The relative contribution of the nervous system, hormones, and metabolites to the total insulin response during a meal in the rat

The cephalic phase of insulin release is modulated by IL-1β

The initial cephalic phase of insulin secretion is mediated through the vagus nerve and is not due to glycemic stimulation of pancreatic β cells. Recently, IL-1β was shown to stimulate postprandial insulin secretion. Here, we describe that this incretin-like effect of IL-1β involves neuronal transmission. Furthermore, we found that cephalic phase insulin release was mediated by IL-1β originating from microglia. Moreover, IL-1β activated the vagus nerve to induce insulin secretion and regulated the activity of the hypothalamus in response to cephalic stimulation. Notably, cephalic phase insulin release was impaired in obesity, in both mice and humans, and in mice, this was due to dysregulated IL-1β signaling. Our findings attribute a regulatory role to IL-1β in the integration of nutrient-derived sensory information, subsequent neuronally mediated insulin secretion, and the dysregulation of autonomic cephalic phase responses in obesity.

How neural mediation of anticipatory and compensatory insulin release helps us tolerate food

Learned anticipatory and compensatory responses allow the animal and human to maintain metabolic homeostasis during periods of nutritional challenges, either acutely within each meal or chronically during periods of overnutrition. This paper discusses the role of neurally-mediated anticipatory responses in humans and their role in glucoregulation, focusing on cephalic phase insulin and pancreatic polypeptide release as well as compensatory insulin release during the etiology of insulin resistance. The necessary stimuli required to elicit CPIR and vagal activation are discussed and the role of CPIR and vagal efferent activation in intra-meal metabolic homeostasis and during chronic nutritional challenges are reviewed.

Starch with high amylose and low in vitro digestibility increases short-chain fatty acid absorption, reduces peak insulin secretion, and modulates incretin secretion in pigs

Diets containing different starch types affect peripheral glucose and insulin responses. However, the role of starch chemistry in kinetics of nutrient absorption and insulin and incretin secretion is poorly understood. Four portal vein-catheterized pigs (35.0 ± 0.2 kg body weight) consumed 4 diets containing 70% purified starch [0-63.2% amylose content and 0.22 (slowly) to 1.06%/min (rapidly) maximum rate of in vitro digestion] for 7-d periods in a 4 × 4 Latin square. On d 7, blood was collected for 12 h postprandial with simultaneous blood flow measurement for determining the net portal appearance (NPA) of nutrients and hormones. The NPA of glucose, insulin, C-peptide, and glucose-dependent insulinotropic polypeptide (GIP) during 0-4 h postprandial were lower (P < 0.05) and those of butyrate and total SCFA were higher (P < 0.05) when pigs consumed the diet containing slowly digestible compared with rapidly digestible starch. The peak NPA of insulin occurred prior to that of glucose when pigs consumed diets containing rapidly digestible starch. The kinetics of insulin secretion had a linear positive relation with kinetics of NPA of glucose (R(2) = 0.50; P < 0.01). In conclusion, starch with high amylose and low in vitro digestibility decreases the kinetics of glucose absorption and insulin and GIP secretion and increases SCFA absorption and glucagon-like peptide-1 secretion. In conclusion, starch with high amylose content and a lower rate and extent of in vitro digestion decreased glucose absorption and insulin secretion and increased SCFA absorption.

Autonomic involvement in the permanent metabolic programming of hyperinsulinemia in the high-carbohydrate rat model

Exposure to a high-carbohydrate (HC) milk formula during the suckling period results in permanent metabolic programming of hyperinsulinemia in HC rats. Previous studies have shown that hyperinsulinemia in HC rats involves a programmed hyperresponsiveness to glucose. However, the immediate onset and persistence of enhanced insulin secretion throughout life suggests a role for numerous factors that control insulin secretion. Present in vivo and in vitro studies have shown a role for altered autonomic activity, including increased parasympathetic and decreased sympathetic activities, in the maintenance of hyperinsulinemia in 100-day-old HC rats. HC rats were shown to be more sensitive to cholinergic-induced potentiation of glucose-stimulated insulin secretion (GSIS) in response to acetylcholine and showed increased sensitivity to blockade of cholinergic-induced insulin secretion by the muscarinic-type 3 receptor-specific antagonist 4-diphenylacetoxy-N-methylpiperidine. In addition, HC rats were less sensitive to adrenergic-induced inhibition of insulin secretion by oxymetazoline, whereas treatment with yohimbine resulted in increased GSIS. Furthermore, HC rats showed greater reductions in plasma insulin levels after vagotomy, as well as an attenuation of yohimbine-induced potentiation of GSIS, suggesting that yohimbine-mediated changes are mediated by parasympathetic activity. Changes in autonomic regulation of GSIS are supported by increased mRNA levels of the parasympathetic signaling molecules muscarinic-type 3 receptor, phospholipase Cbeta1, and protein kinase C-alpha and decreased levels of alpha(2a)-adrenergic receptors in islets from adult HC rats. In conclusion, metabolic programming of hyperinsulinemia throughout adulthood of HC rats involves changes in autonomic activity in response to the HC dietary intervention in the suckling period.

Role of the autonomic nervous system in the development of hyperinsulinemia by high-carbohydrate formula feeding to neonatal rats

An early dietary intervention in the form of a high-carbohydrate (HC) milk formula in neonatal rat pups results in immediate onset of hyperinsulinemia. While increased insulin secretion in HC rats has been shown to be related to hypersensitivity to glucose, the immediate onset of hyperinsulinemia and its persistence throughout the suckling period suggest involvement of multiple systems that enhance insulin secretion in response to increased demand. Evidence presented here in 12-day-old HC rats indicates that altered activity of the autonomic nervous system contributes to enhanced insulin secretory responses to glucose stimulation through increased parasympathetic and decreased sympathetic signaling. Both in vivo and in vitro studies have shown that HC rats secrete significantly higher levels of insulin in response to glucose in the presence of acetylcholine, a cholinergic agonist, while sensitivity to inhibition of insulin secretion by oxymetazoline, an alpha(2a)-adrenergic receptor (alpha(2a)AR) agonist, was reduced. In addition, HC rats showed increased sensitivity to blockade of cholinergic-induced insulin secretion by the muscarinic type 3 receptor (M3R) antagonist 4-diphenylacetoxy-N-methylpiperidine methobromide, as well as increased potentiation of glucose-stimulated insulin secretion by treatment with yohimbine. Increases in islets levels of M3R, phospholipase C-beta1, and protein kinase Calpha mRNAs, as well as decreased alpha(2a)AR mRNA, in 12-day-old HC rats provide a mechanistic connection to the changes in insulin secretion seen in HC rats. In conclusion, altered autonomic regulation of insulin secretion, due to the HC nutritional intervention, contributes to the development of hyperinsulinemia in 12-day-old HC rats.

Quantitative trait loci for carbohydrate and total energy intake on mouse chromosome 17: congenic strain confirmation and candidate gene analyses (Glo1, Glp1r)

Quantitative trait loci (QTL) for carbohydrate (Mnic1) and total energy (Kcal2) intake on proximal mouse chromosome 17 were identified previously from a C57BL/6J (B6) X CAST/Ei (CAST) intercross. Here we report that a new congenic strain developed in our laboratory has confirmed this complex locus by recapitulating the original linked phenotypes: B6.CAST-17 homozygous congenic mice consumed more carbohydrate (27%) and total energy (17%) compared with littermate wild-type mice. Positional gene candidates with relevance to carbohydrate metabolism, glyoxalase I (Glo1) and glucagon-like peptide-1 receptor (Glp1r), were evaluated. Glo1 expression was upregulated in liver and hypothalamus of congenic mice when compared with B6 mice. Analyses of Glp1r mRNA and protein expression revealed tissue-specific strain differences in pancreas (congenic>B6) and stomach (B6>congenic). These results suggest the possibility of separate mechanisms for enhanced insulin synthesis and gastric accommodation in the presence of high carbohydrate intake and larger food volume, respectively. Sequence analysis of Glp1r found a G insert at nt position 1349, which results in earlier termination of the open reading frame, thus revealing an error in the public sequence. Consequently, the predicted length of GLP-1R is 463 aa compared with 489 aa, as previously reported. Also, we found a polymorphism in Glp1r between parental strains that alters the amino acid sequence. Variation in Glp1r could influence nutrient intake in this model through changes in the regulatory or protein coding regions of the gene. These congenic mice offer a powerful tool for investigating gene interactions in the control of food intake.

The vagus is inhibitory of the late postprandial insulin secretion in conscious pigs

The vagus is involved in the cephalic phase of insulin secretion but its role in the meal absorption phase of insulin release remains to be defined. The aim of this study was therefore to evaluate the role of the vagus in the early and the late meal absorption phases of insulin secretion. In six pigs, venous insulin profiles were compared in intact animals, after ventral or dorsal vagal trunk section, and after section of both vagal trunks (truncal vagotomy). Since gastric emptying could be modified by vagotomy, it was recorded concomitantly by gamma scintigraphy. Semi-solid (porridge) and liquid (glucose 10%) meals were tested. Truncal vagotomy significantly increased insulin release compare to intact animals after glucose (63.8%) and porridge (174.4%) meals in the early and the late absorption phases of insulin secretion, respectively. For the glucose meal, this effect could be explained by a vagally mediated change in gastric emptying rate, since insulin concentrations for a similar amount of nutrient propelled to the duodenum were not different in intact and truncal vagotomized animals. In contrast, after the porridge meal, truncal vagotomy was associated with a second, later occurring increase in circulating insulin, which could not be explained by changes in gastric emptying rate. These results demonstrate for the first time an inhibitory role of the vagus in the late meal absorption phase of insulin release.

The vagus is inhibitory of insulin secretion under fasting conditions

The involvement of the vagus in the insulin response during the early phase of absorption of a meal has been demonstrated recently. The extent of this vagal influence was investigated during fasting in an anesthetized porcine model. Portal and systemic insulin were evaluated together with glycemia during cooling and sectioning of both cervical vagal trunks in 12 splanchnicotomized or sham-operated pigs. In sham-operated animals, portal and systemic insulin were significantly and reversibly increased by cooling (173 and 123%, respectively). Portal insulin peaked 20 min after the onset of cooling but declined slowly while cooling was still activated. In contrast, systemic insulin was increased evenly along cooling. Section of the vagus was also associated with a portal and systemic insulin increase (144 and 117%) but to a lesser extent than cooling. In both treatments, portal and systemic insulin increases were either reduced (vagal cooling) or eliminated (vagal section) in splanchnicotomized animals. We conclude that the vagus exerts an inhibitory activity on interdigestive insulin secretion that is partly mediated by the splanchnic nerves.

Parasympathetic and sympathetic control of the pancreas: a role for the suprachiasmatic nucleus and other hypothalamic centers that are involved in the regulation of food intake

To reveal brain regions and transmitter systems involved in control of pancreatic hormone secretion, specific vagal and sympathetic denervation were combined with injection of a retrograde transsynaptic tracer, pseudorabies virus (PRV), into the pancreas. After sympathetic or vagal transsection first-order neurons were revealed in the dorsal motor nucleus of the vagus (DMV) or in preganglionic spinal cord neurons (SPN), respectively. Careful timing of the survival of the animals allowed the detection of cell groups in immediate control of these DMV or SPN neurons. A far larger number of cell groups is involved in the control of DMV than of SPN neurons. Examples are given of a high level of interaction between the sympathetic and parasympathetic nervous system. Several cell groups project to both branches of the autonomic nervous system, sometimes even the same neurotransmitter is used, e.g., oxytocin neurons in the paraventricular nucleus and melanin-concentrating hormone and orexin neurons in the lateral hypothalamus project to both the DMV and SPN neurons. Moreover, the appearance of third-order neurons located in the sympathetic SPN after complete sympathectomy and in the DMV after complete vagotomy illustrates the possibility that motor neurons of the sympathetic and parasympathetic system may exchange information by means of interneurons. The presence of second-order neurons in prefrontal, gustatory, and piriform cortex may provide an anatomic basis for the involvement of these cortices in the cephalic insulin response. The observation that second-order neurons in both vagal and sympathetic control of the pancreas contain neuropeptides that are known to play a role in food intake indicates a direct association between behavioral and autonomic functions. Finally, the observation of third-order neurons in the suprachiasmatic nucleus and ventromedial hypothalamus shows the modulatory action of the time of the day and metabolic state, respectively.

Activation of the parasympathetic nervous system is necessary for normal meal-induced insulin secretion in rhesus macaques

Meal-induced insulin secretion is thought to be regulated primarily by absorbed nutrients and incretin hormones released from the gastrointestinal tract. In addition, the parasympathetic nervous system (PNS) is known to mediate preabsorptive, or cephalic phase, insulin secretion. Despite evidence that the PNS remains activated during the absorptive phase of the meal, its role in mediating postprandial insulin secretion has not been established. To study the role of the PNS in absorptive phase insulin release, we measured plasma concentrations of glucose as well as islet hormones and incretins in six healthy rhesus monkeys before and for 60 min after meals while they were infused with saline (control), atropine (muscarinic blockade), or trimethaphan (nicotinic blockade). During the infusion of saline, plasma levels of glucose, pancreatic polypeptide (PP), insulin, glucose-dependent insulinotropic polypeptide, and glucagon-like peptide-1 increased promptly after meal ingestion and remained elevated throughout the 60 min of the study. The PP response was nearly abolished in animals treated with trimethaphan, indicating functional blockade of PNS input to the islet, and in contrast to the control study, there were minimal changes in plasma concentrations of glucose, incretin hormones, and insulin. Because trimethaphan inhibited glycemic and incretin stimuli in addition to blocking PNS input to the islet, it was not possible to discern the relative roles of these factors in the stimulation of insulin secretion. Atropine also significantly decreased PNS transmission to the islet, as reflected by PP levels similar to those observed with trimethaphan. Unlike the trimethaphan study, plasma glucose levels rose normally during atropine treatment and were similar to those in the control study over the course of the experiments (114 +/- 22 and 132 +/- 23 mmol/L.60 min, respectively). In addition, the rise in plasma glucagon-like peptide-1 following the meal was not suppressed by atropine, and the glucose-dependent insulinotropic polypeptide responses were only modestly decreased. Despite the significant increases in circulating glucose and incretins, plasma insulin levels were greatly attenuated by atropine, so that the 60 min responses were more comparable to those during trimethaphan treatment than to those in the control study (atropine, 3,576 +/- 1,284; trimethaphan, 4,128 +/- 2,616; control, 15,834 +/- 5,586 pmol/L.60 min; P: < 0.05). Thus, muscarinic blockade markedly suppressed the meal-induced insulin response despite normal postprandial glycemia and significant elevations of incretins. These results indicate that activation of the PNS during the absorptive phase of meals contributes significantly to the postprandial insulin secretory response.

Prolonged effects of modified sham feeding on energy substrate mobilization

BACKGROUND

Vagal stimulation in response to nutrients is reported to elicit an array of digestive and endocrine responses, including an alteration in postprandial lipid metabolism.

OBJECTIVE

The objective of this study was to assess whether neural stimulation could alter hormone and substrate metabolism during the late postprandial phase, with implications for body fat mobilization.

DESIGN

Vagal stimulation was achieved by using the modified sham feeding (MSF) technique, in which nutrients are chewed and tasted but not swallowed. Ten healthy subjects were studied on 3 separate occasions, 4 wk apart. Five hours after a high-fat breakfast (56 g fat), the subjects were given 1 of 3 test meals allocated in random order: water, a lunch containing a modest amount of fat (38 g), or MSF (38 g fat). Blood was collected for 3 h poststimulus for hormone and metabolite analyses.

RESULTS

Plasma insulin and pancreatic polypeptide concentrations peaked at 250% and 209% of baseline concentrations within 15 min of MSF. The plasma glucose concentration increased significantly (P = 0.038) in parallel with the changes observed in the plasma insulin concentration. The nonesterified fatty acid concentration was significantly suppressed (P: = 0.006); maximum suppression occurred at a mean time of 114 min after MSF. This fall in nonesterified fatty acid was accompanied by a fall in the plasma glucagon concentration from 122 to 85 pmol/L (P = 0.018) at a mean time of 113 min after MSF.

CONCLUSIONS

Effects on substrate metabolism after MSF in the postprandial state differ from those usually reported in the postabsorptive state. The effects of MSF were prolonged beyond the period of the cephalic response and these may be relevant for longer-term metabolic regulation.

Meal patterns and meal-induced metabolic changes in calves fed milk ad lib

The feeding behavior of 11 calves fed milk ad lib was characterized and analyzed at the age of 5 weeks, and the short-term changes in the plasma concentrations of various metabolites (glucose, lactate, free fatty acids, triglycerides, beta-hydroxybutyrate) and insulin in relation to a representative spontaneous milk meal were measured during the following week. In a 6-day period, the calves consumed 287 (=86%) of a total of 335 milk meals during the light phase from 0500-2200 [on average, 4.4 +/- 0.5 (mean +/- SEM) meals]. The meal size and duration during light were 2.0 +/- 0.3 kg and 5.3 +/- 0.3 min, respectively. However, only 0.7 +/- 0.1 milk meals of similar size and duration were consumed during the dark phase. The plasma concentrations of insulin and glucose increased in response to the spontaneous milk meal and remained elevated for at least 2 h after meal end. The plasma concentrations of triglycerides, free fatty acids, and beta-hydroxybutyrate also increased after meal termination, and remained elevated until 40 min (triglycerides, free fatty acids) and 60 min (beta-hydroxybutyrate) after meal end, respectively. The observed spontaneous milk intake patterns were similar to the natural suckling behavior described for calves, suggesting that the conditions of the present experiment did not disrupt the animals' natural feeding behavior. Some of the profound metabolic changes in relation to a spontaneous milk meal might contribute to the control of milk intake in calves, but further experiments are necessary to test this idea.

Muscarinic receptors and insulin concentration in the rat pancreas after chronic alcohol intake and cholinergic stimulation

The effects of chronic alcohol intake and carbachol stimulation on pancreatic muscarinic receptor binding and insulin concentrations were studied in the rat pancreas. There was a strong correlation between the number of muscarinic receptors and the concentration of insulin in the pancreas. The concentration of insulin decreased in the pancreas after long-term ethanol exposure and increased after carbachol stimulation. These results indicate that the secretion of insulin is mediated via the muscarinic receptor pathway, and that the changes in the number of muscarinic receptors may have a role in insulin deficiency after long-term alcohol consumption.

Role of the liver in the metabolic control of eating: what we know--and what we do not know

Profound metal-related changes in the supply of metabolites to t he liver and in the hepatic metabolism occur, and there is ample evidence that neural signals from hepatic metabolic sensors can affect eating. Hepatic afferent nerves presumably represent glucosensors which contribute to the control of eating by monitoring their own glucose utilization. Yet, the nature of the putative sensors that respond to the oxidation of other metabolites than glucose had not been identified. ATP and sodium pump activity may link hepatic oxidative metabolism and membrane potential, because hepatic phosphate-trapping by 2,5-anhydro-mannitol, and inhibition of sodium pump activity by ouabain is associated with a stimulation of eating. Hepatocyte membrane potential is also subject to changes in transmembranal potassium flow through volumetrically controlled membranal potassium channels. Yet it is unknown if and how hepatocytes are linked to afferent nerves. It is also unclear how the effects of glucagon and insulin fit into the hepatic metabolic control of eating. Glucagon appears to induce satiety through its actions in the liver, but the involved mechanism is still unclear. Recent studies suggest that insulin, which has mainly been explored as a centrally acting long-term satiety signal, has an immediate effect on meal size, but is presently unknown whether an hepatic action of insulin is involved.

Synergistic interaction of glucose and neurohumoral agonists to stimulate islet phosphoinositide hydrolysis

The interaction between neurohumoral agonists and glucose to stimulate phosphoinositide (PI)-specific phospholipase C (PLC) and insulin release was examined. In freshly isolated rat islets, maximal glucose (40 mM), cholecystokinin (CCK; 300 nM), or carbachol (CCh; 1 mM) stimulated PI hydrolysis 6.5-, 9.8-, and 5.7-fold, respectively, above basal. The combination of glucose and CCK or of glucose and CCh, but not of CCK and CCh, synergistically increased PI hydrolysis 23.2- and 21.6-fold, respectively, indicating that these secretagogues activate PLC by distinct pathways and that there is an interaction between them. This synergy was maximal at physiological concentrations of stimulatory glucose (8-10 mM) and was paralleled by a marked synergistic stimulation of insulin secretion. The enhanced PI response was partially Ca2+ dependent and may involve the activation of distinct isozymes of PLC, which we identify in islets. These studies demonstrate for the first time a unique and highly sensitive synergistic interaction between glucose and neurohumoral agonists to stimulate PI hydrolysis, and they suggest that enhanced PI hydrolysis is important in the potentiation of glucose- and neurohumoral-stimulated insulin secretion.

Postprandial thermogenesis and conditioned taste aversion or preference

Postprandial thermogenesis is under the control of the autonomic nervous system and alimentary conditioned stimuli change sympathetic and parasympathetic activity. Here we studied the effect of conditioned taste aversion on postprandial thermogenesis in rats. Two groups of animals were used, rats of the first group were controls, these were placed on a standard diet and, for some days, on two other different diets: one thiamine-free and the other thiamine-rich. Each diet had a different taste. The treated animals belonged to the second group, these were fed with the same three diets but for different lengths of times: thiamine-free diet for the first 5 wk afterwards, with thiamine-rich diet for 3 wk, and finally with laboratory standard diet for a few days. After a preference test with the three familiar diets, oxygen consumption rate and brown adipose tissue temperature were evaluated three times in both groups after ingestion of a test meal, each time with one of the three different diets. The preference test was unvaried for the three different familiar foods in controls, while the treated animals showed a lower preference for thiamine-free food than for the other two. Treated rats had a significantly higher increase in O2 consumption rate than controls. In this group intake of thiamine-free food induced a significantly lower increase in O2 consumption than the other two. The increase in brown adipose tissue temperature was also higher in treated than in control animals but in treated rats this was lower after intake of thiamine-free food than after the intake of the other two.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin secretion by rat islet isografts of a defined endocrine volume after transplantation to three different sites

We have analysed the graft function of rat islet isografts of identical and well-defined endocrine volumes after transplantation to three different sites (kidney, liver and spleen). Graft endocrine mass was determined by measuring the total islet volume prior to transplantation and was chosen to be similar to the endocrine volume in the normal adult rat pancreas. Graft function was tested in unanaesthetized, unstressed rats by the responses to glucose infusion and to a meal. All transplanted animals returned to normoglycaemia within one week after transplantation. At one month, basal glucose and insulin levels were similar to controls in rats with grafts to the spleen, but higher in rats with grafts to the kidney or liver. Irrespective of the transplantation site, recipients had higher glucose and lower insulin levels than controls in response to glucose infusion, but in response to a meal these differences from normal were less obvious. Finally, recipients showed both an acute insulin response to glucose infusion as well as a pre-absorptive insulin release after food ingestion, irrespective of the transplantation site. Our findings indicate that the insulin response to glucose infusion and to a meal is quantitatively reduced, but qualitatively intact after transplantation to the kidney, liver or spleen.

Morphology and distribution of efferent vagal innervation of rat pancreas as revealed with anterograde transport of Dil

Vagal efferent innervation of the pancreas was labeled by anterograde transport of Dil injected into the dorsal motor nucleus (dmnX). While over the entire organ only 19 +/- 3 (or 8 +/- 1%) of the 231 +/- 17 interlobular ganglia received Dil-labeled vagal fibers and terminals, the proximal duodenal lobe (or head) was significantly more densely innervated. Laser scanning confocal microscopy revealed further morphological details of the vagal terminals and their target ganglion cells. No vagal fibers or terminals were found in islets and acinar tissue.

Inhibitory effects of cholecystokinin develop through interaction with duodenal signals

I previously reported that cholecystokinin octapeptide (CCK-8) had little effect on feeding motivation as indexed by runway performance, but that substantial impairment resulted from the interaction of CCK-8 and prefeeding of sucrose. If the sucrose was sham-fed, potentiation of the effect of CCK-8 was seen only after large doses. The current experiments examined whether synergy between 1 microgram/kg CCK-8 and prefeeding could be reproduced using gastric, duodenal, or hepatic-portal infusions. Gastric and duodenal infusions of 30% sucrose were effective; in conjunction with CCK-8 they decreased running speed as substantially as prefeeding did. The duodenal effect was specific. Glucose appeared to be as effective as sucrose, whereas equi-osmotic saline and mannitol were not. Intraportal glucose was relatively ineffective; interaction with CCK-8 was seen only after very large infusions, and the resulting running speeds did not approach those seen after prefeeding or gastrointestinal infusions. These results suggest that decreased feeding motivation after 1 microgram/kg CCK-8 develops through synergy between this peptide and a carbohydrate-sensitive signal generated within the small intestine.

Thyroidal dependence of glucose-induced insulin secretion in starved rats

To elucidate the possible role of thyroid hormone in insulin secretion during starvation, we investigated the glucose-induced insulin release in T3-administered starved rats. The rats were fasted for 72 hr, and one-half of the starved rats were administered subcutaneous 3,5,3'-triiodothyronine (5.0 micrograms/kg) at 12, 36, and 60 hr. In an intravenous glucose (0.5 g/kg) infusion test, insulin response was significantly lower in starved rats than in fed controls. This decreased insulin was restored by T3 administration. These results suggest the thyroidal dependence of insulin response to glucose in short-term starvation.

Ventromedial hypothalamic and paraventricular nucleus lesions damage a common system to produce hyperphagia

We investigated the anatomical basis of paraventricular (PVN) and ventromedial (VMH) hypothalamic hyperphagia. Asymmetrical electrolytic lesions, damaging the VMH and PVN contralaterally, produced significant hyperphagia and weight gains (mean = 257.2 g) almost three times those of controls (89.8 g) during 56 postsurgical days. Weight gain in these rats was not significantly different from that in rats with bilateral lesions of the VMH (277.2 g) or PVN (188.2 g). Combined bilateral destruction of the PVN and VMH produced weight gain (272.8 g) almost identical to that seen after bilateral VMH lesions alone. The lack of additivity of these combined lesions and the effectiveness of the asymmetrical lesions are consistent with the hypothesis that lesions of either of these two regions damage a longitudinally running system to produce elevated food intake and body weight. Cell bodies of this system may lie within the PVN and send efferent projections through the VMH. Hyperinsulinemia developed only in rats with bilateral damage in the VMH. Thus, hypothalamic hyperphagia and hyperinsulinemia appear to be dissociable, reflecting damage to separate neural systems.

Hyperglycemia and hyperinsulinemia increase glucose utilization in fetal rat tissues

In vivo measurement of glucose utilization by individual tissues of 19-day rat fetuses have been performed using radioactive 2-deoxy-D-glucose technique. In the basal state, glucose metabolic index was 13.6 +/- 0.5 ng.min-1.mg-1 for the whole fetus, 21 +/- 1 in the hindlimb muscles, 13 +/- 2 in the liver, and 16 +/- 2 in the brain, whereas the fetal heart had the highest value: 62 +/- 5 ng.min-1.mg-1. To raise the fetal glycemia, the basal maternal blood glucose concentration of 0.78 +/- 0.02 g/l was elevated to 1.04 +/- 0.02 g/l by mean of hyperglycemic clamps. The fetal hyperglycemia increased glucose metabolic index by 30-100% over basal values in all the tissues tested except in the brain. To raise fetal insulinemia, maternal euglycemic clamp with supraphysiological insulin concentrations were performed, then a fraction (1%) of exogenous insulin crossed the placenta. Fetal plasma insulin concentrations were thus elevated to 180 +/- 32 and 255 +/- 23 microU/ml. The fetal heart increased significantly its glucose metabolic index in response to the lower insulin level. Glucose metabolic index in hindlimb muscles and liver was increased by 50-100% for the highest insulin level, whereas the brain was unaffected by exogenous insulin. We conclude that glucose metabolic index is stimulated by physiological hyperglycemia in individual fetal tissues and that fetal tissues (heart, liver, and muscle) are sensitive to exogenous insulin.

Effect of porcine gastric inhibitory polypeptide on beta-cell function in type I and type II diabetes mellitus

The effect of highly purified natural porcine GIP on C-peptide release was examined in six type I (insulin-dependent) diabetics (IDD) with residual beta-cell function, six type II non-insulin-dependent) diabetics (NIDD), and six normal subjects. All subjects were normal weight. From -120 minutes to 180 minutes glucose or insulin was infused IV to achieve a constant plasma glucose level of 8 mmol/L. On two separate days GIP (2 pmol/kg/min) or isotonic NaCl at random were infused from 0 to 30 minutes. After 10 minutes of GIP infusion plasma IR-GIP concentrations were in the physiologic postprandial range. At 30 minutes a further increase in IR-GIP to supraphysiologic levels occurred. In all subjects plasma, C-peptide increased more after 10 minutes of GIP infusion (IDD, 0.48 +/- 0.05; NIDD, 0.79 +/- 0.11; normal subjects, 2.27 +/- 0.29 nmol/L) than on the corresponding day with NaCl infusion (IDD, 0.35 +/- 0.03; NIDD, 0.62 +/- 0.08; normal subjects, 1.22 +/- 0.13 nmol/L, P less than .05 for all). The responses of the diabetics were significantly lower than that of the normal subjects (P less than .001 for both groups). No further increase in C-peptide occurred during the remaining 20 minutes of the GIP infusion in the diabetic subjects (IDD, 0.49 +/- 0.05; NIDD, 0.83 +/- 0.10 nmol/L). In the presence of a plasma glucose concentration of 8 mmol/L, physiologic concentrations of porcine GIP caused an immediate but impaired beta-cell response in IDD and NIDD patients.

Role of autonomic nervous system in postprandial thermogenesis in dogs

Two phases in postprandial thermogenesis have been recently identified in dogs; an initial cephalic phase lasting 45 min and a subsequent digestive phase occurring after 45 min. The objective of this study was to determine the role of the autonomic nervous system during these two phases in dogs. O2 uptake (VO2) as well as respiratory quotient (RQ) were monitored at least 1 h before and 2 h after a meal of 1,034 kcal under the following infusion conditions: saline, propranolol, atropine, propranolol plus atropine, phenoxybenzamine, and propranolol plus phenoxybenzamine. The initial peak value for VO2 increase was 100% in the cephalic phase and 40% in the digestive phase during the saline infusion. The VO2 response during the initial phase was 58 and 56% less with propranolol and atropine, respectively, compared with the control experiment. A 36% decrease in the VO2 response was found during the digestive phase with propranolol, whereas it was abolished by atropine. Propranolol and atropine given together decreased the VO2 response during the cephalic phase by 27% and abolished it completely in the digestive phase. Phenoxybenzamine did not affect the VO2 or RQ responses during the whole period and when given in combination with propranolol the same result as propranolol alone was found. These findings indicate that both the parasympathetic and sympathetic nervous system participate in the control of postprandial thermogenesis during both the cephalic and digestive phases.

Effect of insulin on in vivo glucose utilization in individual tissues of anesthetized lactating rats

Glucose utilization rate has been measured in skeletal muscles, white adipose tissue, and mammary gland of anesthetized nonlactating and lactating rats. During lactation, basal glucose utilization is decreased by 40% in periovarian white adipose tissue and by 65% in epitrochlearis and extensor digitorum longus but not in soleus muscle. This may be related to the lower blood glucose and plasma insulin concentrations observed during lactation. Basal glucose utilization rate in the mammary gland was, respectively, 18 +/- 2 and 350 +/- 50 micrograms/min in nonlactating and lactating rats. During the euglycemic hyperinsulinemic clamp, a physiological increment in plasma insulin concentration (231 +/- 18 in lactating vs. 306 +/- 24 microU/ml in nonlactating rats) induces a similar increase in glucose utilization rate in skeletal muscles (except soleus) and white adipose tissue in the two groups of rats. Furthermore this low increase in plasma insulin concentration does not alter mammary glucose utilization rate in nonlactating rats but induces the same increase (sevenfold over basal) as a maximal insulin concentration in lactating rats. These data show that the active mammary gland is the most insulin-sensitive tissue of the lactating rat that has been tested. The overall increase in insulin sensitivity and responsiveness that has been described in lactating rats can then mainly be attributed to the presence of the active mammary gland.

Increased insulin sensitivity and responsiveness during lactation in rats

In 12-day lactating rats blood glucose and plasma insulin were decreased by, respectively, 20 and 35% when compared with nonlactating rats, despite a 25% increase of their glucose turnover rate. Then, by using the euglycemic hyperinsulinemic clamp technique, dose-response curves for the effects of insulin on glucose production and utilization in lactating and nonlactating rats were performed. Glucose production rate was totally suppressed at 250 microU/ml of insulin in lactating rats and for plasma insulin concentrations higher than 500 microU/ml in nonlactating rats. Plasma insulin level inducing half-maximal inhibition of glucose production was decreased by 60% during lactation. The maximal effect of insulin on glucose utilization rate and glucose metabolic clearance rate was, respectively, increased 1.5- and 2.4-fold during lactation and was obtained for plasma insulin concentrations lower in lactating than in nonlactating rats (250 vs. 500 microU/ml). Insulin concentrations inducing half-maximal stimulation of glucose utilization and glucose metabolic clearance were decreased by 50% during lactation. In conclusion, this study has shown that insulin sensitivity and responsiveness of liver and peripheral tissues are improved at peak lactation in the rat.

Palatability and postprandial thermogenesis in dogs

The role of palatability on postprandial thermogenesis was determined in six mongrel dogs of approximately 16 kg. Oxygen uptake (VO2) was continuously monitored by indirect calorimetry for 1 h before and 2 h after a standard meal of 1,390 kcal. In the first experiment each dog was given access to the meal, which was ingested within 5 min. In a second experiment the same meal was sham fed and collected into an esophageal pouch. In a third experiment the dogs were tube fed. In the first experiment a biphasic response was found in the postprandial increase in metabolic rate; a first phase lasting approximately 40 min and the other from 40 to 125 min. In the second experiment the increase in VO2 was comparable with that of the first experiment for the first 40 min but almost abolished for the remaining period; the sight and smell of food alone produced a similar effect. In the third experiment tube feeding caused a small increase in VO2, which was four times smaller than that found in both the first and the second experiment during the initial phase. However, during the second phase the increase in VO2 was comparable with that of experiment 1. An initial phase of feeding lasting approximately 40 min is identified with food palatability, whereas the second phase would correspond to a large extent to specific dynamic action of food.