Insulin's effect on bile flow and lipid excretion during euglycemia and hypoglycemia

Dynamics and determinants of human plasma bile acid profiles during dietary challenges

In recent years, bile acids (BA) have received great interest due to their pleiotropic biological activity and the presence of plasma membrane-bound and nuclear receptors. Moreover, BA in blood have been identified by metabolite screening approaches as biomarkers that are associated with various diseases and even with a human longevity phenotype. With the growing interest in the microbiota contribution to the health-disease trajectory, BA that undergo deconjugation and other modifications by bacteria in the large intestine have become a prime target as a microbiome diversity modifier. We here profiled BA by a quantitative and a semiquantitative approach in 15 healthy and phenotypically very similar young individuals for over a 36-h fasting period, an oral glucose tolerance test (OGTT), and an oral lipid tolerance test (OLTT). We demonstrate a remarkable heterogeneity of the responses and describe the different dynamics of the plasma changes that likely originate from different routes by which BA enters the peripheral blood, and that may represent a direct secretion from the liver into the blood and a route that reaches the blood as a spill-over after passing from the gallbladder through the intestine and the portal system. We discuss the finding that an individual transport process involved in the passage of BA could be a critical determinant in the kinetics of plasma appearance and the overall phenotypic variability found.

Macronutrients and insulin resistance in cholesterol gallstone disease

Cholelithiasis is a major source of digestive morbidity worldwide. Cholesterol stones account for the majority of gallstones in the United States and other Western countries. The pathogenesis of cholesterol gallstone disease is multifactorial with key factors including cholesterol supersaturation of bile, altered biliary motility, and nucleation and growth of cholesterol crystals. Increasing evidence suggests that many, but not all, causative factors of cholesterol gallstones are related to insulin resistance which, in association with obesity, has reached an epidemic level worldwide. Experimental studies show that hyperinsulinemia, a key feature of insulin resistance, may cause increased hepatic cholesterol secretion and cholesterol supersaturation of bile and gallbladder dysmotility, and thereby may enhance gallstone formation. Insulin resistance syndrome can be modified by environmental factors, including dietary factors. The impact of diet on insulin sensitivity is mediated by both dietary composition and its energy content. The contribution of specific dietary elements to the prevalence and incidence of cholesterol gallstone disease has been explored in animal and human studies. There is considerable evidence to suggest that different types of fatty acids, independent of the total amount of fat consumption, affect insulin sensitivity and cholesterol gallstone disease differently. The effects of salt intake, consumption of protein and carbohydrates, and alcohol drinking on insulin resistance are controversial. Additional intervention trials and controlled experimental feeding studies are needed to further clarify these relationships and to provide useful prophylactic and therapeutic strategies.

Pioglitazone Increases Gallbladder Volume in Insulin-Resistant Obese Mice

BACKGROUND

Both obesity and diabetes are associated with an increased incidence of gallstones. Recent animal and human data from our laboratory suggest that insulin resistance is associated with increased gallbladder volume and/or impaired gallbladder emptying. Pioglitazone is a thiazolidinedione that has been shown to improve insulin resistance. Therefore, we tested the hypothesis that pioglitazone would improve insulin resistance, decrease resting gallbladder volume and improve gallbladder response to neurotransmitters in insulin-resistant obese mice fed a 25% carbohydrate diet.

MATERIALS AND METHODS

Twenty eight-week-old insulin-resistant obese (Lep(ob)) mice fed a 25% carbohydrate diet for 4 weeks. Half of the animals had 0.3 g/kg pioglitazone added to their diet. At 12 weeks all animals were fasted and underwent cholecystectomy. Gallbladder volume and weight were measured, and fresh gallbladders were placed in a muscle bath to assess response to acetylcholine (ACh 10(-5)M), neuropeptide Y (NPY 10(-8,-7,-6)M) and cholecystokinin (CCK 10(-10,-9,-8,-7)M). Serum glucose and insulin were measured, and HOMA Index, a measure of insulin resistance, was calculated.

RESULTS

Fasting serum insulin and HOMA Index were significantly decreased (P < 0.03), but gallbladder volume was significantly increased (P < 0.03) in the pioglitazone treated group. Pioglitazone did not alter gallbladder weight or response to ACh, NPY, or CCK.

CONCLUSION

These data suggest that in insulin-resistant obese mice pioglitazone 1) lowers insulin-resistance, 2) increases resting gallbladder volume, and 3) does not alter gallbladder response to neurotransmitters. Therefore, we conclude that pioglitazone, while improving insulin resistance, paradoxically increases gallbladder volume and, thereby, may increase the propensity for gallstone formation by enhancing gallbladder stasis.

The rationale and management of hyperglycemia for in-patients with cardiovascular disease: time for change

There is increasing evidence that aggressive glycemic control for patients admitted into the hospital improves clinical outcomes, especially for patients with cardiovascular disease. There appear to be a variety of mechanisms for this. Although hyperglycemia has been shown to result in poor wound healing and more infectious complications, especially after cardiac surgical procedures, what has become clear is that the treatment of hyperglycemia with i.v. glucose, insulin, and potassium (GIK) results in better clinical outcomes even in patients without diabetes. The mechanisms for this are not year clear, but could be related to the insulin administration, perhaps due to suppression of various cytokines or free fatty acids. The practical use of insulin in these patients requires basic understanding of the use of both i.v. and s.c. insulin. Although there are several appropriate options for both of these routes of administration, it is critical that all caregivers involved in this population's care are knowledgeable about insulin strategies.

Insulin inhibits secretin-induced ductal secretion by activation of PKC alpha and inhibition of PKA activity

Insulin stimulates canalicular bile flow by interaction with hepatocytes. Insulin regulates the function of a number of epithelia through activation and membrane translocation of Ca(2+)-dependent PKC isoforms. No information exists regarding insulin regulation of ductal bile secretion. The aim of the study was to determine the role and mechanisms of action of insulin in the regulation of cholangiocyte secretion in BDL rats. We determined the subcellular localization of insulin receptor in cholangiocytes. We measured the effect of insulin on (1) secretin-stimulated cAMP levels in cholangiocytes and duct expansion in intrahepatic bile duct units (IBDUs) in the absence or presence of BAPTA/AM, H7 or rottlerin and (2) bile flow. We evaluated (1) if insulin effects are associated with activation of PKC alpha and (2) if activation of PKC causes inhibition of secretin-stimulated cAMP levels and PKA activity. We found insulin receptors only in the apical domain of cholangiocytes. Insulin inhibited secretin-induced choleresis and secretin-stimulated cholangiocyte cAMP levels. Insulin inhibited secretin-induced secretion in IBDUs when applied at the basolateral membrane or microinjected into IBDU lumen. Insulin inhibitory effects on cholangiocyte secretion were blocked by BAPTA/AM and H7. Insulin induced activation of PKC alpha, which decreased secretin-stimulated cAMP and PKA activity. In conclusion, insulin inhibited secretin-induced ductal secretion of BDL rats through activation of PKC and inhibition of secretin-stimulated cAMP and PKA activity. In conclusion, insulin counter-regulates cholangiocyte secretory processes in the BDL model, which is characterized by cholangiocyte proliferation.

Intensive insulin therapy in critically ill patients

BACKGROUND

Hyperglycemia and insulin resistance are common in critically ill patients, even if they have not previously had diabetes. Whether the normalization of blood glucose levels with insulin therapy improves the prognosis for such patients is not known.

METHODS

We performed a prospective, randomized, controlled study involving adults admitted to our surgical intensive care unit who were receiving mechanical ventilation. On admission, patients were randomly assigned to receive intensive insulin therapy (maintenance of blood glucose at a level between 80 and 110 mg per deciliter [4.4 and 6.1 mmol per liter]) or conventional treatment (infusion of insulin only if the blood glucose level exceeded 215 mg per deciliter [11.9 mmol per liter] and maintenance of glucose at a level between 180 and 200 mg per deciliter [10.0 and 11.1 mmol per liter]).

RESULTS

At 12 months, with a total of 1548 patients enrolled, intensive insulin therapy reduced mortality during intensive care from 8.0 percent with conventional treatment to 4.6 percent (P<0.04, with adjustment for sequential analyses). The benefit of intensive insulin therapy was attributable to its effect on mortality among patients who remained in the intensive care unit for more than five days (20.2 percent with conventional treatment, as compared with 10.6 percent with intensive insulin therapy, P=0.005). The greatest reduction in mortality involved deaths due to multiple-organ failure with a proven septic focus. Intensive insulin therapy also reduced overall in-hospital mortality by 34 percent, bloodstream infections by 46 percent, acute renal failure requiring dialysis or hemofiltration by 41 percent, the median number of red-cell transfusions by 50 percent, and critical-illness polyneuropathy by 44 percent, and patients receiving intensive therapy were less likely to require prolonged mechanical ventilation and intensive care.

CONCLUSIONS

Intensive insulin therapy to maintain blood glucose at or below 110 mg per deciliter reduces morbidity and mortality among critically ill patients in the surgical intensive care unit.

Effect of insulin on basal and cholecystokinin-stimulated gallbladder motility in humans

BACKGROUND/AIMS

Acute hyperglycemia inhibits gallbladder contraction. In non-diabetic subjects this inhibitory effect may result from endogenous hyperinsulinemia. Therefore we investigated the effects of acute hyperglycemia and euglycemic hyperinsulinemia on basal and cholecystokinin-stimulated gallbladder motility.

METHODS

Gallbladder volume (ultrasonography) and duodenal bilirubin output were studied simultaneously in nine healthy volunteers (age 20-52 years) on 3 separate occasions in random order during: (a) saline infusion (control), (b) hyperglycemic hyperinsulinemic clamping (HG; plasma glucose at 15 mmol/l), and (c) euglycemic hyperinsulinemic clamping (HI; plasma insulin at 150 mU/l, glucose at 4-5 mmol/l). After a 2-h basal clamp period, cholecystokinin was infused intravenously for 60 min at 0.25 IDU x kg(-1) x h(-1), followed by another 60 min at 0.5 IDU x kg(-1) x h(-1).

RESULTS

HI and HG significantly (p<0.05) reduced basal duodenal bilirubin output compared to control, while basal gallbladder volume did not change. At the low dose cholecystokinin, gallbladder emptying during HG (25+/-3%) and HI (39+/-4%) was significantly (p<0.01) reduced compared to control (61+/-4%). The inhibitory effect of HG was significantly (p<0.05) stronger compared to HI. Duodenal bilirubin output during the low dose cholecystokinin was significantly (p<0.05) reduced by HG, but not by HI. No inhibitory effect of HG and HI on gallbladder emptying and duodenal bilirubin output was observed with the high dose of cholecystokinin.

CONCLUSIONS

In healthy subjects acute hyperglycemia and euglycemic hyperinsulinemia reduce basal duodenal bilirubin output and inhibit gallbladder emptying stimulated by low dose cholecystokinin. These results suggest that insulin is involved in the inhibitory effect of hyperglycemia on basal and cholecystokinin-stimulated gallbladder motility.

Hyperglycemia-induced cholestasis in the isolated perfused rat liver

In a previous report we showed that cholestasis in diabetic rats is due in part to hyperglycemia. To gain information about the mechanism responsible for this phenomenon, bile flow was studied in isolated perfused rat livers. The perfusion media were modified erythrocyte-free Krebs-Henseleit solutions. Under these experimental conditions, no cholestasis was observed in isolated rat livers obtained from rats treated with streptozotocin (6 mg/100 gm body wt) 6 days before the experiments. We then proceeded to use normal animals. The composition of the perfusion media was modified to maintain the osmolality even after increasing D-glucose concentrations from 0 to 35 mmol/L. Bile flow was not affected with doses up to 15 mmol/L D-glucose. Beyond a threshold value for plasma D-glucose concentrations between 15 and 20 mmol/L, cholestasis was observed. Using D-glucose analogs such as L-glucose and 3-O-methyl-D-glucose, bile flow was also reduced (by 0.54 and 0.53 microliters/min/gm liver, respectively). Isosmotic sucrose-containing perfusion media were also observed to impair bile flow (by 0.66 microliters/min/gm liver). However, i-erythritol and D-mannitol failed to inhibit bile formation. The study of bile/plasma concentration ratios determined using tracer amounts of radioactive sugars indicated that this value was much lower for cholestatic sucrose (0.11) and L-glucose (0.31) than for noncholestatic i-erythritol (0.99) and D-mannitol (0.98). Cholestasis was partly reversed after induction by 35 mmol/L D-glucose if perfusion media were replaced by sugar-free ones, but also by media containing 25 mmol/L D-glucose. Insulin given during the perfusion with sugar-free media was observed to have no effect on bile flow.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of phlorizin on hepatic bile production before and during glucose infusion

During intravenous infusion of glucose, bile secretion is reduced (cholestasis), indicating that hepatocellular metabolism of glucose could have harmful effects on the liver. Phlorizin has been identified as a compound capable of impeding glucose uptake of liver cells. To examine whether phlorizin had any effect on glucose-associated cholestasis, three groups of experiments were performed on anaesthetized pigs. In group I phlorizin (100 mg/kg body wt) during normoglycaemia stimulated bicarbonate-dependent bile secretion by 56 +/- 4%. After phlorizin, hyperglycaemia decreased both bile acid- and bicarbonate-dependent bile secretion by 37 +/- 4%. But after the glucose load normalization of plasma glucose concentration increased the bicarbonate-dependent fraction by 38 +/- 4%. In group II phlorizin (100 mg/kg body wt, infused intravenously) during hyperglycaemia stimulated bicarbonate-dependent bile secretion by 35 +/- 5%. In group III bile secretion was continuously stimulated by infusion of Na-taurocholate. Hyperglycaemia reduced bicarbonate-dependent bile secretion by 33 +/- 4%, but after phlorizin both bile acid- and bicarbonate-dependent bile secretion increased on average by 121 +/- 8%. The osmotic effect of hyperglycaemia cannot be blocked by phlorizin, but judged by the effect on bile secretion, phlorizin may decrease the cholestatic effect induced by metabolism of glucose. Phlorizin could be an interesting compound for use in solutions for organ preservation.

Biliary response to glucagon in humans

Glucagon has been demonstrated to have profound effect on biliary secretion in several species. Glucagon's biliary effects were studied in humans following biliary tract surgery. Nine patients underwent common bile duct exploration and insertion of a balloon-occludable t tube. An aliquot of the collected sample was kept and the enterohepatic circulation was maintained by reinfusion of the collected bile via the distal t-tube port. Glucagon increased bile flow and decreased cholesterol and phospholipid output during stable bile acid output. Furthermore high-performance liquid chromatographic analysis of bile acid profiles revealed no significant changes in bile salt species or conjugation after glucagon infusion. Glucagon is probably important in the physiologic regulation of biliary secretion in humans.

Beta-adrenergic agonists inhibit gastric acid and pepsin secretion through somatostatin release in dogs

The purpose of the present study was to evaluate whether the inhibitory effects of beta-adrenergic agonists on gastric secretory activity in vivo could be mediated through a local release of somatostatin. The gastric secretion was measured during continuous stimulation with pentagastrin (1 microgram/kg/h). The infusion of isoprenaline (beta 1 + beta 2), salmefamol (beta 2), and somatostatin produced inhibitory effects on both acid and pepsin secretion. The reaction patterns were similar for isoprenaline and somatostatin, whereas salmefamol induced an inhibition of longer duration and with dissimilar dose-response kinetics. The gastric somatostatin release was significantly increased after infusion of both beta-adrenergic agonists and somatostatin, with patterns similar to those obtained for the secretory inhibition. There was a significant correlation between the somatostatin release and the acid and pepsin secretion during infusion of the secretory inhibitors but not in the control state. This study shows that beta-adrenergic agonists have inhibitory effects on gastric secretion in vivo similar to those of somatostatin. Both somatostatin and the beta-adrenergic agonists stimulated the release of somatostatin from the gastric mucosa. beta-Adrenergic antagonists were without effects. Somatostatin thereby fulfils the requirements for an endogenous mediator of the beta-adrenergic inhibition.

Role of glucose reabsorption from bile on hyperglycaemia-induced cholestasis in the rabbit

The effect of glucose administration on bile secretion of glucose and bile flow and composition was studied in the rabbit. After intravenous glucose infusion at 83 mumol/kg/min a mean bile concentration of 12.7 +/- 1.8 mg/dl was reached. Intraportal administration of phlorizin enhanced bile glucose concentration to 169.6 +/- 18.1 mg/dl, suggesting the presence of a system for transferring glucose from bile to liver in the biliary tree of the rabbit. A significant correlation between bile flow and plasma glucose levels could be demonstrated. A cholestatic effect appeared in glucose-infused rabbits with a decrease in bile flow by about 40% during the second hour of infusion. Both bile acid and inorganic electrolyte output were significantly lowered. Cholestasis was maintained after phlorizin administration. Possible explanations for this effect are discussed.