Impaired myocellular Ca2+ cycling in protein phosphatase PP2A-B56α knockout mice is normalized by β-adrenergic stimulation

The activity of protein phosphatase 2A (PP2A) is determined by the expression and localization of the regulatory B-subunits. PP2A-B56α is the dominant isoform of the B′-family in the heart. Its role in regulating the cardiac response to β-adrenergic stimulation is not yet fully understood. We therefore generated mice deficient in B56α to test the functional cardiac effects in response to catecholamine administration versus corresponding WT mice. We found the decrease in basal PP2A activity in hearts of KO mice was accompanied by a counter-regulatory increase in the expression of B′ subunits (β and γ) and higher phosphorylation of sarcoplasmic reticulum Ca²⁺ regulatory and myofilament proteins. The higher phosphorylation levels were associated with enhanced intraventricular pressure and relaxation in catheterized KO mice. In contrast, at the cellular level, we detected depressed Ca²⁺ transient and sarcomere shortening parameters in KO mice at basal conditions. Consistently, the peak amplitude of the L-type Ca²⁺ current was reduced and the inactivation kinetics of I_CaL were prolonged in KO cardiomyocytes. However, we show β-adrenergic stimulation resulted in a comparable peak amplitude of Ca²⁺ transients and myocellular contraction between KO and WT cardiomyocytes. Therefore, we propose higher isoprenaline-induced Ca²⁺ spark frequencies might facilitate the normalized Ca²⁺ signaling in KO cardiomyocytes. In addition, the application of isoprenaline was associated with unchanged L-type Ca²⁺ current parameters between both groups. Our data suggest an important influence of PP2A-B56α on the regulation of Ca²⁺ signaling and contractility in response to β-adrenergic stimulation in the myocardium.

Phenotyping of Mice with Heart Specific Overexpression of A2A-Adenosine Receptors: Evidence for Cardioprotective Effects of A2A-Adenosine Receptors

Background: Adenosine can be produced in the heart and acts on cardiac adenosine receptors. One of these receptors is the A_2A-adenosine receptor (A_2A-AR).

Methods and Results: To better understand its role in cardiac function, we generated and characterized mice (A_2A-TG) which overexpress the human A_2A-AR in cardiomyocytes. In isolated atrial preparations from A_2A-TG but not from WT, CGS 21680, an A_2A-AR agonist, exerted positive inotropic and chronotropic effects. In ventricular preparations from A_2A-TG but not WT, CGS 21680 increased the cAMP content and the phosphorylation state of phospholamban and of the inhibitory subunit of troponin in A_2A-TG but not WT. Protein expression of phospholamban, SERCA, triadin, and junctin was unchanged in A_2A-TG compared to WT. Protein expression of the α-subunit of the stimulatory G-protein was lower in A_2A-TG than in WT but expression of the α-subunit of the inhibitory G-protein was higher in A_2A-TG than in WT. While basal hemodynamic parameters like left intraventricular pressure and echocardiographic parameters like the systolic diameter of the interventricular septum were higher in A_2A-TG than in WT, after β-adrenergic stimulation these differences disappeared. Interestingly, A_2A-TG hearts sustained global ischemia better than WT.

Conclusion: We have successfully generated transgenic mice with cardiospecific overexpression of a functional A_2A-AR. This receptor is able to increase cardiac function per se and after receptor stimulation. It is speculated that this receptor may be useful to sustain contractility in failing human hearts and upon ischemia and reperfusion.

Sphingosine-1-Phosphate Receptor 1 Regulates Cardiac Function by Modulating Ca2+ Sensitivity and Na+/H+ Exchange and Mediates Protection by Ischemic Preconditioning

Background

Sphingosine‐1‐phosphate plays vital roles in cardiomyocyte physiology, myocardial ischemia–reperfusion injury, and ischemic preconditioning. The function of the cardiomyocyte sphingosine‐1‐phosphate receptor 1 (S1P₁) in vivo is unknown.

Methods and Results

Cardiomyocyte‐restricted deletion of S1P₁ in mice (S1P₁^αMHCCre) resulted in progressive cardiomyopathy, compromised response to dobutamine, and premature death. Isolated cardiomyocytes from S1P₁^αMHCCre mice revealed reduced diastolic and systolic Ca²⁺ concentrations that were secondary to reduced intracellular Na⁺ and caused by suppressed activity of the sarcolemmal Na⁺/H⁺ exchanger NHE‐1 in the absence of S1P₁. This scenario was successfully reproduced in wild‐type cardiomyocytes by pharmacological inhibition of S1P₁ or sphingosine kinases. Furthermore, Sarcomere shortening of S1P₁^αMHCCre cardiomyocytes was intact, but sarcomere relaxation was attenuated and Ca²⁺ sensitivity increased, respectively. This went along with reduced phosphorylation of regulatory myofilament proteins such as myosin light chain 2, myosin‐binding protein C, and troponin I. In addition, S1P₁ mediated the inhibitory effect of exogenous sphingosine‐1‐phosphate on β‐adrenergic–induced cardiomyocyte contractility by inhibiting the adenylate cyclase. Furthermore, ischemic precondtioning was abolished in S1P₁^αMHCCre mice and was accompanied by defective Akt activation during preconditioning.

Conclusions

Tonic S1P₁ signaling by endogenous sphingosine‐1‐phosphate contributes to intracellular Ca²⁺ homeostasis by maintaining basal NHE‐1 activity and controls simultaneously myofibril Ca²⁺ sensitivity through its inhibitory effect on adenylate cyclase. Cardioprotection by ischemic precondtioning depends on intact S1P₁ signaling. These key findings on S1P₁ functions in cardiac physiology may offer novel therapeutic approaches to cardiac diseases.

[The EHEC O104:H4 outbreak in Germany 2011 - lessons learned?!]

The EHEC O104:H4 outbreak 2011 in Germany provided numerous insights into the recognition and control of such epidemic situations. Food-borne outbreaks and their related dynamics may lead to a critical burden of disease and an eventual capacity overload of the medical care system. Possible difficulties in the microbiological diagnostics of new or significantly altered infectious agents may result in a delayed detection of the outbreak as well as the launching of interventional measures. Besides an early notification of the local public health office by the affected institutions, in which a complete electronic procedure and additional sentinel or surveillance instruments (e. g., in emergency departments of hospitals) may be of great help, an interdisciplinary cooperation of the local public health and food safety agencies is the key to an effective outbreak control. Corresponding organizations on the state and federal level should support the investigation process by microbiological diagnostics and advanced epidemiological analysis as well as examination of the food chains. Finally, successful crisis communication relies on "speaking with one voice" (not necessarily one person). Immediate, transparent, appropriate and honest information of the general public concerning the reasons, consequences and (counter-) measures of a crisis are the best means to keep the trust of the population and to counteract the otherwise inevitable speculations.

Critical role of transcription factor cyclic AMP response element modulator in beta1-adrenoceptor-mediated cardiac dysfunction

BACKGROUND

Chronic stimulation of the beta(1)-adrenoceptor (beta(1)AR) plays a crucial role in the pathogenesis of heart failure; however, underlying mechanisms remain to be elucidated. The regulation by transcription factors cAMP response element-binding protein (CREB) and cyclic AMP response element modulator (CREM) represents a fundamental mechanism of cyclic AMP-dependent gene control possibly implicated in beta(1)AR-mediated cardiac deterioration.

METHODS AND RESULTS

We studied the role of CREM in beta(1)AR-mediated cardiac effects, comparing transgenic mice with heart-directed expression of beta(1)AR in the absence and presence of functional CREM. CREM inactivation protected from cardiomyocyte hypertrophy, fibrosis, and left ventricular dysfunction in beta(1)AR-overexpressing mice. Transcriptome and proteome analysis revealed a set of predicted CREB/CREM target genes including the cardiac ryanodine receptor, tropomyosin 1alpha, and cardiac alpha-actin as altered on the mRNA or protein level along with the improved phenotype in CREM-deficient beta(1)AR-transgenic hearts.

CONCLUSIONS

The results imply the regulation of genes by CREM as an important mechanism of beta(1)AR-induced cardiac damage in mice.

Cardiomyocyte-specific inactivation of transcription factor CREB in mice

The transcription factor cAMP response element (CRE)-binding protein (CREB, Creb1) plays a critical role in regulating gene expression in response to activation of the cAMP-dependent signaling pathway, which is implicated in the pathophysiology of heart failure. Using the Cre-loxP system, we generated mice with a cardiomyocyte-specific inactivation of CREB and studied in this model whether CREB is critical for cardiac function. CREB-deficient mice were viable and displayed neither changes in cardiac morphology nor alterations of basal or isoproterenol-stimulated left ventricular function in vivo or of important cardiac regulatory proteins. Since CREB was proposed as a negative regulator of cardiomyocyte apoptosis by enhancing the expression of the antiapoptotic protein Bcl-2, we analyzed the fragmentation of DNA, the activity of caspases 3/7 and the expression of Bcl-2 and did not observe any differences between CREB-deficient and CREB-normal hearts. Our results suggest that the presence of CREB is not critical for normal cardiac function in mice.

Glucagon-stimulated but not isoproterenol-stimulated glucose formation inhibition by interleukin-6 in primary cultured rat hepatocytes

During prolonged sepsis, impairment of glucose supply by the liver leads to hypoglycemia. Our aim was to investigate whether proinflammatory cytokine interleukin-6, a major mediator of the hepatic acute phase reaction, could contribute to this impairment by inhibiting hepatic glucose production stimulated by glucagon or isoproterenol in rat hepatocytes. Interleukin-6 inhibited the stimulation of glucose formation from glycogen by glucagon but not by isoproterenol in cultured rat hepatocytes. This was confirmed in the perfused rat liver. In cultured hepatocytes, the increase in cyclic adenosine-3',5'-monophosphate formation by glucagon was inhibited by interleukin-6, which was probably due to attenuation of glucagon binding to the glucagon receptor. The increase in cyclic adenosine-3',5'-monophosphate stimulated by isoproterenol was not affected by interleukin-6. However, the cytokine inhibited both expression of the key gluconeogenic control enzyme, phosphoenolpyruvate carboxykinase, stimulated by glucagon and isoproterenol. Thus, while increased glucose demand during the acute-phase reaction might initially be accomplished by catecholamine-mediated stimulation of glucose formation from glycogen, inhibition of gluconeogenesis by interleukin-6 may contribute to the impairment of glucose homeostasis during the prolonged acute phase reaction.

Mice without the regulator gene Rsc1A1 exhibit increased Na+-D-glucose cotransport in small intestine and develop obesity

The product of the intronless single copy gene RSC1A1, named RS1, is an intracellular 617-amino-acid protein that is involved in the regulation of the Na(+)-d-glucose cotransporter SGLT1. We generated and characterized RS1 knockout (RS1(-/-) mice. In the small intestines of RS1(-/-) mice, the SGLT1 protein was up-regulated sevenfold compared to that of wild-type mice but was not changed in the kidneys. The up-regulation of SGLT1 was posttranscriptional. Small intestinal d-glucose uptake measured in jointly perfused small bowel and liver was increased twofold compared to that of the wild-type, with increased peak concentrations of d-glucose in the portal vein. At birth, the weights of RS1(-/-) and wild-type mice were similar. At the age of 3 months, male RS1(-/-) mice had 5% higher weights and 15% higher food intakes, whereas their energy expenditures and serum leptin concentrations were similar to those of wild-type mice. At the age of 5 months, male and female RS1(-/-) mice were obese, with 30% increased body weight, 80% increased total fat, and 30% increased serum cholesterol. At this age, serum leptin was increased, whereas food intake was the same as for wild-type mice. The data suggest that the removal of RS1 leads to leptin-independent up-regulation of food intake, which causes obesity.

Na(+)-D-glucose cotransporter in muscle capillaries increases glucose permeability

By immunohistochemistry, we demonstrated the localization of the Na(+)-D-glucose cotransporter SGLT1 in capillaries of rat heart and skeletal muscle, but not in capillaries of small intestine and submandibular gland. mRNA of SGLT1 was identified in skeletal muscle and primary cultured coronary endothelial cells. The functional relevance of SGLT1 for glucose transport across capillary walls in muscle was tested by measuring the extraction of D-glucose from the perfusate during non-recirculating perfusion of isolated rat hindlimbs. In this model, D-glucose extraction from the perfusate is increased by insulin which accelerates D-glucose uptake into myocytes by increasing the concentration of glucose transporter GLUT4 in the plasma membrane. The insulin-induced increase of D-glucose extraction from the perfusate was abolished after blocking SGLT1 with the specific inhibitor phlorizin. The data show that SGLT1 in capillaries of skeletal muscle is required for the action of insulin on D-glucose supply of myocytes.

Normal kinetics of intestinal glucose absorption in the absence of GLUT2: evidence for a transport pathway requiring glucose phosphorylation and transfer into the endoplasmic reticulum

Glucose is absorbed through the intestine by a transepithelial transport system initiated at the apical membrane by the cotransporter SGLT-1; intracellular glucose is then assumed to diffuse across the basolateral membrane through GLUT2. Here, we evaluated the impact of GLUT2 gene inactivation on this transepithelial transport process. We report that the kinetics of transepithelial glucose transport, as assessed in oral glucose tolerance tests, was identical in the presence or absence of GLUT2; that the transport was transcellular because it could be inhibited by the SGLT-1 inhibitor phlorizin, and that it could not be explained by overexpression of another known glucose transporter. By using an isolated intestine perfusion system, we demonstrated that the rate of transepithelial transport was similar in control and GLUT2(-/-) intestine and that it was increased to the same extent by cAMP in both situations. However, in the absence, but not in the presence, of GLUT2, the transport was inhibited dose-dependently by the glucose-6-phosphate translocase inhibitor S4048. Furthermore, whereas transport of [(14)C]glucose proceeded with the same kinetics in control and GLUT2(-/-) intestine, [(14)C]3-O-methylglucose was transported in intestine of control but not of mutant mice. Together our data demonstrate the existence of a transepithelial glucose transport system in GLUT2(-/-) intestine that requires glucose phosphorylation and transfer of glucose-6-phosphate into the endoplasmic reticulum. Glucose may then be released out of the cells by a membrane traffic-based pathway similar to the one we previously described in GLUT2-null hepatocytes.

Dilated bile canaliculi and attenuated decrease of nerve-dependent bile secretion in connexin32-deficient mouse liver

Gap junction channels in the rodent liver are composed of connexin26 (Cx26) and connexin32 (Cx32) proteins. Gap junctional intercellular communication in the mouse liver enhances the effects of hormonal or sympathetic stimulation of glucose release from glycogen stores. To determine whether contraction of bile canaliculi and bile secretion are dependent on the function of gap junction channels, we compared wild-type and connexin32-deficient mice. Confocal laser scanning microscopy of the wild-type mouse liver confirmed the close association of connexin26 and -32 proteins with the zona occludens-1 protein and actin filaments of the bile canaliculi. The decrease of bile flow after electrical stimulation of sympathetic nerves in the perfused liver was attenuated in the Cx32-deficient liver compared with wild-type controls. The amount of secreted bile, however, was similar in wild-type and Cx32-deficient livers. Furthermore, Cx32-deficient mice exhibited dilated bile canaliculi, suggesting that the contraction of bile canaliculi could be impaired in these animals.

Stimulation by portal insulin of intestinal glucose absorption via hepatoenteral nerves and prostaglandin E2 in the isolated, jointly perfused small intestine and liver of the rat

Insulin infused into the portal vein acutely enhanced intestinal glucose and galactose absorption via the sodium-dependent glucose cotransporter-1 in the isolated, jointly perfused small intestine and liver of the rat. Atropine and tetrodotoxin infused into the superior mesenteric artery completely prevented the portal insulin-dependent increase in intestinal glucose absorption, and carbachol caused an increase similar to that of portal insulin. Thus, a signal was transmitted against the bloodstream in a retrograde direction from the portal vein to the small intestine via hepatoenteral cholinergic nerves. The intracellular messenger in the enterocytes was cAMP, and the link between the muscarinic receptors, which do not increase cAMP concentrations, and adenylate cyclase was found to be prostaglandin E2.

Impaired stimulation of intestinal glucose absorption by portal insulin via hepatoenteral nerves in chronically ethanol-intoxicated rats

In the isolated, jointly perfused small intestine and liver of rats insulin, infused into the portal vein, induced an increase in intestinal glucose absorption via hepatoenteral cholinergic nerves. The possible loss of function of these nerves due to ethanol-induced neuropathy was investigated with 6 weeks ethanol-fed rats. Portal insulin or arterial carbachol failed to increase intestinal glucose absorption but cAMP still did so. The intact stimulatory effect of cAMP indicated an undisturbed capacity of the enterocytes. The loss of action of portal insulin and of arterial carbachol can be explained by the impairment of the hepatoenteral nerves in line with an ethanol-induced neuropathy.

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.

Role of hepatic, intrahepatic and hepatoenteral nerves in the regulation of carbohydrate metabolism and hemodynamics of the liver and intestine

The liver as an effector organ is the major glucose reservoir, the utilization of which is controlled by hormones but also by hepatic sympathetic nerves. The liver as a sensory organ detects a glucose concentration gradient between the hepatic artery and the portal vein by intrahepatic sensory-effector nerves, generating a cholinergic signal for an insulin-dependent net hepatic glucose uptake. The liver senses the insulin concentration by hepatoenteral sensory-effector nerves, generating a cholinergic signal to increase glucose absorption in the intestine and thus its coordinated utilization in liver, muscle and adipose tissue.

Acute increase, stimulated by prostaglandin E2, in glucose absorption via the sodium dependent glucose transporter-1 in rat intestine

BACKGROUND/AIMS

Acute stimulation by cAMP of the sodium dependent glucose cotransporter SGLT1 has previously been shown. As prostaglandin E2 (PGE2) increases intracellular cAMP concentrations via its receptor subtypes EP2R and EP4R, it was investigated whether PGE2 could enhance intestinal glucose absorption.

METHODS

The action of PGE2 on carbohydrate absorption in the ex situ perfused rat small intestine and on 3-O-[14C]methylglucose uptake in isolated villus tip enterocytes was determined. Expression of mRNA for the PGE2 receptor subtypes 1-4 was assayed in enterocytes by reverse transcriptase polymerase chain reaction (RT-PCR).

RESULTS

In the perfused small intestine, PGE2 acutely increased absorption of glucose and galactose, but not fructose (which is not a substrate for SGLT1); in isolated enterocytes it stimulated 3-O-[14C]methylglucose uptake. The 3-O-[14C]methylglucose uptake could be inhibited by the cAMP antagonist RpcAMPS and the specific inhibitor of SGLT1, phlorizin. High levels of EP2R mRNA and EP4R mRNA were detected in villus tip enterocytes.

CONCLUSION

PGE2 acutely increased glucose and galactose absorption by the small intestine via the SGLT1, with cAMP serving as the second messenger. PGE2 acted directly on the enterocytes, as the stimulation was still observed in isolated enterocytes and RT-PCR detected mRNA for the cAMP-increasing PGE2 receptors EP2R and EP4R.

Connexin 32 gap junctions enhance stimulation of glucose output by glucagon and noradrenaline in mouse liver

Gap junctions connect neighboring cells via intercellular channels composed of connexins (Cx). Connexin 32 (Cx32) is the main connexin in hepatocytes. Gap junctions propagate a signal from periportal to perivenous hepatocytes generated by electrical stimulation of sympathetic liver nerves. Therefore, it was the aim of this study to examine the involvement of hepatocellular gap junctions in hormonal regulation. In perfused livers from wild-type mice and Cx32-deficient mice, the stimulation of glucose release by varying noradrenaline and glucagon concentrations was investigated. At saturating hormone concentrations, glucose release was the same in wild-type and Cx32-deficient livers. However, glucose output was significantly smaller in Cx32-deficient than wild-type livers at half-maximally effective hormone concentrations. Because the two hormones circulate at less than half-saturating concentrations and because they are degraded during passage of blood through the liver, they lose efficiency from the periportal to the perivenous zone. In wild-type livers, this decrease in efficiency can be partially compensated by intercellular signal propagation through gap junctions, resulting in higher hormone actions than in Cx32-deficient livers. It is concluded that gap junctions are not only involved in intercellular propagation of nervous, but also of hormonal signals from periportal to perivenous hepatocytes.

Enteric glucagon 37 rather than pancreatic glucagon 29 stimulates glucose absorption in rat intestine

BACKGROUND & AIMS

Glucose and galactose are absorbed by the small intestine via the sodium-dependent glucose transporter 1 (SGLT1) and fructose via the facilitated glucose transporter 5. A stimulatory effect of enteric glucagon 37 and pancreatic glucagon 29 on intestinal carbohydrate absorption has been shown. However, only glucagon 37 is released after nutrient uptake and would thus fit into a regulatory circuit of nutrient-dependent hormone release that enhances carbohydrate absorption. Therefore, the aim of the present study was to evaluate whether glucagon 37 rather than glucagon 29 is the physiological stimulus of intestinal glucose absorption.

METHODS

We examined the effects of glucagon 37, glucagon 29, and dibutyryl adenosine 3',5'-cyclic monophosphate on intestinal carbohydrate absorption and hepatic glucose output in the isolated perfused small intestine, isolated enterocytes, and isolated perfused liver of the rat.

RESULTS

Reciprocal dose-response curves for the effects of the two hormones in intestine and liver were demonstrated: glucagon 37 was one order of magnitude more potent than glucagon 29 in increasing intestinal absorption of glucose via the SGLT1. In contrast, glucagon 29 more efficiently stimulated hepatic glucose release. The intracellular messenger was shown to be adenosine 3',5'-cyclic monophosphate.

CONCLUSIONS

Glucagon 37 rather than glucagon 29 is the physiological stimulus of intestinal glucose absorption and exerts its effect via a specific glucagon 37 receptor.

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.

A new role for enteric glucagon-37: acute stimulation of glucose absorption in rat small intestine

Glucagon-37 is secreted by intestinal L-cells following carbohydrate uptake. It is known to inhibit gastric acid secretion (hence also named oxyntomodulin) and appears to increase intracellular cyclic AMP concentrations. Since cyclic AMP could enhance intestinal glucose absorption, a possible stimulatory effect of glucagon-37 on glucose transport was examined. Glucagon-37 acutely increased glucose absorption in the isolated, vascularly perfused small intestine and in isolated enterocytes of the rat. In these cells the stimulation by glucagon-37 could be completely blocked by the cAMP antagonist Rp-cAMPS and was therefore mediated by cAMP. The stimulation of intestinal glucose absorption by glucagon-37 appears to be a major new physiological function.

Sensing by intrahepatic muscarinic nerves of a portal-arterial glucose concentration gradient as a signal for insulin-dependent glucose uptake in the perfused rat liver

In vivo, insulin increases net hepatic glucose uptake efficiently only in the presence of a portal-arterial glucose gradient. In isolated perfused rat livers supplied with a glucose gradient (portal 10 mM/arterial 5 mM) insulin-induced glucose uptake was blocked by atropine; in livers not supplied with the gradient (portal = arterial 5 mM) insulin-dependent glucose uptake was elicited by acetylcholine. Apparently, the gradient was sensed and transformed into a metabolic signal by intrahepatic nerves, releasing acetylcholine to muscarinic receptors.

Impairment of metabolic hepatic nerve action by chronic but not acute ethanol intoxication studied in isolated perfused rat liver

BACKGROUND/AIMS

Liver carbohydrate metabolism and blood flow are regulated by hepatic nerves and hormones such as glucagon, insulin or catecholamines. Acute and chronic application of alcohol are known to depress the function of central and peripheral nerves. The extent of inhibition of the autonomic nervous system is not well characterized; thus, the possible impairment of hepatic nerve function by acute and chronic application of ethanol was investigated.

METHODS

Rat livers were perfused simultaneously via both the portal vein and hepatic artery. Hepatic nerves were stimulated electrically for 2 min (20 Hz, 20 V, 2 ms). As a control, noradrenaline (1 microM) was infused into the portal vein for 2 minutes.

RESULTS

During acute application of ethanol in portal concentrations of 50, 150 and 300 mM, which elevated basal glucose release, stimulation of hepatic nerves as well as portal noradrenaline infusion caused the same increase in glucose output and decrease in portal and arterial flow as in controls. Following chronic application of ethanol by feeding rats the Lieber-DeCarli liquid diet containing 5% (v/v) ethanol for 4 and 6 weeks, only nerve stimulation caused a significantly reduced enhancement of glucose output (50%, p < 0.025), whereas portal noradrenaline was as effective as in controls. Noradrenaline overflow was significantly reduced following nerve stimulation.

CONCLUSION

The decrease in nerve stimulation-dependent glucose output and noradrenaline overflow in chronically ethanol fed rats indicates an impaired function of hepatic nerves.

Defective propagation of signals generated by sympathetic nerve stimulation in the liver of connexin32-deficient mice

The gap junctional protein connexin32 is expressed in hepatocytes, exocrine pancreatic cells, Schwann cells, and other cell types. We have inactivated the connexin32 gene by homologous recombination in the mouse genome and have generated homozygous connexin32-deficient mice that were viable and fertile but weighed on the average approximately 17% less than wild-type controls. Electrical stimulation of sympathetic nerves in connexin32-deficient liver triggered a 78% lower amount of glucose mobilization from glycogen stores, when compared with wild-type liver. Thus, connexin32-containing gap junctions are essential in mouse liver for maximal intercellular propagation of the noradrenaline signal from the periportal (upstream) area, where it is received from sympathetic nerve endings, to perivenous (downstream) hepatocytes. In connexin32-defective liver, the amount of connexin26 protein expressed was found to be lower than in wild-type liver, and the total area of gap junction plaques was approximately 1000-fold smaller than in wild-type liver. In contrast to patients with connexin32 defects suffering from X chromosome-linked Charcot-Marie-Tooth disease (CMTX) due to demyelination in Schwann cells of peripheral nerves, connexin32-deficient mice did not show neurological abnormalities when analyzed at 3 months of age. It is possible, however, that they may develop neurodegenerative symptoms at older age.

Acute increase by portal insulin in intestinal glucose absorption via hepatoenteral nerves in the rat

BACKGROUND & AIMS

Insulin exerts a strict short-term control of glucose disappearance by glucose storage as well as degradation in the liver and peripheral insulin target tissues, but an acute control of glucose appearance by glucose absorption in the intestine is as yet unknown. The aim of the present study was to evaluate, whether insulin acutely modulates intestinal glucose absorption.

METHODS

In the isolated, nonrecirculating joint perfusion of the small bowel and liver of the rat via the celiac trunc and the superior mesenteric artery, glucose absorption was examined without and with infusion of insulin via the portal vein.

RESULTS

Portal insulin enhanced acutely intestinal glucose absorption. This thus far unknown stimulatory effect of portal insulin was dose-dependent and detectable at physiological insulin concentrations. Atropine infused into the superior mesenteric artery completely prevented the insulin-dependent increase in intestinal glucose absorption, and carbachol caused a similar increase as portal insulin.

CONCLUSIONS

Portal insulin dose-dependently generated a signal in the liver or portal vein. This signal was transmitted in a retrograde direction against the blood stream in the portal vein to the small intestine via hepatoenteral muscarinic nerves. This signal markedly increased intestinal glucose absorption.

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

The consequences of autonomic diabetic neuropathy, a common complication of chronic diabetes mellitus, have been studied mainly with regard to heart and stomach function. Since the autonomic nervous system also regulates liver carbohydrate metabolism and haemodynamics via hepatic nerves, it was the purpose of this study to examine the function of hepatic nerves in chronically diabetic rats. Diabetes was induced by i.p. injection of streptozotocin. Rat livers were perfused via both portal vein and hepatic artery. Hepatic nerves were stimulated for 2 min using a platinum electrode placed around the portal vein and the hepatic artery; in an additional stimulation phase noradrenaline was infused into the portal vein. Stimulation of hepatic nerves as well as portal noradrenaline infusion increased hepatic glucose output and reduced flow in control and in acutely (48-h) diabetic animals, which still had almost normal glycogen content. In addition stimulation also caused an overflow of noradrenaline into the caval vein. However, nerve stimulation neither increased glucose output nor decreased flow in 4-month diabetic rats. In these rats noradrenaline overflow was nearly completely abolished and hepatic glycogen content was markedly depleted. Portal noradrenaline infusion in chronically diabetic rats reduced flow to a similar extent as in controls, yet the increase in glucose output was diminished. The lack of nerve stimulation-dependent glucose output, flow reduction and noradrenaline overflow is indicative of a profound loss of function of hepatic autonomic nerves in chronically diabetic rats.

Acute actions of insulin-like growth factor II on glucose metabolism in adult rats

The metabolic potency of recombinant human insulin-like growth factor II was studied in anaesthetized adult rats by obtaining dose-response curves for the hypoglycaemic action and for the stimulation of glucose metabolism during euglycaemic clamping. Compared to insulin, about 50 times higher doses of insulin-like growth factor II were required to result in identical in vivo responses, with half-maximally effective serum concentrations for the stimulation of glucose disposal during clamp studies of about 0.8 and 50 pmol/ml, respectively. A similar difference in potency was observed for the dose-dependent stimulatory actions on glucose metabolism in individual target tissues. Half-maximally effective serum concentrations in the range of 0.8 to 3.0 pmol/ml for insulin and of 40 to 70 pmol/ml for insulin-like growth factor II were seen to be required for 2-deoxyglucose uptake, glycogen formation in skeletal muscle and lipogenesis in epididymal fat. Maximal responses were identical with both peptides. These data suggest that in vivo acute metabolic actions of insulin-like growth factor II on carbohydrate metabolism occurred through insulin receptors.

Mechanism of insulin resistance in CCl4-induced cirrhosis of rats

Insulin action was studied in rats with CCl4/phenobarbital-induced cirrhosis of the liver using the euglycemic hyperinsulinemic clamp technique coupled with isotopic measurement of individual tissue glucose uptake, glycogen formation, and lipogenesis. In cirrhotic rats, dose response curves showed a reduction of insulin-stimulated total body glucose disposal of about 30%. Insulin action on tissue glucose uptake and initial phosphorylation (assessed with [3H]2-deoxyglucose) were unchanged; however, incorporation of [14C]glucose into lipids and particularly into glycogen was reduced substantially (being most pronounced in skeletal muscle and diaphragm) at maximally as well as half-maximally effective serum insulin concentrations during euglycemic clamping. At identical IV insulin infusion rates, steady-state serum insulin concentrations were elevated up to fourfold in cirrhotic animals. Antilipolytic action of insulin was unaltered. These data suggest that the principal metabolic pathway affected in insulin resistance of rats with experimental cirrhosis appeared to be insulin-stimulated glycogen formation in muscle tissues.

In vivo metabolic action of insulin-like growth factor I in adult rats

The acute metabolic actions of insulin-like growth factor I were studied in anaesthetized adult rats and its potency was compared to that of insulin. Following an i.v. bolus injection of insulin-like growth factor I a dose-dependent decrease of blood glucose and serum non-esterified fatty acid concentrations was noted with a potency of about 2% that of insulin. Stimulation of total body glucose disposal during euglycaemic clamping required approximately 50 times higher insulin-like growth factor I serum concentrations to achieve an identical half-maximal response. A similar difference in potency was observed for the stimulatory action on 2-deoxyglucose uptake and on glycogen formation in skeletal muscle. Lipogenesis in epididymal fat pads was increased dose-dependently by both hormones requiring approximately 30 times higher half-maximally effective serum concentrations of insulin-like growth factor I. These data demonstrate that insulin-like growth factor I exerted acute insulin-like metabolic actions in vivo with low potency. These effects were probably mediated via insulin receptors. A preferential stimulation of glucose metabolism in skeletal muscle was not observed.

In vivo metabolic activity of des-(B26-B30)-insulin-B25-amide and related analogues in the rat

Metabolic potency of des-(B26-B30)-insulin-B25-amide, [TyrB25]des- (B26-B30)-insulin-B25-amide and [HisB25]des-(B26-B30)-insulin-B25-amide was studied in anaesthetized rats. Compared to insulin, full potency for des-(B26-B30)-insulin-B25-amide and an enhanced potency for both substituted analogues has been described previously on rat adipocytes in vitro. Hypoglycaemic effects following i.v. injection of all of these analogues were almost identical to those of native insulin with a half-maximal effective dose of approximately 3 nmol.kg-1. Stimulation of glucose metabolism during euglycaemic hyperinsulin-/analogueaemic clamp studies was indistinguishable from that of the native hormone with a maximal stimulation of approximately 19 mg.kg-1.min-1 and half-maximal effective hormone concentrations of approximately 1 pmol.ml-1. Analogue action on individual peripheral tissues estimated by the uptake of 2-deoxyglucose as well as stimulation of lipogenesis in epididymal fat was not different to that of insulin. These data demonstrate that C-terminal amidation of des-(B26-B30)-insulin results in a shortened molecule with full in vivo metabolic potency. When substituting phenylalanine in position B25 by tyrosine or histidine, the insulin-identical potency is preserved.