Dynamics of insulin release from the foetal and neonatal rat pancreas

Polycomb Repressive Complexes: Shaping Pancreatic Beta-Cell Destiny in Development and Metabolic Disease

Pancreatic beta-cells secrete the hormone insulin, which is essential for the regulation of systemic glucose homeostasis. Insufficiency of insulin due to loss of functional beta-cells results in diabetes. Epigenetic mechanisms orchestrate the stage-specific transcriptional programs that guide the differentiation, functional maturation, growth, and adaptation of beta-cells in response to growth and metabolic signals throughout life. Primary among these mechanisms is regulation by the Polycomb Repressive Complexes (PRC) that direct gene-expression via histone modifications. PRC dependent histone modifications are pliable and provide a degree of epigenetic plasticity to cellular processes. Their modulation dictates the spatio-temporal control of gene-expression patterns underlying beta-cell homeostasis. Emerging evidence shows that dysregulation of PRC-dependent epigenetic control is also a hallmark of beta-cell failure in diabetes. This minireview focuses on the multifaceted contributions of PRC modules in the specification and maintenance of terminally differentiated beta-cell phenotype, as well as beta-cell growth and adaptation. We discuss the interaction of PRC regulation with different signaling pathways and mechanisms that control functional beta-cell mass. We also highlight recent advances in our understanding of the epigenetic regulation of beta-cell homeostasis through the lens of beta-cell pathologies, namely diabetes and insulinomas, and the translational relevance of these findings. Using high-resolution epigenetic profiling and epigenetic engineering, future work is likely to elucidate the PRC regulome in beta-cell adaptation versus failure in response to metabolic challenges and identify opportunities for therapeutic interventions.

DNA Methylation Patterning and the Regulation of Beta Cell Homeostasis

Pancreatic beta cells play a central role in regulating glucose homeostasis by secreting the hormone insulin. Failure of beta cells due to reduced function and mass and the resulting insulin insufficiency can drive the dysregulation of glycemic control, causing diabetes. Epigenetic regulation by DNA methylation is central to shaping the gene expression patterns that define the fully functional beta cell phenotype and regulate beta cell growth. Establishment of stage-specific DNA methylation guides beta cell differentiation during fetal development, while faithful restoration of these signatures during DNA replication ensures the maintenance of beta cell identity and function in postnatal life. Lineage-specific transcription factor networks interact with methylated DNA at specific genomic regions to enhance the regulatory specificity and ensure the stability of gene expression patterns. Recent genome-wide DNA methylation profiling studies comparing islets from diabetic and non-diabetic human subjects demonstrate the perturbation of beta cell DNA methylation patterns, corresponding to the dysregulation of gene expression associated with mature beta cell state in diabetes. This article will discuss the molecular underpinnings of shaping the islet DNA methylation landscape, its mechanistic role in the specification and maintenance of the functional beta cell phenotype, and its dysregulation in diabetes. We will also review recent advances in utilizing beta cell specific DNA methylation patterns for the development of biomarkers for diabetes, and targeting DNA methylation to develop translational approaches for supplementing the functional beta cell mass deficit in diabetes.

Expression of the Protein Tyrosine Phosphatase-like Protein IA-2 During Pancreatic Islet Development

A tyrosine phosphatase-like protein, IA-2, is a major autoantigen in Type 1 diabetes but its role in islet function is unclear. Tyrosine phosphorylation mediates regulation of cellular processes such as exocytosis, cell growth, and cell differentiation. To investigate the potential involvement of IA-2 in islet differentiation and insulin secretion, we analyzed by immunohistochemistry expression of IA-2 during islet development in fetal rats and during the maturation of insulin secretory responses after birth. In the fetus, IA-2 immunoreactivity was detected in primitive islets positive for insulin and glucagon at 12 days' gestation. Subsequently, IA-2 was only weakly detectable in the fetal pancreas. In neonatal rat, a progressive increase in IA-2 immunoreactivity was observed in islets from very low levels at 1 day of age to moderate labeling at 10 days. In the adult, relatively high levels of IA-2 were detected in islets, with heterogeneous expression in individual cells within each islet. IA-2 marks a population of endocrine cells that transiently appear early in pancreatic ontogeny. Islet IA-2 expression reappears after birth concomitant with the development of mature insulin secretory responses, consistent with a role for this protein in regulated hormone secretion.

Ontogenic pattern of nucleobindin-2/nesfatin-1 expression in the gastroenteropancreatic tissues and serum of Sprague Dawley rats

Nesfatin-1 is a novel metabolic hormone that has glucose-responsive insulinotropic actions. Islet β-cells and gastrointestinal tissues have been reported as abundant sources of nesfatin-1 and its precursor hormone nucleobindin-2 (NUCB2). While nesfatin-1 is emerging as a multifunctional hormone, there are no reports on the developmental expression of NUCB2/nesfatin-1. The main objective of this study was to examine the ontogenic expression of NUCB2 mRNA, and NUCB2/nesfatin-1 immunoreactivity in the pancreas, stomach and duodenum, and the circulating levels NUCB2/nesfatin-1 in Sprague Dawley rats. In addition, we also determined the co-localization of NUCB2/nesfatin-1 and insulin immunoreactivity during development. NUCB2/nesfatin-1 immunoreactivity was found in the rat stomach from postnatal days 13-27. Furthermore, NUCB2/nesfatin-1 immunoreactivity was also detected in the enteroendocrine cells of the duodenum at postnatal days 13 and 27. Duodenal NUCB2 mRNA expression at postnatal day 27 was highest. Serum NUCB2/nesfatin-1 levels on embryonic day 21 and postnatal day 1 were lower than serum NUCB2/nesfatin-1 levels of adults and neonates at postnatal days 13, 20 and 27, gradually increasing with growth, suggesting an increase in its production and secretion from tissues including the gastrointestinal tract and pancreas. Our findings indicate that NUCB2/nesfatin-1 colocalizes with insulin in the islet β-cells at all developmental stages, but the percentage of colocalization varies in an age-dependent manner. These findings suggest that NUCB2/nesfatin-1 has potential age- and tissue-specific role in the developmental physiology of rats during growth.

Pig islets for clinical islet xenotransplantation

PURPOSE OF REVIEW

Allogeneic islet transplantation faces difficulties because organ shortage is recurrent; several pancreas donors are often needed to treat one diabetic recipient; and the intrahepatic site of islet implantation may not be the most appropriate one. Another source of insulin-producing cells, therefore, would be of major interest, and pigs represent a possible and serious source for obtaining such cells.

RECENT FINDINGS

Pig islet grafts may appear difficult because of the species barrier, but recent studies demonstrate that pig islets may function in diabetic primates for at least 6 months.

SUMMARY

Pig islet xenotransplantation, however, must still overcome the selection of a suitable pig donor to translate preclinical findings into clinical applications. This review summarizes the actual acquired knowledge of pig islet transplantation in primates to select the 'ideal' pig donor.

Pig islet xenotransplantation into non-human primate model

Allogeneic islet transplantation faces difficulties because (1) organ shortage is recurrent; (2) several pancreas donors are often needed to treat one diabetic recipient; and (3) the intrahepatic site of islet implantation may not be the most appropriate site. Another source of insulin-producing cells, therefore, would be of major interest, and pigs represent a possible and serious source for obtaining such cells. Pig islet grafts may seem difficult because of the species barrier, but recent reports demonstrate that pig islets may function in primates for at least 6 months. Pig islet xenotransplantation, however, must still overcome several hurdles before becoming clinically applicable. The actual consensus is to produce more preclinical data in the pig-to-primate model as a necessary requirement to envisage any pig-to-human transplantation of islets; therefore, a summary of the actual acquired knowledge of pig islet transplantation in primates seemed useful and is summarized in this overview.

Pig-to-nonhuman primate islet xenotransplantation: a review of current problems

Islet allotransplantation has been shown to have potential as a treatment for type 1 diabetic patients. Xenotransplantation, using the pig as a donor, offers the possibility of an unlimited number of islets. This comprehensive review focuses on experience obtained in pig-to-nonhuman primate models, particularly with regard to the different types of islets (fetal, neonatal, adult) and isolation procedures used, and the methods to determine islet viability. The advantages and disadvantages of the methods to induce diabetes (pancreatectomy, streptozotocin) are discussed. Experience in pig-to-nonhuman primate islet transplantation studies is reviewed, including discussion of the possible mechanisms of rejection and the immunosuppressive regimens used. The research carried out to date has led to workable animal models to study islet xenotransplantation, but several questions regarding methodology remain unanswered, and details of these practicalities require to be adequately addressed. The encouraging porcine islet survival reported recently provides an indicator for future immunosuppressive regimens.

Induction of pancreatic duct cells of neonatal rats into insulin-producing cells with fetal bovine serum: a natural protocol and its use for patch clamp experiments

AIM

To induce the pancreatic duct cells into endocrine cells with a new natural protocol for electrophysiological study.

METHODS

The pancreatic duct cells of neonatal rats were isolated, cultured and induced into endocrine cells with 15% fetal bovine serum for a period of 20 d. During this period, insulin secretion, MTT value, and morphological change of neonatal and adult pancreatic islet cells were comparatively investigated. Pancreatic beta-cells were identified by morphological and electrophysiological characteristics, while ATP sensitive potassium channels (K(ATP)), voltage-dependent potassium channels (K(V)), and voltage-dependent calcium channels (K(CA)) in beta-cells were identified by patch clamp technique.

RESULTS

After incubation with fetal bovine serum, the neonatal duct cells budded out, changed from duct-like cells into islet clusters. In the first 4 d, MTT value and insulin secretion increased slowly (MTT value from 0.024+/-0.003 to 0.028+/-0.003, insulin secretion from 2.6+/-0.6 to 3.1+/-0.8 mIU/L). Then MTT value and insulin secretion increased quickly from d 5 to d 10 (MTT value from 0.028+/-0.003 to 0.052+/-0.008, insulin secretion from 3.1+/-0.8 to 18.3+/-2.6 mIU/L), then reached high plateau (MTT value >0.052+/-0.008, insulin secretion >18.3+/-2.6 mIU/L). In contrast, for the isolated adult pancreatic islet cells, both insulin release and MTT value were stable in the first 4 d (MTT value from 0.029+/-0.01 to 0.031+/-0.011, insulin secretion from 13.9+/-3.1 to 14.3+/-3.3 mIU/L), but afterwards they reduced gradually (MTT value <0.031+/-0.011, insulin secretion <8.2+/-1.5 mIU/L), and the pancreatic islet cells became dispersed, broken or atrophied correspondingly. The differentiated neonatal cells were identified as pancreatic islet cells by dithizone staining method, and pancreatic beta-cells were further identified by both morphological features and electrophysiological characteristics, i.e. the existence of recording currents from K(ATP), K(V), and K(CA).

CONCLUSION

Islet cells differentiated from neonatal pancreatic duct cells with the new natural protocol are more advantageous in performing patch clamp study over the isolated adult pancreatic islet cells.

In vivo proliferation of differentiated pancreatic islet beta cells in transgenic mice expressing mutated cyclin-dependent kinase 4

AIMS/HYPOTHESIS

It has previously been hypothesised that highly differentiated endocrine cells do not proliferate or regenerate. However, recent studies have revealed that cyclin-dependent kinase 4 (CDK4) is necessary for the proliferation of pancreatic islet beta cells. The aim of this study was to determine whether activation of CDK4 can potentially be used as a radical treatment for diabetes without malignant transformation.

METHODS

We generated transgenic mice expressing mutant CDK4 under the control of the insulin promoter to examine the effect of activated CDK4 overexpression in the postnatal development of pancreatic islets.

RESULTS

In the transgenic mice, total CDK4 protein expression was increased by up to 5-fold, with a concomitant increase in CDK4 activity indicated by the detection of phosphorylated Rb protein in pancreatic islets. Histopathologically, many cells tested positive for proliferating cell nuclear antigen, and pancreatic islets displayed hyperplasia due to the extreme proliferation of beta cells containing a large number of insulin granules. Pancreatic islet alpha, delta and PP cells did not increase. Over an 18-month observation period, the transgenic mice did not develop insulinoma. Levels of expression of GLUT1 and c-myc were comparable to those in the littermates of the transgenic mice. GLUT2 expression was identified in the pancreatic islets of transgenic mice. No significant differences in telomerase activities were detected between transgenic mice and their littermates. Transgenic mice were superior to their littermates in terms of glucose tolerance and insulin secretion in response to the intraperitoneal injection of glucose, and hypoglycaemia was not observed.

CONCLUSIONS/INTERPRETATION

Activated CDK4 stimulates postnatal pancreatic beta cell proliferation, during which the highly differentiated phenotypes of pancreatic islet beta cells are preserved without malignant transformation.

Different regulated expression of the tyrosine phosphatase-like proteins IA-2 and phogrin by glucose and insulin in pancreatic islets: relationship to development of insulin secretory responses in early life

IA-2 and phogrin are tyrosine phosphatase-like proteins that may mediate interactions between secretory granules and cytoskeleton in islets and neuroendocrine tissues. We investigated factors that regulate IA-2 and phogrin expression and their relationship to maturation of insulin secretory responses that occur after birth. Islet content of IA-2, but not phogrin, increased during the first 10 days of life in rats, when insulin secretion in response to glucose increased to adult levels. In cultured 5-day-old rat islets, IA-2 protein and mRNA was increased by glucose and agents that potentiate insulin secretion by the cAMP pathway. Addition of insulin increased IA-2 protein levels and insulin biosynthesis without affecting IA-2 mRNA. Blocking insulin secretion with diazoxide or insulin action with insulin receptor antibodies inhibited glucose-induced increases in IA-2 protein, but not those of mRNA. Phogrin expression was unchanged by all agents. Thus, IA-2 is regulated at the mRNA level by glucose and elevated cAMP, whereas locally secreted insulin modulates IA-2 protein levels by stimulating biosynthesis. In contrast, phogrin expression is insensitive to factors that modify beta-cell function. These results demonstrate differential regulation of two closely related secretory granule components and identify IA-2 as a granule membrane protein subject to autocrine regulation by insulin.

Early-life programming of susceptibility to dysregulation of glucose metabolism and the development of Type 2 diabetes mellitus

There is increasing epidemiological evidence in humans which associates low birthweight with later metabolic disorders, including insulin resistance and glucose intolerance. There is evidence that nutritional and hormonal factors (e.g. maternal protein restriction, exposure to excess maternal glucocorticoids) markedly influence intra-uterine growth and development. A picture is also emerging of the biochemical and physiological mechanisms that may underlie these effects. This review focuses on recent research directed towards understanding the molecular basis of the relationship between indices of poor early growth and the subsequent development of glucose intolerance and Type 2 diabetes mellitus using animal models that attempt to recreate the process of programming via an adverse intra-uterine or neonatal environment. Emphasis is on the chain of events and potential mechanisms by which adverse adaptations affect pancreatic-beta-cell insulin secretion and the sensitivity to insulin of key metabolic processes, including hepatic glucose production, skeletal-muscle glucose disposal and adipose-tissue lipolysis. Unravelling the molecular details involved in metabolic programming may provide new insights into the pathogenesis of impaired glucoregulation and Type 2 diabetes.

Separate and joint effects of arginine and glucose on ovine fetal insulin secretion

To determine separate and joint effects of increases (delta) in fetal plasma concentrations of arginine (Af) and glucose (Gf) on fetal insulin (If) secretion (delta If), 15 late-gestation fetal sheep were given 5-min arginine bolus infusions (40, 86, 144, 201, and 402 mumol/kg estimated fetal wt) at 90 min of 120 min steady-state glucose clamps (basal Gf, basal + 0.6 mM Gf, and basal + 1.1 mM Gr), producing absolute and percent increases above basal Af of 25.8 +/- 1.3 microM (+33%), 50.9 +/- 6.3 microM (+66%), 83.8 +/- 7.1 microM (+108%), 122.1 +/- 9.4 microM (+156%), and 302.2 +/- 28.2 microM (+386%), respectively. Acute hyperglycemia alone produced an increase above basal If of 9 +/- I microU/ml (+80%) and 19 +/- 2 microU/ml (+170%) after basal + 0.6 mM Gf and basal + 1.1 mM Gf, respectively. Increasing values of delta Af showed separate but lesser effects on delta If, which were significant only at very high values of Af (> 100% above mean normal Af) unless marked hyperglycemia (1.5- to 2-fold normal) was also present, demonstrating joint effects of delta Af and delta Gf on delta If according to a best-fit inverse polynomial response surface. We conclude that physiological increases in Af at normal glucose concentrations are not a potent-stimulus to insulin secretion in fetal sheep.

Large scale isolation, growth, and function of porcine neonatal islet cells

Based upon existing methods of isolating fetal porcine islet tissue, a simple, reliable procedure was developed for the preparation of porcine neonatal islet cell aggregates with a reproducible and defined cellular composition. After 9 d of in vitro culture, tissue from one neonatal pig pancreas yielded approximately 50,000 islet cell aggregates, consisting of primarily epithelial cells (57%) and pancreatic endocrine cells (35%). During the culture period, the total beta cell mass decreased initially, but subsequently increased 1.5-fold between days 3 and 9. Transplantation of grafts consisting of 3 x 10(5) beta cells (1,000 aggregated) under the kidney capsule of alloxan-diabetic nude mice corrected hyperglycemia in 75% (10/13) of the animals, whereas, 100% (20/20) of recipients implanted with 6 x 10(5) beta cells (2,000 aggregates) achieved euglycemia within 8 wk posttransplantation. Nephrectomy of the graft bearing kidney at 14 wk posttransplantation resulted in hyperglycemia in all recipients, and examination of the grafts revealed the presence of numerous well-granulated insulin- and glucagon-containing cells. The cellular insulin content of these grafts was 20 to 30-fold higher than at the time of transplantation. These results indicate that the neonatal porcine pancrease can be used as a source of large numbers of viable islet cells, which have the potential for growth both in vitro and in vivo, and exhibit the metabolic capacity to correct diabetes in nude mice.

A critical period in the development of the insulin secretory response to glucose in fetal rat pancreas

Insulin secretion by fetal rat pancreas was studied at 19.5 and 20.5 days of gestation. Over this 24-hour period, the response to glucose changed rapidly from one that is insensitive to the calcium channel antagonist nitrendipine but markedly enhanced by the presence of the inhibitor of fatty acid oxidation 2-bromostearate, to one that is larger, sensitive to nitrendipine but now not enhanced by 2-bromostearate. The 19.5-day pancreas that is not affected by nitrendipine when responding to glucose alone, is inhibited by nitrendipine when the response to glucose is enhanced by 2-bromostearate. The data suggest a possible metabolic change in the developing B-cell in which fatty acid oxidation is decreased, glucose oxidation increased, and a change in stimulus-secretion coupling from a (KATP) channel-independent mechanism alone, to a combination of that system with the (KATP) channel-dependent system. This could be achieved by a simple increase in the strength of the signal from glucose metabolism.

Potassium permeability of fetal rat pancreatic islets: abnormal sensitivity to glucose

Potassium channels of fetal rat islets have been recently reported to be inadequately regulated by stimulation with glucose when compared to islets of adult rats. Though in patch clamp experiments the properties of their KATP-channels were shown to be comparable to those from adult rats, until now no closure could be demonstrated with the technique measuring the 86Rb+ efflux. Using this technique, in the presence of a basal (3 mM) glucose concentration the 86Rb+ efflux was completely insensitive to a stimulation with glucose (5.6 mM) or tolbutamide. In contrast, in islets perifused in the absence of glucose the introduction of a low glucose concentration (3 mM) or stimulation with tolbutamide alone inhibited the 86Rb+ efflux, confirming the presence of functioning KATP-channels. The absolute value of the 86Rb+ efflux rate in the absence of glucose was, however, much lower in fetal rat islets as normally observed in adult rat islets. Apart from this, the ATP content of fetal rat islets remained unchanged at either glucose concentration tested. It is suggested that in islets of fetal rats a K+ permeability is present and can be inhibited by glucose and tolbutamide but in contrast to islets of adult rats the K+ efflux is already maximally inhibited in the presence of 3 mM glucose. This may be one reason why pancreatic islets of fetal rats do not respond to glucose-stimulation with an adequate calcium uptake and insulin release.

Growth factors and the regulation of pre- and postnatal growth

Peptide growth factors represent a largely paracrine level of intercellular communication that is basic to the process of life. Growth factors are present in the ovum and are amongst the first products expressed by the embryonic genome. They function as both signals and progression factors for embryonic tissue growth, induction, differentiation, maturation and function. While a widespread tissue expression is demonstrable during fetal development, and in certain postnatal tissues such as the epiphyseal growth plate, growth factor presence in the adult is restricted to tissues sharing rapid cellular turnover such as ovary. However, a transient re-expression of peptide growth factors occurs during adult tissue repair. In addition to mitogenic peptides such as IGFs or EGF, the family of growth factors also includes physiological growth inhibitors such as TGF beta and certain neuropeptides. Insulin is mitogenic in the early embryo and evidence is presented to support a continuation of this role, under defined nutritional conditions, in late gestation. The importance of insulin to pre- and postnatal growth has prompted an expanding literature dealing with the interactions of nutrients, hormones and growth factors during the growth and functional maturation of the islets of Langerhans. While the expression of growth factors in the early embryo is apparently autonomous, some, such as IGFs, become increasingly dependent on nutrient, insulin and GH availability during fetal development and in childhood growth. This has resulted in circulating IGF I and II determinations becoming useful diagnostic markers of endocrine-based growth disorder and nitrogen balance.

Insulin release from pancreatic islets of fetal rats mediated by leucine b-BCH, tolbutamide, glibenclamide, arginine, potassium chloride, and theophylline does not require stimulation of Ca2+ net uptake

In pancreatic islets of fetal rats the effect of glucose (3 and 16.7 mM), glyceraldehyde (10 mM), leucine (20 mM), b-BCH (20 mM), tolbutamide (100 micrograms/ml), glibenclamide (0.5 and 5.0 micrograms/ml) arginine (20 mM), KCl (20 mM) and theophylline (2.5 mM) on 45Ca2+ net uptake and secretion of insulin was studied. All compounds tested failed to stimulate 45Ca2+ net uptake. However, in contrast to glucose and glyceraldehyde, leucine, b-BCH, tolbutamide, glibenclamide, arginine, KCl and theophylline significantly stimulated release of insulin. This effect could not be inhibited by the calcium antagonist verapamil (20 microM). Elevation of the glucose concentration from 3 to 5.6 mM did not alter 86Rb+ efflux of fetal rat islets but inhibited 86Rb+ efflux of adult rat islets. Stimulation of 86Rb+ efflux with tolbutamide (100 micrograms/ml), leucine (20 mM) or b-BCH (20 mM) in the presence of 3 mM glucose was also ineffective in fetal rat islets. Our data suggest that stimulation of calcium uptake via the voltage dependent calcium channel is not possible in the fetal state. They also provide evidence that stimulators of insulin release which are thought not to act through their metabolism, initiate insulin secretion from fetal islets by a mechanism which is different from stimulation of calcium influx.

Failure of glucose to elicit a normal secretory response in fetal pancreatic beta cells results from glucose insensitivity of the ATP-regulated K+ channels

Fetal pancreatic beta cells demonstrate a deficient insulin release in response to glucose, but the underlying mechanism at the cellular level is unknown. By using beta cells from 21-day fetal rats we made an attempt to clarify the mechanism(s) behind this reduced glucose response. In addition to measuring insulin release, glucose metabolism, and cellular ATP content, ATP-regulated K+ channels (G channels) and voltage-activated Ca2+ currents were investigated with the patch-clamp technique. It was thus demonstrated that the ATP-regulated K+ channels in fetal beta cells were not sensitive to glucose but otherwise had similar characteristics as those of adult beta cells. Also, the characteristics of the voltage-activated Ca2+ currents were similar in adult and fetal beta cells. However, as judged from measurements of both glucose oxidation and glucose utilization, glucose metabolism was impaired in fetal beta cells. In addition, there was no increase in the ATP content, even when the cells were stimulated for 30 min. It is therefore concluded that the attenuated glucose-induced insulin release in fetal pancreatic beta cells is due to an immature glucose metabolism resulting in impaired regulation of the ATP-sensitive K+ channels. These findings may be relevant to the understanding of the deficient stimulus-secretion coupling associated with non-insulin-dependent diabetes.

Effects of cholecystokinin octapeptide and carbachol on neonatal insulin secretory dynamics

The effects of glucose, sulfated cholecystokinin-octapeptide (CCK-8), or carbachol on insulin secretory dynamics were studied in pancreatic islets isolated from 1- and 3-day-old neonatal rats. When challenged with glucose, 1-day islets responded with a definite first phase and elevated secretion during the latter part of the stimulation period; 3-day islets had a first phase and a rising, sustained second phase. The presence of stimulatory concentrations of CCK-8 or carbachol in addition to glucose caused dramatic changes in the release pattern in both islet populations. In 1-day islets, carbachol stimulated mainly first phase secretion whereas CCK-8 enhanced first phase release and produced a definite second phase response. The two secretagogues increased significantly both phases of release in 3-day islets with no differences between the two agents in their effects. These results indicate that CCK-8 and carbachol differentially stimulate neonatal insulin secretion, possibly through different steps in the stimulus-secretion pathway. They also suggest that the cellular mechanism for second phase release is present in 1-day islets and can be activated by CCK-8.

Nonenzymatic isolation and culture of adult islets from atrophic pancreata of copper-deficient rats: a morphologic analysis

The purpose of this study was to develop a nonenzymatic method of isolating adult islets using atrophied pancreata from copper-deficient rats and to analyze their morphologic characteristics and behavior in culture. This unusual model of isolation was studied because islets remain intact in the course of dietary copper deficiency while the acinar glandular component of the pancreas undergoes selective atrophy and lipomatosis. Small fragments containing islets were readily microdissected from atrophied glands and placed in culture. Within 24 h the fragments congealed into small irregular- to spherical-shaped masses within which the darker profile of islets could be distinguished. Within a period of 3 to 5 d, islet tissue began to bud from the lipocytic mass until by Day 7 spherical aggregates of intact islet tissue separated from the residual fragments. Subsequent to further in vitro treatment, these islets could be maintained as free viable spherical masses if periodically agitated, as attached stationary islets which developed monolayer growth if left undisturbed and as aggregated masses of islet tissue forming megaislets if combined in small groups. Grouped islets treated with actinomycin D and cycloheximide did not exhibit aggregation when incubated with these inhibitors. This suggests that megaislet formation was an active process requiring protein-RNA synthesis rather than passive clumping or aggregation that can accompany metabolically altered or dying islets undergoing cellular shedding and adhesion. Immunohistochemical localization demonstrated that insulin, glucagon, somatostatin, and pancreatic polypeptide-immunoreactive cell types were present within the islets derived from this technique. The cellular topography of these islets was not unlike that described by others for islets cultured from enzymatic isolation. This culture model may serve as a resource for mature, viable islets isolated without mechanical or enzymatic disaggregation which can have attenuating effects on islet function.

Studies on insulin secretion by monolayer cultures of normal and tumorous human pancreatic cells. Effects of glucose, somatostatin and SMS 201-995

Recently, somatostatin analogs have been introduced which can be used clinically in the treatment of tumorous or functional hypoglycemia. In the present study we investigated in vitro the regulation, the degree of autonomy and the sensitivity to natural somatostatin and its analog SMS 201-995 of insulin secretion by monolayer cultures of human pancreatic cells obtained from patients with insulinomas and from a newborn with nesidioblastosis. All cultures released insulin upon the addition of dibutyryl-cAMP and calcium, demonstrating their intact viability. Insulin secretion from nontumorous pancreatic cells surrounding an insulinoma was dose-dependently stimulated by glucose. In contrast, insulin release by B cells from a patient with nesidioblastosis and from 2 insulinomas was not stimulated by the addition of glucose. Native somatostatin (SRIF) and the synthetic analog SMS 201-995 inhibited insulin secretion from all cultures. The inhibitory effects of SRIF and SMS in the culture from the nesidioblastosis tissue, could be reversed by the addition of 11.2 mmol glucose/l, but not in one of the insulinoma cultures. This demonstrates that some sensitivity to glucose is present in B cells from the nesidioblastosis tissue, despite the unresponsiveness to glucose alone. Insulin release by insulinoma cells was blocked by somatostatin, while it was inhibited to some extent only in the cultures of nontumor B cells and of cells from the nesidioblastosis tissue. In conclusion, it was shown that insulin release by the cultured B cells obtained from several pathological conditions differed with regard to the autonomy of hormone release (glucose sensitivity) and the sensitivity to somatostatin and its analog.

Failure of glucose to affect 86rubidium efflux and 45calcium uptake of fetal rat pancreatic islets

Ion movements and insulin secretion of pancreatic islets of adult and fetal rats have been studied at three glucose concentrations. In islets of adult rats, 86Rb efflux is maximally decreased by 5.6 mM-glucose. 16.7 mM-glucose caused a biphasic efflux pattern which may be due to glucose-stimulated Ca uptake. In islets of fetal rats elevation of the glucose concentration from 3 to 5.6 or 16.7 mM does not cause a change of 86Rb efflux, and the fractional efflux from fetal islets in the presence of 3 mM-glucose is similar to that from adult rat islets in the presence of 5.6 mM-glucose. Elevation of the glucose concentration from 3 to 16.7 mM is not associated with an increase in 45Ca uptake into fetal islets, although this change in glucose concentration doubles 45Ca uptake into adult islets. When challenged with 16.7 mM-glucose, fetal islets exhibit no insulin secretory response; however, they do respond to theophylline. It is concluded that the failure of fetal islets to exhibit an insulin-secretory response when challenged with glucose might be related to the inability of glucose to affect 86Rb efflux and Ca uptake. The present data are discussed in light of differences between pancreatic islets of fetal and adult rats with respect to the redox state of pyridine nucleotides, thiols and glucose metabolism.

Pentose phosphate shunt, pyridine nucleotides, glutathione, and insulin secretion of fetal islets

In rat fetal islets it was tested whether their failure to respond to glucose with insulin secretion might be due to inadequate changes of the redox state of pyridine nucleotides and of glutathione. In islets of newborn (5 days) and adult (3 mo) rats elevation of glucose produced an increase in insulin secretion, pentose phosphate shunt (PPS) activity, and NADPH/NADP, NADH/NAD, and GSH/GSSG ratios. An increase in the NADH/NAD ratio was also observed in islets of fetal rats, but in contrast to islets of newborns and adults no increase in insulin release, PPS activity, and the GSH/GSSG ratio was observed. However, at all glucose concentrations tested islets of fetal rats exhibited a high NADPH/NADP ratio similar to the ratio of adult rats in the presence of 16.7 mM glucose. It is suggested that in fetal islets there exists a lack of hydrogen transfer from NADPH to GSSG. The high NADPH/NADP ratio may in turn suppress PPS activity. It is possible that the missing insulin release of fetal islets in response to glucose is at least in part due to the fact that the oxidation-reduction state of the GSH/GSSG system also does not respond to the elevation of the glucose concentration.

Hyperinsulinemic hypoglycemia of the neonate associated with persistent fetal histology and function of the pancreas

Early in development, the fetal pancreas is characterized by the presence of two distinct generations of endocrine cells and a B-Cell mass that is unresponsive to acute changes in circulating glucose levels. Near the end of intrauterine development, the normal pancreas has "matured" and contains a single generation of endocrine cells and B-Cells that are responsive to changes in glucose concentrations. Recent microscopic examination of resected pancreatic tissue from an infant with hyperinsulinemic hypoglycermia revealed a combination of all three of the currently accepted findings in this neonatal condition: hyperplasia, adenomatosis, and nesidioblastosis. These observations prompted the following hypothesis: When compared to the usual histology of the developing pancreas, nesidioblastosis may be interpreted as an abnormal continuation of normal proliferation of endocrine cells; hyperplasia may be a specific overproduction of the Secondary Islands of Langerhans; and adenomatosis may be an abnormal continuation or overgrowth of the Primary Island of Langerhans. Such extrapolation suggests that infants with hyperinsulinemic hypoglycemia may represent a failure in the normal histological and functional maturation of the endocrine portion of the fetal pancreas.

Fetal carbohydrate metabolism: its clinical importance

A basic understanding of fetal nutrition and metabolism is essential in the clinical management of the obstetric patient. The fetus depends upon a constant infusion of glucose for energy production and growth. Maternal glucose is the prime source of this nutrient. Alterations in maternal carbohydrate homeostasis will lead to changes in fetal metabolism. In diabetes mellitus, hyperglycemia may produce hyperinsulinemia and macrosomia. The growth-retarded fetus may have a decreased supply of maternal glucose and reduced amounts of hepatic glycogen and adipose tissue. The fetus must depend upon these stores for survival during periods of intrauterine hypoxia. In the newborn period, hypothermia and hypoxia may rapidly deplete energy reserves. With this information, the clinician may more knowledgeably manage dietary demands in the antepartum patient, fetal distress during labor, and the immediate newborn period.

Extracellular bicarbonate ions and insulin secretion

The role of bicarbonate ions in insulin release was studied with incubated and perifused isolated rat islets of Langerhans. In the absence of NaHC03, the early phase of glucose-induced secretion was completely abolished and the second phase inhibited by approximately 65%. The insulinotropic effect of the sugar was totally restored after reintroduction of the ion in the medium. The monophasic secretory after reintroduction of the ion in the medium. The monophasic secretory response to tolbutamide was also markedly diminished by omission of NaHC03, WHereas the release evoked by a high concentration of K+ was very little affected. CO2 wwas unable to substitute for HC03minus, but small concentrations of the anion (3to 5mM) WEre sufficient to ensure a normal response to glucose.