Christopher J. Ramnanan

Physiologic action of glucagon on liver glucose metabolism

Glucagon is a primary regulator of hepatic glucose production (HGP) in vivo during fasting, exercise and hypoglycaemia. Glucagon also plays a role in limiting hepatic glucose uptake and producing the hyperglycaemic phenotype associated with insulin deficiency and insulin resistance. In response to a physiological rise in glucagon, HGP is rapidly stimulated. This increase in HGP is entirely attributable to an enhancement of glycogenolysis, with little to no acute effect on gluconeogenesis. This dramatic rise in glycogenolysis in response to hyperglucagonemia wanes with time. A component of this waning effect is known to be independent of hyperglycemia, though the molecular basis for this tachyphylaxis is not fully understood. In the overnight fasted state, the presence of basal glucagon secretion is essential in countering the suppressive effects of basal insulin, resulting in the maintenance of appropriate levels of glycogenolysis, fasting HGP and blood glucose. The enhancement of glycogenolysis in response to elevated glucagon is critical in the life‐preserving counterregulatory response to hypoglycaemia, as well as a key factor in providing adequate circulating glucose for working muscle during exercise. Finally, glucagon has a key role in promoting the catabolic consequences associated with states of deficient insulin action, which supports the therapeutic potential in developing glucagon receptor antagonists or inhibitors of glucagon secretion.

Brain insulin action augments hepatic glycogen synthesis without suppressing glucose production or gluconeogenesis in dogs

In rodents, acute brain insulin action reduces blood glucose levels by suppressing the expression of enzymes in the hepatic gluconeogenic pathway, thereby reducing gluconeogenesis and endogenous glucose production (EGP). Whether a similar mechanism is functional in large animals, including humans, is unknown. Here, we demonstrated that in canines, physiologic brain hyperinsulinemia brought about by infusion of insulin into the head arteries (during a pancreatic clamp to maintain basal hepatic insulin and glucagon levels) activated hypothalamic Akt, altered STAT3 signaling in the liver, and suppressed hepatic gluconeogenic gene expression without altering EGP or gluconeogenesis. Rather, brain hyperinsulinemia slowly caused a modest reduction in net hepatic glucose output (NHGO) that was attributable to increased net hepatic glucose uptake and glycogen synthesis. This was associated with decreased levels of glycogen synthase kinase 3β (GSK3β) protein and mRNA and with decreased glycogen synthase phosphorylation, changes that were blocked by hypothalamic PI3K inhibition. Therefore, we conclude that the canine brain senses physiologic elevations in plasma insulin, and that this in turn regulates genetic events in the liver. In the context of basal insulin and glucagon levels at the liver, this input augments hepatic glucose uptake and glycogen synthesis, reducing NHGO without altering EGP.

Molecular Characterization of Insulin-Mediated Suppression of Hepatic Glucose Production In Vivo

OBJECTIVE Insulin-mediated suppression of hepatic glucose production (HGP) is associated with sensitive intracellular signaling and molecular inhibition of gluconeogenic (GNG) enzyme mRNA expression. We determined, for the first time, the time course and relevance (to metabolic flux) of these molecular events during physiological hyperinsulinemia in vivo in a large animal model. RESEARCH DESIGN AND METHODS 24 h fasted dogs were infused with somatostatin, while insulin (basal or 8× basal) and glucagon (basal) were replaced intraportally. Euglycemia was maintained and glucose metabolism was assessed using tracer, 2H2O, and arterio-venous difference techniques. Studies were terminated at different time points to evaluate insulin signaling and enzyme regulation in the liver. RESULTS Hyperinsulinemia reduced HGP due to a rapid transition from net glycogen breakdown to synthesis, which was associated with an increase in glycogen synthase and a decrease in glycogen phosphorylase activity. Thirty minutes of hyperinsulinemia resulted in an increase in phospho-FOXO1, a decrease in GNG enzyme mRNA expression, an increase in F2,6P2, a decrease in fat oxidation, and a transient decrease in net GNG flux. Net GNG flux was restored to basal by 4 h, despite a substantial reduction in PEPCK protein, as gluconeogenically-derived carbon was redirected from lactate efflux to glycogen deposition. CONCLUSIONS In response to acute physiologic hyperinsulinemia, 1) HGP is suppressed primarily through modulation of glycogen metabolism; 2) a transient reduction in net GNG flux occurs and is explained by increased glycolysis resulting from increased F2,6P2 and decreased fat oxidation; and 3) net GNG flux is not ultimately inhibited by the rise in insulin, despite eventual reduction in PEPCK protein, supporting the concept that PEPCK has poor control strength over the gluconeogenic pathway in vivo.

Effects of Insulin on the Metabolic Control of Hepatic Gluconeogenesis In Vivo

OBJECTIVE Insulin represses the expression of gluconeogenic genes at the mRNA level, but the hormone appears to have only weak inhibitory effects in vivo. The aims of this study were 1) to determine the maximal physiologic effect of insulin, 2) to determine the relative importance of its effects on gluconeogenic regulatory sites, and 3) to correlate those changes with alterations at the cellular level. RESEARCH DESIGN AND METHODS Conscious 60-h fasted canines were studied at three insulin levels (near basal, 4×, or 16×) during a 5-h euglycemic clamp. Pancreatic hormones were controlled using somatostatin with portal insulin and glucagon infusions. Glucose metabolism was assessed using the arteriovenous difference technique, and molecular signals were assessed. RESULTS Insulin reduced gluconeogenic flux to glucose-6-phosphate (G6P) but only at the near-maximal physiological level (16× basal). The effect was modest compared with its inhibitory effect on net hepatic glycogenolysis, occurred within 30 min, and was associated with a marked decrease in hepatic fat oxidation, increased liver fructose 2,6-bisphosphate level, and reductions in lactate, glycerol, and amino acid extraction. No further diminution in gluconeogenic flux to G6P occurred over the remaining 4.5 h of the study, despite a marked decrease in PEPCK content, suggesting poor control strength for this enzyme in gluconeogenic regulation in canines. CONCLUSIONS Gluconeogenic flux can be rapidly inhibited by high insulin levels in canines. Initially decreased hepatic lactate extraction is important, and later reduced gluconeogenic precursor availability plays a role. Changes in PEPCK appear to have little or no acute effect on gluconeogenic flux.

Suppression of Na+/K+-ATPase activity during estivation in the land snail Otala lactea.

SUMMARY Entry into the hypometabolic state of estivation requires a coordinated suppression of the rate of cellular ATP turnover, including both ATP-generating and ATP-consuming reactions. As one of the largest consumers of cellular ATP, the plasma membrane Na+/K+-ATPase is a potentially key target for regulation during estivation. Na+/K+-ATPase was investigated in foot muscle and hepatopancreas of the land snail Otala lactea, comparing active and estivating states. In both tissues enzyme properties changed significantly during estivation: maximal activity was reduced by about one-third, affinity for Mg.ATP was reduced (Km was 40% higher), and activation energy (derived from Arrhenius plots) was increased by ∼45%. Foot muscle Na+/K+-ATPase from estivated snails also showed an 80% increase in Km Na+ and a 60% increase in Ka Mg2+ as compared with active snails, whereas hepatopancreas Na+/K+-ATPase showed a 70% increase in I50 K+ during estivation. Western blotting with antibodies recognizing the alpha subunit of Na+/K+-ATPase showed no change in the amount of enzyme protein during estivation. Instead, the estivation-responsive change in Na+/K+-ATPase activity was linked to posttranslational modification. In vitro incubations manipulating endogenous kinase and phosphatase activities indicated that Na+/K+-ATPase from estivating snails was a high phosphate, low activity form, whereas dephosphorylation returned the enzyme to a high activity state characteristic of active snails. Treatment with protein kinases A, C or G could all mediate changes in enzyme properties in vitro that mimicked the effect of estivation, whereas treatments with protein phosphatase 1 or 2A had the opposite effect. Reversible phosphorylation control of Na+/K+-ATPase can provide the means of coordinating ATP use by this ion pump with the rates of ATP generation by catabolic pathways in estivating snails.

Evidence against a Physiologic Role for Acute Changes in CNS Insulin Action in the Rapid Regulation of Hepatic Glucose Production

This Perspective will discuss the physiologic relevance of data that suggest CNS insulin action is required for the rapid suppression of hepatic glucose production. It will also review data from experiments on the conscious dog, which show that although the canine brain can sense insulin and, thereby, regulate hepatic glucoregulatory enzyme expression, CNS insulin action is not essential for the rapid suppression of glucose production caused by the hormone. Insulins direct hepatic effects are dominant, thus it appears that insulins central effects are redundant in the acute regulation of hepatic glucose metabolism.

Interaction Between the Central and Peripheral Effects of Insulin in Controlling Hepatic Glucose Metabolism in the Conscious Dog

The importance of hypothalamic insulin action to the regulation of hepatic glucose metabolism in the presence of a normal liver/brain insulin ratio (3:1) is unknown. Thus, we assessed the role of central insulin action in the response of the liver to normal physiologic hyperinsulinemia over 4 h. Using a pancreatic clamp, hepatic portal vein insulin delivery was increased three- or eightfold in the conscious dog. Insulin action was studied in the presence or absence of intracerebroventricularly mediated blockade of hypothalamic insulin action. Euglycemia was maintained, and glucagon was clamped at basal. Both the molecular and metabolic aspects of insulin action were assessed. Blockade of hypothalamic insulin signaling did not alter the insulin-mediated suppression of hepatic gluconeogenic gene transcription but blunted the induction of glucokinase gene transcription and completely blocked the inhibition of glycogen synthase kinase-3β gene transcription. Thus, central and peripheral insulin action combined to control some, but not other, hepatic enzyme programs. Nevertheless, inhibition of hypothalamic insulin action did not alter the effects of the hormone on hepatic glucose flux (production or uptake). These data indicate that brain insulin action is not a determinant of the rapid (<4 h) inhibition of hepatic glucose metabolism caused by normal physiologic hyperinsulinemia in this large animal model.

Effect of 11β-hydroxysteroid dehydrogenase-1 inhibition on hepatic glucose metabolism in the conscious dog

Inactive cortisone is converted to active cortisol within the liver by 11 beta-hydroxysteroid dehydrogenase-1 (11 beta-HSD1), and impaired regulation of this process may be related to increased hepatic glucose production (HGP) in individuals with type 2 diabetes. The primary aim of this study was to investigate the effect of acute 11 beta-HSD1 inhibition on HGP and fat metabolism during insulin deficiency. Sixteen conscious, 42-h-fasted, lean, healthy dogs were studied. Somatostatin was infused to create insulin deficiency, and the animals were treated with a specific 11 beta-HSD1 inhibitor (compound 531) or placebo for 5 h. 11 beta-HSD1 inhibition completely suppressed hepatic cortisol production, and this attenuated the increase in HGP that occurred during insulin deficiency. PEPCK and glucose-6-phosphatase expression were decreased when 11 beta-HSD1 was inhibited, but gluconeogenic flux was unchanged, implying an effect on glycogenolysis. Since inhibition of hepatic cortisol production reduces HGP during insulin deficiency, 11 beta-HSD1 is a potential therapeutic target for the treatment of excess glucose production that occurs in diabetes.

A review of literature on medical students and scholarly research: experiences, attitudes, and outcomes.

Purpose The aim of medical student research programs is to develop interest in and competencies related to scholarly research within future physicians. Although schools invest in these programs, there is currently no consensus regarding what benefits they confer. The goal of this review is to characterize students’ perceptions of research programs during medical school as well as the outcomes attributed to these programs to provide recommendations for their optimization. Method In June 2013, the authors reviewed the literature (1950–June 2013) and identified 20 reports that provided original data delineating undergraduate medical students’ primarily self-reported experiences with, outcomes related to, and attitudes toward research. Results Students generally perceive their medical school research experiences to be positive in terms of stimulating research interest and developing scholarly research abilities. The majority of students author at least one article, and first-author publications occur more frequently as formal research experiences lengthen. Elective experiences do not differ from mandatory experiences in terms of student satisfaction or productivity. Several studies uncovered negative student perceptions regarding their research experience, including too little acknowledgment, time, and faculty interaction. Published studies were deficient in characterizing effects on future research engagement in participants. Conclusions These findings suggest that formal medical student research programs can be optimized by improving the recognition of student effort, promoting student–mentor interaction, and allowing students the option to increase the duration of the research experience. Future studies are needed to determine whether these programs affect research participation and productivity later in participants’ careers.

Advances in medical education and practice: student perceptions of the flipped classroom

The flipped classroom (FC) approach to teaching has been increasingly employed in undergraduate medical education in recent years. In FC applications, students are first exposed to content via online resources. Subsequent face-to-face class time can then be devoted to student-centered activities that promote active learning. Although the FC has been well received by students in other contexts, the perceptions of medical students regarding this innovation are unclear. This review serves as an early exploration into medical student perceptions of benefits and limitations of the FC. Medical students have generally expressed strong appreciation for the pre-class preparation activities (especially when facilitated by concise, readily accessed online tools) as well as for interactive, engaging small group classroom activities. Some students have expressed concerns with the FC and noted that suboptimal student preparation and insufficient direction and structure during active learning sessions may limit the student-centered benefits. Although students generally perceive that FC approaches can improve their learning and knowledge, this has not been conclusively shown via performances on assessment tools, which may be related to caveats with the assessment tools used. In any case, lifelong self-directed learning skills are perceived by medical students to be enhanced by the FC. In conclusion, medical students have generally expressed strong satisfaction with early applications of the FC to undergraduate medical education, and generally prefer this method to lecture-based instruction.