A morphological basis for intercellular communication between alpha- and beta-cells in the endocrine pancreas

Nanomaterial journey in the gut: from intestinal mucosal interaction to systemic transport

Engineered nanomaterials (NMs) are commonly utilized in food additives, cosmetics, and therapeutic applications due to their advantageous properties. Consequently, humans are frequently exposed to exogenous nanomaterials through oral ingestion, thus making the intestinal mucosal system a primary site for these particles. Understanding the interactions between nanomaterials and the intestinal mucosal system is crucial for harnessing their therapeutic potential and mitigating potential health risks from unintended exposure. This review aims to elucidate recent advancements in the dual effects of nanomaterials on the intestinal mucosal system. Upon entering the gut lumen, nanomaterials will interact with diverse intestinal components, including trillions of gut microbiota, mucus layer, intestinal epithelial cells (IECs), and the intestinal immune system. Additionally, the systemic fate and transportation of nanomaterials to distal organs, such as central nervous system, are also highlighted. These interactions result in a distinct biological effect of nanomaterials on the multilayer structure of intestine, thus displaying complex journeys and outcomes of nanomaterials in the living body. This in-depth exploration of the in vivo destiny and immunological implications of nanomaterials encountering the intestine has the potential to propel advancements in oral drug delivery techniques and motivate future investigations in novel toxicology research.

Intercellular Communication in the Islet of Langerhans in Health and Disease

Blood glucose homeostasis requires proper function of pancreatic islets, which secrete insulin, glucagon, and somatostatin from the β-, α-, and δ-cells, respectively. Each islet cell type is equipped with intrinsic mechanisms for glucose sensing and secretory actions, but these intrinsic mechanisms alone cannot explain the observed secretory profiles from intact islets. Regulation of secretion involves interconnected mechanisms among and between islet cell types. Islet cells lose their normal functional signatures and secretory behaviors upon dispersal as compared to intact islets and in vivo. In dispersed islet cells, the glucose response of insulin secretion is attenuated from that seen from whole islets, coordinated oscillations in membrane potential and intracellular Ca2+ activity, as well as the two-phase insulin secretion profile, are missing, and glucagon secretion displays higher basal secretion profile and a reverse glucose-dependent response from that of intact islets. These observations highlight the critical roles of intercellular communication within the pancreatic islet, and how these communication pathways are crucial for proper hormonal and nonhormonal secretion and glucose homeostasis. Further, misregulated secretions of islet secretory products that arise from defective intercellular islet communication are implicated in diabetes. Intercellular communication within the islet environment comprises multiple mechanisms, including electrical synapses from gap junctional coupling, paracrine interactions among neighboring cells, and direct cell-to-cell contacts in the form of juxtacrine signaling. In this article, we describe the various mechanisms that contribute to proper islet function for each islet cell type and how intercellular islet communications are coordinated among the same and different islet cell types. © 2021 American Physiological Society. Compr Physiol 11:2191-2225, 2021.

Lelio Orci, the modern master of morphology

Lelio Orci has made seminal contributions to our understanding of pancreatic islet structure and function. He introduced quantitative criteria to structural analysis in the study of endocrine pancreas in a series of works performed in collaboration with Albert Renold, Roger Unger, and Donald Steiner. Orci has moved islet cell morphology from the primitive era of histochemistry and electron microscopy into the modern era of cell biology, applying the most advanced techniques and covering every aspect of normal and pathological structure-function relationships. In collaboration with James Rothman in New York and Randy Schekman in Berkley, Orci discovered that the transport steps from the endoplasmic reticulum to the Golgi complex, and within the Golgi, are mediated by two sets of vesicles coated with protein envelopes different from clathrin.

Generation of Human Stem Cell-Derived Pancreatic Organoids (POs) for Regenerative Medicine

Insulin-dependent diabetes mellitus or type 1 diabetes mellitus (T1DM) is an auto-immune condition characterized by the loss of pancreatic β-cells. The curative approach for highly selected patients is the pancreas or the pancreatic islet transplantation. Nevertheless, these options are limited by a growing shortage of donor organs and by the requirement of immunosuppression.Xenotransplantation of porcine islets has been extensively investigated. Nevertheless, the strong xenoimmunity and the risk of transmission of porcine endogenous retroviruses, have limited their application in clinic. Generation of β-like cells from stem cells is one of the most promising strategies in regenerative medicine. Embryonic, and more recently, adult stem cells are currently the most promising cell sources exploited to generate functional β-cells in vitro. A number of studies demonstrated that stem cells could generate functional pancreatic organoids (POs), able to restore normoglycemia when implanted in different preclinical diabetic models. Nevertheless, a gradual loss of function and cell dead are commonly detected when POs are transplanted in immunocompetent animals. So far, the main issue to be solved is the post-transplanted islet loss, due to the host immune attack. To avoid this hurdle, nanotechnology has provided a number of polymers currently under investigation for islet micro and macro-encapsulation. These new approaches, besides conferring PO immune protection, are able to supply oxygen and nutrients and to preserve PO morphology and long-term viability.Herein, we summarize the current knowledge on bioengineered POs and the stem cell differentiation platforms. We also discuss the in vitro strategies used to generate functional POs, and the protocols currently used to confer immune-protection against the host immune attack (micro- and macro-encapsulation). In addition, the most relevant ongoing clinical trials, and the most relevant hurdles met to move towards clinical application are revised.

Regulation of islet glucagon secretion: Beyond calcium

The islet of Langerhans plays a key role in glucose homeostasis through regulated secretion of the hormones insulin and glucagon. Islet research has focused on the insulin-secreting β-cells, even though aberrant glucagon secretion from α-cells also contributes to the aetiology of diabetes. Despite its importance, the mechanisms controlling glucagon secretion remain controversial. Proper α-cell function requires the islet milieu, where β- and δ-cells drive and constrain α-cell dynamics. The response of glucagon to glucose is similar between isolated islets and that measured in vivo, so it appears that the glucose dependence requires only islet-intrinsic factors and not input from blood flow or the nervous system. Elevated intracellular free Ca²⁺ is needed for α-cell exocytosis, but interpreting Ca²⁺ data is tricky since it is heterogeneous among α-cells at all physiological glucose levels. Total Ca²⁺ activity in α-cells increases slightly with glucose, so Ca²⁺ may serve a permissive, rather than regulatory, role in glucagon secretion. On the other hand, cAMP is a more promising candidate for controlling glucagon secretion and is itself driven by paracrine signalling from β- and δ-cells. Another pathway, juxtacrine signalling through the α-cell EphA receptors, stimulated by β-cell ephrin ligands, leads to a tonic inhibition of glucagon secretion. We discuss potential combinations of Ca²⁺ , cAMP, paracrine and juxtacrine factors in the regulation of glucagon secretion, focusing on recent data in the literature that might unify the field towards a quantitative understanding of α-cell function.

Gap junction proteins are key drivers of endocrine function

It has long been known that the main secretory cells of exocrine and endocrine glands are connected by gap junctions, made by a variety of connexin species that ensure their electrical and metabolic coupling. Experiments in culture systems and animal models have since provided increasing evidence that connexin signaling contributes to control the biosynthesis and release of secretory products, as well as to the life and death of secretory cells. More recently, genetic studies have further provided the first lines of evidence that connexins also control the function of human glands, which are central to the pathogenesis of major endocrine diseases. Here, we summarize the recent information gathered on connexin signaling in these systems, since the last reviews on the topic, with particular regard to the pancreatic beta cells which produce insulin, and the renal cells which produce renin. These cells are keys to the development of various forms of diabetes and hypertension, respectively, and combine to account for the exploding, worldwide prevalence of the metabolic syndrome. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

How to design the surface of peptide-loaded nanoparticles for efficient oral bioavailability?

The oral administration of proteins is a current challenge to be faced in the field of therapeutics. There is currently much interest in nanocarriers since they can enhance oral bioavailability. For lack of a clear definition, the key characteristics of nanoparticles have been highlighted. Specific surface area is one of these characteristics and represents a huge source of energy that can be used to control the biological fate of the carrier. The review discusses nanocarrier stability, mucus interaction and absorption through the intestinal epithelium. The protein corona, which has raised interest over the last decade, is also discussed. The universal ideal surface is a myth and over-coated carriers are not a solution either. Besides, common excipients can be useful on several targets. The suitable design should rather take into account the composition, structure and behavior of unmodified nanomaterials.

Identification of proliferative and mature β-cells in the islets of Langerhans

Insulin-dependent diabetes is a complex multifactorial disorder characterized by loss or dysfunction of β-cells. Pancreatic β-cells differ in size, glucose responsiveness, insulin secretion and precursor cell potential; understanding the mechanisms that underlie this functional heterogeneity might make it possible to develop new regenerative approaches. Here we show that Fltp (also known as Flattop and Cfap126), a Wnt/planar cell polarity (PCP) effector and reporter gene acts as a marker gene that subdivides endocrine cells into two subpopulations and distinguishes proliferation-competent from mature β-cells with distinct molecular, physiological and ultrastructural features. Genetic lineage tracing revealed that endocrine subpopulations from Fltp-negative and -positive lineages react differently to physiological and pathological changes. The expression of Fltp increases when endocrine cells cluster together to form polarized and mature 3D islet mini-organs. We show that 3D architecture and Wnt/PCP ligands are sufficient to trigger β-cell maturation. By contrast, the Wnt/PCP effector Fltp is not necessary for β-cell development, proliferation or maturation. We conclude that 3D architecture and Wnt/PCP signalling underlie functional β-cell heterogeneity and induce β-cell maturation. The identification of Fltp as a marker for endocrine subpopulations sheds light on the molecular underpinnings of islet cell heterogeneity and plasticity and might enable targeting of endocrine subpopulations for the regeneration of functional β-cell mass in diabetic patients.

Role of Connexins and Pannexins in the Pancreas

The pancreas produces enzymes with a digestive function and hormones with a metabolic function, which are produced by distinct cell types of acini and islets, respectively. Within these units, secretory cells coordinate their functioning by exchanging information via signals that flow in the intercellular spaces and are generated either at distance (several neural and hormonal inputs) or nearby the pancreatic cells themselves (inputs mediated by membrane ionic-specific channels and by ionic- and metabolite-permeant pannexin channels and connexin "hemichannels"). Pancreatic secretory cells further interact via the extracellular matrix of the pancreas (inputs mediated by integrins) and directly with neighboring cells, by mechanisms that do not require extracellular mediators (inputs mediated by gap and tight junction channels). Here, we review the expression and function of the connexins and pannexins that are expressed by the main secretory cells of the exocrine and endocrine pancreatic cells. Available data show that the patterns of expression of these proteins differ in acini and islets, supporting distinct functions in the physiological secretion of pancreatic enzymes and hormones. Circumstantial evidence further suggests that alterations in the signaling provided by these proteins are involved in pancreatic diseases.

Cellular communication and heterogeneity in pancreatic islet insulin secretion dynamics

Coordinated pulses of electrical activity and insulin secretion are a hallmark of the islet of Langerhans. These coordinated behaviors are lost when β cells are dissociated, which also leads to increased insulin secretion at low glucose levels. Islets without gap junctions exhibit asynchronous electrical activity similar to dispersed cells, but their secretion at low glucose levels is still clamped off, putatively by a juxtacrine mechanism. Mice lacking β cell gap junctions have near-normal average insulin levels, but are glucose intolerant due to reduced first-phase and pulsatile insulin secretion, illustrating the importance of temporal dynamics. Here, we review the quantitative data on islet synchronization and the current mathematical models that have been developed to explain these behaviors and generate greater understanding of the underlying mechanisms.

In vitro reconstitution of pancreatic islets

The lack of transplantable pancreatic islets is a serious problem that affects the treatment of patients with type 1 diabetes mellitus. Beta cells can be induced from various sources of stem or progenitor cells, including induced pluripotent stem cells in the near future; however, the reconstitution of islets from β cells in culture dishes is challenging. The generation of highly functional islets may require three-dimensional spherical cultures that resemble intact islets. This review discusses recent advances in the reconstitution of islets. Several factors affect the reconstitution of pseudoislets with higher functions, such as architectural similarity, cell-to-cell contact, and the production method. The actual transplantation of naked or encapsulated pseudoislets and islet-like cell clusters from various stem cell sources is also discussed. Advancing our understanding of the methods used to reconstitute pseudoislets should expand the range of potential strategies available for developing de novo islets for therapeutic applications.

Connexins and β-cell functions

Proper functioning of pancreatic islets requires that numerous β-cells are properly coordinated. With evolution, many mechanisms have converged, which now allow individual β-cells to sense the state of activity of their neighbors as well as the changes taking place in the extracellular medium, and to regulate accordingly their own function. Here, we review one such mechanism for intercellular coordination, which depends on connexins. These integral membrane proteins accumulate at sites of close apposition between adjacent islet cell membranes, referred to as gap junctions. Recent evidence demonstrates that connexin-dependent signaling is relevant for the in vivo control of insulin biosynthesis and release, as well as for the survival of β-cells under stressing conditions. The data suggest that alterations of this signaling may be implicated in the β-cell alterations which characterize most forms of diabetes, raising the tantalizing possibility that targeting of the direct intercellular communications β-cells establish within each pancreatic islet may provide a novel, therapeutically useful strategy.

Protein-mediated interactions of pancreatic islet cells

The islets of Langerhans collectively form the endocrine pancreas, the organ that is soley responsible for insulin secretion in mammals, and which plays a prominent role in the control of circulating glucose and metabolism. Normal function of these islets implies the coordination of different types of endocrine cells, noticeably of the beta cells which produce insulin. Given that an appropriate secretion of this hormone is vital to the organism, a number of mechanisms have been selected during evolution, which now converge to coordinate beta cell functions. Among these, several mechanisms depend on different families of integral membrane proteins, which ensure direct (cadherins, N-CAM, occludin, and claudins) and paracrine communications (pannexins) between beta cells, and between these cells and the other islet cell types. Also, other proteins (integrins) provide communication of the different islet cell types with the materials that form the islet basal laminae and extracellular matrix. Here, we review what is known about these proteins and their signaling in pancreatic β -cells, with particular emphasis on the signaling provided by Cx36, given that this is the integral membrane protein involved in cell-to-cell communication, which has so far been mostly investigated for effects on beta cell functions.

Hyaluronan and hyaluronan binding proteins are normal components of mouse pancreatic islets and are differentially expressed by islet endocrine cell types

The pancreatic islet comprises endocrine, vascular, and neuronal cells. Signaling among these cell types is critical for islet function. The extracellular matrix (ECM) is a key regulator of cell-cell signals, and while some islet ECM components have been identified, much remains unknown regarding its composition. We investigated whether hyaluronan, a common ECM component that may mediate inflammatory events, and molecules that bind hyaluronan such as versican, tumor necrosis factor-stimulated gene 6 (TSG-6), and components of inter-α-trypsin inhibitor (IαI), heavy chains 1 and 2 (ITIH1/ITIH2), and bikunin, are normally produced in the pancreatic islet. Mouse pancreata and isolated islets were obtained for microscopy (with both paraformaldehyde and Carnoy's fixation) and mRNA. Hyaluronan was present predominantly in the peri-islet ECM, and hyaluronan synthase isoforms 1 and 3 were also expressed in islets. Versican was produced in α cells; TSG-6 in α and β cells; bikunin in α, β, and δ cells; and ITIH1/ITIH2 predominantly in β cells. Our findings demonstrate that hyaluronan, versican, TSG-6, and IαI are normal islet components and that different islet endocrine cell types contribute these ECM components. Thus, dysfunction of either α or β cells likely alters islet ECM composition and could thereby further disrupt islet function.

Selective expression of CYP2A13 in human pancreatic α-islet cells

Exposure to cigarette smoke is an etiological factor of human pancreatic cancer and has been associated with an increased risk of pancreatic diseases, including pancreatitis and diabetes. The toxicants in cigarette smoke can reach pancreatic tissue, and most of the toxicants require cytochrome P450 (P450)-mediated metabolic activation to exert their toxicity. Among all the human P450 enzymes, CYP2A13 is the most efficient enzyme in the metabolic activation of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), a major tobacco-specific toxicant and a suspected human carcinogen. It also metabolically activates 4-aminobiphenyl, another toxicant in cigarette smoke. Immunohistochemical analysis in this study demonstrated that CYP2A13 was selectively expressed in the islets but not in the exocrine portion of adult human pancreas. Further study using dual immunofluorescence labeling technique showed that CYP2A13 protein was mainly expressed in the α-islet but not in β-islet cells. The selective expression of CYP2A13 in human pancreatic α-islet cells suggests that these islet cells could be damaged by the toxicants existing in cigarette smoke through CYP2A13-mediated in situ metabolic activation. Our result provides a mechanistic insight for human pancreatic diseases that have been associated with cigarette smoke exposure.

Paracrine interactions within islets of Langerhans

Glucose supply fluctuates between meal and fasting periods and its consumption by the body varies greatly depending on bodily metabolism. Pancreatic islets of Langerhans secrete various endocrine hormones including insulin and glucagon to keep blood glucose level relatively constant. Additionally, islet hormones regulate activity of neighboring cells as local autocrine or paracrine modulators. Moreover, islet cells release neurotransmitters such as glutamate and γ-aminobutyric acid (GABA) to gain more precise regulation of hormones release kinetics. Excitatory glutamate is co-released with glucagon from α-cells and activates glutamate receptors in the neighboring cells. GABA released from β-cells was shown to inhibit α-cells but to activate β-cells by acting GABA(A) receptors. This review summarizes the recent progress in understanding the paracrine/autocrine interactions in islets.

Adhesion molecule CADM1 contributes to gap junctional communication among pancreatic islet α-cells and prevents their excessive secretion of glucagon

Cell adhesion molecule-1 (CADM1) is a recently identified adhesion molecule of pancreatic islet α-cells that mediates nerve-α-cell interactions via trans-homophilic binding and serves anatomical units for the autonomic control of glucagon secretion. CADM1 also mediates attachment between adjacent α-cells. Since gap junctional intercellular communication (GJIC) among islet cells is essential for islet hormone secretion, we examined whether CADM1 promotes GJIC among α-cells and subsequently participates in glucagon secretion regulation. Dye transfer assays using αTC6 mouse α-cells, which endogenously express CADM1, supported this possibility; efficient cell-to-cell spread of gap junction-permeable dye was detected in clusters of αTC6 cells transfected with nonspecific, but not with CADM1-targeting, siRNA. Immunocytochemical analysis of connexin 36, a major component of the gap junction among αTC6 cells, revealed that it was localized exclusively to the cell membrane in CADM1-non-targeted αTC6 cells, but diffusely to the cytoplasm in CADM1-targeted cells. Next, we incubated CADM1-targeted and non-targeted αTC6 cells in a medium containing 1 mM glucose and 200 mM arginine for 30 min to induce glucagon secretion, and found that the targeted cells secreted three times more glucagon than did the non-targeted. We conducted similar experiments using pancreatic islets that were freshly isolated from wild-type and CADM1-knockout mice, and expressed glucagon secretion as ratios relative to baseline values. The increase in ratio was larger in CADM1-knockout islets than in wild-type islets. These results suggest that CADM1 may serve as a volume limiter of glucagon secretion by sustaining α-cell attachment necessary for efficient GJIC.

Connexin-dependent signaling in neuro-hormonal systems

The advent of multicellular organisms was accompanied by the development of short- and long-range chemical signalling systems, including those provided by the nervous and endocrine systems. In turn, the cells of these two systems have developed mechanisms for interacting with both adjacent and distant cells. With evolution, such mechanisms have diversified to become integrated in a complex regulatory network, whereby individual endocrine and neuro-endocrine cells sense the state of activity of their neighbors and, accordingly, regulate their own level of functioning. A consistent feature of this network is the expression of connexin-made channels between the (neuro)hormone-producing cells of all endocrine glands and secretory regions of the central nervous system so far investigated in vertebrates. This review summarizes the distribution of connexins in the mammalian (neuro)endocrine systems, and what we know about the participation of these proteins on hormone secretion, the life of the producing cells, and the action of (neuro)hormones on specific targets. The data gathered since the last reviews on the topic are summarized, with particular emphasis on the roles of Cx36 in the function of the insulin-producing beta cells of the endocrine pancreas, and of Cx40 in that of the renin-producing juxta-glomerular epithelioid cells of the kidney cortex. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Connexins: key mediators of endocrine function

The appearance of multicellular organisms imposed the development of several mechanisms for cell-to-cell communication, whereby different types of cells coordinate their function. Some of these mechanisms depend on the intercellular diffusion of signal molecules in the extracellular spaces, whereas others require cell-to-cell contact. Among the latter mechanisms, those provided by the proteins of the connexin family are widespread in most tissues. Connexin signaling is achieved via direct exchanges of cytosolic molecules between adjacent cells at gap junctions, for cell-to-cell coupling, and possibly also involves the formation of membrane "hemi-channels," for the extracellular release of cytosolic signals, direct interactions between connexins and other cell proteins, and coordinated influence on the expression of multiple genes. Connexin signaling appears to be an obligatory attribute of all multicellular exocrine and endocrine glands. Specifically, the experimental evidence we review here points to a direct participation of the Cx36 isoform in the function of the insulin-producing β-cells of the endocrine pancreas, and of the Cx40 isoform in the function of the renin-producing juxtaglomerular epithelioid cells of the kidney cortex.

Glucose regulates free cytosolic Zn²⁺ concentration, Slc39 (ZiP), and metallothionein gene expression in primary pancreatic islet β-cells

Zn²⁺ is an important cofactor for insulin biosynthesis and storage in pancreatic β-cells. Correspondingly, polymorphisms in the SLC30A8 gene, encoding the secretory granule Zn²⁺ transporter ZnT8, are associated with type 2 diabetes risk. Using a genetically engineered (FRET)-based sensor (eCALWY-4), we show here that elevated glucose time-dependently increases free cytosolic Zn²⁺ ([Zn²⁺](cyt)) in mouse pancreatic β-cells. These changes become highly significant (853 ± 96 pm versus 452 ± 42 pm, p < 0.001) after 24 h and are associated with increased expression of the Zn²⁺ importer family members Slc39a6, Slc39a7, and Slc39a8, and decreased expression of metallothionein 1 and 2. Arguing that altered expression of the above genes is not due to altered [Zn²⁺](cyt), elevation of extracellular (and intracellular) [Zn²⁺] failed to mimic the effects of high glucose. By contrast, increases in intracellular cAMP prompted by 3-isobutyl-1-methylxanthine and forskolin partially mimicked the effects of glucose on metallothionein, although not ZiP, gene expression. Modulation of intracellular Ca²⁺ and insulin secretion with pharmacological agents (tolbutamide and diazoxide) suggested a possible role for changes in these parameters in the regulation of Slc39a6 and Slc39a7 but not Slc39a8, nor metallothionein expression. In summary, 1) glucose induces increases in [Zn²⁺](cyt), which are then likely to facilitate the processing and/or the storage of insulin and its cocrystallization with Zn²⁺, and 2) these increases are associated with elevated expression of zinc importers. Conversely, a chronic increase in [Zn²⁺](cyt) following sustained hyperglycemia may contribute to β-cell dysfunction and death in some forms of diabetes.

Characteristics and functions of {alpha}-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors expressed in mouse pancreatic {alpha}-cells

Pancreatic islet cells use neurotransmitters such as l-glutamate to regulate hormone secretion. We determined which cell types in mouse pancreatic islets express ionotropic glutamate receptor channels (iGluRs) and describe the detailed biophysical properties and physiological roles of these receptors. Currents through iGluRs and the resulting membrane depolarization were measured with patch-clamp methods. Ca(2+) influx through voltage-gated Ca(2+) channels and Ca(2+)-evoked exocytosis were detected by Ca(2+) imaging and carbon-fiber microamperometry. Whereas iGluR2 glutamate receptor immunoreactivity was detected using specific antibodies in immunocytochemically identified mouse alpha- and beta-cells, functional iGluRs were detected only in the alpha-cells. Fast application of l-glutamate to cells elicited rapidly activating and desensitizing inward currents at -60 mV. By functional criteria, the currents were identified as alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors. They were activated and desensitized by AMPA, and were activated only weakly by kainate. The desensitization by AMPA was inhibited by cyclothiazide, and the currents were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Islet iGluRs showed nonselective cation permeability with a low Ca(2+) permeability (P(Ca)/P(Na) = 0.16). Activation of the AMPA receptors induced a sequence of cellular actions in alpha-cells: 1) depolarization of the membrane by 27 +/- 3 mV, 2) rise in intracellular Ca(2+) mainly mediated by voltage-gated Ca(2+) channels activated during the membrane depolarization, and 3) increase of exocytosis by the Ca(2+) rise. In conclusion, iGluRs expressed in mouse alpha-cells resemble the low Ca(2+)-permeable AMPA receptor in brain and can stimulate exocytosis.

Islet-cell-to-cell communication as basis for normal insulin secretion

The emergence of pancreatic islets has necessitated the development of a signalling system for the intra- and inter-islet coordination of beta cells. With evolution, this system has evolved into a complex regulatory network of partially cross-talking pathways, whereby individual cells sense the state of activity of their neighbours and, accordingly, regulate their own level of functioning. A consistent feature of this network in vertebrates is the expression of connexin (Cx)-36-made cell-to-cell channels, which cluster at gap junction domains of the cell membrane, and which adjacent beta cells use to share cytoplasmic ions and small metabolites within individual islets. This chapter reviews what is known about Cx36, and the mechanism whereby this beta-cell connexin significantly regulates insulin secretion. It further outlines other less established functions of the protein and evaluates its potential relevance for the development of novel therapeutic approaches to diabetes.

Pro-protein convertases in intermediary metabolism: islet hormones, brain/gut hormones and integrated physiology

Many peptide hormones implicated in the regulation of intermediary metabolism arise from larger precursors called prohormones. These precursors are cut into pieces by proprotein convertases, more precisely those called prohormone convertases (PCs) that cleave at the C terminus of basic doublets. The remaining basic amino acids are eliminated by a specialized carboxypeptidase, leading to the active hormone. This processing may provide, from a single precursor, several peptides with different biological activities depending on the site(s) of cleavage on the precursor. When the processing is tissue-specific, this mechanism allows to produce, from a single protein, different sets of hormones depending on the tissue considered, leading to novel regulatory processes. The archetype of such a pluripotent prohormone in the field of intermediary metabolism is pro-glucagon that, when cut by PC1 in intestinal L cells, produces four different peptides with different specificities [glicentin, oxyntomodulin (OXM), glucagon-like peptide-1, and glucagon-like peptide-2], whereas, when cut by PC2 in the alpha cells of the endocrine pancreas, glucagon is produced and, through the supplementary action of NRD convertase, a fragment of glucagon (miniglucagon) with original properties.

Islet alpha-cells do not influence insulin secretion from beta-cells through cell-cell contact

Interactions between the endocrine cells in islets of Langerhans influence their secretory function, and disruption of islet structure results in impaired insulin secretory responses to both nutrient and non-nutrient stimuli. We have previously demonstrated that insulin-secreting MIN6 cells show enhanced secretory responses when grown as islet-like structures (pseudoislets) suggesting that homotypic cell-cell interactions between beta-cells are important for normal function. We have now extended this experimental model to study the role of heterotypic interactions between insulin-expressing and glucagon-expressing cells by measuring the organization and secretory function of pseudoislets formed from MIN6 and alphaTC1 cells. The direct alpha-cell to beta-cell contact in the heterogenous MIN6/alphaTC1 pseudoislets was sufficient to enable the formation of anatomically correct islet-like structures, with a central core of MIN6 cells surrounded by a periphery of alphaTC1 cells. However, the presence of alphaTC1 cells had no detectable effect on insulin secretory responses to nutrient or non-nutrient stimuli. In contrast, exogenous glucagon enhanced insulin secretion, in accordance with a paracrine role for alpha-cell-derived glucagon in the regulation of insulin secretion rather than direct, contact-mediated effects of alpha-cells on neighbouring beta-cells.

Homotypic cell contact enhances insulin but not glucagon secretion

Intra-islet interactions influence beta-cell function, and disruption of islet architecture results in a reduction in glucose-induced insulin secretion, whereas re-aggregation improves secretory responsiveness. Our studies on MIN6 cells have shown that by configuring beta-cells as three-dimensional islet-like structures there is a marked improvement in glucose-induced insulin secretion compared to that of their monolayer equivalents. In the present study, we have used the mouse glucagon-secreting alphaTC1 cell line to see whether homotypic interactions are important in the regulation of glucagon secretion from alpha-cells. We found no significant difference in the secretory responses of alphaTC1 cells maintained as monolayers or as cell clusters. We also found that different cell adhesion molecules are involved in cell interactions between alpha- and beta-cells; MIN6 cells express ECAD, whereas alphaTC1 cells express NCAM. ECAD is necessary for cell cluster formation by MIN6 cells but not by alphaTC1 cells, whereas NCAM is not needed for the formation of cell clusters in either cell line.

Association of connexin36 with zonula occludens-1 in HeLa cells, betaTC-3 cells, pancreas, and adrenal gland

The PDZ domain-containing protein zonula occludens-1 (ZO-1), a well-established component of tight junctions, has recently been shown to interact with various connexin proteins that form gap junctions. We investigated the association of connexin36 (Cx36) with ZO-1 in various cultured cells and tissues. Punctate immunofluorescence labeling for Cx36 was detected in Cx36-transfected HeLa cells, betaTC-3 cells, pancreatic islets, and adrenal medulla. Immunofluorescence for ZO-1 was also punctate in cells and tissues, and was colocalized with Cx36 at points of cell-cell contact. Immunoprecipitation of either Cx36 or ZO-1 from cell lysates and tissue homogenates resulted in immunoblot detection of ZO-1 or Cx36, respectively, in immunoprecipitates. A 14-amino acid peptide corresponding to the carboxy-terminus of Cx36 showed binding capacity to the PDZ1 domain of ZO-1, which was eliminated after removal of the last 4 carboxy-terminus amino acids. Low micromolar concentrations of the 14-amino acid peptide produced up to 85% inhibition of Cx36 interaction with the PDZ1 domain of ZO-1. These results provide evidence for molecular interaction between Cx36 and ZO-1 in vitro, and in vivo, and suggest that the interference with Cx36/ZO-1 interaction by short carboxy-terminus peptides of Cx36 may be of value for functional studies of this interaction.

A reappraisal of the blood glucose homeostat which comprehensively explains the type 2 diabetes mellitus-syndrome X complex

Blood glucose concentrations are unaffected by exercise despite very high rates of glucose flux. The plasma ionised calcium levels are even more tightly controlled after meals and during lactation. This implies 'integral control'. However, pairs of integral counterregulatory controllers (e.g. insulin and glucagon, or calcitonin and parathyroid hormone) cannot operate on the same controlled variable, unless there is some form of mutual inhibition. Flip-flop functional coupling between pancreatic alpha- and beta-cells via gap junctions may provide such a mechanism. Secretion of a common inhibitory chromogranin by the parathyroids and the thyroidal C-cells provides another. Here we describe how the insulin:glucagon flip-flop controller can be complemented by growth hormone, despite both being integral controllers. Homeostatic conflict is prevented by somatostatin-28 secretion from both the hypothalamus and the pancreatic islets. Our synthesis of the information pertaining to the glucose homeostat that has accumulated in the literature predicts that disruption of the flip-flop mechanism by the accumulation of amyloid in the pancreatic islets in type 2 diabetes mellitus will lead to hyperglucagonaemia, hyperinsulinaemia, insulin resistance, glucose intolerance and impaired insulin responsiveness to elevated blood glucose levels. It explains syndrome X (or metabolic syndrome) as incipient type 2 diabetes in which the glucose control system, while impaired, can still maintain blood glucose at the desired level. It also explains why it is characterised by high plasma insulin levels and low plasma growth hormone levels, despite normoglycaemia, and how this leads to central obesity, dyslipidaemia and cardiovascular disease in both syndrome X and type 2 diabetes.

On-line analysis of gap junctions reveals more efficient electrical than dye coupling between islet cells

Pancreatic beta-cells constitute a well-communicating multicellular network that permits a coordinated and synchronized signal transmission within the islet of Langerhans that is necessary for proper insulin release. Gap junctions are the molecular keys that mediate functional cellular connections, which are responsible for electrical and metabolic coupling in the majority of cell types. Although the role of gap junctions in beta-cell electrical coupling is well documented, metabolic communication is still a matter of discussion. Here, we have addressed this issue by use of a fluorescence recovery after photobleaching (FRAP) approach. This technique has been validated as a reliable and noninvasive approach to monitor functional gap junctions in real time. We show that control pancreatic islet cells did not exchange a gap junction-permeant molecule in either clustered cells or intact islets of Langerhans under conditions that allowed cell-to-cell exchange of current-carrying ions. Conversely, we have detected that the same probe was extensively transferred between islet cells of transgenic mice expressing connexin 32 (Cx32) that have enhanced junctional coupling properties. The results indicate that the electrical coupling of native islet cells is more extensive than dye communication. Dye-coupling domains in islet cells appear more restricted than previously inferred with other methods.

Cellular function in multicellular system for hormone-secretion: electrophysiological aspect of studies on alpha-, beta- and delta-cells of the pancreatic islet

We review a new method to explore the cellular functions in multicellular system by application of the perforated patch-clamp technique to intact pancreatic islet of Langerhans. Using this approach, the integrity of the islet is preserved and intercellular communication via gap junctions and paracrine processes are maintained. By using low-resistance patch electrodes, rapid current responses can be monitored under voltage-clamp control. We have applied this methodology to answer questions not resolved by patch-clamp experiments on isolated single insulin-secreting beta-cells. First, the role of a K(+)-current dependent on Ca(2+)-influx for the termination of burst of action potentials in beta-cells could be documented. Neither the current, nor the bursting pattern of electrical activity is preserved in isolated beta-cells. Second, the conductance of gap junctions (approximately 1 nS) between beta-cells was determined. Third, electrical properties of glucagon-producing alpha- and somatostatin-secreting delta-cells and the different mechanisms for glucose-sensing in these cells could be explored. The findings emanating from these experiments may have implications for neuroscience research such as the mechanism of oscillatory electrical activity in general and processes involved in the glucose-sensing in some neurons, which response to changes of blood glucose concentration.

Miniglucagon (glucagon 19-29): a novel regulator of the pancreatic islet physiology

Miniglucagon, the COOH-terminal (19-29) fragment processed from glucagon, is a potent and efficient inhibitor of insulin secretion from the MIN 6 beta-cell line. Using the rat isolated-perfused pancreas, we investigated the inhibitory effect of miniglucagon on insulin secretion and evaluated the existence of an inhibitory tone exerted by this peptide inside the islet. Miniglucagon dose-dependently inhibited insulin secretion stimulated by 8.3 mol/l glucose, with no change in the perfusion flow rate. A concentration of 1 nmol/l miniglucagon had a significant inhibitory effect on a 1 nmol/l glucagon-like peptide 1 (7-36) amide-potentiated insulin secretion. A decrease in extracellular glucose concentration simultaneously stimulated glucagon and miniglucagon secretion from pancreatic alpha-cells. Using confocal and electron microscopy analysis, we observed that miniglucagon is colocalized with glucagon in mature secretory granules of alpha-cells. Perfusion of an anti-miniglucagon antiserum directed against the biologically active moiety of the peptide resulted in a more pronounced effect of a glucose challenge on insulin secretion, indicating that miniglucagon exerts a local inhibitory tone on beta-cells. We concluded that miniglucagon is a novel local regulator of the pancreatic islet physiology and that any abnormal inhibitory tone exerted by this peptide on the beta-cell would result in an impaired insulin secretion, as observed in type 2 diabetes.

The chromogranins and the counter-regulatory hormones: do they make homeostatic sense?

1. The chromogranins are ubiquitous proteins which are co-stored and co-secreted with many peptide hormones. All appear to be powerful inhibitors of endocrine secretions. This poses a problem. 2. When endocrine glands are involved in the efferent limbs of homeostatic loops, they are message transmitters. The self-inhibition caused by the co-secretion of a chromogranin will, on the face of it, erase the message. 3. Pairs of counter-regulatory homeostatic hormones also present a problem. 4. If both members of the pair have clearly defined set points, as suggested by their 'time integral' (or 'growth with time') responsiveness to deviations from set point, then, if the two set points are not exactly the same, one or other member will always register an error, leading, eventually, to an overwhelmingly large and unnecessary response. 5. Our model eliminates both paradoxes, and emphasizes the importance of counter-regulation and the co-secretion of chromogranins in 'zero steady-state error' (ZSSE) homeostasis. 6. If hormone A is secreted into the blood in progressively increasing amounts when [Q], the plasma concentration of substance Q, is low, and in decreasing amounts when [Q] is high; and hormone B responds in the opposite manner, then there will be a [Q], designated [Q]p, at which the secretory rate increase, or decrease, of the two hormones is exactly the same. 7. If, in addition, the secretion of both hormones is stimulated by low plasma chromogranin levels, [Cg], but inhibited by high [Cg] then there will be a different [Q]p for every chromogranin concentration in the blood. 8. At one of these points (at a unique [Q] and [Cg]) the concentration of neither hormone will increase or decrease. This is the equilibrium point to which, according to our model, the system always returns regardless of disturbances within physiological limits. 9. This is robust ZSSE control.

Magnitude and modulation of pancreatic beta-cell gap junction electrical conductance in situ

The parallel gap junction electrical conductance between a beta-cell and its nearest neighbors was measured by using an intracellular microelectrode to clamp the voltage of a beta-cell within a bursting islet of Langerhans. The holding current records consisted of bursts of inward current due to the synchronized oscillations in membrane potential of the surrounding cells. The membrane potential record of the impaled cell, obtained in current clamp mode, was used to estimate the behavior of the surrounding cells during voltage clamp, and the coupling conductance was calculated by dividing the magnitude of the current bursts by that of the voltage bursts. The histogram of coupling conductance magnitude from 26 cells was bimodal with peaks at 2.5 and 3.5 nS, indicating heterogeneity in extent of electrical communication within the islet of Langerhans. Gap junction conductance reversibly decreased when the temperature was lowered from 37 to 30 degrees C and when the extracellular calcium concentration was raised from 2.56 to 7.56 mM. The coupling conductance decreased slightly during the active phase of the burst. Activation of adenylate cyclase with forskolin (10 microM) resulted in an increase in cell-to-cell electrical coupling. We conclude that beta-cell gap junction conductance can be measured in situ under near physiological conditions. Furthermore, the magnitude and physiological regulation of beta-cell gap junction conductance suggest that intercellular electrical communication plays an important role in the function of the endocrine pancreas.

Isolated pancreatic islets of the rat: an immunohistochemical and morphometric study

Although there is a recent increase in the use of the isolated pancreatic islets of the rat in the transplantation and functional studies, there has been no detailed quantitative assessment on the size and cellular constituents of islets after the isolation procedure. The present work was undertaken to study the size classes of the isolated islets and the morphometry of their cellular populations. Islets of the rat pancreas were isolated by using the intraductal collagenase digestion technique, the most commonly used procedure for the isolation of pancreatic islets. Different endocrine cells of the isolated islets were stained by immunoperoxidase staining techniques. The distribution of the cellular constituents of the isolated islets was similar to that of the intact islets of the normal pancreas; A, D, and PP cells were peripherally arranged around the centrally located B cells. However, morphometric quantitative study showed that the percent volume and percent number of A, D, and PP cells of the isolated islets were lower than those of the corresponding intact ones. Further, the mean true diameter of the isolated islets was lower than that of the intact ones. These data indicate loss of islet cells during the process of isolation. Most of the lost cells were from the periphery of islets. This may provide an explanation for the incomplete metabolic control and recurrence of hyperglycemia encountered after isolated islet transplantation in the treatment of diabetes mellitus. It seems that further refinements of the isolation techniques are necessary to obtain islet tissue with total cellular integrity, before a complete success in transplantation could be achieved.

Gap-junction quantification in biological tissues: freeze-fracture replicas versus thin sections

The relative efficiency of freeze-fracture replicas versus thin sections for the visualization and quantification of gap junctions in biological tissues has been evaluated. Both methods may underestimate gap-junction number--thin sections for reasons of tissue resolution and freeze-fracture replicas due to the mechanics of the fracturing process. Freeze-fracture misses gap junctions in regions of plasma membrane which are highly contoured, such as the overlapping basal cell processes of Drosophila imaginal wing discs and the interdigitating lateral membrane plications of intercalated discs in cardiac tissue. If the missed gap junctions are relatively large, as they are in both of these examples, freeze-fracture significantly underestimates the total gap-junctional area. Thin sections may miss small gap junctions, but in tissues which contain a range of gap-junction sizes the lost junctions constitute a relatively small fraction of the total junctional area. In neoplastic imaginal wing discs, thin sections were as efficient as freeze-fracture replicas in identifying even the smallest gap junctions. Although freeze-fracture may be the better technique for the qualitative and quantitative documentation of small gap junctions in tissues with relatively flat to gently contoured plasma membranes and thin sections may be the superior method for gap-junction quantification in tissues containing a range of gap-junctional sizes and highly contoured cellular processes, the data suggest that a combination of the two approaches should be utilized whenever possible.

Altered anomeric specificity of glucose-induced insulin release in rabbits with duct-ligated pancreas

Ligation of the pancreatic duct in rabbits provokes a decrease in the insulin and glucagon content of the pancreas, and may lead to chronic hyperglycemia. The insulin secretory behavior of the perfused pancreas is perturbed in duct-ligated animals, and this is illustrated in several respects: 1. The steady-state insulin output evoked by L-leucine (10 mM) is higher in duct-ligated than control rabbits; 2. In the presence of the amino acid, the response to D-glucose is characterized by a delayed onset, the absence of an early secretory peak, and a sluggish return towards basal value upon removal of the hexose from the perfusate; and 3. Whereas control rabbits display a higher secretory response to alpha- than beta-D-glucose, such is no more the case in duct-ligated rabbits. The perturbation of the anomeric specificity in secretory response is most obvious in diabetic duct-ligated rabbits, in which case beta-D-glucose stimulates insulin release more efficiently than alpha-D-glucose. In both control and duct-ligated rabbits, however, the alpha-anomer is more potent than the beta-anomer in suppressing leucine-stimulated glucagon secretion. These findings are compatible with the view that chronic hyperglycemia leads to alteration in the anomeric preference of the pancreatic B-cell for alpha-D-glucose, possibly as a result of the nonenzymatic glycation of glycolytic enzymes in insulin-producing, but not glucagon-producing, islet cells.

Glucagon gene transcription is negatively regulated by insulin in a hamster islet cell line

Complex interrelationships exist between the four pancreatic islet cell types and their respective secretory products, insulin, glucagon, somatostatin, and pancreatic polypeptide. These hormones are known to interact with the different islet cells and modulate their functions. Insulin inhibits glucagon secretion from the A cell both in vivo and in vitro and, in states of insulin deficiency, high glucagon levels are observed that are normalized by insulin replacement. To determine if insulin also regulates glucagon biosynthesis, we studied its effects on glucagon gene expression. Our studies indicate that insulin, in a dose-dependent fashion decreases steady-state glucagon mRNA levels in a clonal hamster islet cell line, In-R1-G9; this decrease is secondary to an inhibition of glucagon gene transcription as assessed by transcriptional run-on assays and does not involve detectable changes in mRNA stability. Inhibition of glucagon gene transcription is accompanied by corresponding decreases in glucagon immunoreactivity in both cell extracts and medium. We conclude that insulin may not only regulate glucagon secretion but also glucagon gene expression.

The anterograde and retrograde infusion of glucagon antibodies suggests that A cells are vascularly perfused before D cells within the rat islet

We have suggested that the order of cellular vascular perfusion within the islet is important in the regulation of islet hormone secretion. Anatomically, the A and D cells appear to be randomly dispersed throughout the mantle. Although islet capillary blood flow is known to be from the B-cell core to the A- and D-cell mantle, it has not yet been established whether the cells of the mantle may influence one another vascularly. Rat pancreata were perfused in vitro anterogradely and retrogradely with or without glucagon antibody in order to determine the order of cellular perfusion and interaction between the A and D cells in the islet mantle. Anterograde infusion of glucagon antibody did not affect insulin secretion, but rapidly decreased somatostatin secretion -46 +/- 8%, (p less than 0.005). Retrograde infusion of glucagon antibody decreased insulin secretion (-27 +/- 8%, p less than 0.005) but had no effect upon somatostatin secretion. This study not only confirms a core to mantle islet perfusion but also establishes that the A cell precedes the D cell in the terms of vascular perfusion. Thus within the islet, vascular borne insulin regulates the release of glucagon, which in turn, regulates the release of somatostatin. Somatostatin is vascularly neutral owing to its downstream position in the sequence (B to A to D) of cellular perfusion.

The gap junction: a channel for multiple functions?

Gap junctions are specialized membrane structures that enable the intercytoplasmic exchange of small molecules and ions between contacting cells. During the past decade, biophysical and structural analyses of the junctional channel have considerably increased our understanding of the pharmacological properties and gating mechanisms of gap junctions. Despite this impressive amount of work, until recently the physiological role of these ubiquitous intercellular pathways has remained speculative in most tissues. This review summarizes the most recent information obtained on the structure of the gap junction by molecular cloning of the major protein components and emphasizes the growing evidence for their functional role in adult tissues formed by highly differentiated secretory cells. The relevance of cell-to-cell coupling for the co-ordinated function of the exocrine and endocrine pancreas is discussed.

The order of islet microvascular cellular perfusion is B----A----D in the perfused rat pancreas

In order to determine whether microvascular blood flow is important in the regulation of intra-islet cellular interactions, rat pancreata were isolated and perfused in vitro, both anterogradely or retrogradely, with and without anti-insulin or anti-somatostatin gamma-globulin. Expressed as percent change, anterograde infusion of insulin antibody increased efflux concentrations of glucagon (110 +/- 20%, P less than 0.0005) and somatostatin (2,112 +/- 73%, P less than 0.0005) above their respective control. Retrograde infusion of insulin antibody did not affect efflux concentrations of glucagon (P less than 0.50) or somatostatin (P less than 0.50). The anterograde infusion of anti-somatostatin antibody had no effect upon insulin (P less than 0.50) or glucagon (P less than 0.50) efflux concentrations, whereas retrograde anti-somatostatin antibody infusion produced immediate increases in efflux concentrations of both insulin (115 +/- 33%, P less than 0.0005) and glucagon (77 +/- 8%, P less than 0.0005). These results strongly suggest that (a) the vascular compartment is important in the regulation of intra-islet cellular interactions and further suggest that (b) the order of islet cellular perfusion and interaction is from the B cell core outward to the mantle, and (c) the mantle is further subordered with the majority of D cells downstream or distal to the majority of A cells. Thus, in the vascular compartment, B cells inhibit A-cell secretion and A cells stimulate D-cell secretion.

Voltage-activated currents in guinea pig pancreatic alpha 2 cells. Evidence for Ca2+-dependent action potentials

Glucagon-secreting alpha 2 cells were isolated from guinea pig pancreatic islets and used for electrophysiological studies of voltage-activated ionic conductances using the patch-clamp technique. The alpha 2 cells differed from beta cells in producing action potentials in the absence of glucose. The frequency of these potentials increased after addition of 10 mM arginine but remained unaffected in the presence of 5-20 mM glucose. When studying the conductances underlying the action potentials, we identified a delayed rectifying K+ current, an Na+ current, and a Ca2+ current. The K+ current activated above -20 mV and then increased with the applied voltage. The Na+ current developed at potentials above -50 mV and reached a maximal peak amplitude of 550 pA during depolarizing pulses to -15 mV. The Na+ current inactivated rapidly (tau h approximately 0.7 ms at 0 mV). Half-maximal steady state inactivation was attained at -58 mV, and currents could no longer be elicited after conditioning pulses to potentials above -40 mV. The Ca2+ current first became detectable at -50 mV and reached a maximal amplitude of 90 pA (in extracellular [Ca2+] = 2.6 mM) at about -10 mV. Unlike the Na+ current, it inactivated little or not at all. Membrane potential measurements demonstrated that both the Ca2+ and Na+ currents contribute to the generation of the action potential. Whereas there was an absolute requirement of extracellular Ca2+ for action potentials to be elicited at all, suppression of the much larger Na+ current only reduced the upstroke velocity of the spikes. It is suggested that this behavior reflects the participation of a low-threshold Ca2+ conductance in the pacemaking of alpha 2 cells.

Anomeric specificity of glucose-induced somatostatin secretion

In isolated perfused rat pancreases, the alpha-anomer of D-glucose is more potent than beta-D-glucose not solely in stimulating insulin release and suppressing glucagon output, but also in causing somatostatin secretion.

Morphology and endocrine production of cells in the islets of Langerhans of the Chacma baboon

Biopsies of the pancreas head, tail, and uncinate regions of 6 Chacma baboons (Papio ursinus) were processed for ultrastructural and immunocytochemical (ICC) studies using avidin-biotin peroxidase label for light microscopy (LM) and immunogold for electron microscopy (EM). Survey 0.5 micron sections of Spurrs resin embedded tissue revealed areas of suitable islets. Thin 100-nm sections were then cut and stained from the osmicated blocks for ultrastructural studies. For ICC investigations, 1 micron sections were immunolabeled for LM before areas were selected for thin sectioning for ultrastructural immunolabeling. The baboon endocrine pancreas ultrastructure was found to be similar to that of other mammals with minor differences in islet and secretory granule size and shape and in electron opacity of the secretory granule cores. Insulin glucagon, somatostatin and pancreatic polypeptide (PP) producing cells were described. A small number of cells were seen to contain both glucagon and PP and some D cells were observed to contain a few granules with both the appearance and immunoreactivity of A cell secretory granules. Statistical analysis of 100 secretory granule diameters of each of the 4 cell types in 6 baboons revealed significant differences (p less than 0.001) in size between all but those of the A and D cells. The insulin precursor subunit, C-peptide, and the glucagon precursor, glicentin, were each found together with the final hormone product in their respective secretory granules. The precursors were often located toward the periphery of the secretory granule, suggesting that the conversion of precursor to active hormone may be membrane associated. A nonrandom topographical association was observed between A and D cells, suggesting a strong functional implication.

Glucose alters configuration of gap junctions between pancreatic islet cells

In rat pancreatic islets, gap junctional subunits (GJS) occur under two different configurations, namely in linear single strands and in polygonal particle aggregates. The present freeze-fracture study demonstrates that GJS can rapidly (dis)assemble into one of these membrane specializations without changes in their total number. Isolation of the pancreatic gland and its perfusion at 2.8 mM glucose is accompanied by a decrease in polygonally packed GJS from 46 to 16%. A rise in medium glucose concentration is followed, within 10 min, by a dose-dependent increase in the percent polygonal particles. This glucose effect on gap junction configuration is calcium dependent and reversible upon glucose removal; it is still entirely detectable when protein synthesis is blocked by cycloheximide. These results indicate that islet gap junctions are dynamic structures that rapidly adjust their configuration to extracellular regulators of beta-cell function. In the light of previous observations, it is suggested that this rapid (dis)assembly of gap junctional structures be considered as a component in the ionic and metabolic coupling between insulin-containing beta-cells of the pancreas.