Synthesis and release of proinsulin and insulin by isolated rat islets of Langerhans.

Isolated rat islets of Langerhans were incubated for 60, 120, and 180 min and the incorporation of leucine-(3)H into proinsulin and insulin moieties was followed. Synthesis and release of these hormones could be followed by separate extractions of islets and incubation media. RELEASE OF NEWLY SYNTHESIZED PROINSULIN AND INSULIN OCCURRED UNDER THE FOLLOWING CONDITIONS: (a) incubation for greater than 60 min; (b) glucose concentrations above 5.3 mmoles/liter; (c) incubation with 5 mM dibutyryl cyclic AMP or theophylline in 5.3 mM glucose (potentiated by 16 mM glucose); and (d) incubation with 5 mM tolbutamide and 16 mM glucose. Synthesis of proinsulin and insulin was enhanced by time of incubation, high glucose concentrations, by dibutyryl cyclic AMP or theophylline, and by tolbutamide only at 16 mM glucose. Synthesis was totally inhibited by tolbutamide at 5.3 mM glucose.

Decreased glucagon receptors in diabetic rat hepatocytes. Evidence for regulation of glucagon receptors by hyperglucagonemia.

The effects of endogenous and exogenous hyperglucagonemia on the specific binding of glucagon to hepatocyte receptors was studied, as was the response of cAMP to glucagon. In streptozotocin diabetic rats, blood glucose and plasma glucagon increased and plasma insulin decreased as compared with controls. Insulin treatment in diabetic rats restored blood glucose and plasma glucagon toward normal and elevated plasma insulin. Specific binding of (125)I-glucagon to isolated hepatocytes (10(6) cells) decreased in diabetic rats (8.17+/-0.38%) compared to controls (14.05+/-0.87%) and was restored by insulin treatment (12.25+/-0.93%). Specific binding of (125)I-insulin in controls was 7.30+/-10.16%; it increased in diabetic rats to 12.50+/-0.86%, and decreased in diabetic rats after insulin treatment (9.08+/-0.87%). Scatchard analysis and the competition plots of the data indicate that decreased glucagon binding and increased insulin binding in diabetes were due to change in the number of receptors rather than a change in their affinity. Hepatocyte cAMP response to glucagon (0.25-5.0 ng/ml) was almost abolished in diabetic rats and was restored with insulin treatment. Specific glucagon binding by hepatocytes from chronically hyperglucagonemic (glucagon injected) rats was decreased (P < 0.005) to 8.76+/-0.61% compared with controls (13.20+/-0.74%) and acutely hyperglucagonemic animals (13.53+/-1.33%). The decreased binding was associated with a 70% decrease in hepatocyte cAMP response to glucagon compared with a normal response in acutely hyperglucagonemic rats.These data appear to support the concept of receptor regulation by ambient hormone level. In both endogenous and exogenous hyperglucagonemia, however, there was a disproportionately large decrease in cAMP response to glucagon compared to the decrease in glucagon binding.

Specific biologic effects of intestinal glucagon-like materials.

It has been demonstrated that gastrointestinal extracts contain substances which react immunologically with antibodies prepared to pancreatic glucagon. These extracts have been termed intestinal GLI for glucagon-like immunoreactivity, or enteroglucagon. To determine whether GLI has specific biological effects, studies were designed using the criterion of effect with antiglucagon antibodies. These antibodies did not cross-react with either secretin or pancreozymin. Rat intestinal extracts were prepared and filtered on Sephadex G-50 columns eluted in 0.02 M ammonium carbonate buffer pH 8.8. Two peaks of GLI (I, II) were consistently found, and the in vitro effects of these peaks on two biological systems were tested: (a) immunoreactive insulin (IRI) release by rat pancreas pieces, and (b) free fatty acid (FFA) release and 3',5'-cyclic adenosine monophosphate (cAMP) levels in adipose tissue. Both GLI peaks increased IRI release in the absence of glucose and also enhanced the glucose effects. Antiglucagon antibody suppressed only peak II GLI activity. Both peaks increased FFA release and cAMP levels in adipose tissue. Only peak II GLI activity was suppressed by antibody. These findings support a specific IRI-releasing and lipolytic action for Peak II GLI. Hypotheses are presented concerning the structure and possible physiologic role of peak II GLI.

Studies on the biological activity of porcine proinsulin.

The biological activity of purified porcine proinsulin was investigated in rats. In vivo studies revealed that proinsulin produced a hypoglycemic response similar to insulin but of lesser magnitude. Hypophysectomized and adrenalectomized animals proved to be more sensitive to proinsulin than normal. In vitro studies with rat hemidiaphragm were consistent with the in vivo findings. No competition with insulin action could be demonstrated. Experiments were carried out to determine whether proinsulin is converted to intermediate forms or insulin as a requisite to its biological activity. Labeled proinsulin injected in vivo or incubated in vitro remained intact by a variety of techniques (Sephadex column chromatography and polyacrylamide-gel electrophoresis). An inhibitory action of Kunitz pancreatic trypsin inhibitor on proinsulin action in vitro was confirmed. No clarification of this effect could be ascertained.

Altered beta-endorphin, Met- and Leu-enkephalins, and enkephalin-containing peptides in pancreas and pituitary of genetically obese diabetic (db/db) mice during development of diabetic syndrome.

We have recently shown that in addition to beta-endorphin the opioid peptides Met- and Leu-enkephalin and their apparent precursors are localized in islet endocrine cells of the rat pancreas. To begin evaluating a possible role for these pancreatic opiates in the pathophysiology of genetic diabetes in rodents, immunoreactive beta-endorphin and Met- and Leu-enkephalins were measured in acetic acid extracts of pancreas and pituitary of C57BL/KsJ db/db mice and their lean littermates. Groups of animals were studied during three phases of development of the diabetic syndrome in the mutant mice: at 4 (hyperinsulinemic and prediabetic); 6, 9, and 12 (frankly obese and diabetic); and 30 (hypoinsulinemic) wk of age. Elevations or decreases (P less than .05) were found in db/db mice (vs. lean littermates) as follows: pituitary content of Met-enkephalin was twofold higher at all ages studied; pituitary free Leu-enkephalin was lower at 4 wk and reversed to higher at 6-30 wk; pancreatic beta-endorphin was 30% lower at 4 wk and reversed to threefold higher at 6-12 wk; Met- and Leu-enkephalin-containing larger peptides were elevated at one or more points between 6 and 12 wk in both the pancreas and the pituitary. Thus, the onset of overt obesity between 4 and 6 wk of age was accompanied by a marked rise in both pancreatic beta-endorphin and pituitary Leu-enkephalin; similar elevations in these parameters have been reported previously in C57BL/6J ob/ob mice at approximately 12 wk of age.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Characteristics of the interaction of the glucagon receptor, cAMP, and insulin secretion in parent cells and clone 5F of a cultured rat insulinoma.

Rat insulinoma cells, which grow in culture and secrete insulin, were used to study the mechanism of stimulation of insulin release by glucagon. The parent cell line (RIN-m) and a clone that secretes high levels of insulin (5F) had been shown to possess specific receptors for glucagon. Glucagon (1 microM) stimulated a rapid increase in cyclic adenosine 3':5'-monophosphate (cAMP) that was followed by an increase in insulin secretion in both cell lines. The concentration of glucagon necessary for half-maximal stimulation of cAMP was 50 nM in parent and approximately 0.5 microM in 5F, whereas the concentration required to inhibit binding by 50% was 0.5 nM and 30 nM, respectively. In 5F, the dose-response relationships for cAMP and insulin secretion were superimposable. The glucagon effects on insulin secretion and cAMP did not require either glucose or amino acids in the incubation media. No refractoriness to glucagon stimulation of cAMP or insulin was noted. It may be concluded that there are significant differences between glucagon binding and glucagon responses in parent cells and clone 5F, there are glucagon receptors that are not coupled to adenylate cyclase, and cAMP mediates glucagon-stimulated insulin release.

Identification of human mononuclear leukocytes bearing receptors for somatostatin and glucagon.

Mononuclear leukocytes (MNL) were isolated from human blood by Ficoll-Hypaque. These cells were further separated into lymphocyte (L) and monocyte (M) enriched fractions. L contained 99% lymphocytes and M contained 74% monocytes, a threefold enrichment over MNL. Specific binding to somatostatin, glucagon, and insulin was measured in the three fractions. Binding of all three hormones in the M fraction was increased by a factor of 3 compared with MNL and was linear with cell number. Binding of glucagon and insulin to the L fraction was very low while, in contrast, somatostatin binding was substantial and linear with lymphocyte number. Autoradiography confirmed the binding of glucagon to monocytes and of somatostatin to both monocytes and lymphocytes. Somatostatin is the first of the peptide hormones shown to bind to both types of circulating mononuclear cells, perhaps complicating quantification of somatostatin binding in disease states in which differential alteration of binding of lymphocytes or monocytes might occur.

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.

Insulin, glucagon, and somatostatin receptors on cultured cells and clones from rat islet cell tumor.

Cells grown in culture from rat islet cell tumor (parent cells) and clones obtained from them were used in this study. Parent cells secreted primarily insulin and somatostatin with very small quantities of glucagon. The clones, based on hormone content and secretion, were divided into three phenotypic groups: insulin secreting, somatostatin secreting, and nonsecreting clones. Specific receptors for insulin, glucagon, and somatostatin were demonstrated on parent cells and clones. Parent cells bound 4.12 +/- 0.46% insulin, 2.11 +/- 0.29% glucagon, and 2.49 +/- 1.24% somatostatin per 2 x 10(6) cells. Characteristic hormone binding patterns were observed in insulin secreting versus somatostatin secreting clones. Insulin secreting versus somatostatin secreting clones. Insulin secreting clones bound less insulin than somatostatin and other noninsulin-secreting clones (P less than 0.02). In contrast, somatostatin secreting clones bound more somatostatin than non-somatostatin-secreting clones (P less than 0.05). Somatostatin-secreting clones had a significantly greater number of receptors for all three hormones. The difficulties involved in the interpretation of the quantitative aspects of binding in the presence of continued hormone secretion are discussed. Nonetheless, the presence of receptors on the cells for hormones secreted by the same cells strongly suggests autoregulation. The apparent low affinity of some of these receptors and the presence of receptors for all three islet cell hormones on all islet cells supports the likelihood of paracrine controls.

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)

Reversal of somatostatin inhibition of insulin and glucagon secretion.

These studies were designed to elucidate the mechanism of inhibitory action of somatostatin (SRIF) on glucagon (IRG) and insulin (IRI) secretion. Studies were carried out in the unrecirculated isolated rat pancreas perfusion with arginine 19.2 mM and glucose 5.5 mM as stimulus primarily for IRG but also IRI secretion. The effects of excess Ca++ (15.2 mEq./L.) and excess K+ (12.8 mEq./L.) on IRG, IRI, and the SRIF-inhibited pancreas were studied. Ca++ excess in five perfusions strikingly stimulated IRG secretion (+92 per cent) but only stabilized IRI secretion compared with control perfusions. K+ excess (in seven perfusions) markedly inhibited IRG secretion (-39 per cent) while stimulating IRI secretion (+16 per cent). Restoration of normal concentrations of K+ resulted in a rebound of IRG to levels 120 per cent that of controls. SRIF, at concentrations from 0.1-20 ng./ml., produced inhibition of both IRG and IRI. In 11 perfusions, with SRIF at 10 ng./ml., IRG decreased more than IRI (-75.2 per cent IRG and -46.9 per cent IRI). In five perfusions, addition of Ca++ (15.2 mEq./L.) 10 minutes after SRIF was started resulted in a reversal of IRG inhibition to 69.4 per cent and IRI to 73.2 per cent of the arginine controls. The reversal by Ca++ of SRIF effect on IRG was greater at higher concentrations of Ca++, suggesting some form of competition. In four perfusions, excess K+ reversed SRIF-induced IRI inhibition to 79.6 per cent that of controls but had no effect on IRG inhibition. Studies in vitro with isolated islets revealed that SRIF (2 mug./ml.) inhibited 45Ca uptake of islets as did epinephrine (10(-5) M). It was concluded that SRIF-induced inhibition of hormone release appears related to an action on Ca++ uptake.

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)

The effect of fasting on tissue cyclic cAMP and plasma glucagon in the obese hyperglycemic mouse.

The effect of 48 h of fasting in C57B1/6J-ob/ob and +/+ mice on body weight (BW), blood glucose (BG), serum immunreactive insulin (IRI), plasma immunoreactive glucagon (IRG) and on tissue levels of cyclic adenosine monophosphate (cAMP) were studied. Both groups of mice lost weight and demonstrated a decrease in BG and IRI with fasting. However, the BG and IRI of the ob/ob animals were initially highter and remained higher than those of the 2% of their initial weight while the +/+ lost 14 %. The +/+ mice exhibited an increase in cAMP levels in skeletal muscle, fat and liver with fasting, while the ob/ob mice had increased levels of cAMP in fat, but not in muscle. They also had a paradoxical decrease in liver cAMP levels with fasting, and associated with this was the lack of stimulation of glycogenolysis. Glycogenolysis was significant in the livers of fasted +/+ mice. The plasma IRG levels of the fed ob/ob mice were significantly higher (1.8) times) than those of the fed +/+ mice. Islet cAMP levels were decreased with fasting in ob/ob mice. However, the levels were significantly higher in 48-h faster ob/ob mice compared to the fasted +/+ group. The apparent paradoxical response to fasting observed in the livers of the ob/ob mice remains unexplained.

Role of the hypophysis in regulation of pancreatic and gastric somatostatin.

Since hypophysectomy and GH deficiency are associated with decreases in hypothalamic content and release of SRIF, it was of interest to determine whether these hormonal alterations also affect peripheral tissue levels of SRIF. Hypophysectomized (hypox) rats were studied at various times after surgery and compared with age-matched controls. Pancreatic, gastric, and hypothalamic SRIF levels were measured by RIA and expressed as nanograms per mg protein or nanograms per organ. Decreased levels of hypothalamic SRIF were observed in hypox animals at all time periods after surgery. In contrast, pancreatic SRIF concentrations increased within 1 week of hypophysectomy, and the tissue content increased as much as 3-fold after 20 weeks. Measurement of the SRIF content of isolated rat islets of Langerhans revealed a 67% increased content/islet in hypox rats compared with controls. The gastric SRIF concentration was not changed early, but subsequently, the total organ content was significantly decreased compared with that in controls. The changes in stomach and pancreas SRIF contents became more marked with duration of pituitary deficiency. Studies in genetically dwarfed Snell mice, lacking primarily GH but also other anterior pituitary hormones, were similar to the findings noted in hypox rats; the SRIF concentration was significantly increased in the pancreas and decreased in the stomach and hypothalamus. It is probable that deficiencies in other hormones as well as GH are involved in producing the changes in pancreatic SRIF in hypox and dwarfed animals. This contention was supported in that replacement of T3 (5 micrograms/kg . day) reduced pancreatic SRIF concentration by 30%, while GH plus T3 produced a significantly greater (60%) decrease in pancreatic SRIF in hypox rats.

Expression of glucagon receptors on T- and B- lymphoblasts: comparison with insulin receptors.

Activation of T- and B-lymphocytes by a variety of immunological stimuli has been reported to induce specific insulin receptors. The purpose of the present work was to determine whether glucagon receptors are also induced in activated cells. Studies of glucagon and insulin receptors were carried out using normal human mononuclear cells activated by phytohemagglutinin or T-cell growth factor (TCGF), as well as established B- and T-lymphoblastoid cell lines. With phytohemagglutinin, glucagon and insulin binding increased 15- and 36-fold, respectively, and peaked after 5 days in parallel with the rise in thymidine incorporation. Increased binding was associated with an increase in the number of receptors, most marked for insulin, though affinity for the insulin receptor was decreased. Normal human mononuclear cells cultured with TCGF showed an early modest rise in insulin binding due to increased receptor number, without a change in affinity, and a striking and progressive rise up to 50-fold in glucagon binding due to both increased receptor number and affinity. The differences in receptor response to these T-cell mitogens suggest that TCGF selects out a T-lymphoblast subset with very high glucagon receptors. B- and T-lymphoblastoid cells showed patterns of glucagon and insulin receptors that appear to be characteristic for each cell type. Glucagon binding was 7-fold higher (P less than 0.01), while inulin binding was 7-fold lower (P less than 0.01) in T- vs. B-lymphoblastoid cells. T-Cell lines had twice the number of glucagon receptors, whereas B-lines had 4-fold the number of insulin receptors, with much greater affinity for insulin compared with T-line insulin receptors. Induction of both insulin and glucagon receptors on activated lymphoblasts suggests that these receptors may play a significant role in cell function.

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.

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)

Neuropeptide content in pancreas and pituitary of obese and diabetes mutant mice: strain and sex differences.

The nature of the primary genetic defects in ob/ob and db/db mice are unknown. Both the obese (ob) and diabetes (db) mutations produce similar, multicomponent obese-hyperinsulinemic syndromes when maintained in the same strain of mouse. In an attempt to find differences between these mutations in neuroendocrine function affecting the islets of Langerhans or the pituitary, tissue content of four neuropeptides that are known to be capable of influencing the rate of insulin secretion was examined in obese (ob/ob) and diabetes (db/db) mice. In the first study, C57BL/6Job/ob and control males were studied at 3, 4, and 11 weeks of age. In the second study, db/db mice of both sexes and two inbred strains (C57BL/6J and C57BL/KsJ), which differ markedly in the severity of expression of the diabetes phenotype, were studied at 3 weeks of age, before the development of hyperglycemia and secondary consequences thereof. Immunoreactive peptides were measured in acetic acid extracts of pancreas and pituitary. No differences between male ob/ob and db/db mice of the C57BL/6J strain were found. Marked sex differences in lean control mice were found at 3 weeks of age in pancreatic Met-enkephalin-LI and galanin-LI (with two- to threefold higher content in males). Low pancreatic content (50% to 70% lower than in control mice) of galanin-LI, Met-enkephalin-LI and Leu-enkephalin-LI was associated with hyperinsulinemia in male B6 ob/ob and db/db mice at 3 weeks of age, though not in B6 db/db females and not in BKs db/db mice of either sex.(ABSTRACT TRUNCATED AT 250 WORDS)