Insulin exerts metabolic and growth-promoting effects by a direct action on the liver in vivo: clarification of the functional significance of the portal vascular link between the beta cells of the pancreatic islets and the liver

Soluble and insoluble fibers in ostrich nutrition: influences on growth performance and blood biochemical indices during different ages

This experiment was undertaken to evaluate the effects of different levels of soluble and insoluble fibers on growth performance and blood indices in ostrich at the age of 1 to 6 months. Thirty 30-day-old ostriches (males and females) were randomly assigned to five treatments with six replicates. There were five dietary treatments consisted of (1) basal diet; (2 and 3) diets containing 2 and 4% more soluble fibers (SF) than basal diet, and (4 and 5) diets containing 2 and 4% more insoluble fibers (ISF) than the basal diet. The dietary treatments had no significant influence on growth performance of ostriches. By increasing the levels of SF and ISF, the concentration of blood glucose (P < 0.0001) and high-density lipoprotein (HDL, P = 0.0046) increased, while the plasma concentration of triglyceride (P < 0.0001) and very-low-density lipoprotein (VLDL, P = 0.0006) decreased. The plasma enzyme activity of alanine aminotransferase (ALT) decreased in ostriches fed diet containing 4% more SF as compared with control (P < 0.0006). Body weight gain (BWG) at the starter phase (3-4 months) was higher than the pre-starter (1-2 months) and grower (5-6 months) phases (P < 0.0001). The highest concentration of blood glucose was observed at 2 months of age while the lowest magnitude was found at 4 months of age (P < 0.0001). The plasma cholesterol concentration increased at 4 months and decreased at 6 months as compared with 2 months (P < 0.0001). The concentration of HDL and LDL at 4 months of age was higher than two other ages (P < 0.0001). The blood concentration of triglyceride (P < 0.0001), VLDL (P < 0.0001), and ALT (P = 0.0005) decreased as ostriches grew up. It could be concluded that increasing the levels of soluble and insoluble fibers in the diet up to 4% more than previous recommendation may not have negative effects on growth performances and blood indices in ostrich.

aThe characteristics of glucose metabolism in the sulfonylurea receptor 1 knockout rat model

Background

Sulfonylurea receptor 1 (SUR1) is primarily responsible for glucose regulation in normal conditions. Here, we sought to investigate the glucose metabolism characteristics of SUR1^−/− rats.

Methods

The TALEN technique was used to construct a SUR1 gene deficiency rat model. Rats were grouped by SUR1 gene knockout or not and sex difference. Body weight; glucose metabolism indicators, including IPGTT, IPITT, glycogen contents and so on; and other molecule changes were examined.

Results

Insulin secretion was significantly inhibited by knocking out the SUR1 gene. SUR1^−/− rats showed lower body weights compared to wild-type rats, and even SUR1^−/− males weighed less than wild-type females. Upon SUR1 gene knockout, the rats showed a peculiar plasma glucose profile. During IPGTT, plasma glucose levels were significantly elevated in SUR1^−/− rats at 15 min, which could be explained by SUR1 mainly working in the first phase of insulin secretion. Moreover, SUR1^−/− male rats showed obviously impaired glucose tolerance than before and a better insulin sensitivity in the 12th week compared with females, which might be related with excess androgen secretion in adulthood. Increased glycogen content and GLUT4 expression and the inactivation of GSK3 were also observed in SUR1^−/− rats, which suggested an enhancement of insulin sensitivity.

Conclusions

These results reconfirm the role of SUR1 in systemic glucose metabolism. More importantly, our SUR1^−/− rat model might be applied in other fields, such as for exploring other hypoglycaemic functions of sulfonylureas.

Electronic supplementary material

The online version of this article (10.1186/s10020-018-0067-9) contains supplementary material, which is available to authorized users.

Novel Mechanisms for IGF-I Regulation by Glucagon in Carp Hepatocytes: Up-Regulation of HNF1α and CREB Expression via Signaling Crosstalk for IGF-I Gene Transcription

Glucagon, a key hormone for glucose homeostasis, can exert functional crosstalk with somatotropic axis via modification of IGF-I expression. However, its effect on IGF-I regulation is highly variable in different studies and the mechanisms involved are largely unknown. Using grass carp as a model, the signal transduction and transcriptional mechanisms for IGF-I regulation by glucagon were examined in Cyprinid species. As a first step, the carp HNF1α, a liver-enriched transcription factor, was cloned and confirmed to be a single-copy gene expressed in the liver. In grass carp hepatocytes, glucagon treatment could elevate IGF-I, HNF1α, and CREB mRNA levels, induce CREB phosphorylation, and up-regulate HNF1α and CREB protein expression. The effects on IGF-I, HNF1α, and CREB gene expression were mediated by cAMP/PKA and PLC/IP₃/PKC pathways with differential coupling with the MAPK and PI3K/Akt cascades. During the process, protein:protein interaction between HNF1α and CREB and recruitment of RNA Pol-II to IGF-I promoter also occurred with a rise in IGF-I primary transcript level. In parallel study to examine grass carp IGF-I promoter activity expressed in αT3 cells, similar pathways for post-receptor signaling were also confirmed in glucagon-induced IGF-I promoter activation and the trans-activating effect by glucagon was mediated by the binding sites for HNF1α and CREB located in the proximal region of IGF-I promoter. Our findings, as a whole, shed light on a previously undescribed mechanism for glucagon-induced IGF-I gene expression by increasing HNF1α and CREB production via functional crosstalk of post-receptor signaling. Probably, by protein:protein interaction between the two transcription factors and subsequent transactivation via their respective cis-acting elements in the IGF-I promoter, IGF-I gene transcription can be initiated by glucagon at the hepatic level.

The growth hormone signaling system: Insights into coordinating the anabolic and catabolic actions of growth hormone

Although growth hormone (GH) is a multifunctional factor that coordinates various aspects of feeding, reproduction, osmoregulation, and immune system function, perhaps two of its most studied actions are the regulation of growth and metabolism, particularly lipid metabolism. In this review, we describe the major growth-promoting and lipid metabolic actions of GH and then discuss how the GH system regulates these actions. Numerous intrinsic and extrinsic factors provide information about the metabolic status of the organism and influence the production of release of GH. The actions of GH are mediated by GH receptors (GHR), which are widely distributed among tissues. Teleosts possess multiple forms of GHRs that arose through the evolution of this group. Modulation of tissue responsiveness to GH is regulated by molecular and functional expression of GHRs, and in teleosts GHR subtypes, by various factors that reflect the metabolic and growth status of the organism, including nutritional state. The action of GH is propagated by the linkage of GHRs to several cellular effector systems, including JAK-STAT, ERK, PI3K-Akt, and PKC. The differential activation of these pathways, which is governed by nutrient status, underlies GH stimulation of growth or GH stimulation of lipolysis. Taken together, the multi-functional actions of GH are determined by the distribution and abundance of GHRs (and GHR subtypes in teleosts) as well as by the GHR-effector system linkages.

Mechanisms Underlying the Synergistic Action of Insulin and Growth Hormone on IGF-I and -II Expression in Grass Carp Hepatocytes

In mammals, insulin is known to modify growth hormone (GH)-induced IGF-I expression at the hepatic level, which also contributes to the functional crosstalk between energy homeostasis and somatotropic axis. However, the studies on the comparative aspects of this phenomenon are limited and the mechanisms involved have not been fully characterized. Using a serum-free culture of grass carp hepatoctyes, the functional interaction between GH and insulin on hepatic expression of IGF-I and -II was examined in a fish model. In carp hepatocytes, GH could up-regulate IGF-I and -II mRNA expression via the JAK₂/STAT₅, MEK/ERK and PI3K/Akt pathways. These stimulatory effects were mimicked by insulin via activation of the PI3K/Akt but not MEK/ERK and P38 MAPK cascades. Although insulin did not activate JAK₂ and STAT₅ at hepatocyte level, insulin-induced IGF-I and -II mRNA expression were highly dependent on the normal functioning of JAK₂/STAT₅ pathway. In parallel experiments, insulin co-treatment was found to markedly enhance IGF-I and -II responses induced by GH and these potentiating effects were mediated by insulin receptor (InsR) but not IGF-I receptor. Interestingly, co-treatment with GH also enhanced insulin-induced InsR phosphorylation with a current elevation in protein:protein interaction between GH receptor and phosphorylated InsR and these stimulatory effects were noted with further enhancement in STAT₅, ERK_1/2 and Akt phosphorylation at hepatocyte level. Consistent with these findings, the potentiating effects of GH and insulin co-treatment on IGF-I and -II mRNA expression were found to be suppressed/abolished by inhibiting JAK₂/STAT₅, MEK/ERK and PI3K/Akt but not P38 MAPK pathways. These results, as a whole, suggest that insulin and GH can act in a synergistic manner in the carp liver to up-regulate IGF-I and -II expression through protein:protein interaction at the receptor level followed by potentiation in post-receptor signaling.

Metabolic, anabolic, and mitogenic insulin responses: A tissue-specific perspective for insulin receptor activators

Insulin acts as the major regulator of the fasting-to-fed metabolic transition by altering substrate metabolism, promoting energy storage, and helping activate protein synthesis. In addition to its glucoregulatory and other metabolic properties, insulin can also act as a growth factor. The metabolic and mitogenic responses to insulin are regulated by divergent post-receptor signaling mechanisms downstream from the activated insulin receptor (IR). However, the anabolic and growth-promoting properties of insulin require tissue-specific inter-relationships between the two pathways, and the nature and scope of insulin-regulated processes vary greatly across tissues. Understanding the nuances of this interplay between metabolic and growth-regulating properties of insulin would have important implications for development of novel insulin and IR modulator therapies that stimulate insulin receptor activation in both pathway- and tissue-specific manners. This review will provide a unique perspective focusing on the roles of "metabolic" and "mitogenic" actions of insulin signaling in various tissues, and how these networks should be considered when evaluating selective pharmacologic approaches to prevent or treat metabolic disease.

Differential regulation of Igf1 and Igf2 mRNA levels in tilapia hepatocytes: effects of insulin and cortisol on GH sensitivity

Igf1 and Igf2 stimulate growth and development of vertebrates. In mammals, liver-derived endocrine Igf1 mediates the growth promoting effects of GH during postnatal life, whereas Igf2 stimulates placental and fetal growth and is not regulated by GH. Insulin enhances Igf1 production by the mammalian liver directly, and by increasing hepatocyte sensitivity to GH. We examined the regulation of igf1 and igf2 mRNA levels by GH, insulin, and cortisol, and the effects of insulin and cortisol on GH sensitivity in primary cultured hepatocytes of tilapia, a cichlid teleost. GH increased mRNA levels of both igf1 and igf2 in a concentration-related and biphasic manner over the physiological range, with a greater effect on igf2 mRNA level. Insulin increased basal igf2 mRNA level, and strongly increased GH-stimulated igf2 mRNA level, but slightly reduced basal igf1 mRNA level and did not affect GH-stimulated igf1 mRNA level. Cortisol inhibited GH stimulation of igf1, but increased GH stimulation of igf2 mRNA level. The synergistic effect of insulin and GH on igf2 mRNA level was confirmed in vivo. These results indicate that insulin and cortisol differentially modulate the response of igf1 and igf2 mRNA to GH in tilapia hepatocytes, and suggest that the regulation of liver Igf2 production differs between fish and mammals. Regulation of liver Igf2 production in fish appears to be similar to regulation of liver Igf1 production in mammals.

Insulin-associated weight gain in diabetes--causes, effects and coping strategies

Insulin therapy or intensification of insulin therapy commonly results in weight gain in both type 1 and type 2 diabetes. This weight gain can be excessive, adversely affecting cardiovascular risk profile. The spectre of weight gain can increase diabetic morbidity and mortality when it acts as a psychological barrier to the initiation or intensification of insulin, or affects adherence with prescribed regimens. Insulin-associated weight gain may result from a reduction of blood glucose to levels below the renal threshold without a compensatory reduction in calorie intake, a defensive or unconscious increase in calorie intake caused by the fear or experience of hypoglycaemia, or the 'unphysiological' pharmacokinetic and metabolic profiles that follow subcutaneous administration. There is, however, scope for limiting insulin-associated weight gain. Strategies include limiting dose by increasing insulin sensitivity through diet and exercise or by using adjunctive anorectic or insulin-sparing pharmacotherapies such as pramlintide or metformin. Insulin replacement regimens that attempt to mimic physiological norms should also enable insulin to be dosed with maximum efficiency. The novel acylated analogue, insulin detemir, appears to lack the usual propensity for causing weight gain. Elucidation of the pharmacological mechanisms underlying this property might help clarify the mechanisms linking insulin with weight regulation.

Nutritional assessment of somatolactin function in gilthead sea bream (Sparus aurata): concurrent changes in somatotropic axis and pancreatic hormones

The role of somatolactin (SL) in the regulation of energy homeostasis in gilthead sea bream (Sparus aurata) has been analysed. First, a down-regulation of plasma SL levels in response to gross shifts in dietary amino acid profile and the graded replacement of fish meal by plant protein sources (50%, 75% and 100%) has been observed. Thus, the impaired growth performance with changes in dietary amino acid profile and dietary protein source was accompanied by a decrease in plasma SL levels, which also decreased over the course of the post-prandial period irrespective of dietary nitrogen source. Secondly, we examined the effect of SL and growth hormone (GH) administration on voluntary feed intake. A single intraperitoneal injection of recombinant gilthead sea bream SL (0.1 microg/g fish) evoked a short-term inhibition of feed intake, whereas the same dose of GH exerted a marked enhancement of feed intake that still persisted 1 week later. Further, we addressed the effect of arginine (Arg) injection upon SL and related metabolic hormones (GH, insulin-like growth factor-I (IGF-I), insulin and glucagon) in fish fed diets with different nitrogen sources. A consistent effect of Arg injection (6.6 micromol/g fish) on plasma GH and IGF-I levels was not found regardless of dietary treatment. In contrast, the insulinotropic effect of Arg was found irrespective of dietary treatment, although the up-regulation of plasma glucagon and glucose levels was more persistent in fish fed a fish meal based diet (diet FM) than in those fed a plant protein diet with a 75% replacement (diet PP75). In the same way, a persistent and two-fold increase in plasma SL levels was observed in fish fed diet FM, whereas no effect was found in fish fed diet PP75. Taken together, these findings provide additional evidence for a role of SL as a marker of energy status, which may be perceived by fish as a daily and seasonal signal of abundant energy at a precise calendar time.

Insulin detemir, does a new century bring a better basal insulin?

The treatment of diabetes was revolutionised shortly after the turn of the twentieth century by the extraction and purification of insulin. Methods to protract (i.e. prolong) the action of insulin were developed in the 1930s; little changed in the technology of insulin protraction until the turn of this century when, with renewed interest in the importance of basal insulin in controlling diabetes and thus preventing or delaying complications, technology advanced again. Two new long-acting insulin analogues have come to the market; some may be familiar with insulin glargine, which has been widely used for some years now. This review attempts to describe the novel method of protraction that insulin detemir (launched last summer) employs by albumin binding, to discuss the possible therapeutic benefits of this method of protraction and to describe the findings of studies comparing insulin detemir with other currently available long-acting insulin preparations. The intention of this article is not to review all of the currently available long-acting insulin analogues.

Time course of the GH/IGF axis response to fasting and increased ration in chinook salmon (Oncorhynchus tshawytscha)

Body growth in vertebrates is chiefly regulated by the GH/IGF axis. Pituitary growth hormone (GH) stimulates liver insulin-like growth factor-I (IGF-I) production. During fasting, plasma IGF-I levels decline due to the development of liver GH resistance, while GH levels generally increase. In mammals, decreased insulin during fasting is thought to cause liver GH resistance. However, the sequence of events in the GH/IGF axis response to fasting is not well characterized, especially in non-mammalian vertebrates. We assessed the time course of the GH/IGF axis response to fasting and increased ration in chinook salmon. Fish were placed on Fasting, Increased, or Control rations, and sampled daily for 4 days and at more widely spaced intervals through 29 days. Plasma IGF-I, GH, insulin, and 41 kDa IGF binding protein (putative salmon IGFBP-3), and liver IGF-I gene expression were measured. Control and Increased ration fish did not differ strongly. Plasma IGF-I and 41 kDa IGFBP were significantly lower in Fasted versus Control fish from day 4 onward, and liver IGF-I gene expression was significantly lower from day 6 onward. Liver IGF-I gene expression and plasma IGF-I levels were correlated. Plasma insulin was lower in Fasted fish from day 6 onward. There was a trend toward increased GH in Fasted fish on days 1-2, and GH was significantly increased Fasted fish from day 3 onward. Fasted GH first increased (days 1-3) to a plateau of 10-20 ng/ml (days 4-12) and then increased dramatically (days 15-29), suggesting that the GH response to fasting had three phases. The early increase in GH, followed by the decrease in plasma IGF-I after 4 days, suggests that GH resistance developed within 4 days.

Insulin gene transfer with adenovirus vector via the spleen safely and effectively improves posthepatectomized conditions in diabetic rats

BACKGROUND

We examined administration methods of adenovirus vector carrying human insulin gene (AxCAIns), which could safely and effectively enhance liver regeneration in diabetic rats after hepatectomy.

METHODS

Male Wistar rats were made diabetic with streptozotocin and subjected to 70% partial hepatectomy. AxCAIns was administrated into the spleen, the portal vein, the peritoneal cavity, or the femoral muscle. Liver regeneration and damage, and nutritional conditions were compared among the groups which were different in the administration methods of AxCAIns.

RESULTS

Intrasplenic administration of AxCAIns enhanced liver regeneration, improving nutritional conditions without liver damage. In contrast, intraportal administration enhanced liver regeneration but caused hypoglycemia with liver damage. Neither intraperitoneal nor intramuscular administration produced detectable serum levels of human c-peptide, and did not enhance liver regeneration.

CONCLUSIONS

In conclusion, data showed that intrasplenic administration of AxCAIns, rather than other methods, effectively enhanced liver regeneration without liver damage and improved nutritional conditions after hepatectomy in diabetic rats. It is suggested that insulin gene transfer with AxCAIns via the spleen may safely and effectively improve posthepatectomized conditions in inslinopenic patients.

Chronic intermittent intravenous insulin therapy: a new frontier in diabetes therapy

The limited success achieved in controlling diabetes and its complications with conventional insulin therapy suggests the need for reevaluation of the appropriateness of insulin administration protocols. Indeed, conventional subcutaneous insulin administration produces slowly changing blood insulin levels and suboptimal hepatocyte insulinization resulting in impaired hepatic capacity for processing incoming dietary glucose. The novel approach to insulin administration known as chronic intermittent intravenous insulin therapy (CIIIT) delivers insulin in a pulsatile fashion and achieves physiological insulin concentration in the portal vein. Done as a weekly outpatient procedure combined with daily intensive subcutaneous insulin therapy, this procedure has been shown to (1) significantly improve glycemic control while decreasing the incidence of hypoglycemic events, (2) improve hypertension control, (3) slow the progression of overt diabetic nephropathy, and (4) reverse some manifestations of diabetic autonomic neuropathy (e.g., abnormal circadian blood pressure pattern, severe postural hypotension, and hypoglycemia unawareness).

Regulation of plasma insulin-like growth factor-I levels in brown trout (Salmo trutta)

In this study we report that the use of a heterologous radioimmunoassay (RIA) is valid for the detection of insulin-like growth factor-I (IGF-I) levels in plasma of a variety of fish species. Parallelism between standard curves and plasma dilutions were observed and the standard curve obtained with mammalian IGF-I presented the same characteristics as that obtained with coho salmon recombinant IGF-I. The RIA was biologically validated since total plasma IGF-I values were significantly modified by different experimental conditions. Hyperinsulinemia induced either by arginine or insulin injection was accompanied by increases in IGF-I plasma levels in brown trout (Salmo trutta). In contrast, parallel decreases in insulin and IGF-I circulating levels were observed after 45 days of fasting and 20 days after a single streptozotocin injection. Administration of arginine in fasted fish led to a relative increase in insulin and IGF-I plasma concentrations, while arginine injection in fish previously treated with streptozotocin increased IGF-I levels only. The above data suggest that insulin, together with other factors, may act to increase the levels of IGF-I in plasma.

Indirect effect of insulin to suppress endogenous glucose production is dominant, even with hyperglucagonemia

Suppression of endogenous glucose production (EGP) is one of insulin's primary metabolic effects and failure of this action is a major contributor to fasting hyperglycemia of type 2 diabetes mellitus. Classically, insulin was thought to suppress the liver directly, via hyperinsulinemia in the portal vein. Recently, however, we and others have demonstrated that at least part, and possibly most of insulin's action to suppress EGP is normally mediated via an extrahepatic (i.e., indirect) mechanism. We have suggested that this mechanism involves insulin suppression of adipocyte lipolysis, leading to lowered FFA and reduced EGP ("Single Gateway Hypothesis"). Previous studies of the indirect insulin effect from this laboratory were done under conditions of lowered portal glucagon. Because of the possibility that the direct (i.e., portal) effect of insulin may have been underestimated with hypoglucagonemia, these studies examined the relative importance of portal insulin, versus peripheral insulin (administered at one-half the dose to equalize peripheral insulin levels) at four rates of portal glucagon infusion: 0, 0.65 (under-), 1.5 (basal-), and 3.0 ng/kg per min (over-replacement). Portal versus peripheral insulin suppressed steady-state EGP to the same extent (52%), confirming that the primary effect of insulin to suppress EGP is via the peripheral mechanism. This conclusion was maintained regardless of portal glucagonemia, although there was some evidence for an increase in the direct insulin effect at hyperglucagonemia. The indirect effect of insulin is the primary mechanism of steady-state EGP suppression under normal conditions. The direct effect increases with hyperglucagonemia; however, the indirect effect remains predominant even under those conditions.

Hepatic insulin gene expression as treatment for type 1 diabetes mellitus in rats

Type 1 diabetes mellitus is caused by a lack of insulin that results from the autoimmune destruction of the pancreatic beta-cells. Severe diabetes, if not controlled by periodic insulin injections, can lead to ketoacidosis and death. We have previously shown that sustained low level production of insulin in the liver of diabetic rats prevented their death from complications of diabetes. To test the hypothesis that there is a window of serum insulin concentrations that can prevent ketoacidosis without significant risk of hypoglycemia secondary to hyperinsulinemia, rats were infused with various doses of a recombinant retrovirus encoding an engineered rat preproinsulin-1 gene. The gene was engineered to allow processing into mature insulin by the protease furin. At the lower doses tested, fatal ketoacidosis was prevented, but the rats exhibited nonfasting hyperglycemia. At intermediate doses, which resulted in serum insulin concentrations of 1.6 mg/ml, the rats achieved near-normoglycemia and no serum ketones. These rats did not exhibit hypoglycemia even during a 24-h fast. At high virus doses, the animals achieved nonfasting normoglycemia but exhibited hypoglycemia during the fast. In conclusion, we have defined a therapeutic window of hepatic insulin expression that provides protection against ketoacidosis without significant risk of hypoglycemia. This window of sustained hepatic insulin expression might permit its development into a novel treatment modality for the prevention of ketoacidosis in patients with severe insulin-dependent diabetes mellitus.

Growth in the diabetic child

Insulin has an important role in the GH:IGF-I axis, and its absence leads to major endocrine disturbances, particularly during puberty. Rapid growth in the prediabetic phase occurs and may be a risk factor. Final height reflects gender and the age of onset more than specific growth factors. The influence of hepatic insulin on IGF-I bioactivity has important implications in diabetic adolescents.

Restoration of lactation in bromocriptine-treated rats by prolactin replacement: comparison of constant versus pulsatile infusion and intrahepatic versus intrajugular routes of delivery

The effectiveness of pulsed vs constant infusion of ovine(o) prolactin (PRL), given by different schedules, at restoring lactation in PRL-suppressed rats was compared, and the possibility that the liver participates in the restorative effects of the infused hormone was investigated. Lactating dams were given subcutaneous injections of bromocriptine (BC) between days 7 and 12 postpartum to suppress endogenous PRL secretion. Osmotic minipumps were used to infuse the oPRL into either the jugular vein or the hepatic portal vein. The latter route would expose the liver to higher concentrations of PRL than would intrajugular infusion. Constant infusion of oPRL in different doses was, overall, more effective at restoring lactation (i.e. litter weight gain) than was giving pulses, regardless of the site of delivery. Infusion of the PRL at 100 micrograms/rat/day in pulses of 1h duration was ineffective at frequencies of either 4 or 8/day, whereas pulses of 2h duration were effective at both of these frequencies. Infusing that dose of oPRL was equally effective whether it was given in 4 or 8 pulses/day of 2 h duration. Intrahepatic infusion of oPRL was not more effective than intrajugular delivery regardless of the schedule of administration. These results indicate that pulse duration is a more important determinant of the effectiveness of the galactopoietic action of PRL in the lactating rat than is pulse frequency. No evidence was obtained that the liver participates in the galactopoietic effects of PRL.

Importance of peripheral insulin levels for insulin-induced suppression of glucose production in depancreatized dogs

It is generally believed that glucose production (GP) cannot be adequately suppressed in insulin-treated diabetes because the portal-peripheral insulin gradient is absent. To determine whether suppression of GP in diabetes depends on portal insulin levels, we performed 3-h glucose and specific activity clamps in moderately hyperglycemic (10 mM) depancreatized dogs, using three protocols: (a) 54 pmol.kg-1 bolus + 5.4 pmol.kg-1.min-1 portal insulin infusion (n = 7; peripheral insulin = 170 +/- 51 pM); (b) an equimolar peripheral infusion (n = 7; peripheral insulin = 294 +/- 28 pM, P < 0.001); and (c) a half-dose peripheral infusion (n = 7), which gave comparable (157 +/- 13 pM) insulinemia to that seen in protocol 1. Glucose production, use (GU) and cycling (GC) were measured using HPLC-purified 6-[3H]- and 2-[3H]glucose. Consistent with the higher peripheral insulinemia, peripheral infusion was more effective than equimolar portal infusion in increasing GU. Unexpectedly, it was also more potent in suppressing GP (73 +/- 7 vs. 55 +/- 7% suppression between 120 and 180 min, P < 0.001). At matched peripheral insulinemia (protocols 2 and 3), not only stimulation of GU, but also suppression of GP was the same (55 +/- 7 vs. 63 +/- 4%). In the diabetic dogs at 10 mM glucose, GC was threefold higher than normal but failed to decrease with insulin infusion by either route. Glycerol, alanine, FFA, and glucagon levels decreased proportionally to peripheral insulinemia. However, the decrease in glucagon was not significantly greater in protocol 2 than in 1 or 3. When we combined all protocols, we found a correlation between the decrements in glycerol and FFAs and the decrease in GP (r = 0.6, P < 0.01). In conclusion, when suprabasal insulin levels in the physiological postprandial range are provided to moderately hyperglycemic depancreatized dogs, suppression of GP appears to be more dependent on peripheral than portal insulin concentrations and may be mainly mediated by limitation of the flow of precursors and energy substrates for gluconeogenesis and by the suppressive effect of insulin on glucagon secretion. These results suggest that a portal-peripheral insulin gradient might not be necessary to effectively suppress postprandial GP in insulin-treated diabetics.

Elevated plasma insulin-like growth factor binding protein-1 levels in type 1 (insulin-dependent) diabetic patients with peripheral neuropathy

Previous studies have suggested that nerve regeneration may be defective in patients with diabetic polyneuropathy. Since insulin-like growth factor I (IGF-I) has been shown to stimulate nerve regeneration, and IGF binding protein-1 is acutely regulated by plasma insulin we have investigated the relationships between plasma IGF-I, IGFBP-1, glucose and insulin in Type 1 (insulin-dependent) diabetic patients with peripheral polyneuropathy. Plasma samples were taken at hourly intervals over an 11-h period (08.00-19.00 hours) in order to characterise secretory profiles for 15 Type 1 diabetic patients (eight neuropathic and seven non-neuropathic) and eight non-diabetic control subjects. In the non-diabetic subjects, mean plasma IGF-I levels were stable throughout the 11-h period with a range of 97 micrograms/l-169 micrograms/l. In contrast, mean plasma IGFBP-1 levels declined steadily from a high level of 1.99 micrograms/l at 08.00 hours to approximately one half (0.86 microgram/l) at 15.00 hours. Comparison of areas under the curves revealed significant negative correlations between IGFBP-1 and glucose (-0.88, p = 0.01), IGFBP-1 and insulin (-0.75, p = 0.016), and IGFBP-1 and IGF-I (-0.68, p = 0.03). A significant positive correlation was found between insulin and IGF-I (+0.89, p = 0.001). The diabetic patients had markedly elevated plasma IGFBP-1 levels (area under curve, p = 0.01) and lower plasma IGF-I levels (p = 0.033) even though these patients were hyperinsulinaemic throughout the study period.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin regulates insulin-like growth factor I mRNA in rat hepatocytes

To evaluate the regulatory role of growth hormone (GH) and insulin on insulin-like growth factor I (IGF-I) mRNA levels, we employed primary rat hepatocytes. Cells were incubated for 16 h with 10 nM insulin, 10 nM GH, or a combination thereof, and IGF-I mRNA levels were analyzed by Northern blotting. Insulin results in 2.5-fold and GH in 3.8-fold higher IGF-I mRNA levels than hormone-free controls, and a combination of insulin and GH had an additive effect (6.7-fold). The effect of 10 nM insulin was constant at variable GH concentrations. Therefore, GH and insulin affect IGF-I mRNA levels independently of each other. The half-maximal effective dose of insulin was 4.7 X 10(-10) M, and, in kinetic experiments, insulin was effective within 2 h. These findings demonstrate that insulin modulates hepatic IGF-I production by a direct regulation of the transcript levels of IGF-I.

Differential expression of insulin-like growth factor-I and insulin-like growth factor binding protein-1 in the diabetic rat

Multiple factors contribute to the growth retardation which is a characteristic feature of uncontrolled diabetes. In this report we have examined the effects of streptozotocin-induced (STZ) diabetes on expression of insulin-like growth factor-I (IGF-I) and insulin-like growth factor binding protein-1 (IGFBP-1) in various tissues. As early as 7 days after STZ administration there was a modest reduction in IGF-I mRNA abundance. The reduction (10-30%) was of similar magnitude in each of the 7 tissues examined; liver, kidney, lung, diaphragm, quadraceps, heart and adipose tissue. However, the reduction achieved statistical significance only in the lung (p less than 0.05) and diaphragm (p less than 0.01). A further reduction in IGF-I mRNA abundance was seen in many tissues, 32 and 91 days after STZ administration. In contrast to the decrease in IGF-I mRNA, IGFBP-1 mRNA was significantly increased in the liver and kidney of diabetic rats. IGFBP-1 mRNA was detectable at only very low levels in other tissues but was increased in diabetic rats compared non-diabetic rats. In diabetic rats, a highly significant correlation (R = 0.75, p less than 0.001) between hepatic IGFBP-1 mRNA and glucose was observed whereas there was no significant correlation between serum glucose and hepatic IGF-I mRNA abundance (R = 0.24, p = NS). Treatment of diabetic rats with insulin resulted in a small, non significant increase in hepatic and renal IGF-I mRNA and a significant decrease in renal IGFBP-1 mRNA abundance. The observations reported here are consistent with the hypothesis that diminished IGF-I expression and inhibition of available IGF-I by increased levels of IGFBP-1 may explain the impaired growth seen in diabetic animals.

Somatomedin-C (SM-C). Study in diabetic patients with and without retinopathy

In the present study we evaluated somatomedin-C (Sm-C) plasma levels in diabetic patients, with and without retinopathy. One hundred and thirty four diabetic patients (65 type I and 69 type II) and 90 controls, strictly matched for age and sex, were enrolled in the study. Ophthalmoscopy and fluorescein angiography allowed to distinguish: 49 patients without retinopathy, 45 patients with background retinopathy, and 40 with proliferative retinopathy. Growth hormone (GH) and Sm-C plasma levels were measured using a pool of 20-24 blood samples over 24h. Sm-C levels in type I (0.62 +/- 0.11 U/ml) and type II (0.56 +/- 0.09 U/ml) patients were significantly decreased (p less than 0.01) when compared to controls (0.89 +/- 0.30 U/ml). The mean daily secretion of GH was significantly (p less than 0.01) greater in diabetic patients (7.8 +/- 2.6 ng/ml) than in controls (4.1 +/- 1.5 ng/ml), but no correlation was found between Sm-C and GH (r = 0.15; p = n.s.). Our findings did not show any correlation between Sm-C plasma levels and either the existence of retinopathy, regardless of the degree of microvascular damage, or duration of the disease, or degree of metabolic control, as evaluated by HbA1c.

The effect of acute and chronic insulin administration on insulin-like growth factor-I expression in the pituitary-intact and hypophysectomised rat

Although growth hormone is known to be the main regulator of insulin-like growth factor-I, insulin has also been shown to play a role in regulating serum insulin-like growth factor I levels in diabetic animals. While this effect is thought to be due to correction of metabolic perturbations, some studies have suggest that insulin may have a direct effect on growth and/or insulin-like growth factor-I levels. We have examined the effects of acute and chronic insulin administration to non-diabetic, pituitary-intact and hypophysectomised rats. Rats were injected intraperitoneally with insulin as an acute bolus (10 U) or a chronic subcutanious infusion (low dose; 2.4 U/day, high dose; 12 U/day) over 5 days. Insulin-like growth factor-I mRNA was quantitated by Northern and slot blots of RNA from various tissues. A small (less than 2-fold) but significant increase (p less than 0.05) was seen in hepatic insulin-like growth factor-I mRNA abundance in pituitary-intact rats following acute insulin injection and chronic low dose insulin infusion. An increase in insulin-like growth factor-I mRNA levels was also seen in other tissues including diaphragm, lung, kidney and heart. A significant increase (p less than 0.05) in serum insulin-like growth factor-I levels was also observed 6 h after insulin injection. In contrast, in pituitary-intact rats which received high dose insulin infusion and were hypoglycaemic at the time of death, tissue levels of insulin-like growth factor-I mRNA were reduced compared to saline-treated control groups.(ABSTRACT TRUNCATED AT 250 WORDS)