Pancreatic Content Of Immunoreactive Insulin Research Articles

Beta-cell glucokinase deficiency and hyperglycemia are associated with reduced islet amyloid deposition in a mouse model of type 2 diabetes.

Type 2 diabetes is characterized by impaired beta-cell function, hyperglycemia, and islet amyloid deposition. The primary constituent of islet amyloid is the 37-amino acid beta-cell product called islet amyloid polypeptide (IAPP) or amylin. To study mechanisms of islet amyloid formation, we developed a transgenic mouse model that produces and secretes the amyloidogenic human IAPP (hIAPP) molecule and have shown that 81% of male transgenic mice develop islet amyloid after 14 months on a high-fat diet. To test whether impaired beta-cell function and hyperglycemia could enhance islet amyloid formation, we cross-bred our hIAPP transgenic mice with beta-cell glucokinase-knockout mice (GKKO) that have impaired glucose-mediated insulin secretion and fasting hyperglycemia. The resulting new (hIAPPxGKKO) line of mice had higher basal plasma glucose concentrations than the hIAPP transgenic mice at 3, 6, and 12 months of age (P < 0.05), as did GKKO mice compared with hIAPP transgenic mice at 6 and 12 months of age (P < 0.05). Basal plasma immunoreactive insulin (IRI) levels were lower in hIAPP x GKKO mice than in hIAPP transgenic mice at 6 months of age (P < 0.05). The area under the glucose curve in response to an intraperitoneal glucose challenge (1 g/kg body weight) was larger in hIAPPxGKKO mice than in hIAPP transgenic mice at 3, 6, and 12 months of age (P < 0.005) and in GKKO mice compared with hIAPP transgenic mice at 6 and 12 months of age (P < 0.005). The area under the IRI curve was lower in hIAPPxGKKO mice at 6 and 12 months of age (P < 0.05) than in hIAPP transgenic mice and in GKKO mice compared with hIAPP transgenic mice at 12 months of age (P < 0.05). Despite the presence of hyperglycemia, hIAPPxGKKO mice had a lower incidence (4 of 17 vs. 12 of 19, P < 0.05) and amount (0.40 +/- 0.24 vs. 1.2 +/- 0.3 arbitrary units, P < 0.05) of islet amyloid than hIAPP transgenic mice had. As expected, no islet amyloid was observed in GKKO mice lacking the hIAPP transgene (0 of 13). There was no difference in pancreatic content of IRI and hIAPP among the three groups of mice. Thus, despite the presence of impaired islet function and hyperglycemia, hIAPPxGKKO mice had a decreased incidence and quantity of islet amyloid. Therefore, our data suggest that impaired beta-cell glucose metabolism or hyperglycemia are not likely to contribute to islet amyloid formation in diabetes. Furthermore, this finding may explain the lack of progression of glycemia in patients with maturity-onset diabetes of the young.

Oophorectomy promotes islet amyloid formation in a transgenic mouse model of Type II diabetes.

In Type II (non-insulin-dependent) diabetes mellitus, amyloid depletes islet mass. We previously found that 81% of male human islet amyloid polypeptide (IAPP) transgenic mice but only 11% of female mice developed islet amyloid, suggesting that either testosterone promotes or ovarian products protect against amyloid deposition. We did a bilateral oophorectomy or sham procedure in female human IAPP transgenic mice (n = 11 and n = 8, respectively) and in female non-transgenic mice (n = 7 and n = 9, respectively) at 6-8 weeks of age. Animals were followed for 1 year on a 9% fat (w/w) diet. Before we killed them we measured, fasting plasma human IAPP and did an intraperitoneal glucose tolerance test. Pancreatic content of IAPP and immunoreactive insulin (IRI) were estimated and pancreata were analysed for islet amyloid. No amyloid was detected in either the sham-operated transgenic mice or, as expected, in both groups of non-transgenic mice. In strong contrast, 7 of 11 (64%) oophorectomized mice developed islet amyloid (p < 0.05). Amyloid deposition in the oophorectomized transgenic mice was not associated with any differences in incremental body weight, fasting human IAPP concentrations or glucose tolerance between the groups. Furthermore, pancreatic content of mouse IAPP, human IAPP and immunoreactive insulin did not differ between groups. Oophorectomy is associated with an enhancement of islet amyloid formation in the absence of changes in glucose tolerance, circulating IAPP or pancreatic content of IRI, mouse or human IAPP. Thus, the early stages of islet amyloidogenesis seem to be independent of glucose tolerance, with ovarian products having a protective role.

Long-term administration of a sucrose-rich diet to normal rats: Relationship between metabolic and hormonal profiles and morphological changes in the endocrine pancreas

The aim of the present investigation was to study normal rats fed a sucrose-rich diet (SRD) for a prolonged period (up to 30 weeks) (1) to obtain additional data on the hormonal and metabolic patterns induced by this treatment and (2) to provide information on changes taking place in the pancreatic islet cell populations. We found that long-term feeding with a SRD resulted in a steady state of hypertriglyceridemia and hyperglycemia in which insulin levels remained unchanged and unable to compensate for the increased demands of the developing metabolic changes. The endocrine pancreas showed a significant increase of both islet number and B-cell area, as well as changes in the profile of islet cell distribution. However, these changes were not accompanied by an increase in the pancreatic content of immunoreactive insulin (IRI). It may therefore be postulated that the newly emerged B-cell mass has some sort of derangement with the increased insulin demand resulting from insulin resistance induced by the long-term SRD feeding. Thus, feeding a SRD to normal rats may prove to be an attractive animal model for studying the role of environmental nutritional factors in the unsettled issue of the relationship between insulin resistance and relative insulin deficiency. The model might provide key information for understanding the pathophysiology of human diseases such as type II diabetes, dyslipidemia, and a number of entities included in so-called syndrome X.

Glycation of insulin in the islets of Langerhans of normal and diabetic animals.

The glycation of immunoreactive insulin (IRI) was assessed in extracts of pancreas and islets from control and hyperglycemic animal models. Glycated and nonglycated IRI were separated by affinity chromatography and quantified by radioimmunoassay. Hydrocortisone-treated Wistar rats (80 mg x kg-1 x day-1 and obese hyperglycemic (ob/ob) mice showed significant increases in plasma glucose (P < 0.001), percentage glycated hemoglobin (P < 0.001), plasma IRI (P < 0.01), and total pancreatic IRI content (P < 0.01), compared with their respective controls. These diabetic groups also demonstrated significant increases (P < 0.05) in the percentage of glycated pancreatic IRI above the controls. Streptozotocin-treated (200 mg/kg) Swiss TO mice exhibited significant increases in plasma glucose (P < 0.001), glycated hemoglobin (P < 0.001), and percentage glycated pancreatic IRI (P < 0.05), compared with untreated controls, despite a marked decrease in both plasma IRI (P < 0.001) and total pancreatic IRI content (P < 0.001). Significant elevations in the percentage of glycated IRI were also observed in islets isolated from obese hyperglycemic (ob/ob) mice (P < 0.001), compared with islets from lean controls, and when lean mouse islets were cultured in hyperglycemic media for 24 h (33.3 vs. 5.6 mmol/l D-glucose; P < 0.001). The contribution of glycated plus nonglycated insulin and proinsulin to the total IRI was estimated in lean and obese mouse pancreatic extracts following high-performance liquid chromatography separation. The contribution of proinsulin to the total IRI was approximately 10%. Proinsulin represented 27-28% of the total glycated IRI. These data indicate that the glycation of insulin and proinsulin occurs within the pancreatic islets and is elevated in both insulin-deficient and insulin-resistant diabetic animal models.

Pancreatic Expression and Secretion of Human Islet Amyloid Polypeptide in a Transgenic Mouse

Islet amyloid polypeptide (IAPP) is a secretory product of the pancreatic beta-cell, which is the primary constituent of the islet amyloid that develops in type II diabetes. To study the role the inherent amyloidogenicity of human IAPP (hIAPP) plays in the formation of islet amyloid deposits and to investigate a possible hormonal role for IAPP, transgenic mice expressing hIAPP were developed. The transgene was composed of a fragment of an hIAPP cDNA linked to the rat insulin II promoter. One line of transgenic mice expressed the transgene and synthesized hIAPP in their pancreatic islets. IAPP-like immunoreactivity in pancreatic extracts and plasma were two- to threefold greater in the transgenic mice compared with nontransgenic control mice. Although plasma concentrations of immunoreactive insulin (IRI) and glucose were equal in transgenic and control mice, the pancreatic content of IRI was nearly twofold greater in the transgenic animals, and proinsulin mRNA was significantly elevated, suggesting increased rates of insulin biosynthesis. Pancreatic samples obtained from transgenic mice up to 19 months of age had no evidence of islet amyloid. These results indicate that an increased level of synthesis of the amyloidogenic hIAPP is not sufficient to cause islet amyloid deposition. However, the increased synthesis and storage of insulin in the islets of the transgenic mice are consistent with either a direct regulatory effect of IAPP on the beta-cell or indirect stimulation of insulin production through IAPP-induced insulin resistance.

Susceptibility to neonatal streptozotocin-induced diabetes in spontaneously hypertensive rats.

We studied the difference in the susceptibility to neonatal streptozotocin (STZ) diabetes between spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY). Two-day-old female SHR and WKY were injected intraperitoneally with 75.0 mg/kg of STZ or vehicle for control. Hyperglycemia developed in both strains at 4 days of age, but SHR were more hyperglycemic. Overt hyperglycemia developed in SHR with aging after a partial recovery from initial hyperglycemia at 10 days of age, whereas WKY did not develop significant hyperglycemia except shortly after STZ treatment. Percentage of insulin-positive B cells in total islet cells and pancreatic immunoreactive insulin (IRI) content were measured at 4 days, 10 days, 4 weeks, and 12 weeks of age. B cells per islet and pancreatic IRI content were significantly reduced in STZ-treated groups as compared with control in both SHR and WKY at 4 days of age, but later they increased significantly with aging in both strains. However, the reduction in pancreatic IRI content relative to control was significantly greater in SHR than in WKY from 4 days (-94.5 +/- 3.5%, -84.1 +/- 4.8%; p < 0.01) to 12 weeks (-97.1 +/- 2.1%, -28.0 +/- 2.5%; p < 0.05), and the reduction in B cells per islet was also greater in SHR at 4 weeks of age. These results indicated that the initial destruction of pancreatic B cells induced by STZ was greater, and the following regeneration was less in SHR than in WKY. The association of the susceptibility to neonatal STZ diabetes with the development of genetic hypertension in SHR remained to be elucidated.

Effect of aging on glucose tolerance in spontaneously hypertensive rats.

We studied the age-related changes of glucose tolerance in female spontaneously hypertensive rats (SHR) that did not become obese with aging. Oral glucose tolerance test was performed in young (3 months), middle-aged (6 to 11 months), and aged (26 months) SHR. Fasting plasma glucose was significantly lower in aged SHR than in young SHR. The increase in plasma glucose after glucose administration over fasting level was significantly higher in aged SHR than in middle-aged SHR, but did not differ between young and aged rats. Pancreatic islet size and pancreatic immunoreactive insulin content were similar between young and aged SHR. The present study demonstrated that glucose tolerance did not deteriorate in SHR with aging, while genetic hypertension persisted. This suggests that the persistence of hypertension per se may not affect glucose tolerance in SHR.

Adaptation to supraphysiologic levels of insulin gene expression in transgenic mice: evidence for the importance of posttranscriptional regulation.

Insulin production was studied in transgenic mice expressing the human insulin gene under the control of its own promoter. Glucose homeostasis during a 48-h fast was similar in control and transgenic mice, with comparable levels of serum immunoreactive insulin. Northern blot and primer extension analyses indicated that more than twice as much insulin mRNA is present in pancreata from transgenic mice. Primer extension analysis using oligonucleotides specific for mouse insulins I and II or for human insulin, showed that the excess insulin mRNA was due solely to expression of the foreign, human insulin gene. The ratio of mRNA for mouse insulin I and II was unaffected by coexpression of human insulin. There were coordinate changes in the levels of all three mRNA during the 48-h fast, or after a 24-h fast followed by 24-h refeed. Despite the supraphysiologic levels of insulin mRNA in the transgenic mice, their pancreatic content of immunoreactive insulin was not significantly different from controls. The comparison of the relative levels of human and mouse insulin mRNAs with their peptide counterparts (separated by HPLC) indicates that the efficiency of insulin production from mouse insulin mRNA is greater than that from human, stressing the importance of posttranscriptional regulatory events in the overall maintenance of pancreatic insulin content.

Correlation between residual β-cell function and age at onset of spontaneous diabetes in Long Evans Tokushima Lean rats

We studied the natural course of disease in spontaneously diabetic rats, Long Evans Tokushima Lean (LETL) rats, to determine whether it showed similar pathogenetic heterogeneity to that of patients with insulin-dependent diabetes mellitus (IDDM) with regard to the relationships between age at onset, rapidity of disease progress, and degree of β-cell function at the time of its manifestation. Type 1 diabetes developed in 35 rats (6.3%) between 40 and 140 days of age. Eight rats that became diabetic at age 69 days or less were more severely ketotic at the time of first detection of glycosuria and showed more rapid deterioration than seven rats that became diabetic later after birth (mean plasma 3-hydroxybutyrate levels, 4,707 ± 1,215 pmol/L v 1,390 ± 859 pmol/L; mean ± SEM, P < .01). The mean plasma levels and pancreatic content of immunoreactive insulin (IRI) of the early onset rats, 47 ± 13 pmol/L and 19 ± 12 pmol/g tissue weight, were significantly lowere ( P < .01) than the corresponding values of the late-onset rats, 262 ± 52 pmol/L and 348 ± 87 pmol/g tissue weight, respectively. Both values were markedly lower than the mean values of 25 nondiabetic LETL rats, 976 ± 122 pmol/L and 3,488 ± 628 pmol/g tissue weight. Plasma immunoreactive glucagon (IRG) levels were significantly increased in the diabetic groups (early onset, 57 ± 13 pmol/L; late-onset, 51 ± 12 pmol/L; nondiabetic, 18 ± 1 pmol/L; P < .01). These changes in pancreatic hormone levels of the early onset and late-onset rats were compatible with the histological features of their pancreatic islets. The titers of plasma competitive insulin autoantibodies (CIAA) were within the normal range in all but two diabetic LETL rats. The changes in blood chemistry, pancreatic hormone content, and histology of the pancreatic islets of 13 cyclophosphamide-induced diabetic LETL rats were similar to but less marked than those of the spontaneously diabetic rats with regard to the relationship between age at onset and β-cell function at the time of manifestation of disease. These results demonstrate that LETL rats with spontaneous or cyclophosphamide-induced type 1 diabetes showed similar pathogenetic heterogeneity to that of IDDM patients with respect to the relationship between age at onset and the rapidity of progress of disease; pancreatic β cells tend to be more rapidly and severely damaged in early onset than in late-onset diabetic LETL rats.

Probucol attenuated hyperglycemia in multiple low-dose streptozotocin-induced diabetic mice

The present studies were undertaken to examine the effects of probucol on the protection against pancreatic beta-cell damage by multiple low-dose streptozotocin (STZ: 40 mg/kg, ip). The degree of hyperglycemia at 7, 14 and 17 days after STZ injection was attenuated by probucol. Serum immunoreactive insulin (IRI) levels were increased in the rats fed probucol containing diet at Day 14 and 17. Serum IRI levels after intraperitoneal injection of 2.0 g/kg glucose was reduced in STZ mice and the reduction of serum IRI levels was attenuated in the rats fed probucol, accompanied with a significant reduction of the degree of hyperglycemia after bolus of glucose. Probucol attenuated the reduction of pancreatic IRI content by STZ. The percentage of Thy 1.2-positive splenocytes was increased by STZ and probucol reduced the percentage of Thy 1.2-positive splenocytes, although there were no differences in the populations of splenocytes, positive with Lyt 2 or L3/T4. These data suggest that probucol has a protective action against pancreatic insulitis by multiple low-dose STZ administration.

Pancreatic cellular infiltrates in autoimmune-prone New Zealand Black mice.

Healthy New Zealand Black (NZB) mice of both sexes (age 19-31 weeks) were studied to determine the magnitude of pancreatic beta cell injury related to mononuclear cell infiltration of islets. This investigation was undertaken following the description of spontaneous cellular immune reactions against islets in four strains of autoimmune-prone mice, including NZB mice [1]. Studies included complete autopsies with histological examination, determination of the pancreatic content of immunoreactive insulin, and the measurement of the plasma glucose concentration. Mononuclear cell infiltrates were identified in the lung, liver, kidney, salivary gland, mesentery, and pancreas. In the latter site, the infiltrates were situated in fibrous septae about ducts, ductules, and venules rather than islets. Only islets contiguous to infiltrates were involved, and then but focally. Insulitis, as manifest by the envelopment and permeation of islets by mononuclear cells, was not observed. In none was there a significant reduction of beta cells or pancreatic insulin content, neither was hyperglycaemia manifest. This study reveals that, although NZB mice are subject to autoimmune phenomena and widespread mononuclear cell infiltrates, beta cell injury and insulitis are not consistent features of this strain.

Protective actions of taurine against streptozotocin-induced hyperglycemia

The effect of continuous administration of taurine (2-aminoethanesulfonic acid) on streptozotocin (STZ)-induced hyperglycemia was examined. Acute administration of STZ (250 mg/kg, i.v.) to ddy strain mice induced transient but significant increase in the serum level of glucose and decrease in the pancreatic immunoreactive insulin (IRI) content at 24 hr after the administration. These changes induced by acute STZ administration were suppressed significantly by the oral pretreatment of animals with taurine (7–9 g/kg/day, for 5–7 days). Similar preventive effects were also observed when methionine or cysteine was orally administered for 7 days prior to the sacrifice of STZ administered animals, whereas isoleucine and valine had not such preventive properties. Since the administration of these three amino acids, which have a suppressive effect on STZ-induced hyperglycemia, significantly increased taurine content in the pancreas, it is suggested that the increase in pancreatic taurine may be intimately related to the protective effect against the pancreato-toxic action of STZ. In CD-1 strain mice, one of the STZ-sensitive strains, the continuous administration of taurine not only exhibited significant preventive effects on hyperglycemia and decrease in serum and pancreatic contents of IRI induced by acute administration of STZ (150 mg/kg, i.v.), but also suppressed the continuous hyperglycemia and fall in pancreatic IRI content caused by the repeated administration of STZ (50 mg/kg, i.p., for 5 days). In addition, it was also found that morphological alterations in the islets of Langerhans induced by STZ were considerably less in taurine-treated animals than those found in animals treated with STZ alone. These results suggest that pancreatic taurine may play important physiological roles in maintaining the function and/or integrity of β cells in the islets of Langerhans and protecting these structures from pancreato-toxic substances such as STZ.

Venezuelan Encephalitis Virus–induced Alterations in Carbohydrate Metabolism in Genetically Diabetic Mice

We previously reported in this journal that the TC-83-vaccine strain of Venezuelan encephalitis (VE) virus results in a sustained diminution of glucose-stimulated insulin release in golden Syrian hamsters, persisting as long as 90 days after viral infection. This study was designed to examine the metabolic and pathologic consequences of TC-83 VE virus infection in C57 BL/Ks mice (+/+) and in genetic variants of this strain homozygous (db/db) and heterozygous (db/+) for the diabetic gene, db. Five-week-old mice of each genetic variant were inoculated subcutaneously, in groups of 18, with 100,000 plaque-forming units (PFU) of TC-83 that had not been passaged in chick embryo cells or with diluent (control mice). The clinical course in all three groups of mice following VE inoculation was mild, with a 5 to 10% mortality. By light microscopy, control +/+, TC-83 VE-infected +/+, control db/+, and TC-83 VE-infected db/+ pancreases manifested no appreciable difference in morphology. Uninfected db/db mice showed typical changes, including a definite decrease in the number of aldehyde fuchsin-staining granules in beta cells. TC-83 VE-infected db/db mice exhibited a profound decrease in pancreatic beta cell granulation on aldehyde fuchsin staining. After TC-83 VE inoculation, the most striking alterations in carbohydrate metabolism occurred in db/db mice, which showed further worsening of glucose tolerance 120 min after intraperitoneal glucose as well as significantly decreased basal and glucose-stimulated immunoreactive insulin (IRI) levels. After TC-83 VE infection, pancreatic IRI content was not decreased in +/+ or db/+ mice but was virtually absent in db/db mice. The data support the ability of TC-83 VE to inhibit glucose-stimulated IRI release in three genetic variants of C57 BL/Ks mice. In addition, this model demonstrates the heightened susceptibility of the remaining diabetic beta cells in the db/db mice to subsequent infection with a pancreatropic virus.

Virus-induced pancreatic disease by Venezuelan encephalitis virus. Alterations in glucose tolerance and insulin release.

Viral infections have been implicated in the induction of diabetes mellitus in man and laboratory animals. Since virus-specific immunofluorescence (FA) is detectable in hamster pancreas during the acute phase of Venezuelan encephalitis (VE), experiments were designed to correlate pathologic and virologic events with metabolic studies in VE-infected hamsters. Golden Syrian hamsters were inoculated s.c. in groups of four to 12 with 100,000 plaque-forming units (PFU) of the vaccine strain (TC-83) of VE or 1,000 PFU of the virulent Trinidad strain of VE. Ultrastructurally, during Trinidad infection, mature virions were associated with the cell surfaces and within pancreatic beta cells in contrast to absence of virus-related changes in TC-83-infected hamsters. Virus-specific-FA was noted in islet cells and acinar cells of Trinidad-infected hamsters. VE growth curves demonstrated viral replication in pancreas with both strains. Although ultrastructural and FA changes were much more prominent in Trinidad-infected hamsters in contrast to TC-83-infected hamsters during the first few days of illness, the rapid lethality of the Trinidad-infected group necessitated performing all metabolic studies in TC-83-strain-infected hamsters. Accordingly, for the metabolic studies, glucose tolerance tests (GTT) using 2 mg. or 5 gm./kg. glucose i.p. were performed in groups of hamsters acutely infected two days earlier with the TC-83 vaccine strain and in 24-day and 90-day convalescent hamsters after TC-83 vaccine strain. Samples were obtained for glucose and immunoreactive insulin (IRI) determinations. Glucose intolerance occurred in hamsters in each of the infected groups given 5 gm./kg. glucose except for the 90-day convalescent TC-83 group. Severely decreased IRI responses occurred in the 24-day and 90-day convalescent TC-83 hamsters following both 2- and 5-gm./kg. glucose. Pancreatic IRI content in 24-day convalescent TC-83 hamsters was within normal limits, suggesting a defect in IRI release from the beta cells at this stage of convalescence.

Metabolic and hormonal effects of exercise in the severely streptozotocin-diabetic rat.

Metabolic and hormonal changes during muscular exercise were studied in streptozotocin (65 mg/kg) — diabetic rats. The untreated, non-fasted animals ran in a rotating cage for 30 min, and blood, liver and pancreatic specimens were taken before, immediately after and/or two hrs after exercise. The slight pre-exercise acidosis of the diabetic rats changed during exercise to a marked acidosis due to a rise in blood lactate. High blood glucose fell in 33 out of 54 animals, while hepatic glycogen was low and unchanged. A marked increase in circulating glycerol was found, whereas plasma free fatty acids (FFA) decreased and the fall of FFA was glucose “sparing”. In animals, in which blood glucose decreased, plasma immunoreactive insulin (IRI) rose significantly and the rise correlated with the fall in blood glucose and with the intensity of work. Exercise did not affect the low pancreatic content of IRI. The results add weight to the suggestion that in severe, acute diabetes muscular work can mobilize IRI, perhaps of extrapancreatic origin, which helps to explain the decrease in circulating glucose.

Diabetogenic action of streptozotocin: relationship of dose to metabolic response

The relationship between the dose of intravenously administered streptozotocin (a N-nitroso derivative of glucosamine) and the diabetogenic response has been explored by use of the following indices of diabetogenic action: serum glucose, urine volume, and glycosuria, ketonuria, serum immunoreactive insulin (IRI), and pancreatic IRI content. Diabetogenic activity could be demonstrated between the doses of 25 and 100 mg/kg, all indices used showing some degree of correlation with the dose administered. Ketonuria was only seen with the largest dose, 100 mg/kg. The most striking and precise correlation was that between the dose and the pancreatic IRI content 24 hr after administration of the drug, and it is suggested that this represents a convenient test system either for both related and unrelated beta cytotoxic compounds or for screening for modifying agents or antidiabetic substances of a novel type. Ability to produce graded depletion of pancreatic IRI storage capacity led to an analysis of the relationship between pancreatic IRI content and deranged carbohydrate metabolism. Abnormal glucose tolerance and insulin response were seen when pancreatic IRI was depleted by about one-third, while fasting hyperglycemia and gross glycosuria occurred when the depletion had reached two-thirds and three-quarters, respectively. The mild yet persistent anomaly produced by the lowest effective streptozotocin dose, 25 mg/kg, exhibits characteristics resembling the state of chemical diabetes in humans and might thus warrant further study as a possible model. Finally, the loss of the diabetogenic action of streptozotocin by pretreatment with nicotinamide was confirmed and was shown to be a function of the relative doses of nicotinamide and streptozotocin and of the interval between injections.

Studies in spiny mice (Acomys cahirinus): metabolic state and pancreatic insulin release in vitro.

Incidence of diabetes, serum immunoreactive insulin (IRI) levels, and pancreatic IRI content have been studied in a colony of some 700 spiny mice (Acomys cahirinus) held under laboratory conditions with unlimited access to food. The overall incidence of diabetes was about 15 %. Three types of diabetic syndrome could be defined: 1. Intermittent diabetes, where gross glycosuria or hyperglycemia persisted for several months but disappeared spontaneously. 2. Non-ketotic diabetes, usually occurring between 9 and 14 months of age and evolving to ketotic diabetes within 1 to 6 months after discovery. 3. Ketotic diabetes. Only "intermittent" diabetes appeared to be associated with obesity. Gross congenital hyperplasia of the islets of Langerhans occurred in all animals, irrespective of the incidence of diabetes. In non-diabetic spiny mice, both mean serum IRI and pancreatic IRI content were higher than in other strains of mice, variation between individuals being considerable. Serum IRI concentrations were highest in intermittent diabetic animals and lowest in ketotic diabetics. Pancreatic IRI content of intermittent diabetic animals was higher than that observed in any other obese hyperglycemic syndrome studied thus far. In relation to pancreatic IRI content, serum IRI of spiny mice - irrespective of the metabolic state - was lower than that of Swiss mice, NZO or obob mice. Preliminary studies of insulin secretion from pieces of pancreas perifused in vitro suggest that decreased responsiveness of Acomys pancreas to known stimulators of insulin release may be characteristic of our strain of this species.

Measurements of insulin activities in pancreas and serum of mice with spontaneous ("Obese" and "New Zealand Obese") and induced (Goldthioglucose) obesity and hyperglycemia, with considerations on the pathogenesis of the spontaneous syndrome.

1. Insulin-like activity (ILA) and immunoreactive insulin (IRI) of pancreatic extracts and of serum have been measured in lean and obese (obob) mice from the Jackson Memorial Laboratory in Bar Harbor, Maine; in New Zealand obese mice; and in goldthioglucose-obese Swiss mice, and their lean controls. In all three types of mice the relative amounts of “suppressible” and “nonsuppressible” insulin activities of serum have also been established. — 2. Pancreatic content of IRI in (obob) mice is much greater than in their lean sibblings at five months of age, but smaller at 5 weeks, whereas serum IRI and ILA of (obob) mice are greatly in excess of the same activities of serum from their lean littermates at both ages. It is suggested that increased peripheral demand for insulin preceeds increased insulin synthesis and storage in this type of obesity. — 3. In all three strains of mice, obese or lean, the ratio of biologic to immunologic activity of pancreatic extracts was constant and close to unity. In serum, this ratio was also constant but approximately five times greater than unity. These findings do not support the notion that differences in insulin structure might account for differences in the biological activity of insulin in obese and non-obese animals; they are in keeping with the concept of a potentiation of serum insulin activity by some component of serum in all mice. — 4. Approximately 90% of serum ILA in all three types of obese mice studied was suppressed in the presence of anti-insulin serum, although the absolute levels of non-suppressible ILA were similar to those reported for other species. It is unlikely, therefore, that “bound” insulin accounts for a significant part of the increased ILA of serum of any of the three types of obese animals studied. — 5. Present data support the concept of a marked tissue-bound muscular resistance to insulin action as an important, possibly primary pathogenetic feature in (obob) mice. A similar, but much less marked muscular resistance can be demonstrated in NZO mice, whereas muscular resistance to insulin action is totally absent in goldthioglucose obese Swiss mice after a 24 hour fast.