Opioid peptides in rat islets of Langerhans. Immunoreactive met- and leu-enkephalins and BAM-22P.

Previous studies have shown that met- and leu-enkephalins are present in extracts of whole pancreas obtained from guinea pigs and human cadavers. The present studies demonstrate that immunoreactive methionine (met)- and leucine (leu)-enkephalins present in rat pancreas are localized in islets of Langerhans. Immunohistochemical staining of fixed, whole pancreas indicated that only islet endocrine cells were heavily stained when any of four different met- and leu-enkephalin-directed antisera or an anti-BAM-22P (bovine adrenal medulla docosapeptide) antiserum was used. The peptides were characterized by a combination of gel-filtration chromatography, high-performance liquid chromatography (HPLC), and specific radioimmunoassay. Free met-enkephalin content in extracts of rat islets was 90-fold enriched over content in extracts of whole pancreas (1.72 +/- 0.35 versus 0.019 +/- 0.007 pmol/mg protein). Treatment with trypsin and carboxy-peptidase-B of high-molecular-weight peptides extracted from pancreas or islets resulted in release of additional met-enkephalin immunoreactivity, which was 39-fold enriched in islets compared with pancreas (5.90 +/- 0.58 and 0.153 +/- 0.032 pmol/mg protein, respectively). Total islet content (per milligram protein) of met-enkephalin-containing peptides was similar to that reported elsewhere for bovine hypothalamus. The immunohistochemical data as well as the enrichment of extractable enkephalins in islets compared with whole pancreas indicate that essentially all the met-enkephalin present in pancreas is localized in islets, while the presence of BAM-22P immunoreactivity in islets is consistent with biosynthesis of enkephalins in islet cells via a preprohormone, such as that described in the bovine adrenal medulla and rat brain.

Cytosolic insulin-degrading activity in islet-derived tumor cell lines and in normal rat islets.

RIN-m cells, cultured from a rat insulinoma, not only bind and secrete but also degrade insulin (Diabetes 1982; 31:521-31). The insulin-degrading activity resides in the cytosol and is similar to the insulin-specific proteases previously described in muscle and other tissues. It has an apparent Km of 0.15 microM for porcine insulin in crude cell-free extracts, a competitive inhibition constant for proinsulin that is close to the Km, and a lower but measurable affinity for glucagon. The enzyme is inactive at pHs below 6.0, indicating that it is not lysosomal, is completely inhibited by N-ethylmaleimide, and exhibits apparent competitive inhibition constants (microM) for the following peptides: desoctapeptide insulin, 0.043; guinea pig insulin, 0.048; proinsulin, 0.64; insulin B-chain, 1.17; glucagon, 7.0; and cyclic somatostatin, 8.6. Highly active insulin-degrading activity was found using cell suspensions of 22 cloned and 8 subcloned cell lines derived from RIN-m as well as 11 other continuous cell lines derived from a variety of nonislet tissues of rat, mouse, and human origin. Homogenates of the original rat islet tumor and cytosol of normal rat islets also contained insulin-degrading activity. Although insulin protease is present in a variety of tissues, it may have an additional regulatory function in cells that are actively synthesizing, storing, and secreting insulin.

Reversible impairment of glucose-induced insulin secretion in SHR/N-cp rats. Genetic model of type II diabetes.

The SHR/N-cp rat is a new genetically obese model for non-insulin-dependent diabetes mellitus. Expression of the diabetes is enhanced by a high-sucrose (54%) diet. After 4 wk on the diet, the cp/cp rats weigh significantly more than their +/? controls, have postprandial hyperglycemia (greater than 400 mg/dl), and are hyperinsulinemic, with immunoreactive insulin (IRI) levels 10- to 20-fold greater than controls. Total pancreatic IRI tends to be increased 1.6-fold in the cp/cp rats (although not significantly). There is no increase in pancreatic proinsulin content as a percent of total IRI. Studies of in vitro pancreatic function were carried out with the isolated nonrecirculating perfused pancreas method. The cp/cp rats (n = 10) showed impaired or absent IRI responses to 16.5 mM glucose, whereas +/? rats (n = 9) responded with classic biphasic curves. Comparison of insulin secreted in 20 min revealed a greater than 53% decrease in IRI secretion in cp/cp rats (P less than .05). A paradoxical hypersecretion of IRI at glucose concentrations of 0-2.7 mM was noted in cp/cp but not lean rats, i.e., 1.8 +/- 0.2 mU/min IRI in cp/cp rats vs. 0.04 +/- 0.007 mU/min in +/? rats. Perfusion of pancreases for 45 min with buffers containing no glucose resulted in restoration of a normal biphasic IRI response to 16.5 mM glucose in the cp/cp rats, whereas response in the lean rats was markedly reduced. Brisk IRI responses to 10 mM arginine in buffers with no glucose also occurred in cp/cp but not +/? rats. Glucagon secretion was relatively suppressed in the cp/cp rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of insulin secretory patterns in obese nondiabetic LA/N-cp and obese diabetic SHR/N-cp rats. Role of hyperglycemia.

Obese diabetic SHR/N-(cp/cp) rats are a genetic model for non-insulin-dependent diabetes mellitus. When SHR/N-cp rats are overtly diabetic, they are hyperinsulinemic and hyperglycemic in the fed state when consuming commercial chow or semipurified high-carbohydrate diets. Obese SHR/N-cp rats were hyperinsulinemic by 4 wk of age, although hyperglycemia did not appear until 3-4 wk later and was exacerbated by a high-sucrose diet (mean +/- SE 1488 +/- 238 microU/ml insulin and 425 +/- 51 mg/dl glucose). The control SHR/N-cp rats (+/?) on the sucrose diet remained lean and normoglycemic. The obese diabetic SHR/N-cp rats showed three alterations in pancreas perfusion data (not present in control rats): 1) paradoxically high insulin secretion at low glucose levels (2.5 mM), 2) secretion of insulin in response to arginine (10 mM) in the absence of glucose, and 3) impaired response of insulin secretion to high glucose (16.7 mM). To determine whether hyperglycemia was responsible for the abnormalities of insulin secretion, perfusion studies were conducted in obese nondiabetic LA/N-cp rats and compared with the SHR/N-cp rats. The obese LA/N-cp rats resembled the corpulent SHR/N-cp rats in every way, except that they were normoglycemic on the sucrose diet. The obese LA/N-cp rats had two of the three alterations in insulin secretion shown by obese SHR/N-cp rats, lacking only the impaired response to high glucose, suggesting that hyperglycemia was required for that defect to occur.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple alterations in insulin responses to glucose in islets from 48-h glucose-infused nondiabetic rats.

To examine the biochemical mechanisms by which hyperglycemia produces insulin secretory abnormalities, we studied isolated islets from control rats and rats infused for 48 h with a 50% glucose solution. To preserve the effects of in vivo hyperglycemia during in vitro handling for islet isolation, our standard isolation procedure utilized buffers containing 16.8 mM glucose. Islets from infused rats released similar amounts of insulin in low or high glucose during first incubations at 37 degrees C (92.4 +/- 7.0 ng.10 islets-1.45 min-1 at 2.8 mM, 84.4 +/- 4.1 ng.10 islets-1.45 min-1 at 16.8 mM) in contrast with control (uninfused) islets (18.6 +/- 2.8 ng.10 islets-1.45 min-1 at 2.8 mM and 109.8 +/- 8.0 ng.10 islets-1.45 min-1 at 16.8 mM glucose) (P less than 0.01). Secretion by islets of glucose-infused rats was lower during 60-min second incubations at 28 mM glucose than in first incubations of the same islets in low glucose (P less than 0.01). This phenomenon is comparable to the paradoxical hypersecretion observed during the first 10-15 min of exposure of glucose-infused pancreas to low-glucose perfusions. Paradoxical secretion in low glucose waned rapidly, so that during second incubations at 37 degrees C, little immunoreactive insulin release occurred at 2.8 mM glucose, despite the persistence of two additional lesions. The glucose-insulin dose-response curves in second incubations showed a leftward shift for glucose-infused islets, with two- to threefold higher secretion at 5.6-8.4 mM glucose than control islets. This is termed sensitization to glucose.(ABSTRACT TRUNCATED AT 250 WORDS)

Decreased plasma enkephalins in copper deficiency in man.

Twenty-four male subjects participated in a study in which the effect of feeding diets low in copper (1.03 mg/day) on plasma opiates was determined. The subjects were fed a low-copper diet for 11 wk with either starch or fructose as a major source of carbohydrate. Feeding low-copper diet decreased serum copper level significantly. In addition, plasma leu- and met-enkephalins decreased significantly while beta-endorphin levels rose. On repletion with copper (3 mg/day) for 3 wk, plasma enkephalins increased while beta-endorphin levels decreased to pretest values. These results suggest that feeding low copper decreases plasma enkephalins, which may reflect a copper-dependent process affecting enkephalin biosynthesis and/or release.

Unique alterations of neuropeptide content in median eminence, amygdala, and dorsal vagal complex of 3- and 6-week-old diabetes mutant mice.

The nature of the genetic defects which define the obese (ob) and diabetes (db) loci in mice remain unknown, but both produce similar syndromes when maintained in the same strain of mice. There is some evidence suggesting a lesion in the central nervous system (CNS) in db/db mice, while ob/ob mice appear to have a primary lesion outside the CNS. In a search for further evidence of a unique central lesion in db/db mice, we have examined neuropeptide content in selected, microdissected brain areas in both of these mutants and lean controls. In order to rule out possible interactions of the db mutation with the genetic background, diabetes mice of both C57BL/KsJ and C57BL/6J strains were studied. When concentrations of nine neuropeptide immunoreactivities were examined in up to seven microdissected areas of the brain, C57BL/6J ob/ob mice showed only one reproducible alteration, a lower content of beta-endorphin-like immunoreactivity (LI) in the preoptic area at both 3 and 6 weeks of age as compared with lean littermates. In contrast, db/db mice of both C57BL/6J and C57BL/KsJ strains exhibited alterations in a total of four peptides in three brain areas: lower concentration of somatostatin-LI in median eminence, higher Met-enkephalin-LI in dorsal vagal complex of the medulla oblongata, higher substance P-LI and lower vasoactive intestinal polypeptide (VIP)-LI in amygdala. The concentrations of the peptides studied in medial basal hypothalamus, lateral hypothalamus, substantia nigra, and preoptic area were not reproducibly altered in db/db mice. These data provide preliminary evidence for unique brain abnormalities in db/db mice in specific areas that are involved in processing of neural signals that can affect the islets of Langerhans, gonadotrophin secretory patterns, and many other visceral functions.(ABSTRACT TRUNCATED AT 250 WORDS)

Genetically obese rats with (SHR/N-cp) and without diabetes (LA/N-cp) share abnormal islet responses to glucose.

To assess the effect of hyperglycemia on the function of islets obtained from obese rats, the behavior of isolated islets from LA/N-corpulent (nondiabetic obese) and SHR/N-corpulent (diabetic obese) male rats was examined and compared. Islets from both genetic models showed a left-shifted glucose dose-response curve for insulin release (concentrations for half-maximal release, 5 to 6 mmol/L v 12 to 13 mmol/L in LA/N lean littermates and 3 mmol/L v 10 mmol/L in lean SHR/N). When insulin release was expressed per unit islet volume, the fourfold to fivefold enlarged islets from both obese diabetic and obese nondiabetic rats showed decreased insulin secretory response in high (16.5 to 28 mmol/L) glucose concentrations, although the decrease was more severe in the diabetic rats. Glucose-stimulated insulin release by islets from both models was relatively resistant to inhibition by 1.2 mmol/L mannoheptulose (eg, 82% +/- 3% inhibition in LA/N lean v 16% +/- 8% in LA/N obese), although nearly complete inhibition was observed with 16 mmol/L mannoheptulose (96% v 85%, NS). Islets of obese diabetic rats were also resistant to the calcium-channel blocker, verapamil, suggesting an abnormal pathway of stimulus-secretion coupling for glucose. Glucose oxidation to carbon dioxide was increased in both obese models at all glucose concentrations when expressed per islet. In data expressed per unit volume, the larger islets from the obese-nondiabetic rats showed a left-shifted dose-response curve with an unchanged maximum rate of glucose oxidation at high (16.5 mmol/L) glucose concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Copper deficiency in rats increases pancreatic enkephalin-containing peptides and insulin.

Free enkephalins (enk) and higher molecular weight enkephalin-containing peptides (enk-c-p) are present in the endocrine pancreas of rats, presumably in B cells. To determine whether these opioid peptides show dynamic alterations as insulin content of pancreas changes, we utilized a copper deficient rat model, in which the exocrine pancreas atrophies and the endocrine pancreas is "intact" and insulin (IRI) content increases. Dietary copper deficiency (-C) was produced in weanling male rats for 4 and 7 weeks. The deficient and copper supplemented (+C) groups were further subdivided to receive all dietary carbohydrate as either 62% fructose (F) or 62% starch (S). -CF rats showed the most severe deficiency. After 7 weeks, total units of pancreatic IRI in -CF were 7.5 +CF 2.1, -CS 7.9 and in +CS 2.8 (p less than 0.001). Pancreatic content of Met5- and Leu5-enk was measured in extracts which were purified on C-18 Seppaks with and without prior treatment with trypsin and carboxypeptidase B. -C animals showed progressive, significant increases in pancreatic content of Leu-enk-c-p, with a decrease in free Leu- and Met-enk (p less than 0.02-0.01). The pancreatic findings are compatible with a co-localization of enkephalins and insulin in the endocrine pancreas and are suggestive of co-regulation.

Developmental patterns for pancreatic opioids in the rat.

Developmental patterns for rat pancreatic opioid peptides and islet hormones were studied from gestational day 20 through adulthood. Fetal tissue was obtained as well as pancreas at birth (day 0), and postnatal days 3, 7, 14, and 21, and 7 weeks. The hormones measured included insulin, glucagon, and somatostatin. The opioids measured were beta-endorphin, Met- and Leu-enkephalins, and the high molecular weight enkephalin precursors. Pancreata were pooled as necessary and extracted (acid alcohol, or hot acetic acid), and opioids were further purified on reversed-phase C-18 (Sep-pak) cartridges. In all instances measurements were made by radioimmunoassays. Precursor peptides were first digested (with trypsin and carboxypeptidase B) prior to immunoassay. All opioids and hormones except the precursors for enkephalins showed a well-defined surge in pancreatic concentration during the first postnatal week. In contrast, the precursors had the highest concentration in the fetus, and by the seventh day of life had decreased by greater than 50%. This progressive decrease may represent maturation of the enkephalin convertase and trypsin-like enzymes in the islets. The opioid and hormonal surges that we have described are similar to the surge in islet concentration of thyroid-releasing hormone (TRH) previously described in neonatal rat islets. It is suggested that these postnatal alterations in opioid and hormone concentration relate to a specific function in the development of the endocrine pancreas.

Autoradiographic localization of [3H]-guanidinoethylmercaptosuccinic acid binding sites in rat islets of Langerhans.

Incubation of unfixed, frozen sections of rat pancreas with tritiated guanidinoethylmercaptosuccinic acid (GEMSA) resulted in a localization of [3H]-GEMSA autoradiographic grains over the islets of Langerhans, suggesting that a carboxypeptidase H-like enzyme occurs in the islet.

Regulation of glucose-6-phosphate dehydrogenase activity during caloric restriction in human adipose tissue.

Glucose-6-phosphate dehydrogenase activity was significantly lower in adipose tissue of human subjects after 7 days of severe caloric restriction on low-carbohydrate diets and had returned to normal values 4 days after the subjects resumed normal diets. Three other enzyme activities (malate-NADP dehydrogenase, oxaloacetate decarboxylating; ATP-citrate lyase and 6-phosphofructokinase) were not significantly affected by these dietary changes. These results are consistent with separate control of glucose-6-phosphate dehydrogenase activity versus other 'lipogenic' enzyme activities in human adipose tissue.

Impaired 6-phosphofructokinase activity in mononuclear leukocytes from patients with type II diabetes mellitus.

Seven cytoplasmic enzyme activities were measured in extracts of mononuclear leukocytes (lymphocytes plus monocytes) obtained from 19 type II diabetic humans and 10 healthy control subjects. 6-Phosphofructokinase activity was significantly decreased in cell extracts from diabetics, while other enzyme activities were similar in diabetics and controls. Since the effects of starvation on enzyme activities are sometimes similar to the effects of diabetes, the studies were repeated in 5 control subjects after a 2-day fast. This short period of starvation did not mimic the effect of diabetes on 6-phosphofructokinase activity. The decreased enzyme activity was not correlated with percent specific insulin binding to monocytes in the same cell preparations nor to clinical variables such as obesity or the broad range of fasting plasma glucose values encountered among the diabetics. We conclude that 6-phosphofructokinase activity in mononuclear leukocytes, as in other tissues, may be a marker for a postreceptor lesion associated with the insulin resistance found in type II diabetes mellitus.

Circulating factors affect 6-phosphofructokinase activity in human adipose tissue.

6-Phosphofructokinase activity was lower in adipose tissue of obese, insulin-resistant subjects studied at stable body weight than in nonobese controls. This difference was absent during weight loss due to an increase in 6-phosphofructokinase activity in the obese subjects; the change suggested possible regulation by circulating factors. In an initial test of this hypothesis, blood serum collected from obese subjects at stable weight and incubated with adipose tissue in vitro produced lower 6-phosphofructokinase activity than did sera from nonobese controls.

Tissue somatostatin levels in three models of genetic obesity in rats.

A potential role for somatostatin (SRIF) in the pathogenesis of the hyperinsulinemia of obese rats was considered. SRIF like immunoreactivity (ng/mg protein) was therefore measured in hot 2 N acetic acid extracts of pancreas, stomach, pituitary, and hypothalamus in tissues obtained from three models of genetic obesity in rats. These models included the obese and lean controls of LA/N-cp, SHR/N-cp, and Zucker rats. To assess the effects of diet on SRIF levels, mixed diets were provided ad lib which contained a carbohydrate as either sucrose or starch. Some groups were fed chow diets. No significant dietary effects on tissue levels of SRIF were obtained. However, two of the three models (Zucker and SHR/N-cp) showed phenotypic effects on SRIF levels in pancreas; namely, obese rats showed a significantly greater concentration of SRIF (P less than 0.0005 and less than 0.0002, respectively) than did the lean littermates. These findings were confirmed by measurement of total pancreas SRIF content. Gastric levels were significantly altered only in the obese Zucker rats (P less than 0.005) where obese tissues had lower concentrations than those of lean animals. However similar directional changes in pancreas and stomach were observed in all models. It is concluded that the hyperinsulinemia of the obese animals studied is not due to absolute deficiency in pancreatic SRIF content. It is postulated however that decreased pancreatic SRIF secretion (paracrine or otherwise) relative to pancreatic insulin content could still play a role.

Identification of SNAP receptors in rat adipose cell membrane fractions and in SNARE complexes co-immunoprecipitated with epitope-tagged N-ethylmaleimide-sensitive fusion protein.

The vesicle-associated membrane proteins [VAMPs; vesicle SNAP receptors (v-SNAREs)] present on GLUT4-enriched vesicles prepared from rat adipose cells [Cain, Trimble and Lienhard (1992) J. Biol. Chem. 267, 11681-11684] have been identified as synaptobrevin 2 (VAMP 2) and cellubrevin (VAMP 3) by using isoform-specific antisera. Additional antisera identify syntaxins 2 and 4 as the predominant target membrane SNAP receptors (t-SNAREs) in the plasma membranes (PM), with syntaxin 3 at one-twentieth the level. Syntaxins 2 and 4 are enriched 5-10-fold in PM compared with low-density microsomes (LDM). Insulin treatment results in an 11-fold increase in immunodetectable GLUT4 in PM and smaller (approx. 2-fold) increases in VAMP 2 and VAMP 3, whereas the subcellular distributions of the syntaxins are not altered by insulin treatment. To determine which of the SNAP receptors (SNAREs) in PM might participate in SNARE complexes with proteins from GLUT4 vesicles, complexes were immunoprecipitated with anti-myc antibody from solubilized membranes after the addition of myc-epitope-tagged N-ethylmaleimide-sensitive fusion protein (NSF) and recombinant alpha-soluble NSF attachment protein (alpha-SNAP). These complexes contain VAMPs 2 and 3 and syntaxin 4, but not syntaxins 2 or 3. Complex formation requires ATP and is disrupted by ATP hydrolysis. When all membrane fractions are prepared from basal cells, few or no VAMPs and no syntaxin 4 are immunoprecipitated in SNARE complexes obtained from LDM alone (or from immunoisolated GLUT4 vesicles). The content of syntaxin 4 depends on the presence of PM, and participation of VAMPs 2 and 3 is enhanced 4-6-fold by the addition of solubilized GLUT4 vesicles to PM. The latter increase is greater than can be explained by the 2-fold higher levels of VAMPs added to the reaction mixture. When all membrane fractions are prepared from insulin-stimulated cells, SNARE complexes formed from PM alone contain similar levels of syntaxin 4 but 5-6-fold higher levels of VAMPs 2 and 3 compared with PM alone from basal cells. Addition of GLUT4 vesicle proteins to PM from insulin-treated cells results in a further 2-fold increase in VAMP 2 recovered in SNARE complexes. Therefore the VAMPs in PM of insulin-treated but not basal cells, and in GLUT4-vesicles from cells in either condition, are in a form that readily forms a SNARE complex with PM t-SNAREs and NSF. Insulin seems to activate PM and/or GLUT4 vesicles so as to increase the efficiency of SNARE complex formation.