRH) and that are no longer GHS responsive. The identification of the GHS-R that is specifically expressed in the hypothalamus, pituitary, and several other discrete brain regions, provides significant evidence for the presence of a third neuroendocrine pathway, in addition to GHRH and somatostatin, with a dominant mode of action in the control of pulsatile GH release.

Publisher Summary This chapter presents the experimental and molecular analysis of corticotropin-releasing factors acting on the fish pituitary. The evidence for adrenocorticotropic hormone (ACTH)-releasing activity in the extracts of goldfish hypothalami was obtained with goldfish pituitary cells in culture and in dexamethasone-blocked goldfish in vivo . Neurosecretory cells of the teleost neurohypophyseal system produce arginine vasotocin (AVT) and isotocin (IST), which also affect ACTH secretion. AVT and IST are nonapeptides structurally homologous to arginine vasopressin (AVP) and the oxytocin of the mammalian neurohypophyseal system. In goldfish anterior pituitary cells in vitro , human AII is a potent ACTH secretagogue and has proved to be about 10 times more potent than salmon AII or human angiotensin I. Urotensin I (UI)-specific binding sites in goldfish kept in fresh water or dilute seawater have been characterized with regard to ligand binding, to variations in binding in the osmotically challenged fish, and also to receptor specificity by means of competitive displacement of 1251-labeled UI.

Publisher Summary Stimulation of pancreatic secretion is controlled by both neural and hormonal factors. All physiologically relevant neurotransmitters and gastrointestinal hormones are either acetylcholine or peptides. The most physiologically relevant neurotransmitters that regulate pancreatic secretion are acetylcholine and vasoactive intestinal peptide (VIP), and in some species, gastfin-releasing peptide. Using the dye-coupled starch polymer method to study secretagogue stimulated amylase release from pancreatic acini is versatile and has been applied to the analysis of the kinetics of enzyme secretion, stoichiometric dose-response relationships, and desensitization of enzyme secretion through the use of sequential incubations and potentiation of stimulated secretion by combining two secretagogues in the same incubation. In pancreatic acinar cells, two forms of desensitization have been described; homologous, in which exposure to a secretagogue attenuates the subsequent response only to secretagogues interacting with the same specific class of receptors, and heterologous, in which exposure to one secretagogue attenuates the subsequent response to secretagogues that interact with other classes of receptors.

Publisher Summary This chapter discusses cAMP and secretagogue interactions in isolated gastric glands. While most evidence supports the hypothesis that the gastric histamine receptor is linked to the formation of cAMP, it is not yet clear whether the increase in cAMP is a necessary and sufficient condition for stimulation of acid secretion. A study described in the chapter characterizes the effect of IMX on cAMP and secretion-related parameters in isolated gastric glands. Isolated gastric glands were prepared from rabbit gastric mucosa by the collagenase digestion method. Oxygen consumption and aminopyrine accumulation were measured and cAMP was assayed with a modified protein-binding assay. It was found that in the absence of added secretagogues, IMX produces increases in tissue cAMP, respiration, and aminopyrine accumulation in a dose-dependent manner with a mean ED50 value (5 × 10−5 M), which appears to be the same for all the three responses. This result supports the view that cAMP levels are a primary determinant of acid secretory activity.

Publisher Summary This chapter analyzes electrophysiological studies on the role of cyclic GMP in pancreatic acinar cell stimulus–secretion coupling. This chapter describes a study that investigated stimulus–secretion coupling from a slightly different angle. The main pancreatic acinar secretagogues have the ability to depolarize the acinar cell membrane. The dose-response curves for the depolarizations are parallel, but the secretagogue concentrations required to evoke half-maximal depolarization vary tremendously from about 10−11 M in the case of CCK to about 10−7 M for Ach. In contrast, the depolarizing action of the amino acid l -alanine seems to be of a different nature. The doseresponse curve for l -alanine-evoked depolarization is not parallel to those of the secretagogues, and the maximal depolarization is larger. Detailed investigations of the mechanisms underlying the actions of the three major groups of secretagogues have revealed that they all have the same mechanism of action. It is also found that dibutyryl cyclic GMP or cyclic GMP itself had no effect on the pancreatic acinar cell membrane potential in concentrations as high as 1 mM. Dibutyryl cyclic GMP had no effect on the input resistance of acinar units.

Recent advances in technology have dramatically increased the resolution with which we may examine many features of biological systems. Intracellular recording and tracer injection techniques allow one to study the function of individual neurons and later characterize the same cells morphologically. In situ hybridization techniques can give us information about messenger RNA levels in single cells. More established techniques such as immunocytochemistry and electron microscopy also provide information at the cellular and even subcellular level. With each of these technological advances we have learned more about the mechanisms underlying cell function. We are also beginning to appreciate the role of heterogeneity among cells in relation to the function of the whole organism. Application of the reverse hemolytic plaque assay to the study of hormone or neurotransmitter secretion shoul dhelp clarify this role. This technique permits accurate quantitation of hormone secreted from a large number of cells. Thus while cells can be studied individually they can also be categorized into functional subpopulations. As discussed in this chapter, many other techniques may be applied on cells which have already been functionally defined with the plaque assay. This should result in a clearer understanding of the roles of secretagogue binding and internalization, activation of second messenger systems, protein synthesis, and the cytoskeleton in hormone secretion. In the plaque assays described in this chapter individual pituitary cells are isolated in culture free from possible interactive effects coming from other cells. While these interactions are no doubt critical to the understanding of the function of the organism as a whole they can result in totally uninterpretable results. In fact, when we have gained some understanding into the functioning of individual cells it should be possible using the plaque assay to study the interactions among cells in a controlled fashion.

PAF-acether, 1-0-alkyl-2-acetyl-sn-glyceryl-3-phosphorylcholine, is a potent platelet- and neutrophil-activating factor and exhibits antihypertensive properties. Specific secretagogue stimuli induce its release from various cells with proinflammatory activity. Recently two metabolic steps involved in its biosynthesis have been shown to be operational in platelets and macrophages. PAF-acether formation is dependent on phospholipase A2 activation, yielding a 2-lyso derivative which, upon acetylation, is transformed into a molecule indistinguishable from PAFacether. Recognition of at least two distinct steps for PAF-acether synthesis as well as the existence of a modulable acetyltransferase activity provides new sites for the pharmacological control of PAF-acether formation.

Publisher Summary This chapter provides an overview of cell membrane receptors for secretagogues on pancreatic acinar cells. The initial step in the mechanism of action of secretagogues on enzyme secretion from pancreatic acinar cells is reversible binding of the secretagogues to receptors located on the outer surface of the plasma membrane of the pancreatic acinar cell. Binding of the secretagogue to its receptor is both necessary and sufficient to initiate the sequence of events that ultimately causes stimulation of pancreatic enzyme secretion. Several different classes of receptors have been identified for secretagogues that act on the pancreatic acinar cells. The various secretagogues that act on the pancreatic acinar cells stimulate enzyme secretion by activating one of two functionally distinct processes. One process involves binding of the secretagogue to its receptors, mobilization of cellular calcium, and after a series of presently undefined steps, stimulation of enzyme secretion. The other process involves binding of the secretagogue to its receptors, activation of adenylate cyclase, increased cellular cyclic adenosine monophosphate (AMP), activation of cyclic AMP-dependent protein kinase, and after a series of presently undefined steps, stimulation of enzyme secretion. Secretagogues that cause mobilization of cellular calcium do not increase cyclic AMP, and secretagogues that increase cellular cyclic AMP do not cause mobilization of cellular calcium.