Oscillations of circulating plasma insulin concentrations in the rat

Hepatalin: the missing link in prediabetes, obesity, and type 2 diabetes

Hepatalin is a hormone secreted by the liver in response to pulses of insulin after a mixed nutrient meal, but only if the liver receives two permissive synergistic feeding signals from the stomach. Hepatalin stimulates glucose uptake and storage as glycogen in skeletal muscle, heart, and kidney but not liver, intestines, or adipocytes. Insulin acts primarily on liver and fat. Reduced hepatalin action results in postprandial hyperglycemia, compensatory elevation of insulin secretion, and a resultant shift in partitioning of nutrient energy storage from glycogen in muscle, to fat. Chronic hepatalin suppression leads to a predictable chronology of dysfunctions, first diagnosable as Absence of Meal-induced Insulin Sensitization (AMIS) which progresses to prediabetes, adiposity, and type 2 diabetes. The focus on nutrient partitioning and the role of hepatalin allows AMIS to be diagnosed, prevented, and treated, including through the use of lifestyle interventions.

A Unifying Organ Model of Pancreatic Insulin Secretion

The secretion of insulin by the pancreas has been the object of much attention over the past several decades. Insulin is known to be secreted by pancreatic β-cells in response to hyperglycemia: its blood concentrations however exhibit both high-frequency (period approx. 10 minutes) and low-frequency oscillations (period approx. 1.5 hours). Furthermore, characteristic insulin secretory response to challenge maneuvers have been described, such as frequency entrainment upon sinusoidal glycemic stimulation; substantial insulin peaks following minimal glucose administration; progressively strengthened insulin secretion response after repeated administration of the same amount of glucose; insulin and glucose characteristic curves after Intra-Venous administration of glucose boli in healthy and pre-diabetic subjects as well as in Type 2 Diabetes Mellitus. Previous modeling of β-cell physiology has been mainly directed to the intracellular chain of events giving rise to single-cell or cell-cluster hormone release oscillations, but the large size, long period and complex morphology of the diverse responses to whole-body glucose stimuli has not yet been coherently explained. Starting with the seminal work of Grodsky it was hypothesized that the population of pancreatic β-cells, possibly functionally aggregated in islets of Langerhans, could be viewed as a set of independent, similar, but not identical controllers (firing units) with distributed functional parameters. The present work shows how a single model based on a population of independent islet controllers can reproduce very closely a diverse array of actually observed experimental results, with the same set of working parameters. The model's success in reproducing a diverse array of experiments implies that, in order to understand the macroscopic behaviour of the endocrine pancreas in regulating glycemia, there is no need to hypothesize intrapancreatic pacemakers, influences between different islets of Langerhans, glycolitic-induced oscillations or β-cell sensitivity to the rate of change of glycemia.

Obesity as an Early Symptom of the AMIS Syndrome

We review evidence that the AMIS (Absence of Meal-induced Insulin Sensitization) syndrome describes a paradigm fundamental to development of obesity. The hypoglycemic response to a pulse of insulin is doubled after a meal as a result of Hepatic Insulin Sensitizing Substance (HISS), released from the liver to act selectively on muscle, heart and kidney. In the absence of HISS action, the hypoglycemic response to insulin is the same as in the fasted state, and only half of what it should be. Postprandial hyperglycemia ensues, with compensatory hyperinsulinemia, resultant hyperlipidemia and elevated free radical stress. Storage of nutrient energy shifts from glycogen in muscle to fat. Chronic AMIS results in adiposity, occurs with age, is accelerated with sucrose supplement, and prevented by a synergistic antioxidant. Exercise reverses AMIS, as do pharmaceuticals that mimic the "feeding signals". The AMIS syndrome develops as a sequence of pathologies based on the consequences of absence of HISS action, including adiposity as the earliest symptom. Cardiac dysfunction, hypertension, hypercholesterolemia, and fatty liver are related to lack of HISS action. The AMIS syndrome hypothesis is mechanistic-based and accounts for the major pathologies associated with prediabetes, obesity, diabetes and metabolic syndrome. AMIS can be diagnosed, prevented and treated.

High-fat diet-induced ultradian leptin and insulin hypersecretion are absent in obesity-resistant rats

OBJECTIVE

Sprague-Dawley rats fed a high-fat diet (HFD) are either obesity prone (OP) or obesity resistant (OR). We tested the hypothesis that differences in the ultradian rhythmic patterns of insulin and ghrelin in OP vs. OR rats promote obesity in OP rats.

RESEARCH METHODS AND PROCEDURES

Rats were fed regular chow or an HFD, and ultradian fluctuations in leptin, insulin, and ghrelin were analyzed in blood samples collected at 5-minute intervals from intrajugular cannulae of freely moving rats.

RESULTS

Regular chow feeding resulted in a slow weight gain accompanied by small increases in insulin and leptin and a decrease in ghrelin discharge, with only the pulse amplitude significantly altered. Similar changes were observed in OR rats, despite HFD consumption. In contrast, OP rats exhibited a high rate of weight gain and marked hyperinsulinemia, hyperleptinemia, and hypoghrelinemia; amplitude was altered, but frequency was stable. In a short-term experiment, HFD elicited similar secretory patterns of smaller magnitude even in the absence of weight gain.

DISCUSSION

We showed that three hormonal signals of disparate origin involved in energy homeostasis were secreted in discrete episodes, and only the pulse amplitude component was vulnerable to age and HFD consumption. Increases in insulin and leptin and decreases in ghrelin pulse amplitude caused by HFD were exaggerated in OP rats relative to OR rats and preceded the weight increase. These findings show that a distinct genetic predisposition in the endocrine organs of OR rats confers protection against high-fat intake-induced ultradian hypersecretion of obesity-promoting hormonal signals.

Inhibition of purinoceptors amplifies glucose-stimulated insulin release with removal of its pulsatility

External ATP has been proposed to be an autocrine regulator of glucose-stimulated insulin secretion and responsible for the synchronization of the Ca2+ rhythmicity in the beta-cells required for a pulsatile release of insulin from the pancreas. The importance of external ATP for glucose-stimulated insulin release was evaluated in rats with the aid of 2-deoxy-N-methyladenosine-3,5-bisphosphate (MRS 2179), an inhibitor of the purinoceptors known to affect the Ca2+ signaling in beta-cells. The concentration of cytoplasmic Ca2+ was measured in single beta-cells and small aggregates with ratiometric fura-2 technique and the release of insulin recorded from isolated islets and the perfused pancreas. Addition of 1 micromol/l ATP induced premature cytoplasmic Ca2+ concentration ([Ca2+]i) oscillations similar to those found in beta-cells exposed to 20 mmol/l glucose. In most experiments, the presence of 10 micromol/l MRS 2179 did not remove the glucose-induced [Ca2+]i rhythmicity in single beta-cells or the synchronization seen in coupled cells. Nevertheless, the same concentration of MRS 2179 promptly interrupted the pulsatility (frequency 0.22 +/- 0.01/min) of insulin secretion, raising the total amounts released from the pancreas. Prolonged exposure of islets to 1 and 10 micromol/l MRS 2179 enhanced insulin secretion at 20 mmol/l glucose 33% (P < 0.05) and 63% (P < 0.01), respectively, without affecting the release at 3 mmol/l glucose. The results support the idea that neural ATP signals entrain the islets into a common rhythm resulting in pulsatile release of insulin and that glucose stimulation of the secretory activity is counteracted by accumulation of inhibitory ATP around the beta-cells.

Pattern of insulin delivery affects hepatic insulin sensitizing substance (HISS) action and insulin resistance

Hepatic insulin sensitizing substance (HISS) action accounts for 55% of the glucose disposal effect of a bolus of insulin in the fed state. To determine the effect of continuous versus pulsatile insulin delivery on HISS action in male Sprague–Dawley rats, insulin sensitivity was assessed using the rapid insulin sensitivity test (RIST) before and after a continuous, pulsatile, or bolus insulin (60 mU/kg i.v.) delivery. There was a significant difference in the RIST index after a continuous insulin infusion (247.9 mg/kg before, 73.2 mg/kg after) but not after 3 pulses where insulin action returned to baseline between pulses (211.6 mg/kg before, 191.0 mg/kg after) or single bolus (205.8 mg/kg before, 189.9 mg/kg after) insulin infusion. If a 3-pulse infusion was timed so that insulin action did not return to baseline between pulses, HISS action was suppressed. Continuous insulin infusion (10–30 min) showed progressive postinfusion blockade of HISS action. To maintain HISS-dependent insulin action, continuous insulin infusions should be avoided.Key words: pulsatile, glucose uptake, RIST, euglycemic clamp, insulin sensitivity.

Loss of entrainment of high-frequency plasma insulin oscillations in type 2 diabetes is likely a glucose-specific beta-cell defect

Spontaneous high-frequency insulin oscillations are easily entrainable to exogenous glucose in vitro and in vivo, but this property is lost in type 2 diabetes (2-DM). We hypothesized that this lack of entrainment in 2-DM would be specific to glucose. This was tested in nine control and ten 2-DM subjects. Serial blood sampling at 1-min intervals was carried out for 60 min in the basal state and for 120 min while small (1-60 mg/kg) boluses of arginine were injected intravenously at exactly 29-min intervals. Samples were analyzed for insulin concentrations, and time series analysis was carried out using spectral analysis. In control subjects, the mean period of basal plasma insulin oscillations was 10.3 +/- 1.3 min and was entrained by arginine to a mean period of 14.9 +/- 0.6 min (P < 0.00001 vs. basal). Similarly, in 2-DM subjects, spontaneous insulin oscillations were entrained by arginine; mean basal insulin period was 10.0 +/- 1.0 min and 14.5 +/- 1.8 min with arginine boluses (P < 0.00001). All of the primary peaks observed in spectral analysis were statistically significant (P < 0.05). Percent total power of primary peaks ranged from 17 to 68%. Thus arginine boluses entrain spontaneous high-frequency insulin oscillations in 2-DM subjects. This represents a distinct and striking difference from the resistance of the beta-cell to glucose entrainment in 2-DM. We conclude that loss of entrainment of spontaneous high-frequency insulin oscillations in 2-DM is likely a glucose-specific manifestation of beta-cell secretory dysfunction.

Primary in vivo oscillations of metabolism in the pancreas

The role of metabolism in the generation of plasma insulin oscillations was investigated by simultaneous in vivo recordings of oxygen tension (pO(2)) in the endocrine and exocrine pancreas and portal blood insulin concentrations in the anesthetized rat. At the start of the experiment, the blood glucose concentration of seven rats was 6.2 +/- 0.1 mmol/l and the arterial blood pressure was 116 +/- 5 mmHg. These values did not differ from those obtained at the end of the experiment. Islet pO(2) was measured by impaling superficially located islets with a miniaturized Clark electrode. The pO(2) measurements revealed slow (0.21 +/- 0.03 min(-1)) with superimposed rapid (3.1 +/- 0.3 min(-1)) oscillations. The average pO(2) was 39 +/- 5 mmHg. Simultaneous recordings of pO(2) in the exocrine pancreas were significantly lower (16 +/- 6 mmHg), but showed a slow and a rapid oscillatory activity with similar frequencies as seen in the endocrine pancreas. Corresponding measurements of portal insulin concentrations revealed insulin oscillations at a frequency of 0.22 +/- 0.02 min(-1). The results are the first in vivo recordings of an oscillatory islet parameter with a frequency corresponding to that of plasma insulin oscillations; they support a primary role of metabolic oscillations in the induction of plasma insulin oscillations.

Pulsatile insulin secretion: detection, regulation, and role in diabetes

Insulin concentrations oscillate at a periodicity of 5-15 min per oscillation. These oscillations are due to coordinate insulin secretory bursts, from millions of islets. The generation of common secretory bursts requires strong within-islet and within-pancreas coordination to synchronize the secretory activity from the beta-cell population. The overall contribution of this pulsatile mechanism dominates and accounts for the majority of insulin release. This review discusses the methods involved in the detection and quantification of periodicities and individual secretory bursts. The mechanism by which overall insulin secretion is regulated through changes in the pulsatile component is discussed for nerves, metabolites, hormones, and drugs. The impaired pulsatile secretion of insulin in type 2 diabetes has resulted in much focus on the impact of the insulin delivery pattern on insulin action, and improved action from oscillatory insulin exposure is demonstrated on liver, muscle, and adipose tissues. Therefore, not only is the dominant regulation of insulin through changes in secretory burst mass and amplitude, but the changes may affect insulin action. Finally, the role of impaired pulsatile release in early type 2 diabetes suggests a predictive value of studies on insulin pulsatility in the development of this disease.

Counterregulatory hormones oscillate during steady-state hypoglycemia

During hypoglycemia, the magnitude of the counterregulatory response depends on the extent of plasma glucose reduction. However, our clinical observations during steady-state hypoglycemia indicate that symptom severity can change independently of plasma glucose concentrations, i.e., symptoms appeared to fluctuate despite stable glucose levels. This study was therefore designed to test the hypothesis that hormonal and symptomatic responses to hypoglycemia are pulsatile. Seven healthy subjects had serial blood sampling at 3-min intervals during 90 min of insulin-induced hypoglycemia. Mean +/- SE plasma glucose levels plateaued at 62 +/- 3 mg/dl. Counterregulatory hormones were significantly elevated (P < 0.05-0. 01, except norepinephrine) and strikingly pulsatile. Cluster analysis revealed pulses of large magnitude in plasma glucagon, epinephrine, and norepinephrine concentrations. Amplitudes were, respectively, 72 +/- 4, 64 +/- 8, and 48 +/- 3% of the mean. Interpeak intervals were 27 +/- 7, 19 +/- 4, and 25 +/- 5 min, respectively. Symptom score and cardiovascular responses were also pulsatile; their peaks were found to coincide with epinephrine peaks. We conclude that hormonal and symptomatic counterregulation in hypoglycemia, while critically driven by plasma glucose levels, is also influenced by an endogenous pulsatility that exists despite steady-state glucose concentrations.

Pulsatile intravenous insulin replacement in streptozotocin diabetic rats is more efficient than continuous delivery: effects on glycaemic control, insulin-mediated glucose metabolism and lipolysis

Short-term exposure of tissues to pulses of insulin generally leads to an enhancement of insulin action. We have investigated the possible beneficial effects of long-term near-physiological continuous vs pulsatile intravenous insulin treatment of insulin-deficient streptozotocin (70 mg/kg) diabetic rats on blood glucose control, in vivo insulin action and in vitro insulin action in isolated adipocytes. First, we determined the 24-h peripheral plasma insulin profiles in normal rats under precisely controlled mealfeeding conditions. Basal plasma insulin levels (40 +/- 9 microU/ml) oscillate with a periodicity of 11.9 +/- 0.9 min (p < 0.05), and an amplitude of 60 +/- 10%. Subsequently, the 24-h insulin profile was mimicked in diabetic (D) rats by a continuous (c) or pulsatile (p) (6-min double, 6-min off) insulin infusion rate for 2 weeks, using a programmable pumpswivel unit. Control (C) rats received vehicle treatment. In Cc, Dc, Cp and Dp daily urinary glucose loss and average plasma glucose levels were 0 +/- 0, 7.5 +/- 4.4, 0 +/- 0, 0.8 +/- 0.4 mmol and 6.7 +/- 0.2, 11.5 +/- 2.7, 6.6 +/- 0.1, 5.9 +/- 1.4 mmol/l, respectively. Hypoglycaemia (< 3 mmol/l) was observed in 10 and 20% of the blood samples collected from Dc and Dp rats, respectively. After 2 weeks of treatment, in vivo peripheral and hepatic insulin action was measured by the hyperinsulinaemic euglycaemic (6 mmol/l) clamp with [3-3H]-glucose infusion. Pre-clamp counter-regulatory hormone levels were similar among rats. Compared to Cc and Cp, Dc showed a reduction in insulin sensitivity and responsiveness for peripheral glucose uptake whereas Dp only showed a reduction in insulin sensitivity. Suppression of hepatic glucose production by insulin was similar among rats. After 2.5 weeks of treatment, epididymal adipocytes were isolated. Specific [125I]-insulin binding, basal and insulin-stimulated [U-14C]-glucose uptake and isoproterenol-stimulated glycerol output were comparable among rat adipocytes. The inhibition of glycerol output by insulin was identical in Cp and Dp (V(max) = 48.6 +/- 6.1 and 42.3 +/- 4.6%) but blunted in Dc vs Cc (V(max) = 8.2 +/- 4.6 vs 44.0 +/- 7.2%, p < 0.01) adipocytes, suggesting a post-binding defect in the antilipolytic action of insulin in Dc rats. In conclusion, long-term near-physiological pulsatile intravenous insulin replacement in insulin-deficient diabetic rats is more efficient than continuous delivery in reducing blood glucose, lowering glucosuria, increasing insulin sensitivity and inhibiting lipolysis.

Coordinate pulsatile insulin secretion by chronic intraportally transplanted islets in the isolated perfused rat liver

In the present studies we sought to address the following questions: do chronically transplanted intrahepatic islets (IHI-Tx) secrete insulin in a coordinate pulsatile manner, and, if so, is reestablishment of this coordinate pulsatility a function of time after transplantation? We studied isolated perfused livers at 10 mM glucose from 27 rats rendered diabetic with streptozotocin and then transplanted with approximately 2 x 10(3) islets, 2 (n = 5), 7 (n = 5), 30 (n = 5), and 200 (n = 12) d after transplantation. 12 out of 12 of the 200-d IHI-Tx secreted insulin in coordinate pulses (frequency 3.9 +/- 0.3 pulses/h, amplitude 15.2 +/- 2.4 nmol/min). In contrast, one out of five 2-d, zero out of five 7-d, and one out of five 30-d IHI-Tx showed pulsatile insulin secretion. Insulin secretion was markedly greater (76 +/- 13 vs 13 +/- 3 nmol/min, P < 0.0001) in the 200-d versus early IHI-Tx. Pentobarbital 25 micrograms/ml had no effect on total (13.9 +/- 3.9 vs 15.9 +/- 3.9 nmol/min), nonpulsatile (12.9 +/- 3.5 vs 14.1 +/- 3.3 nmol/min), or pulsatile (pulse amplitude 17.6 +/- 4.5 vs 20.0 +/- 4.2 nmol/min, pulse frequency 4.1 +/- 0.3 vs 4.0 +/- 0.7 pulses/h) insulin secretion. Using synaptophysin, islet innervation was documented in 12 out of 12 200-d IHI-Tx but in none of the early IHI-Tx. We conclude that established (approximately 200 d) IHI-Tx secrete insulin in a coordinate pulsatile manner and that establishment of coordinate pulsatile insulin secretion by IHI-Tx is accompanied by increased total insulin secretion and is associated with islet reinnervation.

Periodicity of insulin secretion comprises multiple cycles with different duration in perfused rat islets

Insulin secretion from pancreatic islets has been found to be periodic by in vivo and in vitro experiments. The pacemaker which regulates the periodicity may be localized in the central nervous system or in the pancreas, though the precise location and the mechanisms of generating pacing have not been determined. In order to solve these problems, we examined the period of secretory cycles of insulin in isolated islets using a prolonged perfusion system, and investigated the effects of glucose and other agents on these periods. Isolated islets from male Wistar rats were enclosed in a millipore holder and were perfused with MEM containing 1 mg/ml glucose at a flow rate of 0.3 ml/min for 240 min. The effluent was collected at 1-min intervals to measure insulin secretion. The results were analyzed by the maximum entropy method to demonstrate the periodicity of insulin secretion. When islets were perfused with 1 mg/ml glucose, the periodicity comprised five cycles with different duration: 71.5 +/- 14.6 min, 29.8 +/- 3.4 min, 19.2 +/- 1.5 min, 11.6 +/- 2.1 min and 4.3 +/- 0.4 min. This indicates the presence of a pacemaker within the islets, although, in vivo, participation of a higher center to control periodicity has to be taken into account. Further, the presence of a long cycle (71.5 +/- 14.6 min) of insulin secretion which previously has only been observed in vivo was first demonstrated in this in vitro study. The cycles were consistent even in islets which were desensitized to glucose by cultivating in a high glucose medium for 5 days before perfusion.(ABSTRACT TRUNCATED AT 250 WORDS)