Hepatic insulin sensitizing substance: a novel 'sensocrine' mechanism to increase insulin sensitivity in anaesthetized rats

Improvement of insulin sensitivity by a novel drug candidate, BGP-15, in different animal studies

BACKGROUND

Insulin resistance has been recognized as the most significant predictor of further development of type 2 diabetes mellitus (T2DM). Here we investigated the effect of a heat shock protein (HSP) co-inducer, BGP-15, on insulin sensitivity in different insulin-resistant animal models and compared its effect with insulin secretagogues and insulin sensitizers.

METHODS

Insulin sensitivity was assessed by the hyperinsulinemic euglycemic glucose clamp technique in normal and cholesterol-fed rabbits and in healthy Wistar and Goto-Kakizaki (GK) rats in dose-ranging studies. We also examined the effect of BGP-15 on streptozotocin-induced changes in the vasorelaxation of the aorta in Sprague-Dawley rats.

RESULTS

BGP-15 doses of 10 and 30 mg/kg increased insulin sensitivity by 50% and 70%, respectively, in cholesterol-fed but not in normal rabbits. After 5 days of treatment with BGP-15, the glucose infusion rate was increased in a dose-dependent manner in genetically insulin-resistant GK rats. The most effective dose was 20 mg/kg, which showed a 71% increase in insulin sensitivity compared to control group. Administration of BGP-15 protected against streptozotocin-induced changes in vasorelaxation, which was similar to the effect of rosiglitazone.

CONCLUSION

Our results indicate that the insulin-sensitizing effect of BGP-15 is comparable to conventional insulin sensitizers. This might be of clinical utility in the treatment of T2DM.

Insulin resistance occurs in parallel with sensory neuropathy in streptozotocin-induced diabetes in rats: differential response to early vs late insulin supplementation

We investigated whether progressive sensory neuropathy was accompanied by changes in whole-body insulin sensitivity (WBIS) in rats made diabetic by streptozotocin (STZ). The effects of early and late insulin supplementation were also studied. The STZ-treated rats failed to gain weight and exhibited stable hyperglycemia and low plasma insulin levels with a decrease in nerve conduction velocity (NCV) measured in A and C fibers of the saphenous nerve. A decreased sensory neuropeptide (SNP) release such as that of substance P, somatostatin, and calcitonin gene-related peptide determined from organ fluid of tracheal preparations subjected to electrical field stimulation also occurred in diabetic animals. These features were accompanied by a decrease in WBIS measured by hyperinsulinemic-euglycemic glucose clamping and a decrease in insulin-stimulated glucose uptake in cardiac and gastrocnemius muscle. When insulin supplementation with slow-release implants (2 IU/d) was started 4 weeks after STZ injection, blood glucose level normalized. Both insulin sensitivity and sensory nerve function reflected in either NCV or SNP release completely recovered by the 12th post-STZ week. When the insulin implants were applied from the eighth post-STZ week, both WBIS and glucose uptake remained significantly decreased, with a seriously impaired NCV and SNP release with strong hyperglycemia. Late insulin supplementation, however, even by using double implantation from the 10th post-STZ week, was unable to restore blood glucose, WBIS, NCV, and SNP release by the 12th week. Insulin resistance occurs in parallel with sensory neuropathy in STZ-diabetic rats. Both can be improved by early but not late insulin supplementation.

Block by nitrate tolerance of meal-induced insulin sensitization in conscious rabbits

PURPOSE

Hemodynamic nitrate tolerance has been shown to result in an insulin-resistant state. We studied whether nitrate tolerance induced by a 7-day continuous exposure to transdermal nitroglycerin influenced the meal-induced insulin sensitization phenomenon in rabbits.

METHODS

Changes in insulin sensitivity in response to feeding in conscious rabbits were determined by rapid insulin sensitivity test, in both nitrate-tolerant and nitrate-intolerant animals. In a separate series of experiments with anesthetized rabbits with or without nitrate tolerance, the hyperinsulinemic euglycemic glucose clamping methods was used to study the effect of intraportal infusion of cholecystokinin (CCK) on whole-body insulin sensitivity.

RESULTS

Rabbits with normal feeding exhibited a 46 ± 6% increase in insulin sensitivity as compared with their matching fasting controls. A 7-day period of treatment with patches releasing 0.07 mg of nitroglycerin per hour yielded nitrate tolerance and a state of insulin resistance and no increase in insulin sensitivity in response to food. Intraportal infusion of CCK8 (0.3-3.0 μg/kg over 20 minutes) resulted in a dose-dependent increase in insulin sensitivity in normal but not in nitrate-tolerant, fasted anesthetized animals.

CONCLUSIONS

Nitrate tolerance blocks both the meal-induced insulin sensitization phenomenon and the insulin-sensitizing effect of intraportal CCK.

Association between an ASIC3 gene variant and insulin resistance in Taiwanese

BACKGROUND

Acid-sensing ion channel 3 (ASIC3) is a ligand-gated cation channel activated by extracellular protons. ASIC3(-/-) mice exhibit protection against age-dependent glucose intolerance with enhanced insulin sensitivity.

METHODS

To determine the association between ASIC3 genetic polymorphisms and insulin resistance in Taiwanese, 606 unrelated subjects with no history of cardiovascular disease were recruited during routine health examinations.

RESULTS

Six ASIC3 gene polymorphisms were genotyped and only the rs2288646 polymorphism was found associated with insulin resistance. Significantly lower fasting serum insulin levels and homeostasis model assessment of insulin resistance (HOMA-IR) index and significantly higher quantitative insulin sensitivity check index (QUICKI) were observed in subjects carrying the rs2288646-A allele than in the non-carriers (P=0.028, P=0.047 and P=0.031, respectively) after adjustment for age, gender, and body mass index (BMI). Significantly lower frequencies of the rs2288646-A-containing genotypes were found in insulin resistant subjects (P=0.023). By multivariate analysis, rs2288646 genotypes, age, BMI, fasting plasma glucose level, C-reactive protein level, and soluble intercellular adhesive molecule 1 level, were all independently associated with HOMA-IR index and QUICKI (all, P<0.05).

CONCLUSIONS

Our analysis indicated an independent association between an ASIC3 genetic polymorphism and insulin resistance in Taiwanese.

Meal-induced enhancement in insulin sensitivity is not triggered by hyperinsulinemia in rats

Several reports confirmed the phenomenon of postprandial increase in whole-body insulin sensitivity. Although the initial step of this process is unknown, the pivotal role of postprandial hyperinsulinemia has strongly been suggested. The aim of the present study was to determine whether hyperinsulinemia per se induces insulin sensitization in healthy male Wistar rats. Rapid insulin sensitivity test (RIST) were performed in fasted, anesthetized rats before and during stable hyperinsulinemia achieved by hyperinsulinemic euglycemic glucose clamping (HEGC) with insulin infused either through the jugular vein (systemic HEGC) or into the portal circulation (portal HEGC) at a rate of 3 mU/(kg min). Insulin sensitivity expressed by the rapid insulin sensitivity (RIST) index (in milligrams per kilogram) was characterized by the total amount of glucose needed to maintain prestudy blood glucose level succeeding an intravenous bolus infusion of 50 mU/kg insulin over 5 minutes. In fasted animals, the RIST index was 37.4 +/- 3.1 mg/kg. When hyperinsulinemia mimicking the postprandial state was achieved by systemic HEGC, the RIST index (39.7 +/- 10.6 mg/kg) showed no significant changes as compared with the pre-HEGC values. Hyperinsulinemia achieved by portal insulin infusion also failed to modify the RIST index (35.7 +/- 4.3 mg/kg). The results demonstrate that acute hyperinsulinemia, no matter how induced, does not yield any sensitization to the hypoglycemic effect of insulin.

Modulation of bone morphogenetic protein-9 expression and processing by insulin, glucose, and glucocorticoids: possible candidate for hepatic insulin-sensitizing substance

Bone morphogenetic protein 9 (BMP-9), a member of the TGF-beta superfamily predominantly expressed in nonparenchymal liver cells, has been demonstrated to improve glucose homeostasis in diabetic mice. Along with this therapeutic effect, BMP-9 was proposed as a candidate for the hepatic insulin-sensitizing substance (HISS). Whether BMP-9 plays a physiological role in glucose homeostasis is still unknown. In the present study, we show that BMP-9 expression and processing is severely reduced in the liver of insulin-resistant rats. BMP-9 expression and processing was directly stimulated by in situ exposition of the liver to the combination of glucose and insulin and oral glucose in overnight fasted rats. Additionally, prolonged fasting (72 h) abrogated refeeding-induced BMP-9 expression and processing. Previous exposition to dexamethasone, a known inductor of insulin resistance, reduced BMP-9 processing stimulated by the combination of insulin and glucose. Finally, we show that neutralization of BMP-9 with an anti-BMP-9 antibody induces glucose intolerance and insulin resistance in 12-h fasted rats. Collectively, the present results demonstrate that BMP-9 plays an important role in the control of glucose homeostasis of the normal rat. Additionally, BMP-9 is expressed and processed in an HISS-like fashion, which is impaired in the presence of insulin resistance. BMP-9 regulation according to the feeding status and the presence of diabetogenic factors reinforces the hypothesis that BMP-9 might exert the role of HISS in glucose homeostasis physiology.

The inhibitory effect of proglumide on meal-induced insulin sensitization in rats

We studied the role of cholecystokinin in meal-induced insulin sensitization in rats. Experiments were done with fed or fasted male Wistar rats. Whole-body insulin sensitivity was determined by the rapid insulin sensitivity test in either group. The fed animals were more sensitive to the hypoglycemic effect of insulin than those in the fasted group. Single intravenous doses of proglumide, a cholecystokinin-1 receptor antagonist, decreased insulin sensitivity in fed animals in a dose-dependent manner, whereas it was without effect in the fasted state. We conclude that prandial insulin sensitization strongly depends on pathways regulated by cholecystokinin.

Diabetes induced by partial hepatic sensory denervation in conscious rabbits

Exposure of the anterior hepatic plexus to 2% perineurial capsaicin solution over three days caused transient insulin resistance confirmed by hyperinsulinaemic euglycaemic glucose clamping. Three additional perineurial capsaicin treatments divided by 3-month intervals yielded diabetes characterized by an increase in fasting blood glucose and glycated haemoglobin levels. Both insulin sensitivity and glycated haemoglobin level re-normalized over an additional 6-month period. We conclude that chronic partial hepatic sensory denervation produces diabetes in rabbits.

Insulin sensitization induced by oral cicletanine in conscious rabbits

The endogenous insulin sensitizing machinery termed the hepatic insulin sensitizing substance (HISS) mechanism has been shown to be nitrergic and linked to sensory fibers in the anterior hepatic plexus. We studied whether this mechanism could pharmacologically be exploited by cicletanine, a cGMP-PDE inhibitor antihypertensive drug, in conscious rabbits. Whole body insulin sensitivity and peripheral glucose uptake were determined by hyperinsulinaemic euglycaemic glucose clamping, and cardiac radiolabelled deoxyglucose (DOG) uptake in neurogenic, achieved by perineurial capsaicin treatment of the anterior hepatic plexus through defunctionalization of hepatic sensory fibers, and metabolic, induced by dietary hypercholesterolemia, insulin resistance models after single oral doses of cicletanine (3, 10 and 30 mg kg(-1)) or rosiglitazone (3 mg kg(-1)). The effect of cicletanine on cardiac and vascular tissue NO, cGMP, cAMP was measured by means of spin trapping technique and radioimmunoassay, respectively. Insulin sensitivity and peripheral DOG uptake were significantly increased by 10 and 30 mg kg(-1) cicletanine in both healthy and hypercholesterolaemic rabbits, but not in those with neurogenic insulin resistance. Rosiglitazone had no effect in healthy and neurogenic insulin resistant rabbits although it improved insulin sensitivity in hypercholesterolemic animals. The 10 mg kg(-1) cicletanine dose induced no change in either cardiac or vascular tissue NO, cGMP or cAMP concentrations. Nevertheless, at a dose of 30 mg kg(-1) producing an insulin sensitizing effect of approximately the same amplitude as seen with 10 mg kg(-1), the drug significantly increased tissue NO and cGMP concentrations. Oral cicletanine attains its insulin sensitizing effect at doses lower than those necessary to activate the NO-cGMP pathway in the cardiovascular system. This metabolic effect requires functional integrity of hepatic sensory nerves.

Neurogenic insulin resistance in guinea-pigs with cisplatin-induced neuropathy

The aim of the present work was to study whether neurotoxicity produced by cisplatin modified tissue insulin sensitivity in guinea-pigs. One week after selective sensory denervation of the anterior hepatic plexus by means of perineurial 2% capsaicin treatment, hyperinsulinaemic euglycaemic glucose clamp were performed to estimate insulin sensitivity in male guinea-pigs. The guinea-pigs underwent regional sensory denervation of the anterior hepatic plexus exhibited insulin resistance, whereas systemic capsaicin desensitization increased insulin sensitivity. Intraportal administration of L-nitro-arginine methyl ester (L-NAME decreased, whereas capsaicin increased insulin sensitivity. Neither atropine nor acetylcholine produced any significant effect. In animals with preceding regional capsaicin desensitization, none of the pharmacological maneuvers modified the resulting insulin resistant state. Cisplatin pretreatment induced sensory neuropathy and decreased insulin sensitivity. Insulin sensitivity did not change after either regional or systemic capsaicin desensitization in the cisplatin-treated animals. CGRP(8-37), a nonselective calcitonin gene-related peptide (CGRP) antagonist (50 microg/kg i.v.), significantly increased insulin sensitivity in normal animals but only a tendency to insulin sensitization was seen after cisplatin treatment. Cisplatin treatment, similar to regional capsaicin desensitization of the anterior hepatic plexus, produced a significant decrease in insulin-stimulated uptake of 2-deoxy-D [L-14C] glucose in cardiac and gastrocnemius muscle with no effect on percentage suppression of endogenous glucose production by hyperinsulinaemia. We conclude that the majority of cisplatin-induced insulin resistance is related to functional deterioration of the hepatic insulin sensitizing substance (HISS) mechanism.

Hemorrhage results in hepatic insulin-sensitizing substance-dependent insulin resistance mediated by somatostatin in rats

BACKGROUND/AIMS

Acute hemorrhage results in hyperglycemia regulated in a redundant manner by adrenal catecholamines and hepatic sympathetic nerves. In addition, insulin secretion is suppressed and insulin resistance is accounted for completely by elimination of the hepatic insulin-sensitizing substance (HISS) component of insulin action. Blockade of HISS action secondary to blood loss leads to a state known as HISS-dependent insulin resistance (HDIR) which results in a decrease in the glucose disposal action of insulin by 33 +/- 3%.

METHODS

This paper describes nine studies that have explored the neuroendocrine control of HDIR that is produced in response to the stress of blood loss. The rapid insulin sensitivity test (RIST), a transient euglycemic clamp, was used to measure insulin sensitivity. To test the role of the adrenergic system, alpha- and beta-adrenergic receptor antagonists, phentolamine and propranolol, were tested for the ability to block HDIR produced by hemorrhage.

RESULTS

Neither intervention was effective (32 +/- 6 and 36 +/- 3%, respectively). Exogenous somatostatin was shown to produce HDIR that could be blocked by the somatostatin receptor antagonist, cyclosomatostatin. Cyclosomatostatin completely blocked the development of HDIR that occurred following hemorrhage (RIST index 214 +/- 9 control, 218 +/- 9 mg glucose/kg body weight after cyclosomatostatin plus hemorrhage).

CONCLUSIONS

The adrenergic system is not involved in producing HDIR in response to hemorrhage. Somatostatin appears to be the hormonal regulator of this response and it is suggested that the somatostatin derives from a neural origin within the liver.

Glucose and lipid metabolism after liver transplantation in inbred rats: consequences of hepatic denervation

The liver plays a central role in glucose and lipid homeostasis. Because liver transplantation severs the hepatic nerves which influence this function, we hypothesized that insulin resistance and hyperlipidemia develop after liver transplantation, thus increasing the atherosclerotic risk. Therefore, we studied inbred rats 8 months after orthotopic liver transplantation (Tx, n = 39) or laparotomy (sham, n = 37) by either oral glucose tolerance test (Tx, n = 13; sham, n = 8), meal tolerance test (Tx, n = 9; sham, n = 13), or euglycemic hyperinsulinemic clamp with tritiated glucose infusion (Tx, n = 17; sham, n = 16). We found that liver transplantation significantly increased basal hepatic glucose production (HGP) in the clamp study by 20% (37.3 +/- 2.2 vs 31.0 +/- 2.1 micromol kg -1 .min -1 , P < .05) and fasting plasma low-density lipoprotein (LDL) cholesterol by 36% (0.79 +/- 0.06 vs 0.58 +/- 0.05 mmol/L, P < .05). However, it did not affect HGP, total glucose uptake, metabolic clearance rate of insulin, and suppression of plasma nonesterified fatty acids, which were all normal in response to rising plasma insulin concentrations in the dose-response clamp studies. The oral glucose tolerance test and meal tolerance test also showed normal glucose and nonesterified fatty acids homeostasis with adequate pancreatic insulin secretion and hepatic insulin clearance after liver transplantation. The only consequences of liver transplantation are increased basal HGP and plasma LDL cholesterol, which may be caused by persistent vagal denervation of the liver. Although insulin resistance is absent, elevated plasma LDL cholesterol increases the atherosclerotic risk.

Sensory nerve inactivation by resiniferatoxin improves insulin sensitivity in male obese Zucker rats

Recent studies have suggested that sensory nerves may influence insulin secretion and action. The present study investigated the effects of resiniferatoxin (RTX) inactivation of sensory nerves (desensitization) on oral glucose tolerance, insulin secretion and whole body insulin sensitivity in the glucose intolerant, hyperinsulinemic, and insulin-resistant obese Zucker rat. After RTX treatment (0.05 mg/kg RTX sc given at ages 8, 10, and 12 wk), fasting plasma insulin was reduced (P < 0.0005), and oral glucose tolerance was improved (P < 0.005). Pancreas perfusion showed that baseline insulin secretion (7 mM glucose) was lower in RTX-treated rats (P = 0.01). Insulin secretory responsiveness to 20 mM glucose was enhanced in the perfused pancreas of RTX-treated rats (P < 0.005) but unaffected in stimulated, isolated pancreatic islets. At the peak of spontaneous insulin resistance in the obese Zucker rat, insulin sensitivity was substantially improved after RTX treatment, as evidenced by higher glucose infusion rates (GIR) required to maintain euglycemia during a hyperinsulinemic euglycemic (5 mU.kg(-1).min(-1)) clamp (GIR(60-120min): 5.97 +/- 0.62 vs. 11.65 +/- 0.83 mg.kg(-1).min(-1) in RTX-treated rats, P = 0.003). In conclusion, RTX treatment and, hence, sensory nerve desensitization of adult male obese Zucker rats improved oral glucose tolerance by enhancing insulin secretion, and, in particular, by improving insulin sensitivity.

The prandial insulin sensitivity-modifying effect of vagal stimulation in rats

The effect of left cervical vagal nerve stimulation was studied on insulin sensitivity to test the proposed permissive insulin-sensitizing role of hepatic vagal parasympathetic efferent pathways in fasted and fed anesthetized rats. In fed animals, electrical stimulation (square impulses: 25 V, 5 Hz, 0.5 milliseconds over 15 minutes) of the vagal nerve induced hyperglycemia and an increase in plasma insulin immunoreactivity. Atropine (1.0 mg/kg intravenously) induced insulin resistance estimated by rapid insulin sensitivity testing. This was amplified when the vagal nerve was stimulated. The insulin-resistant state developed by fasting was not modified by either treatment with atropine or electrical stimulation. We conclude that both parasympathetic cholinergic and noncholinergic vagal efferents modulate postprandial neurogenic insulin sensitivity adjustments.

Control of hepatocyte metabolism by sympathetic and parasympathetic hepatic nerves

More than any other organ, the liver contributes to maintaining metabolic equilibrium of the body, most importantly of glucose homeostasis. It can store or release large quantities of glucose according to changing demands. This homeostasis is controlled by circulating hormones and direct innervation of the liver by autonomous hepatic nerves. Sympathetic hepatic nerves can increase hepatic glucose output; they appear, however, to contribute little to the stimulation of hepatic glucose output under physiological conditions. Parasympathetic hepatic nerves potentiate the insulin-dependent hepatic glucose extraction when a portal glucose sensor detects prandial glucose delivery from the gut. In addition, they might coordinate the hepatic and extrahepatic glucose utilization to prevent hypoglycemia and, at the same time, warrant efficient disposal of excess glucose.

Asymmetric dimethylarginine: A molecule responsible for the coexistence of insulin resistance and atherosclerosis via dual nitric oxide synthase inhibition

Asymmetric dimethylarginine (ADMA) has been recently identified as the major endogenous inhibitor of soluble nitric oxide synthase. Its systemic accumulation was observed in conjunction with atherosclerosis and several cardiovascular and metabolic diseases. Here, we propose that ADMA causes insulin resistance by the inhibition of the neuronal isoform of nitric oxide synthase, while the simultaneously observed atherosclerosis is the consequence of endothelial nitric oxide synthase (NOS) inhibition. Our hypothesis rests on animal models in which experimental insulin resistance was induced by intraportal administration of non-selective and selective neuronal nitric oxide synthase inhibitors, N-methyl-L-arginine (L-NMMA) or 7-nitroindazole. In these models, loss of hepatic nitric oxide productions is presumed to hinder a very potent insulin sensitizing mechanism referred to as meal induced sensitization that is anatomically linked to the nitrergic fibers of the anterior hepatic plexus. Cause and effect relationship between ADMA and insulin resistance has been proposed previously by others however the nature of this relationship has not been elucidated in detail. In our hypothesis, we suggest that ADMA by inhibiting both the neuronal and the endothelial forms of NOS, results both in insulin resistance and in accelerated atherosclerosis, therefore ADMA is the molecule responsible for the coexistence of these two conditions. We also suggest animal models and human studies to test our hypothesis, the results of which may offer novel approaches in the prevention of insulin resistance and atherosclerosis.

Forty years in capsaicin research for sensory pharmacology and physiology

Capsaicin, the pungent ingredient of chilli peppers has become a "hot" topic in neuroscience with yearly publications over half thousand papers. It is outlined in this survey how this exciting Hungarian research field emerged from almost complete ignorance. From the initial observation of the phenomenon of "capsaicin desensitization", a long-lasting chemoanalgesia and impairment in thermoregulation against heat, the chain of new discoveries which led to the formulation of the existence of a "capsaicin receptor" on C-polymodal nociceptors is briefly summarized. Neurogenic inflammation is mediated by these C-afferents which are supplied by the putative capsaicin receptor and were termed as "capsaicin sensitive" chemoceptive afferents. They opened new avenues in local peptidergic regulation in peripheral tissues. It has been suggested that in contrast to the classical axon reflex theory, the capsaicin-sensitive sensory system subserves a "dual sensory-efferent" function whereby initiation of afferent signals and neuropeptide release are coupled at the same nerve endings. Furthermore, in the skin at threshold stimuli which do not evoke sensation elicit already maximum efferent response as enhanced microcirculation. In isolated organ preparations large scale of new type of peptidergic capsaicin-sensitive neurogenic smooth muscle responses were revealed after the first one was described by ourselves on the guinea-pig ileum in 1978. Recently the "capsaicin receptor" has been cloned and it is now named as the "transient receptor potential vanilloid 1" (TRPV1). Hence, capsaicin research led to the discovery of the first temperature-gated ion channel gated by noxious heat, protons, vanilloids and endogenous ligands as anandamide, N-oleoyldopamine and lipoxygenase products. Another recent achievement is the discovery of a novel "unorthodox" neurohumoral regulatory mechanism mediated by somatostatin. Somatostatin released from the TRPV1-expressing nerve endings reaches the circulation and elicits systemic antiinflammatory and analgesic "sensocrine" functions with counter-regulatory influence e.g. in Freund's adjuvant-induced chronic arthritis. Nociceptors supplied by TRPV1 and sst4 somatostatin receptors has become nowadays promising targets for drug development.

Pre-clinical methods for the determination of insulin sensitivity

We compared the hyperinsulinaemic euglycaemic glucose clamping (HEGC) procedure and the rapid insulin sensitivity test (RIST) to characterize insulin sensitivity in anaesthetized rats. The changes in insulin sensitivity were then supplemented with the direct measurement of insulin-stimulated glucose uptake using tissue accumulation of radioactive 2-deoxyglucose in skeletal muscle samples obtained from animals undergone either procedure. Studies of the recently described endogenous insulin sensitizer mechanism termed hepatic insulin sensitizing (HISS) mechanism, by the two methods yielded data for evaluation. The HISS mechanism is defined as an increase in tissue insulin sensitivity in response to post-prandial hepatic release of an undefined substance through a nitrergic pathway. For the HEGC method, insulin was infused to attain a stable plasma insulin immunoreactivity of 100 microU/ml determined by radioimmunoassay, whereas with the RIST method the HISS mechanism was activated by a 50 mg/kg i.v. insulin bolus. Euglycaemia was kept constant by means of glucose infusion. With the HEGC and the RIST methods, insulin sensitivity was defined as the average rate of glucose infusion and the amount of glucose/kg body weight/40 min (RIST index) infused to maintain euglycaemia and preinvestigation blood glucose level, respectively. During HEGC 16+/-4.2 mg/kg/min glucose was able to maintain euglycaemia, which decreased to 8+/-2.9 (p<0.05) after administration of 10 mg/kg NG-nitro-L-arginine methyl ester (L-NAME) (i.p.), a NO synthase inhibitor. Conversely, the RIST index decreased by 55+/-6.9% (p<0.05) after L-NAME. Similarly, 2-deoxyglucose uptake by the gastrocnemius muscle was decreased by 49.9+/-5.8 (p<0.05) and 52.3+/-7.4% (p<0.05) with the HEGC and the RIST methods, respectively. The results show that both the HEGC and the RIST methods supplemented with tissue radioactive 2-deoxyglucose uptake determinations are appropriate methods to characterize the alteration of insulin sensitivity in context of the HISS mechanism.

Insulin sensitivity is mediated by the activation of the ACh/NO/cGMP pathway in rat liver

The hepatic parasympathetic nerves and hepatic nitric oxide synthase (NOS) are involved in the secretion of a hepatic insulin sensitizing substance (HISS), which mediates peripheral insulin sensitivity. We tested whether binding of ACh to hepatic muscarinic receptors is an upstream event to the synthesis of nitric oxide (NO), which, along with the activation of hepatic guanylate cyclase (GC), permits HISS release. Male Wistar rats (8-9 wk) were anesthetized with pentobarbital sodium (65 mg/kg). Insulin sensitivity was assessed using a euglycemic clamp [the rapid insulin sensitivity test (RIST)]. HISS inhibition was induced by antagonism of muscarinic receptors (atropine, 3 mg/kg i.v.) or by blockade of NOS [NG-nitro-L-arginine methyl ester (L-NAME), 1 mg/kg intraportally (i.p.v.)]. After the blockade, HISS action was tentatively restored using a NOdonor [3-morpholynosydnonimine (SIN-1), 5-10 mg/kg i.p.v.] or ACh (2.5-5 microg.kg(-1).min(-1) .i.p.v.). SIN-1 (10 mg/kg) reversed the inhibition caused by atropine (RIST postatropine 137.7 +/- 8.3 mg glucose/kg; reversed to 288.3 +/- 15.5 mg glucose/kg, n = 6) and by L-NAME (RIST post-L-NAME 152.2 +/- 21.3 mg glucose/kg; reversed to 321.7 +/- 44.7 mg glucose/kg, n = 5). ACh did not reverse HISS inhibition induced by L-NAME. The role of GC in HISS release was assessed using 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 5 nmol/kg i.p.v.), a GC inhibitor that decreased HISS action (control RIST 237.6 +/- 18.6 mg glucose/kg; RIST post-ODQ 111.7 +/- 6.2 mg glucose/kg, n = 5). We propose that hepatic parasympathetic nerves release ACh, leading to hepatic NO synthesis, which activates GC, triggering HISS action.