Biomedical subjects

R L Sorenson

Publications and source records attributed to R L Sorenson.

At least 19 recordsLinked to original sources

PubMed1992

Adaptation of islets of Langerhans to pregnancy: increased islet cell proliferation and insulin secretion correlates with the onset of placental lactogen secretion.

To elucidate the temporal profile of adaptive changes of the islets of Langerhans to the increased insulin demands of pregnancy, we have studied islet cell proliferation and insulin secretion during gestation in the rat. 5-Bromo-2'-deoxyuridine incorporation into dividing islet cells was significantly (P less than 0.05) increased over age-matched controls by day 10, rose continuously to a peak at day 14, and then returned to control levels by day 18. By day 20, cell division was significantly inhibited (P less than 0.05). The pattern of changes in insulin secretory profiles observed with perfused pancreata of pregnant animals was similar to that obtained for islet cell proliferation. Both the threshold of glucose-stimulated insulin secretion and the amount of above threshold insulin secretion began to diverge from controls by day 10. By day 12, the glucose-stimulation threshold was significantly decreased from 5.7 mM glucose to 3.3 mM (P less than 0.05), remained at this low level through day 15, and returned toward normal by day 20. Concomitant with the increased sensitivity of B cells to glucose, the above threshold insulin secretion was significantly increased by day 12 (P less than 0.05), peaked at day 15, and returned to control levels by day 20. This insulin secretory data demonstrates that the increased sensitivity of B cells to glucose is an important component of the adaptation of islets during pregnancy to the increased demand for insulin at physiological concentrations of plasma glucose. To correlate the above changes in islet cell proliferation and insulin secretion with levels of placental lactogen (PL), serum lactogenic hormone activity was measured by Nb2 lymphoma cell replication assays. This analysis revealed the expected biphasic pattern: a midpregnancy peak at day 12, followed by a nadir at day 14, and then continuously elevated levels until term. The bioassay data agreed with the known secretory profiles of rat (r) PL-I (midpregnancy) and rPL-II (late pregnancy). Our results provide the first systematic evaluation of changes in islet function during pregnancy in the rat. In addition, they provide evidence that rPL-I may be the critical hormonal signal which triggers the primary adaptive changes in islet function characteristic of pregnancy. The return to normal values of insulin secretion and inhibition of cell division observed at day 20 in the presence of high concentrations of rPL-II suggests that other inhibitory influences become dominant in the later stages of rat pregnancy.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1991

Role of prolactin versus growth hormone on islet B-cell proliferation in vitro: implications for pregnancy.

This study investigated the effects of homologous rat PRL (rPRL) and rat GH (rGH) on islet growth as indicated by modifications in insulin secretion, islet cell proliferation, and islet volume with neonatal and adult rat islets in vitro. The number of proliferating cells was determined by immunocytochemical staining for 5-bromo-2'-deoxyuridine (BrdU) incorporated into replicating DNA during the final 24 h of culture. When neonatal rat islets were examined by laser scanning confocal microscopy, more than 90% of the BrdU-labeled nuclei were observed in B-cells with insulin immunoreactivity. In neonatal rat islets, rPRL was much more effective than rGH in increasing insulin secretion (3.7-fold vs. 1.4-fold) during the 10 days of culture. The number of BrdU-labeled nuclei per islet was increased from 2.9 +/- 0.4 (n = 77) in control islets to 47.3 +/- 2.7 (n = 95) with rPRL (16.3-fold) and 9.7 +/- 0.8 (n = 84) with rGH (3.3-fold). The effects of rPRL and rGH on both insulin secretion and BrdU labeling were approximately additive. After 10 days, an increased average islet volume was only observed with rPRL. When followed for 36 days, the total amount of islet tissue was unchanged for control islets (1.1-fold), more than doubled with rPRL (2.5-fold), and only slightly increased with rGH (1.4-fold). From the observed rates of islet cell proliferation and increases in islet volumes, doubling times of 23-24 days for rPRL and 89-91 days for rGH can be estimated. Of other proposed islet growth factors, cholecystokinin, epidermal growth factor, platelet-derived growth factor, and 2-aminoisobutyric acid, an increase in insulin secretion and islet cell proliferation was only observed with cholecystokinin in neonatal rat islets. However, both effects were less than 20% of those observed with rPRL. In adult rat islets, rPRL was also more effective than rGH in increasing insulin secretion (1.6-fold vs. 1.2-fold) during the 9 days of culture. The number of BrdU-labeled nuclei per islet was increased from 2.7 +/- 0.5 (n = 96) in control islets to 9.5 +/- 0.6 (n = 175) with rPRL (3.5-fold). In contrast to the neonatal islets, rGH had no effect on the number of BrdU-labeled nuclei per islet in adult rat islets (2.4 +/- 0.3, n = 194).(ABSTRACT TRUNCATED AT 400 WORDS)

Aging↗

PubMed1991

Structural and functional considerations of GABA in islets of Langerhans. Beta-cells and nerves.

gamma-Aminobutyric acid (GABA), a prominent inhibitory neurotransmitter, is present in high concentrations in beta-cells of islets of Langerhans. The GABA shunt enzymes, glutamate decarboxylase (GAD) and GABA transaminase (GABA-T), have also been localized in islet beta-cells. With the recent demonstration that the 64,000-M, antigen associated with insulin-dependent diabetes mellitus is GAD, there is increased interest in understanding the role of GABA in islet function. Only a small component of beta-cell GABA is contained in insulin secretory granules, making it unlikely that GABA, coreleased with insulin, is physiologically significant. Our immunohistochemical study of GABA in beta-cells of intact islets indicates that GABA is associated with a vesicular compartment distinctly different from insulin secretory granules. Whether this compartment represents a releasable pool of GABA has yet to be determined. GAD in beta-cells is associated with a vesicular compartment, similar to the GABA vesicles. In addition, GAD is found in a unique extensive tubular cisternal complex (GAD complex). It is likely that the GABA-GAD vesicles are derived from this GAD-containing complex. Physiological studies on the effect of extracellular GABA on islet hormonal secretion have had variable results. Effects of GABA on insulin, glucagon, and somatostatin secretion have been proposed. The most compelling evidence for GABA regulation of islet hormone secretion comes from studies on somatostatin secretion, where it has an inhibitory effect. We present new evidence demonstrating the presence of GABAergic nerve cell bodies at the periphery of islets with numerous GABA-containing processes extending into the islet mantle. This close association between GABAergic neurons and islet alpha- and delta-cells strongly suggests that GABA inhibition of somatostatin and glucagon secretion is mediated by these neurons. Intracellular beta-cell GABAA and its metabolism may have a role in beta-cell function. New evidence indicates that GABA shunt activity is involved in regulation of insulin secretion. In addition, GABA or its metabolites may regulate proinsulin synthesis. These new observations provide insight into the complex nature of GABAergic neurons and beta-cell GABA in regulation of islet function.

Animals↗

PubMed1989

Glucose-stimulated hormone release in rats bearing streptozotocin/nicotinamide-induced islet adenomas: evidence for slow and fast responders.

Rats injected with streptozotocin and nicotinamide developed grossly visible islet cell tumors of the pancreas. During i.v. glucose tolerance tests, two populations of tumor-bearing rats were identified: fast responders exhibited significantly lower plasma glucose and markedly elevated plasma immunoreactive insulin (IRI) levels relative to those of the controls. In slow responders, the plasma glucose level was significantly elevated up to 2 h after glucose injection, and the plasma IRI level was lower than that of the controls. During in vitro perfusions with glucose at 300 mg/dl (16.7 mM), tumor-bearing pancreata of fast responders released elevated levels of IRI and immunoreactive somatostatin (IRS); after tumor removal, glucose-stimulated release of these hormones returned to control levels. However, during similar perfusions of pancreata from slow responders, the IRI and IRS release did not decrease after tumor removal, suggesting that the nontumorous pancreatic islets rather than the gross tumors of the slow-responder group were the source of the glucose-stimulated hormone release. These studies demonstrate that gross tumors in the two responder subgroups differ in their glucose-stimulated hormone release.

Adenoma, Islet Cell↗

PubMed1989

Effects of islet hormones on amylase secretion and localization of somatostatin binding sites.

The interaction of insulin and somatostatin on amylase secretion was examined in the isolated perfused rat pancreas. Exogenous insulin (10 mU/ml) significantly potentiated cholecystokinin- (CCK; 0.5 mU/ml) stimulated amylase secretion (12.47 +/- 2.9 micrograms/ml, n = 7). Glucose (16.7 mM) stimulated endogenous insulin secretion (523 +/- 66 microU/ml) and also significantly enhanced CCK-stimulated amylase secretion (13.41 +/- 2.8 micrograms/ml, n = 11). When somatostatin was included in the perfusion media, containing insulin and CCK, amylase secretion was reduced to 3.17 +/- 0.83 micrograms/ml (n = 7), a level comparable to that of CCK-stimulated amylase secretion alone. Similarly, addition of exogenous somatostatin to perfusion media, containing 16.7 mM glucose and CCK, reduced amylase secretion to 4.29 +/- 1.09 micrograms/ml (n = 9). The effect of somatostatin and insulin on carbamylcholine-stimulated amylase secretion was also examined. Exogenous insulin (50 mU/ml) potentiated carbamylcholine- (10(-8) M) stimulated amylase secretion, and addition of exogenous somatostatin to the media containing both insulin and carbamylcholine suppressed the insulin potentiation. Uptake of 125I-[Tyr11]somatostatin in the perfused pancreas was saturable as it decreased significantly with the addition of excess unlabeled somatostatin. Autoradiograms revealed uptake of the ligand by both the endocrine islets and the exocrine pancreas with the highest density of grains observed over the acini. These results support the hypothesis that islet peptides modulate the exocrine pancreas, that somatostatin inhibits amylase secretion by inhibiting the action of insulin, and that somatostatin may act directly on the exocrine pancreas via specific receptors on acinar cells.

Amylases↗

PubMed1989

Effect of prolactin versus growth hormone on islet function and the importance of using homologous mammosomatotropic hormones.

The purpose of this study was to determine the effects of homologous rat PRL (rPRL) and rat GH (rGH) on islet B-cell function in neonatal and adult rat islets in vitro. In neonatal rat islets, exposure to rPRL for more than 24 h was necessary for a stimulatory effect on insulin secretion. By day 4, insulin secretion was 3.8-fold greater in the islets cultured with rPRL. rGH had a modest effect on insulin secretion, and this effect was additive with that of rPRL. Both rPRL and rGH increased islet insulin content and [3H]thymidine incorporation. After removal of rPRL, more than 24 h were necessary to detect a reversal in the level of insulin secretion. In addition, even after 5 days without rPRL the previously treated islets still had elevated levels of insulin secretion. In a dose-response study of rPRL on insulin secretion, a detectable effect was observed at 62.5 ng/ml, with a half-maximal effect of approximately 100 ng/ml. Glucose oxidation by neonatal islets was enhanced by rPRL treatment, but not by rGH treatment. In adult rat islets, exposure to rPRL, but not to rGH, enhanced insulin secretion. In contrast, when using heterologous human GH, results similar to those obtained with rPRL, but not rGH, were observed. The results from these experiments indicate that rPRL and rGH have both individual and shared regulatory effects on rat islets. However, it is rPRL and not rGH that has the primary influence on insulin secretion. When interpreting studies examining the effect of GH and PRL on islet function, it is important to consider whether homologous hormones are used.

Animals↗

PubMed1989

Three-dimensional imaging of intact isolated islets of Langerhans with confocal microscopy.

We present a new technique for analyzing the three-dimensional structure of intact isolated islets of Langerhans. Adult rat and human islets were stained with whole-mount immunofluorescence techniques and optically sectioned with a confocal microscope. This has several advantages over traditional methods: 1) the technical difficulties in serial sectioning and handling the large numbers of sections are avoided, 2) optical sectioning by confocal microscopy gives improved resolution and strongly suppresses light from out-of-focus structures, and 3) entire islets can be rapidly imaged for the presence of positive staining. This new technique should facilitate the study of the three-dimensional structure of islets of Langerhans.

Animals↗

PubMed1989

Islet vasculature in atrophic pancreas: evidence for coexisting parallel and serial (insuloacinar) angioarchitecture.

Islet vasculature was studied in rats rendered dietarily copper-deficient, a regimen that effects progressive acinar atrophy while leaving islets and ducts intact. Scanning electron microscopy of corrosion casts produced from Mercox injected rats was used to analyze islet angioarchitecture in the atrophied gland. The results indicate that with the onset of acinar atrophy, the vasculature of islets remained intact despite loss of insuloacinar interconnectivity. With atrophy of the dense acinar vascular matrix, three populations of islets could be distinguished according to their size and vascular structures: (a) All small islets (40-150 microns in diameter) were found to have exclusively serial vasculature since no postcapillary collecting venules were found directly continuous with parallel efferent veins. (b) A second population of intermediate size islets (160-250 microns) was found to have serial vessels in addition to postcapillary collecting venules that were directly continuous with larger interlobular veins indicative of a parallel mode of microcirculation. (c) All larger islets (260-700 microns) were also found to have both serial as well as parallel microcirculatory patterns based upon the coexistence of both types of postcapillary venules within the same islet. In all cases, postcapillary venules, which normally link islet capillaries to peri-insular acinar plexuses, were continuous with veins that extended through lipomatous lobules devoid of acini to link with branches of interlobular veins. It is proposed that the functional integrity of islet blood flow remains in the atrophied pancreas because of preservation of both serial and parallel microcirculation that facilitate normal glucose-stimulated insulin secretion in the atrophied pancreas, as demonstrated in an earlier study. Furthermore, it is suggested that analysis of the atrophied state of the gland provides a valid comparative model with which to study islet angioarchitecture and microcirculation in the normal gland.

Animals↗

PubMed1988

Ultrastructural immunocytochemical localization of L-glutamate decarboxylase and GABA in rat pancreatic zymogen granules.

The ultrastructural immunohistochemical localization of gamma aminobutyric acid (GABA) and its regulating enzymes, L-glutamate decarboxylase (GAD) and gamma aminobutyrate-alpha-ketoglutarate transaminase, was determined utilizing an immunogold post-embedding protocol in pancreatic exocrine tissue. Within the acinar cell, GABA and its biosynthetic enzyme, GAD, were localized in zymogen granules. Quantitative analysis of the GABA immunoreactivity in the acinar cell revealed 1.7 +/- 0.5 gold particles/micron2 over the cytoplasm, 36.6 +/- 14.1 gold particles/micron2 over the zymogen granules, and 2.9 +/- 2.1 gold particles/micron2 over the mitochondria. Quantitative analysis of the distribution of colloidal gold particles, representing glutamate decarboxylase immunoreactivity in the acinar cells, revealed 38.4 +/- 2.5 gold particles/micron2 over the zymogen granules, 4.7 +/- 1.1 gold particles/micron2 over the mitochondria and 6.3 +/- 0.5 gold particles/micron2 over the remainder of the cytoplasm. Substitution of normal sheep serum for the sheep anti-glutamate decarboxylase serum revealed a significant (p less than 0.001) decrease of the colloidal gold particle distribution over the zymogen granules and cytoplasmic compartments of the acini. Gamma aminobutyrate-alpha-ketoglutarate transaminase, the catabolic enzyme for GABA, was not detected in the mitochondria, zymogen granules, and cytoplasm of the acinar cell, suggesting that GABA is not catabolized within the acinar cell. Preabsorption and substitution controls resulted in an absence of labeling. These results suggest that GABA may act extracellularly and/or have a role within the zymogen granule in the exocrine pancreas.

Animals↗

PubMed1988

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.

Animals↗

PubMed1988

Radioimmunoassay for rat pancreatic alpha-amylase and the effect of Phe-Met-Arg-Phe-amide on amylase secretion in the isolated perfused rat pancreas.

In this study a radioimmunoassay was developed to measure secreted amylase from the isolated perfused rat pancreas. Using Sephadex G-75 gel chromatography, rat pancreatic amylase was purified to a single migrating protein band as determined by SDS polyacrylamide gel electrophoresis. Specificity of a rat pancreatic amylase antiserum, raised in rabbits, was determined using immunodiffusion, immunoelectrophoresis, and immunoblotting techniques. Secreted amylase concentrations, obtained using the radioimmunoassay, were not significantly different than those measured with the amylase enzyme assay. The rat pancreatic amylase radioimmunoassay was used to measure the amylase secretion in the isolated perfused rat pancreas. Phe-Met-Arg-Phe-amide (FMRF-NH2) immunoreactivity has been shown to be co-localized with pancreatic polypeptide in the rat pancreatic islet, and evidence suggests that islet peptides modulate amylase secretion from the exocrine pancreas. In the present study, FMRF-NH2 significantly (p less than 0.05) suppressed cholecystokinin (CCK)-stimulated amylase secretion by 55%. The average pancreatic amylase secretion in response to CCK was 10.89 +/- 2.0 micrograms/ml/min (n = 6); with the addition of FMRF-NH2, CCK-stimulated amylase secretion was reduced to 4.79 +/- 1.6 micrograms/ml/min (n = 6). These results are consistent with the insuloacinar hypothesis in that an FMRF-NH2-like substance in the islet may act to modulate the exocrine pancreas.

Animals↗

PubMed1988

Cellular and subcellular immunolocalization of L-glutamate decarboxylase in rat pancreatic islets.

The cellular and subcellular distribution of L-glutamate decarboxylase (GAD), the biosynthetic enzyme for gamma-aminobutyric acid (GABA), was determined immunohistochemically in rat pancreatic islet using light and electron microscopic techniques. The cellular distribution of GAD was determined at the light microscopic level using an elution/re-staining protocol and a computerized digital image processing technique. At this level of resolution, immunofluorescent GAD was observed to be co-localized with immunofluorescent insulin in the islet B-cells and absent in both the A-cells, which contained glucagon, and the D-cells, which contained somatostatin. Subcellular localization of GAD was determined using an electron microscopic, colloidal gold post-embedding protocol and was compared to insulin immunoreactivity in serial sections of the same B-cell. In the same islet B-cell, GAD immunoreactivity appeared predominantly in the extragranular cytoplasm, whereas insulin immunoreactivity was associated with the secretory granules. Quantitative analysis of GAD immunoreactivity in the B-cell revealed 15.3 +/- 1.8 gold particles/micron2 in the cytoplasm, 1.7 +/- 0.2 gold particles/micron2 in the secretory granules, and 0.4 +/- 0.4 gold particles/micron2 in the mitochondria. The results of this study, localization of the biosynthetic enzyme for GABA to the B-cell cytoplasmic compartment and its absence in the secretory granules which contain insulin, are compatible with the hypothesis that GABA functions as an intracellular mediator of B-cell activity.

Animals↗

PubMed1988

Nutrient and hormonal regulation of the threshold of glucose-stimulated insulin secretion in isolated rat pancreases.

This study demonstrates that the threshold of glucose-stimulated insulin secretion can be regulated in vivo by long term hormonal and nutrient modifications. The sensitivity of the pancreatic B-cell to glucose stimulation was determined by examining the pattern of insulin release from pancreases perfused with linear glucose gradients. Male rats infused with ovine PRL for 4 days and rats receiving five hourly injections of glucose had a lower threshold and enhanced rates of insulin release at all stimulatory glucose concentrations. Infusion of bGH for 4 days was without effect on glucose gradient-stimulated insulin release. Fasting the rats for 48 h resulted in an elevation of the threshold and a substantial reduction in the extent of insulin release. To determine possible processes involved in these long term modifications of the threshold of glucose-stimulated insulin secretion, the in vitro effect of potentiators of insulin release was examined. Forskolin, glucagon, cholecystokinin, and carbamylcholine were able to lower the threshold and increase the extent of insulin release. This suggests that the long term regulation of insulin secretion may modulate processes controlling cAMP concentrations and the hydrolysis of phosphoinositides in pancreatic B-cells. Also, the proposed incretin gastric inhibitory polypeptide was capable of lowering the threshold and increasing insulin secretion at stimulatory glucose concentrations. The consequences of a decreased threshold is a markedly enhanced insulin secretion at normal serum glucose concentrations.

Animals↗

PubMed1988

Aldose reductase expression in human diabetic retina and retinal pigment epithelium.

The conversion of glucose to sorbitol by aldose reductase (AR) and its subsequent intracellular accumulation have been implicated in the pathogenesis of diabetic cataracts. There is also evidence linking AR activity with retinal capillary basement membrane thickening in galactosemic rats, suggesting a possible role in diabetic retinopathy. In this study, we explored one feature of this issue by examining diabetic and nondiabetic eyes for immunoreactive AR. AR was immunohistochemically undetectable in the retinal pigment epithelia (RPE) and neural retinas of nondiabetic human eyes. Weak, focal staining for AR was present unilaterally in the RPE of 1 of 11 diabetic patients without pathologic ocular findings and in 43% of diabetic patients with mild ocular findings. Retinal positivity was found (unilaterally) in only 2 of 19 individuals from either of these mildly affected groups. Fifty-five percent of patients with background retinopathy demonstrated AR positivity in the RPE, and half of these expressed AR in the RPE of both eyes. Of the individuals with proliferative diabetic retinopathy, 87.5% showed bilateral staining in the RPE. Retinal positivity was present in 36% of background retinopathy and 75% of proliferative retinopathy cases, demonstrating a positive correlation between AR expression and the severity of the disorder. In weakly staining retinas, only the ganglion cell bodies, nerve fibers, and Müller cells were positive, whereas in intensely staining cases, virtually the entire retina, except for the rod outer segments, was positive. Eyes from patients who had had diabetes less than or equal to 6 yr were negative for AR, but those from long- term-diabetic patients (14-45 yr) manifested positively.(ABSTRACT TRUNCATED AT 250 WORDS)

Aldehyde Reductase↗

PubMed1988

Increased dye coupling in pancreatic islets from rats in late-term pregnancy.

Our previous studies have suggested that elevated lactogen, increased glucose-stimulated insulin secretion, and increased beta-cell coupling are associated. To determine whether this association occurs under conditions of physiologically increased lactogen, we have studied the extent of dye coupling in rat islets during the later stage of pregnancy. These animals have high plasma lactogen levels in the form of placental lactogen, increased plasma insulin, and decreased plasma glucose. The fluorescent tracer, Lucifer yellow CH, was microinjected into central cells of islets from both pregnant and virgin rats, and the extent of transfer was quantitated by determining the projected area of dye spread. Two area measurements were made for each injection, one around the entire discernible fluorescent region ("outer") and another around the distinct brighter region of cells surrounding the injected cell ("inner"). Pregnancy increased dye transfer, as determined by both measurements. The outer area of dye transfer was 9047 +/- 775 microns2 for the islets from pregnant rats and 4699 +/- 391 microns2 for the islets from virgin rats (P less than .001). Similarly, pregnancy increased the inner area of dye transfer, 1447 +/- 161 microns2 for the islets from pregnant rats and 795 +/- 80 microns2 for the islets from virgin rats (P less than .001). These results support the hypothesis that elevated lactogen, increased glucose-stimulated insulin secretion, and increased beta-cell dye coupling are associated under physiological conditions. The study indicates that enhanced beta-cell coupling is part of the structural and functional adaptation that the islets undergo during a subject's pregnancy and demonstrates that the extent of beta-cell coupling is regulated by a physiological condition.

Animals↗

PubMed1987

Ultrastructural localization of gamma amino butyric acid immunoreactivity in B cells of the rat pancreas.

The purpose of this study was to determine the ultrastructural localization of gamma amino butyric acid (GABA) within the B cell of isolated rat islets, particularly with respect to the B cell secretion granules. GABA immunoreactivity was localized ultrastructurally, with colloidal gold, in the B cells and absent in the A cells and D cells. Quantitative analysis of the colloidal gold particle distribution in the B cell revealed 29.5 +/- 5.2 gold particles/micron 2 in the nuclei, 29.3 +/- -6.9 gold particles/micron 2 in the mitochondria and 4.9 +/- 1.0 gold particles/micron 2 in the secretion granules. Particle density in the remainder of the cytoplasm was 41.9 +/- 4.1 particles/micron 2. The paucity of gamma amino butyric acid in the B cell secretion granules differs from observations on gamma amino butyric acidergic neurons, where there is an accumulation of gamma amino butyric acid within the neurosecretory vesicles. These findings indicate that if gamma amino butyric acid is released from the islet, then it is by a nongranular mechanism. In addition, the results are compatible with the hypotheses that gamma amino butyric acid within the B cell functions in the regulation of insulin biosynthesis, and/or functions as an alternative energy source for the B cell through the gamma amino butyric acid shunt.

Animals↗

PubMed1987

Potassium-selective ion channels in a transformed insulin-secreting cell line.

K+ channels in inside-out patches from hamster insulin tumor (HIT) cells were studied using the patch-clamp technique. HIT cells provide a convenient system for the study of ion channels and insulin secretion. They are easy to culture, form gigaohm seals readily and secrete insulin in response to glucose. The properties of the cells changed with the passage number. For cell passage numbers 48 to 56, five different K+-selective channels ranging from 15 to 211 pS in symmetrical 140 mM KCl solutions were distinguished. The channels were characterized by the following features: a channel with a conductance (in symmetrical 140 mM KCl solutions) of 210 pS that was activated by noncyclic purine nucleotides and closed by H+ ions (pH = 6.8); a 211 pS channel that was Ca2+-activated and voltage dependent; a 185 pS channel that was blocked by TEA but was insensitive to quinine or nucleotides; a 130 pS channel that was activated by membrane hyperpolarization; and a small conductance (15 pS) channel that was not obviously affected by any manipulation. As determined by radioimmunoassay, cells from passage number 56 secreted 917 +/- 128 ng/mg cell protein/48 hr of insulin. In contrast, cells from passage number 77 revealed either no channel activity or an occasional nonselective channel, and secreted only 29.4 +/- 8.5 ng/mg cell protein/48 hr of insulin. The nonselective channel found in the passage 77 cells had a conductance of 25 pS in symmetrical 140 mM KCl solutions. Thus, there appears to be a correlation between the presence of functional K+ channels and insulin secretion.

Adenoma, Islet Cell↗

PubMed1987

Decreased glucose stimulation threshold, enhanced insulin secretion, and increased beta cell coupling in islets of prolactin-treated rats.

In order to determine the effect of lactogen on insulin secretion and junctional coupling among islet beta cells, ovine prolactin (oPRL) was infused by Alzet minipumps into female rats for 4 days. This treatment produced an oPRL level of 994 +/- 122 ng/ml which, combined with residual rat PRL (rPRL) (12 +/- 2 ng/ml), represented nearly a 20-fold increase from control (rPRL: 53 +/- 17 ng/ml). In addition, plasma insulin was increased nearly 50% (control: 21.9 +/- 3 microU/ml; experimental: 30.3 +/- 3 microU/ml; p less than 0.05). When pancreata from lactogen-treated and control animals were perfused with linear 30-200 mg/dl glucose gradients, the apparent glucose threshold for insulin secretion in the experimental group was nearly 33% lower than that of the controls (i.e., 70 +/- 4.6 mg/dl vs. 104 +/- 7.5 mg/dl; p less than 0.01). The oPRL treatment also increased dye coupling among beta cells. Central cells in islets isolated from lactogen-treated and control animals were injected with Lucifer Yellow CH to estimate the extent of gap junctional coupling. There was nearly a twofold increase in the projected area of dye transfer per injection in the experimental vs. the controls: 4,607 +/- 575 micron 2 vs. 2,302 +/- 474 micron 2, respectively; p less than 0.02. The effects of oPRL decreased the apparent glucose threshold for insulin release, increased the above-threshold glucose-induced insulin secretion, and increased the extent of dye coupling among beta cells. These changes in insulin secretion and dye coupling closely resemble those observed in islets from pregnant rats.

Animals↗