Loss of regulation by sympathetic hepatic nerves of liver metabolism and haemodynamics in chronically streptozotocin-diabetic rats

Regulation of plasma glucose levels by central dopamine D2 receptors is impaired in type 1 but not type 2 diabetic mouse models

Glucose metabolism is reported to be regulated by the central nervous system, but it is unclear whether this regulation is altered in diabetes. We investigated whether regulation of glucose metabolism by central dopamine D2 receptors is altered in type 1 and type 2 diabetic models. Intracerebroventricular injections of both the dopamine D2 receptor agonist quinpirole and the antagonist l-sulpiride induced hyperglycemia in control mice, but not in streptozotocin (STZ)-induced diabetic mice, a type 1 diabetic model. Hyperglycemia induced by quinpirole or l-sulpiride was diminished following fasting and these drugs did not affect hyperglycemia in the pyruvate tolerance test. In addition, both quinpirole and l-sulpiride increased hepatic glucose-6-phosphatase (G6Pase) mRNA. In STZ-induced diabetic mice, dopamine and dopamine D2 receptor mRNA in the hypothalamus, which regulates glucose homeostasis, were decreased. Hepatic glycogen and G6Pase mRNA were also decreased in STZ-induced diabetic mice. Neither quinpirole nor l-sulpiride increased hepatic G6Pase mRNA in STZ-induced diabetic mice. In diet-induced obesity mice, a type 2 diabetic model, both quinpirole and l-sulpiride induced hyperglycemia, and hypothalamic dopamine and dopamine D2 receptor mRNA were not altered. These results indicate that (i) stimulation or blockade of dopamine D2 receptors causes hyperglycemia by increasing hepatic glycogenolysis, and (ii) stimulation or blockade of dopamine D2 receptors does not affect glucose levels in type 1 but does so in type 2 diabetic models. Moreover, hypothalamic dopaminergic function and hepatic glycogenolysis are decreased in the type 1 diabetic model, which reduces hyperglycemia induced by stimulation or blockade of dopamine D2 receptors.

A Spatial Model of Hepatic Calcium Signaling and Glucose Metabolism Under Autonomic Control Reveals Functional Consequences of Varying Liver Innervation Patterns Across Species

Rapid breakdown of hepatic glycogen stores into glucose plays an important role during intense physical exercise to maintain systemic euglycemia. Hepatic glycogenolysis is governed by several different liver-intrinsic and systemic factors such as hepatic zonation, circulating catecholamines, hepatocellular calcium signaling, hepatic neuroanatomy, and the central nervous system (CNS). Of the factors regulating hepatic glycogenolysis, the extent of lobular innervation varies significantly between humans and rodents. While rodents display very few autonomic nerve terminals in the liver, nearly every hepatic layer in the human liver receives neural input. In the present study, we developed a multi-scale, multi-organ model of hepatic metabolism incorporating liver zonation, lobular scale calcium signaling, hepatic innervation, and direct and peripheral organ-mediated communication between the liver and the CNS. We evaluated the effect of each of these governing factors on the total hepatic glucose output and zonal glycogenolytic patterns within liver lobules during simulated physical exercise. Our simulations revealed that direct neuronal stimulation of the liver and an increase in circulating catecholamines increases hepatic glucose output mediated by mobilization of intracellular calcium stores and lobular scale calcium waves. Comparing simulated glycogenolysis between human-like and rodent-like hepatic innervation patterns (extensive vs. minimal) suggested that propagation of calcium transients across liver lobules acts as a compensatory mechanism to improve hepatic glucose output in sparsely innervated livers. Interestingly, our simulations suggested that catecholamine-driven glycogenolysis is reduced under portal hypertension. However, increased innervation coupled with strong intercellular communication can improve the total hepatic glucose output under portal hypertension. In summary, our modeling and simulation study reveals a complex interplay of intercellular and multi-organ interactions that can lead to differing calcium dynamics and spatial distributions of glycogenolysis at the lobular scale in the liver.

Insulin resistance in the liver: deficiency or excess of insulin?

In insulin-resistant states (obesity, pre-diabetes, and type 2 diabetes), hepatic production of glucose and lipid synthesis are heightened in concert, implying that insulin deficiency and insulin excess coexists in this setting. The fact that insulin may be inadequate or excessive at any one point in differing organs and tissues has many biologic ramifications. In this context the concept of metabolic compartmentalization in the liver is offered herein as one perspective of this paradox. In particular, we focus on the hypothesis that insulin resistance accentuates differences in periportal and perivenous hepatocytes, namely periportal glucose production and perivenous lipid synthesis. Subsequently, excessive production of glucose and accumulation of lipids could be expected in the livers of patients with obesity and insulin resistance. Overall, in this review, we provide our integrative perspective regarding how excessive production of glucose in periportal hepatocytes and accumulation of lipids in perivenous hepatocytes interact in insulin resistant states.

Pivotal role of cAMP in the activation of liver glycogen breakdown in high-fat diet fed mice

AIMS

Liver glycogen catabolism was evaluated in male Swiss mice fed a high-fat diet rich in saturated fatty acids (HFD) or normal fat diet (NFD) during one week.

MAIN METHODS

Liver glycogenolysis (LG) and liver glucose production (LGP) were measured either under basal or stimulated conditions (infusion of glycogenolytic agents). Thus, isolated perfused livers from HFD and NFD mice were infused with glycogenolytic agents, i.e., glucagon, epinephrine, phenylephrine, isoproterenol, adenosine-3'-5'-cyclic monophosphate (cAMP), N(6),2'-O-dibutyryl-cAMP (DB-cAMP), 8-bromoadenosine-cAMP (8-Br-cAMP) or N(6)-monobutyryl-cAMP (N6-MB-cAMP). Moreover, glycemia and liver glycogen content were measured.

KEY FINDINGS

Glycemia, liver glycogen content and basal rate of LGP and LG were not influenced by the HFD. However, LGP and LG were lower (p<0.05) in HFD mice during the infusions of glucagon (1 nM), epinephrine (20 μM) or phenylephrine (20 μM). In contrast, the activation of LGP and LG during the infusion of isoproterenol (20 μM) was not different (HFD vs. NFD). Because glucagon showed the most prominent response, the effect of cAMP, its intracellular mediator, on LGP and LG was investigated. cAMP (150 μM) showed lower activation of LGP and LG in the HFD group. However, the activation of LGP and LG was not influenced by HFD whether DB-cAMP (3 μM), 8-Br-cAMP (3 μM) or N6-MB-cAMP (3 μM) were used.

SIGNIFICANCE

The activation of LGP and LG depends on the intracellular availability of cAMP. It can be concluded that cAMP played a pivotal role on the activation of LG in high-fat diet fed mice.

Autonomic neuropathy in experimental models of diabetes mellitus

Autonomic neuropathy complicates diabetes by increasing patient morbidity and mortality. Surprisingly, considering its importance, development and exploitation of animal models has lagged behind the wealth of information collected for somatic symmetrical sensory neuropathy. Nonetheless, animal studies have resulted in a variety of insights into the pathogenesis, neuropathology, and pathophysiology of diabetic autonomic neuropathy (DAN) with significant and, in some cases, remarkable correspondence between rodent models and human disease. Particularly in the study of alimentary dysfunction, findings in intrinsic intramural ganglia, interstitial cells of Cajal and the extrinsic parasympathetic and sympathetic ganglia serving the bowel vie for recognition as the chief mechanism. A body of work focused on neuropathologic findings in experimental animals and human subjects has demonstrated that axonal and dendritic pathology in sympathetic ganglia with relative neuron preservation represents one of the neuropathologic hallmarks of DAN but it is unlikely to represent the entire story. There is a surprising selectivity of the diabetic process for subpopulations of neurons and nerve terminals within intramural, parasympathetic, and sympathetic ganglia and innervation of end organs, afflicting some while sparing others, and differing between vascular and other targets within individual end organs. Rather than resulting from a simple deficit in one limb of an effector pathway, autonomic dysfunction may proceed from the inability to integrate portions of several complex pathways. The selectivity of the diabetic process appears to confound a simple global explanation (e.g., ischemia) of DAN. Although the search for a single unifying pathogenetic hypothesis continues, it is possible that autonomic neuropathy will have multiple pathogenetic mechanisms whose interplay may require therapies consisting of a cocktail of drugs. The role of multiple neurotrophic substances, antioxidants (general or pathway specific), inhibitors of formation of advanced glycosylation end products and drugs affecting the polyol pathway may be complex and therapeutic elements may have both salutary and untoward effects. This review has attempted to present the background and current findings and hypotheses, focusing on autonomic elements including and beyond the typical parasympathetic and sympathetic nervous systems to include visceral sensory and enteric nervous systems.

Investigation of glycemia recovery with oral administration of glycerol, pyruvate, and L-lactate during long-term, insulin-induced hypoglycemia

AIM

The acute effect of oral administration of isolated or combined glycerol, pyruvate, and L-lactate on glycemia recovery (GR) during long-term, insulin-induced hypoglycemia (IIH) was compared.

METHODS

Glycemia of 24 h-fasted rats that received intraperitoneal injection (1.0 U/kg) of regular insulin (IIH group) or saline (COG group) and, 15, 150, or 165 min later, oral saline (control IIH), glycerol (100 mg/kg), pyruvate (100 mg/kg), L-lactate (100 mg/kg), or combined glycerol+pyruvate+L-lactate (each 33.3 or 100 mg/kg) was compared. In addition, for comparative purposes, a group that received glucose (100 mg/kg) was included. Glycemia was measured 180 min after insulin or saline injection. To investigate the participation of the hepatic availability of gluconeogenic substrates to GR, livers from IIH and COG rats that received physiological or supraphysiological concentrations of isolated or combined glycerol, pyruvate, and L-lactate were compared. Liver experiments were done 180 min after insulin or saline injection.

RESULTS

Oral glycerol, pyruvate, and L-lactate (isolated or combined) or glucose promoted GR. Moreover, the best GR was obtained with combined glycerol+pyruvate+L-lactate (100 mg/kg). In agreement, livers that received supraphysiological concentrations of glycerol, pyruvate, and L-lactate (isolated or combined) showed higher glucose release than livers that received physiological concentrations of these substances (isolated or combined).

CONCLUSION

The best GR obtained with combined administration of glycerol, pyruvate, and L-lactate (100 mg/kg) during long-term IIH was a consequence of the higher liver availability of these substances associated with a maintained liver ability to produce glucose from gluconeogenic substrates.

Gluconeogenesis and ketogenesis in perfused liver of rats submitted to short-term insulin-induced hypoglycaemia

Gluconeogenesis and ketogenesis of in situ rat perfused liver submitted to short-term insulin-induced hypoglycaemia (IIH) were investigated. For this purpose, 24-h fasted rats that received intraperitoneal (ip) regular insulin (1.0 U kg(-1)) or saline were compared. The studies were performed 30 min after insulin (IIH group) or saline (COG group) injection. For gluconeogenesis studies, livers from the IIH and COG groups were perfused with increasing concentrations (from basal blood concentrations until saturating concentration) of glycerol, L-lactate (Lac) or pyruvate (Pyr). Livers of the IIH group showed maintained efficiency to produce glucose from glycerol and higher efficiency to produce glucose from Lac and Pyr. In agreement with these results the oral administration of glycerol (100 mg kg(-1)), Lac (100 mg kg(-1)), Pyr (100 mg kg(-1)) or glycerol (100 mg kg(-1)) + Lac (100 mg kg(-1)) + Pyr (100 mg kg(-1)) promoted glycaemia recovery. It can be inferred that the increased portal availability of Lac, Pyr and glycerol could help glycaemia recovery by a mechanism mediated, partly at least, by a maintained (glycerol) or increased (Lac and Pyr) hepatic efficiency to produce glucose. Moreover, in spite of the fact that insulin inhibits ketogenesis, the capacity of the liver to produce ketone bodies from octanoate during IIH was maintained.

Vagal cooling and concomitant portal norepinephrine infusion do not reduce net hepatic glucose uptake in conscious dogs

We examined the role of efferent neural signaling in regulation of net hepatic glucose uptake (NHGU) in two groups of conscious dogs with hollow perfusable coils around their vagus nerves, using tracer and arteriovenous difference techniques. Somatostatin, intraportal insulin and glucagon at fourfold basal and basal rates, and intraportal glucose at 3.8 mg.kg(-1).min(-1) were infused continuously. From 0 to 90 min [period 1 (P1)], the coils were perfused with a 37 degrees C solution. During period 2 [P2; 90-150 min in group 1 (n = 3); 90-180 min in group 2 (n = 6)], the coils were perfused with -15 degrees C solution to eliminate vagal signaling, and the coils were subsequently perfused with 37 degrees C solution during period 3 (P3). In addition, group 2 received an intraportal infusion of norepinephrine at 16 ng.kg(-1).min(-1) during P2. The effectiveness of vagal suppression was demonstrated by the increase in heart rate during P2 (111 +/- 17, 167 +/- 16, and 105 +/- 13 beats/min in group 1 and 71 +/- 6, 200 +/- 11, and 76 +/- 6 beats/min in group 2 during P1-P3, respectively) and by prolapse of the third eyelid during P2. Arterial plasma glucose, insulin, and glucagon concentrations; hepatic blood flow; and hepatic glucose load did not change significantly during P1-P3. NHGU during P1-P3 was 2.7 +/- 0.4, 4.1 +/- 0.6, and 4.0 +/- 1.2 mg.kg(-1).min(-1) in group 1 and 5.0 +/- 0.9, 5.6 +/- 0.7, and 6.1 +/- 0.9 mg.kg(-1).min(-1) in group 2 (not significant among periods). Interruption of vagal signaling with or without intraportal infusion of norepinephrine to augment sympathetic tone did not suppress NHGU during portal glucose delivery, suggesting the portal signal stimulates NHGU independently of vagal efferent flow.

Time sequence of changes in the responsiveness of glycogen breakdown to adrenergic agonists in perfused liver of rats with insulin-induced hypoglycemia

The time-course changes of the responsiveness of glycogen breakdown to alpha- and ss-adrenergic agonists during insulin-induced hypoglycemia (IIH) were investigated. Blood glucose levels were decreased prior to the alteration in the hepatic responsiveness to adrenergic agonists. The activation of hepatic glucose production and glycogenolysis by phenylephrine (2 microM) and isoproterenol (20 microM) was decreased in IIH. The changes in the responsiveness of glycogen catabolism were first observed for isoproterenol and later for phenylephrine. Hepatic ss-adrenergic receptors showed a higher degree of adrenergic desensitization than did alpha-receptors. Liver glycogen synthase activity, glycogen content and the catabolic effect of dibutyryl cyclic AMP (the ss-receptor second messenger) were not affected by IIH.

Impaired stimulation of intestinal glucose absorption via hepatoenteral nerves in streptozotocin-diabetic rats

In an ex situ organ perfusion system, that of the isolated nonrecirculating joint perfusion of rat small intestine and liver, insulin infused into the portal vein increased intestinal glucose absorption. This insulin action against the bloodstream can be blocked by TTX, indicating a propagation of the insulin signal via hepatoenteral nerves, which conforms with previous studies with atropine and carbachol. Insulin action could also be mimicked by dibutyryl cAMP (DBcAMP) acting directly on the absorptive enterocytes. Because autonomic neuropathy is a common late complication of diabetes mellitus, the possible impairment of these nerves in the diabetic state was studied in streptozotocin-diabetic rats. In the isolated joint intestine-liver perfusion, glucose was applied as a bolus into the lumen; its absorption was measured in the portal vein. In 5-day diabetic as well as in control rats, portal insulin, arterial carbachol, and arterial DBcAMP increased intestinal glucose absorption. In 3-mo diabetic rats portal insulin and arterial carbachol failed to stimulate glucose absorption, whereas arterial DBcAMP still did so, indicating an undisturbed function of the absorptive enterocytes. The lack of an effect of portal insulin and arterial carbachol and the unchanged action of DBcAMP in the chronically diabetic rats indicated that the signaling chain via the hepatoenteral nerves was impaired, which is in line with a diabetic neuropathy.

Impaired glucose sensing by intrahepatic, muscarinic nerves for an insulin-stimulated hepatic glucose uptake in streptozotocin-diabetic rats

Insulin-induced net hepatic glucose uptake depends on the sensing by muscarinic, intrahepatic nerves of a glucose concentration gradient between portal vein and hepatic artery. The function of these intrahepatic nerves was examined in streptozotocin-diabetic rats. In the presence of the glucose gradient insulin induced net glucose uptake in isolated perfused livers from control and acutely diabetic but not from chronically diabetic animals. The neurotransmitter acetylcholine still mimicked the existence of the gradient, excluding a metabolic impairment of livers of chronically diabetic animals. The impairment of the intrahepatic nerves due to diabetic neuropathy could contribute to postprandial hyperglycemia in diabetes mellitus.

Glucokinase is located in secretory granules of pancreatic D-cells

We immunohistochemically examined the distribution of glucokinase in rat pancreatic islets. Glucokinase immunoreactivity under light microscopy was detected in the cytoplasm of somatostatin cells as well as in that of insulin cells. No specific immunoreactivity was detected in glucagon and pancreatic polypeptide cells. In somatostatin cells, glucokinase immunoreactivity was located by electron microscopy exclusively within secretory granules.

Polyarteritis nodosa-like inflammatory vascular changes in the pancreas and mesentery of rats treated with streptozotocin and nicotinamide

Pancreatic endocrine tumours and inflammatory vascular changes resembling those of human polyarteritis nodosa occurred simultaneously in male Holtzman rats after a single injection of streptozotocin and two injections of nicotinamide. The histomorphological appearance of the arterioles resembled that seen in vascular diseases of immunopathogenetic origin. The proximity of the vascular lesions to the hormone-producing tumour suggested that the tumour-related hormone production induced the development of the panarteritis. Streptozotocin-nicotinamide-induced vasculitis in rats was histologically similar to human polyarteritis nodosa. It may therefore provide a good animal model for the human disease.