The effect of fasting, diet, and actinomycin D on insulin secretion in the rat.
01 May 1970-Journal of Clinical Investigation (American Society for Clinical Investigation)-Vol. 49, Iss: 5, pp 881-889
TL;DR: The results suggest that the impairment of glucose-stimulated insulin secretion during fasting and its restoration by refeeding are regulated by changes in a glucose-inducible enzyme system in the pancreatic beta cell.
Abstract: A BST R A C T The present studies were performed to elucidate the mechanisms responsible for the impairment of glucose-stimulated insulin secretion observed in fasting. Rats fasted for 48 hr displayed marked impairment in their insulin secretory response to both oral and intravenous glucose. Glucose-stimulated insulin secretion was restored within 24 hr by refeeding; actinomycin D given before refeeding blocked the expected return of normal glucose-stimulated insulin secretion despite adequate food intake. Fasted rats refed a diet devoid of carbohydrate failed to display a return of normal insulin secretory responsiveness to oral glucose in contrast to rats fed isocalorically a high carbohydrate diet. Differences in insulin secretion in fed, fasted, and fasted-refed rats could not be attributed to changes in pancreatic insulin content. There was no significant difference in the insulin secretory response to aminophylline of fed, fasted, or fasted-refed rats. The intermittent pulsing of fasted rats with hyperglycemic episodes by the injection of small amounts of glucose (500 mg) intraperitoneally every 8 hr ameliorated the impairment of glucose-stimulated insulin secretion characteristic of the fasting state. These results suggest that the impairment of glucose-stimulated insulin secretion during fasting and its restoration by refeeding are regulated by changes in a glucose-inducible enzyme system in the pancreatic beta cell.
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TL;DR: The data suggest a reciprocal relationship between insulin in the extracellular fluid and the concentration of insulin receptors per cell, which is mediated at the target cell itself by intracellular insulin-sensitive regulatory processes and directly affects target-cell sensitivity to hormone.
Abstract: Chronic (5-16 hr) exposure of cultured human lymphocytes to 10-8 M insulin at 37° in vitro produced a decrease in insulin receptor concentrations unaccounted for by simple occupancy of sites; acute exposure (0-2 hr) was without effect. These results reproduced observations in vivo where chronic hyperinsulinemia (e.g., 10-8 M insulin in the circulation of obese insulinresistant hyperglycemic mice) is associated with a substantial reduction in the concentration of insulin receptors per cell, while acute hyperinsulinemia in vivo has no effect on receptor concentration. These data suggest a reciprocal relationship between insulin in the extracellular fluid and the concentration of insulin receptors per cell, which is mediated at the target cell itself by intracellular insulin-sensitive regulatory processes and directly affects target-cell sensitivity to hormone.
1,053 citations
TL;DR: The results establish that in the rat, the high circulating concentration of FFA that accompanies food deprivation is a sine qua non for efficient GSIS when a fast is terminated and raise the possibility that imbalances between the two fuels in vivo could have pathological consequences.
Abstract: We asked whether the well known starvation-induced impairment of glucose-stimulated insulin secretion (GSIS) seen in isolated rat pancreas preparations also applies in vivo. Accordingly, fed and 18-24-h-fasted rats were subjected to an intravenous glucose challenge followed by a hyperglycemic clamp protocol, during which the plasma-insulin concentration was measured. Surprisingly, the acute (5 min) insulin response was equally robust in the two groups. However, after infusion of the antilipolytic agent, nicotinic acid, to ensure low levels of plasma FFA before the glucose load, GSIS was essentially ablated in fasted rats, but unaffected in fed animals. Maintenance of a high plasma FFA concentration by coadministration of Intralipid plus heparin to nicotinic acid-treated rats (fed or fasted), or further elevation of the endogenous FFA level in nonnicotinic acid-treated fasted animals by infusion of etomoxir (to block hepatic fatty acid oxidation), resulted in supranormal GSIS. The in vivo findings were reproduced in studies with the perfused pancreas from fed and fasted rats in which GSIS was examined in the absence and presence of palmitate. The results establish that in the rat, the high circulating concentration of FFA that accompanies food deprivation is a sine qua non for efficient GSIS when a fast is terminated. They also serve to underscore the powerful interaction between glucose and fatty acids in normal beta cell function and raise the possibility that imbalances between the two fuels in vivo could have pathological consequences.
366 citations
TL;DR: The findings reaffirm the essentiality of an elevated plasma FFA concentration for GSIS in the fasted rat and show that the insulinotropic effect of individual fatty acids spans a remarkably broad range, increasing and decreasing dramatically with chain length and degree of unsaturation, respectively.
Abstract: Lowering of the elevated plasma FFA concentration in 18- 24-h fasted rats with nicotinic acid (NA) caused complete ablation of subsequent glucose-stimulated insulin secretion (GSIS). Although the effect of NA was reversed when the fasting level of total FFA was maintained by coinfusion of soybean oil or lard oil (plus heparin), the more saturated animal fat proved to be far more potent in enhancing GSIS. We therefore examined the influence of individual fatty acids on insulin secretion in the perfused rat pancreas. When present in the perfusion fluid at 0.5 mM (in the context of 1% albumin), the fold stimulation of insulin release from the fasted pancreas in response to 12.5 mM glucose was as follows: octanoate (C8:0), 3.4; linoleate (C18:2 cis/cis), 5.3; oleate (C18:1 cis), 9.4; palmitate (C16:0), 16. 2; and stearate (C18:0), 21.0. The equivalent value for palmitoleate (C16:1 cis) was 3.1. A cis--> trans switch of the double bond in the C16:1 and C18:1 fatty acids had only a modest, if any, impact on their potency. A similar profile emerged with regard to basal insulin secretion (3 mM glucose). When a subset of these fatty acids was tested in pancreases from fed animals, the same rank order of effectiveness at both basal and stimulatory levels of glucose was seen. The findings reaffirm the essentiality of an elevated plasma FFA concentration for GSIS in the fasted rat. They also show, however, that the insulinotropic effect of individual fatty acids spans a remarkably broad range, increasing and decreasing dramatically with chain length and degree of unsaturation, respectively. Thus, for any given level of glucose, insulin secretion will be influenced greatly not only by the combined concentration of all circulating (unbound) FFA, but also by the makeup of this FFA pool. Both factors will likely be important considerations in understanding the complex interplay between the nature of dietary fat and whole body insulin, glucose, and lipid dynamics.
357 citations
TL;DR: It is concluded that in the rat other factors besides a rise in nutrient content in the blood produce insulin release in the first minutes after food ingestion.
Abstract: Blood glucose and insulin levels were measured in undisturbed and free-moving rats. The insulin level rises already in the 1st min after the start of food ingestion, whereas the glucose level begins to increase only in the 3rd min if carbohydrate-rich food is eaten. This early rise in insulin level is observed also under conditions in which either carbohydrate-free food or even "food" without any caloric value is offered. The smell of food cannot produce this early insulin response. It is concluded that in the rat other factors besides a rise in nutrient content in the blood produce insulin release in the first minutes after food ingestion.
226 citations
TL;DR: It is concluded that alternative splicing and/or the use of distinct tissue-specific promoters generate structurally distinct mRNA species in liver and islets of Langerhans and that tissue- specific transcription mechanisms result in inducible expression of the glucokinase gene in liver but not in islets during the fasting-refeeding transition.
Abstract: Glucokinase, a key regulatory enzyme of glucose metabolism in mammals, provides an interesting model of tissue-specific gene expression. The single-copy gene is expressed principally in liver, where it gives rise to a 2.4-kilobase mRNA. The islets of Langerhans of the pancreas also contain glucokinase. Using a cDNA complementary to rat liver glucokinase mRNA, we show that normal pancreatic islets and tumoral islet cells contain a glucokinase mRNA species approximately 400 nucleotides longer than hepatic mRNA. Hybridization with synthetic oligonucleotides and primer-extension analysis show that the liver and islet glucokinase mRNAs differ in the 5' region. Glucokinase mRNA is absent from the livers of fasted rats and is strongly induced within hours by an oral glucose load. In contrast, islet glucokinase mRNA is expressed at a constant level during the fasting-refeeding cycle. The level of glucokinase protein in islets measured by immunoblotting is unaffected by fasting and refeeding, whereas a 3-fold increase in the amount of enzyme occurs in liver during the transition from fasting to refeeding. From these data, we conclude (i) that alternative splicing and/or the use of distinct tissue-specific promoters generate structurally distinct mRNA species in liver and islets of Langerhans and (ii) that tissue-specific transcription mechanisms result in inducible expression of the glucokinase gene in liver but not in islets during the fasting-refeeding transition.
188 citations
References
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TL;DR: A two antibody system of insulin assay for immunoassay of insulin induces the production of specific nonprecipitating antibodies, both in experimental animals and in humans.
Abstract: It has been demonstrated that insulin induces the production of specific nonprecipitating antibodies, both in experimental animals and in humans.\" Lysis of sensitized red blood cells, paper electrophoresis,\" and salt precipitation have been used in methods of immunoassay of insulin. In a preliminary report the authors described a two antibody system of insulin assay. The present paper describes the method in greater detail and presents results of assays of rat plasma.
2,590 citations
TL;DR: The present author has been able to obviate the difficulties of the Hawkins and Van Slyke method and yet retain the principle involved, and the resulting method is simple, easy, and accurate.
1,428 citations
TL;DR: Over 50 years ago, Benedict published his extensive monograph on the metabolism of fasting in man, in which he demonstrated that carbohydrate stores provide a small but significant component of the body's fuel for only the first few days.
Abstract: Over 50 years ago, Benedict (2) published his extensive monograph on the metabolism of fasting in man, in which he demonstrated that carbohydrate stores provide a small but significant component of the body's fuel for only the first few days. Thereafter, protein and fat are the sole sources of fuel, the former contributing 15% of the calories and the latter the balance. The primary role of fat as fuel was apparent to Benedict and his contemporaries; it is plentiful and expendable. The significance of the protein requirement, however, was less clear; in fact, it was not fully understood until nearly 20 years later when the obligatory dependence of the central nervous system on glucose was firmly established (3). Since glycogen stores in man were known to approximate only 200 g, it was readily apparent that glucose has to be derived from protein in order to maintain cerebral metabolism during a prolonged fast. More recently, our understanding of the fasted state has been further clarified by the demonstration that free fatty acid is both the major transport form of lipid leaving adipose tissue (4, 5) and a substrate that is
996 citations
TL;DR: In the noninsulin-requiring maturity-onset diabetic, the glycemic insulinogenic stimulus for a given oral glucose load is significantly greater than in normal subjects and accounts for the excessive plasma insulin responses observed late in the course of an oral glucose tolerance test.
Abstract: The plasma insulin responses of normal weight and obese, diabetic, and nondiabetic subjects to intravenous glucose was only 30-40% of that seen after oral glucose, indicating that alimentary mechanism(s) in addition to the arterial blood sugar concentration regulate insulin secretion. Observations made in subjects with diverted portal circulation indicate that the alimentary insulinogenic mechanism is located in the intestinal tract. The insulinogenic potency of the alimentary and glycemic stimuli expressed in terms of insulin secretion per gram of glucose were remarkably similar within each group of individuals. Between these groups, however, there were considerable differences. Obesity, with or without associated diabetes, was associated with a true hypersecretory responsiveness, whereas diabetes was characterized, with or without obesity, by a marked impairment in insulin secretion. The experimental design used in these studies permitted quantitation of the magnitude of the glycemic component of an oral glucose load. As a consequence of impaired insulin secretion, a greater than normal proportion of the oral glucose load escapes initial hepatic extraction in the maturity-onset diabetic and enters the peripheral circulation. Therefore, in the noninsulin-requiring maturity-onset diabetic, the glycemic insulinogenic stimulus for a given oral glucose load is significantly greater than in normal subjects and accounts for the excessive plasma insulin responses observed late in the course of an oral glucose tolerance test.
948 citations
TL;DR: A new method for evaluating glucose metabolism in man using an oral glucose load and the calculation of a blood glucose disappearance rate constant is described, which makes it possible to compare quantitatively the response to oral and intravenous glucose administration in a given individual.
Abstract: This paper describes a new method for evaluating glucose metabolism in man using an oral glucose load. The procedure permits the calculation of a blood glucose disappearance rate constant (K) and thereby makes it possible to compare quantitatively the response to oral and intravenous glucose administration in a given individual. Ten metabolically normal adult humans were studied under carefully controlled conditions. Each received similar amounts (20 g) of glucose both orally and intravenously (2–7 days apart) by constant infusion for 1 hr. The effects on blood glucose disappearance rate constants (K) and plasma insulin concentrations (immunoassay) during and for 1 hr following the infusion were compared. Blood glucose concentrations and K values with the 2 routes of glucose administration were similar. In contrast, plasma insulin responses showed a significant difference: oral glucose resulted in a significant and sustained rise, whereas intravenous glucose was associated with a smaller and transient inc...
848 citations