RESUMEN
The recent identification of the mouse obese (ob) gene, whose product is a approximately 16 kd protein secreted from adipose tissue, and the demonstration that the administration of recombinant OB protein inhibits food intake have led us to report a 1979 pilot study demonstrating an extractable activity from rat adipose tissue that inhibited food intake in the normal mouse. Two hundred grams of rat adipose tissue and 40 gm of rat muscle were extracted with acid-ethanol. The aqueous phase was lyophilized, and a filtered solution of the crude powder was injected subcutaneously daily into normal male mice. Administration of the extract from adipose tissue, but not from muscle tissue, resulted in a significant reduction in food intake and slowing of the rate of increase in body weight over the 10-day experimental period. The quantity of extracted fat represented by the daily dose was equivalent to 1.65 gm of fat administered to a 24-gram mouse. In retrospect this acid-ethanol extractable activity was probably the native OB protein in normal rat adipose tissue.
Asunto(s)
Tejido Adiposo/química , Ingestión de Alimentos/efectos de los fármacos , Extractos de Tejidos/farmacología , Animales , Peso Corporal/efectos de los fármacos , Masculino , Ratones , Ratones Endogámicos BALB C , Ratas , Ratas Sprague-Dawley , Extractos de Tejidos/análisisRESUMEN
In the transition from the fasting to the fed state, plasma glucose levels rise, and the liver converts from an organ producing glucose to one of storage. To determine the effect of glucose on hepatic glucose uptake, radiolabeled glucose, 2-deoxyglucose, and 3-O-methylglucose were injected into perfused rat livers during different nontracer glucose levels, and the concentrations in the outflow were measured. A mathematical model was developed that described the behavior of the injected compounds as they traveled through the liver and was used to simulate and fit the experimental results. The rates of membrane transport, glucokinase, glucose-6-phosphatase, and the consumption of glucose 6-phosphate were estimated. Membrane transport for all of the tracers decreased as nontracer glucose increased, demonstrating competitive inhibition of the glucose transporter. In contrast, the consumption of injected [2-14C]glucose increased when glucose was elevated, demonstrating that glucose caused an activation of enzyme activity that overcame the competitive inhibition of transport and phosphorylation. When glucose was elevated, the rate coefficient of glucokinase did not decrease, indicating that glucokinase was stimulated by glucose. Both changes would lead to the increased glycogen synthesis and decreased glucose production rate observed in vivo during the fasted-to-fed transition.
Asunto(s)
3-O-Metilglucosa/farmacocinética , Desoxiglucosa/farmacocinética , Glucosa/farmacología , Glucosa/farmacocinética , Hígado/metabolismo , Animales , Simulación por Computador , Glucoquinasa/metabolismo , Glucosa-6-Fosfato/metabolismo , Técnicas de Dilución del Indicador , Masculino , Modelos Biológicos , Perfusión , Ratas , Ratas Sprague-DawleyRESUMEN
Cholesteryl ester transfer (CET), plasma, lipoprotein lipid and phospholipid composition were studied in insulin-treated baboons with chronic streptozotocin-induced diabetes. In these diabetic animals, CET measured both as the mass (p < 0.001) and isotopic transfer (p < 0.05) of CE from HDL to the apo B-containing lipoproteins (VLDL+LDL) were significantly accelerated compared to controls and the response closely resembled that recently reported in diabetic humans. No significant differences were present in plasma triglyceride, cholesterol, or HDL-C or in lipoprotein core or surface lipid composition. Thus, despite the fact that they did not display the same spectrum of abnormalities in lipoprotein composition, these insulin-treated diabetic baboons demonstrated an abnormality in CET identical to that described in humans. These findings suggest that this non-human primate may provide a suitable diabetic animal model in which to better characterize the mechanisms that underlie this potentially atherogenic disturbance in lipoprotein transport.
Asunto(s)
Proteínas Portadoras/metabolismo , Diabetes Mellitus Experimental/enzimología , Glicoproteínas , Papio/metabolismo , Animales , Glucemia , Proteínas de Transferencia de Ésteres de Colesterol , Diabetes Mellitus Tipo 1/enzimología , Modelos Animales de Enfermedad , Femenino , Lípidos/sangre , Lipoproteínas/sangre , Masculino , Fosfolípidos/sangreRESUMEN
The platelet-activating factor inhibitor BN50730, a hetrazepine, was injected intraperitoneally daily from 30 days of age into diabetes-prone BB rats. While 96% (22/23) Tween 80 injected control rats developed diabetes, 0.05 mg/kg BN50730 decreased the frequency to 72% (17/24; n.s.) and 0.5 mg/kg to 56% (14/25; p < 0.01). Mean onset age in controls was 81 +/- 9 days (mean +/- SD), but BN50730 delayed onset to 87 +/- 15 days in the low and 93 +/- 12. days (p < 0.01) in high dose rats. The relative degree of insulitis was reduced in both low (p < 0.01) and high (p < 0.05) dose treated groups. Serum insulin in young prediabetic controls decreased from 84 +/- 34 microU/ml to 38 +/- 20 in the 22 rats developing diabetes (p < 0.001). Serum insulin in BN50730-protected compared to unprotected rats was 114 +/- 49 and 32 +/- 22 (p < 0.001) in the low, and 91 +/- 46 and 21 +/- 15 (p < 0.001) microU/ml in the high dose group, respectively. Increased serum insulin correlated with preserved islet beta cells and decreased insulitis. Treatment did not affect thyroiditis. Thus, platelet-activating factor may be involved in insulitis pathogenesis and platelet-activating factor inhibitors may decrease autoimmune beta cell destruction.
Asunto(s)
Enfermedades Autoinmunes/prevención & control , Azepinas/uso terapéutico , Diabetes Mellitus Tipo 1/prevención & control , Factor de Activación Plaquetaria/antagonistas & inhibidores , Ratas Endogámicas BB , Tetrazoles/uso terapéutico , Triazoles , Factores de Edad , Animales , Enfermedades Autoinmunes/sangre , Enfermedades Autoinmunes/genética , Enfermedades Autoinmunes/patología , Azepinas/administración & dosificación , Azepinas/farmacología , Diabetes Mellitus Tipo 1/sangre , Diabetes Mellitus Tipo 1/genética , Diabetes Mellitus Tipo 1/patología , Insulina/sangre , Islotes Pancreáticos/patología , Masculino , Factor de Activación Plaquetaria/fisiología , Ratas , Tetrazoles/administración & dosificación , Tetrazoles/farmacología , Tienopiridinas , Glándula Tiroides/patología , Tiroiditis Autoinmune/sangre , Tiroiditis Autoinmune/genética , Tiroiditis Autoinmune/patologíaRESUMEN
The cellular sequence of intraislet vascular perfusion has been shown to be important in the regulation of islet hormone secretion in the rat and dog islet. In order to test whether a B to A to D sequence of islet cellular perfusion is also present in a nonhuman primate, pancreata from the rhesus monkey, Macaca mulatta, were isolated and perfused in vitro in the presence and absence of anti-insulin gamma globulin. In the presence of the insulin antibody, efflux concentration of insulin decreased rapidly (-95 +/- 1.8%), whereas glucagon and somatostatin concentrations increased (111 +/- 28% and 239 +/- 38%, respectively). These results suggest the presence of a B-A-D cellular sequence of vascular perfusion within the monkey islet. The present results also strongly support the hypothesis that a B-A-D sequence of islet perfusion is important in the regulation of islet hormone secretion and further emphasize the central role of the B-cell in intraislet cellular interactions. The results also suggest that, despite differences in islet anatomy, a B-A-D order of islet cellular perfusion may be the preferred functional sequence among mammalian species.
Asunto(s)
Insulina/inmunología , Islotes Pancreáticos/citología , Páncreas/citología , gammaglobulinas/análisis , Animales , Comunicación Celular , Separación Celular , Femenino , Glucagón/análisis , Glucagón/metabolismo , Infusiones Intravenosas , Insulina/análisis , Insulina/metabolismo , Anticuerpos Insulínicos/administración & dosificación , Islotes Pancreáticos/química , Islotes Pancreáticos/metabolismo , Macaca mulatta , Páncreas/química , Páncreas/metabolismo , Perfusión , Radioinmunoensayo , Somatostatina/análisis , Somatostatina/metabolismo , gammaglobulinas/inmunología , gammaglobulinas/metabolismoRESUMEN
Hepatic glucose production is stimulated in vitro twice as effectively by pulsatile as by continuous glucagon, given equivalent time-averaged doses. Efficacy studies of pulsatile insulin have yielded conflicting results. In the rat hepatoma cell line H-4-II-E-C3, insulin rapidly (t1/2 15 min) inhibits transcription of the gene and lowers mRNA levels for the gluconeogenic enzyme. PEPCK via a receptor-mediated process. We attached H-4-II-E-C3 cells to Cytodex-3 microcarriers and used a perifusion column system to test whether pulsatile insulin is more or less effective than equivalent time-averaged doses of continuous insulin. PEPCK transcription was induced by inclusion of cAMP analogue 8-(4-chlorophenyl-thio)-cAMP (0.1 mM) and dexamethasone (0.5 microM) in the perifusion medium. Three columns were exposed either to continuous, pulsatile, or no insulin. After 3 h, total nucleic acid was extracted, and mRNA(PEPCK) was measured with a sensitive-solution hybridization assay. Continuous insulin inhibited PEPCK expression in a dose-dependent fashion with EC50 1 x 10(-11) M. Equivalent time-averaged amounts of insulin delivered as pulses achieved significant inhibition but less effectively than continuous insulin. The apparent EC50 for pulsatile insulin increased from 2 x 10(-11) M to 5 x 10(-11) M as the oscillatory period was raised from 5 to 20 min, respectively. These observations suggest that insulin-mediated inhibition of PEPCK gene transcription is diminished by a pulsatile mode of administration in marked contrast to the pulse enhancement demonstrated for glucagon-mediated hepatic glucose production.
Asunto(s)
AMP Cíclico/análogos & derivados , AMP Cíclico/fisiología , Dexametasona/farmacología , Insulina/farmacología , Neoplasias Hepáticas Experimentales/enzimología , Hígado/enzimología , Fosfoenolpiruvato Carboxiquinasa (GTP)/genética , ARN Mensajero/genética , Tionucleótidos/farmacología , Animales , Línea Celular , AMP Cíclico/farmacología , Relación Dosis-Respuesta a Droga , Expresión Génica/efectos de los fármacos , Hígado/efectos de los fármacos , Perfusión/métodos , ARN Mensajero/metabolismo , Ratas , Factores de TiempoRESUMEN
Spontaneous in vivo cyclic secretion of insulin and glucagon displays a pulse interval of 10 +/- 0.3 (SE) min and a constant phase relationship in fasting rhesus monkeys. When pancreata from six normal rhesus monkeys were perfused in vitro, the insulin pulse interval averaged 6.3 +/- 0.23 (SE) min. Insulin, glucagon, and somatostatin displayed high-amplitude secretory pulses, and the average pulse interval did not differ among the three islet hormones. The islet pulses are less regular in vitro than in vivo, and the phase relationship among the three hormones is lost. The relative amplitude averaged 142 +/- 10, 110 +/- 18, and 81 +/- 11% of the mean hormone concentrations for insulin, somatostatin, and glucagon, respectively. Similar differences in secretory pattern were observed during perfusion of three baboon pancreata compared with the in vivo pattern in this second primate species. The data suggest that the frequency and phase relationship of the islet pulsatile secretory system is modulated by factors extrinsic to the pancreas in the intact nonhuman primate. The nature of these modulating factors remains to be established. The apparent phase independence of the three islet hormones suggests that each of the major endocrine cell types of the islet possess independent episodic secretory mechanisms.
Asunto(s)
Glucagón/metabolismo , Insulina/metabolismo , Islotes Pancreáticos/metabolismo , Somatostatina/metabolismo , Animales , Glucemia/metabolismo , Ayuno , Glucagón/sangre , Técnicas In Vitro , Insulina/sangre , Secreción de Insulina , Cinética , Macaca mulatta , Papio , Perfusión , Periodicidad , Factores de TiempoRESUMEN
To assess the potential therapeutic use of pulsatile intravenous insulin delivery, five streptozocin-induced diabetic baboons were treated with alternate 3- to 6-wk periods of pulsatile and continuous insulin infusion. Time-averaged insulin concentrations were matched during two pulsatile administration periods (P1 and P2) and an intervening period of continuous insulin administration (C). There were no significant differences among the overall means of four daily glucose determinations performed during the three periods (P1, 5.7 +/- 1 mM; C, 5.6 +/- 0.9 mM; P2, 5.3 +/- 0.9 mM); the mean M value, a measure of the stability of glycemic control (P1, 4 +/- 1.7; C, 3.9 +/- 1.8; P2, 3.6 +/- 1.5); the percentage of glucose values less than 2.8 mM (P1, 13 +/- 8.5%; C, 14 +/- 12%; P2, 13 +/- 9.1%); or the glycosylated hemoglobin levels determined at the end of the P1 and C (7.5 +/- 3.4 and 6.5 +/- 1.8%, respectively [all values are means +/- SD]). Fasting hepatic glucose production was suppressed to a similar degree during pulsatile and continuous insulin infusion (P1, 23 +/- 3 mumol.kg-1.min-1; C, 24 +/- 8 mumol.kg-1.min-1). Arterial glucagon levels were similar during pulsatile and continuous insulin infusion, both in the fasting state (84 +/- 29 and 84 +/- 31 ng/L, respectively) and postprandially (30 +/- 14 and 27 +/- 12 ng/L, respectively). Pulsatile insulin infusion failed to entrain a corresponding glucagon secretory rhythm. These data suggest that the metabolic consequences of long-term pulsatile and continuous insulin infusion in an animal model of human non-insulin-dependent diabetes are comparable.
Asunto(s)
Glucemia/metabolismo , Diabetes Mellitus Experimental/tratamiento farmacológico , Sistemas de Infusión de Insulina , Animales , Colesterol/sangre , Diabetes Mellitus Experimental/sangre , Ayuno , Hemoglobina Glucada/análisis , Insulina/sangre , Masculino , PapioRESUMEN
The BB rat spontaneously develops insulin-dependent diabetes mellitus (IDDM) in association with marked insulitis in the islet of Langerhans. Since platelet-activating factor (PAF-acether) is involved in allergic and inflammatory reactions, we tested a PAF antagonist, Ginkgolide B (BN 52021) for potential effects on islet inflammation and diabetes. Diabetes prone BB/Wor rats were treated daily from weaning at 25 days until 105 days of age with either saline (n = 30, controls), 10 (n = 25, low dose) or 20 (n = 30, high dose) mg/kg body weight of BN 52021. The overall incidence of IDDM was unaffected by treatment. Quantitative analysis of insulin area showed a dose-dependent protection of beta cells by Ginkgolide B, reflected in a 6- (low dose) to 8-fold (high dose) (P less than 0.01-0.005) increase in the insulin/glucagon cell ratio compared to the saline treated rats. Ginkgolide B reduced severe insulitis from 84% in the saline rats developing IDDM to 59% (n.s.) in the low and to 33% (P less than 0.001) in the high dose group. These data suggest that PAF inhibitors may prove useful in immunomodulator therapy of IDDM since beta cells are preserved.
Asunto(s)
Diabetes Mellitus Tipo 1/tratamiento farmacológico , Diterpenos , Islotes Pancreáticos/efectos de los fármacos , Lactonas/farmacología , Pancreatitis/tratamiento farmacológico , Factor de Activación Plaquetaria/antagonistas & inhibidores , Animales , Diabetes Mellitus Tipo 1/complicaciones , Modelos Animales de Enfermedad , Relación Dosis-Respuesta a Droga , Eosinófilos/inmunología , Femenino , Ginkgólidos , Inmunohistoquímica , Islotes Pancreáticos/inmunología , Islotes Pancreáticos/patología , Masculino , Pancreatitis/etiología , Pancreatitis/inmunología , Ratas , Ratas Endogámicas BBRESUMEN
The effect of beta-cell deficiency on the spontaneous pulsatile secretory pattern of the islets of Langerhans was studied in the baboon. Measures of beta-cell function were correlated with the secretory pattern before and at intervals after streptozocin administration. The degree of insulin deficiency was variable and ranged from mild to moderate. Highly regular pulses were less prevalent in baboons compared with rhesus monkeys and humans, but the mean frequency was similar and was not affected by treatment. The principal effect of beta-cell destruction was to proportionately reduce the pulse amplitude of insulin (-39%, P less than .003) without detectable change in pulse frequency, interhormonal phase relationship, or the regularity of pulses. Glucagon-pulse amplitude also fell (-19%, P less than .09), but not significantly. However, glucagon-pulse amplitude was strongly correlated with insulin-pulse amplitude (r = -.59, P less than .002), whereas mean fasting plasma concentrations of insulin and glucagon were not significantly changed after treatment. Because streptozocin affects only the beta-cell, the data indicate a major influence of the insulin pulse on the alpha-cell secretory pulse. The data do not support the presence of a separate pacemaker for the alpha-cell but do not eliminate this possibility. The strong correlation of reduction in insulin-pulse amplitude with increasing fasting glucose and decreasing glucose disappearance lends support to growing evidence that the pattern of insulin secretion is an important determinant of normal glucose homeostasis.
Asunto(s)
Glucagón/metabolismo , Insulina/metabolismo , Islotes Pancreáticos/efectos de los fármacos , Estreptozocina/farmacología , Animales , Glucemia/análisis , Modelos Animales de Enfermedad , Secreción de Insulina , Islotes Pancreáticos/patología , Islotes Pancreáticos/fisiopatología , Masculino , Papio , Factores de TiempoRESUMEN
Hepatic receptors are normally exposed to discrete pulses of insulin and glucagon at intervals of 8 to 16 min. Using a multicolumn system for perifusing hepatocytes, we investigated the effect of this pattern on the normal processing of the insulin receptor. Surface-receptor binding was measured in acid-washed cells harvested from individual columns. The number of high-affinity surface receptors fell to a nadir 1 min after the end of a 3-min square-wave pulse of insulin. The maximum reduction reached 45% of baseline at amplitudes of 1000 microU/ml or above. The number of surface binding sites returned to baseline 15 min after the end of the pulse, but the affinity constant of the high-affinity receptor was unchanged. The reduction of surface binding was dose dependent, with an ED50 of 251 +/- 34 microU/ml. Prolonging the pulse to 60 min did not affect the nadir or the rate of restoration of the surface-receptor population. The change in surface binding was reduced at 15 degrees C and abolished at 4 degrees C. After a pulse, the pattern of change was a period of rapid decline to a nadir (t1/2 less than or equal to 1 min) that persisted for 3-5 min, followed by restoration of surface binding that reached baseline in 10-15 min. This same pattern was present after six ED95 pulses delivered at intervals of 15 min. These data indicate that the internalization of hepatocyte surface receptors and their recycling and reinsertion into the plasma membrane can be entrained to pulses at the physiologic pulse frequency.
Asunto(s)
Insulina/metabolismo , Hígado/metabolismo , Receptor de Insulina/metabolismo , Animales , Supervivencia Celular , Células Cultivadas , Relación Dosis-Respuesta a Droga , Insulina/administración & dosificación , Hígado/citología , Masculino , Ratas , Ratas Endogámicas , Receptor de Insulina/efectos de los fármacosRESUMEN
Although chronic postprandial elevation of branched-chain amino acids (BCAAs) occurs in diabetic subjects and in subjects consuming high-protein diets, the metabolic effects of simultaneously increasing levels of these three amino acids are unclear. In this study, a mixture of the BCAAs was infused intravenously into baboons, beginning 30 minutes after the daily meal and continuing for 200 minutes on four consecutive days. Blood samples were collected on the last day of treatment. Infusion of the BCAAs into fed baboons promoted an increase in peak levels of glucagon, a decrease in postprandial levels of seven amino acids, and an increase in plasma levels of 3-OH butyrate. The ketone body response occurred despite an increase in the plasma ratio of insulin/glucagon in four of the five animals and was not associated with a change in the rate of lipolysis as indicated by plasma glycerol measurements. These findings raise the possibility that ketone bodies are one of the metabolic products of BCAA metabolism induced by high concentrations of leucine or ketoisocaproate. The observation that chronic elevation of BCAAs augments glucagon secretion may explain the parallel increases in plasma glucagon and plasma BCAAs observed in subjects fed high protein diets.
Asunto(s)
Aminoácidos de Cadena Ramificada/farmacología , Glucagón/sangre , Hidroxibutiratos/sangre , Ácido 3-Hidroxibutírico , Animales , Ingestión de Alimentos , Insulina/sangre , Cuerpos Cetónicos/metabolismo , Masculino , PapioRESUMEN
Pulsatile administration of glucagon to perifused rat hepatocytes stimulates hepatocyte glucose production (HGP) more effectively than continuous administration. Having established that this effect was due to delayed relaxation of glucagon-stimulated HGP (t1/2 for decay = 3.54 +/- 0.60 min) we wished to examine the mechanism of response termination. Delayed dissociation of glucagon from its receptor was excluded by the brisk washout of [125I]glucagon from perifusion columns (t1/2 = 1.00 +/- 0.13) and the rapid decay in glucagon-stimulated cAMP released into the perifusion medium (t1/2 = 1.14 +/- 0.12). The relaxation of the HGP response to a pulse of administered cAMP was comparable to the decay in glucagon-stimulated HGP (t1/2 = 3.28 +/- 0.22). Furthermore, the phosphodiesterase inhibitor isobutyl-methylxanthine did not alter the decay of the HGP response to glucagon despite increasing the amplitude of the response (t1/2 = 3.04 +/- 0.36). These data place the rate-limiting step for HGP relaxation distal to cAMP generation and degradation. The decay of the beta-hydroxybutyrate response to a glucagon pulse was not different from the cAMP response (t1/2 = 1.14 +/- 0.23), whereas the decay of gluconeogenesis from lactate was not significantly different from HGP relaxation (t1/2 = 1.94 +/- 0.08). We conclude that rate-limiting events for HGP relaxation occur distal to the second messenger cascade; however, ketogenesis is more closely coupled to the kinetics of cAMP. These results may help to explain the absence of excessive ketosis during fasting in normal humans, who secrete glucagon episodically at 10- to 14-min intervals.
Asunto(s)
Glucagón/farmacología , Glucosa/biosíntesis , Hígado/metabolismo , 1-Metil-3-Isobutilxantina/farmacología , Animales , AMP Cíclico/metabolismo , Glucagón/metabolismo , Gluconeogénesis , Glucógeno/metabolismo , Cinética , Hígado/efectos de los fármacos , Masculino , Perfusión , Ratas , Ratas Endogámicas , Receptores de la Hormona Gastrointestinal/metabolismo , Receptores de GlucagónRESUMEN
We have reported that glucagon administered to perifused rat hepatocytes as a series of pulses at 15-min intervals is a more effective stimulus for hepatocyte glucose production (HGP) than is continuous glucagon infusion. To test whether the efficiency of HGP depends upon the frequency of pulsatile glucagon delivery, we administered glucagon to perifused rat hepatocytes as a series of pulses of fixed amplitude [922 +/- 30 (+/- SE) pg/ml] at eight separate pulse intervals ranging from 3-45 min. Compared to continuous infusion of the same total amount of hormone, pulsatile glucagon administration clearly enhanced HGP in a frequency-dependent fashion. At pulse intervals between 10 and 20 min, pulsatile HGP exceeded continuous HGP by a factor of 1.5-2. This range of optimal intervals compared favorably with the glucagon secretory period of 10 min observed in nonhuman primates and that of 13-20 min observed in humans. We noted a desensitization of the hepatocyte response to glucagon that was directly proportional to the log of the time-averaged hormone concentration. Since the magnitude of the desensitization elicited by pulsatile glucagon delivery exceeded the desensitization elicited by continuous hormone delivery regardless of pulse frequency, differential desensitization could not explain the frequency dependency of the pulse enhancement effect. A mathematical simulation of our data demonstrated that the asymmetry of the HGP waveform elicited by a brief glucagon pulse could account for the observed frequency dependency of HGP. Pulse to pulse summation and the desensitization phenomenon modulated both the magnitude of the pulse enhancement effect and the frequency range over which the effect was manifest. We conclude that the enhancement of HGP by glucagon pulses is a frequency-dependent phenomenon and that the physiological glucagon secretory period optimizes HGP.
Asunto(s)
Glucagón/metabolismo , Gluconeogénesis , Hígado/metabolismo , Animales , Relación Dosis-Respuesta a Droga , Masculino , Matemática , Modelos Biológicos , Periodicidad , Ratas , Ratas Endogámicas , Factores de TiempoRESUMEN
We have reported that in the physiological concentration range pulsatile glucagon delivery (6 pulses in 90 min) is a more effective stimulus of rat hepatocyte glucose production than is continuous infusion of the same amount of hormone (pulsatile EC50 = 186 +/- 41 pg/ml, continuous EC50 = 884 +/- 190 pg/ml). At supraphysiological glucagon concentrations, however, the maximal response to continuous glucagon infusion exceeds the response to pulses (241 +/- 14 vs. 140 +/- 11 mumol X G-1 X 90 min-1). In an effort to explain these observations we derived a model for the 90-min hepatocyte responses to pulsatile and continuous glucagon delivery based on the waveform of the hepatocyte response to a transient glucagon stimulus. The model demonstrated that the time constant for response decay was an important determinant of the relative efficacy of the two patterns of hormone delivery. For the observed decay constant value of 0.132 +/- 0.02 min-1 the model predicted the following dose-response parameters: pulsatile EC50 = 131 pg/ml, Rmax = 119 mumol X G-1 X 90 min-1, continuous EC50 = 656 pg/ml, Rmax = 272 mumol X G-1 X 90 min-1. The ability of a model based only on the kinetics of a single pulse to simulate the observed dose-response relationship suggests that pulsatile stimulation is intrinsically more effective than continuous hormonal stimulation.
Asunto(s)
Glucagón/farmacología , Glucosa/biosíntesis , Hígado/metabolismo , Animales , Células Cultivadas , Relación Dosis-Respuesta a Droga , Glucagón/administración & dosificación , Hígado/citología , Hígado/efectos de los fármacos , Masculino , Modelos Biológicos , Ratas , Ratas Endogámicas , Estimulación Química , Factores de TiempoRESUMEN
Perifused anterior hemipituitaries from one male and 4 female monkeys released GH and PRL in a pulsatile pattern, with mean +/- SE interpulse intervals of 8.2 +/- 0.4 and 8.5 +/- 0.3 min, as determined by a cycle detection computer algorithm. Mean hormone concentrations in the perifusate fractions collected at 2-min intervals were 435 +/- 89 (GH) and 515 +/- 262 (PRL) ng/ml. Pulse amplitudes averaged 74 +/- 16 ng/ml for GH and 189 +/- 89 ng/ml for PRL. These findings suggest the presence of a high frequency pulsatile secretory mechanism within the primate pituitary.
Asunto(s)
Hormona del Crecimiento/metabolismo , Adenohipófisis/metabolismo , Prolactina/metabolismo , Animales , Femenino , Técnicas In Vitro , Macaca nemestrina , Masculino , Perfusión , Periodicidad , Adenohipófisis/efectos de los fármacos , Somatostatina/farmacologíaRESUMEN
Blood concentrations of the branched chain amino acids (BCAAs) are elevated during fasting in healthy subjects and are abnormally high both postprandially and during fasting in diabetic patients. Despite evidence that these amino acids influence brain metabolism and neurotransmitter synthesis, there is little information on the neuroendocrine effects of the BCAAs. This study provides evidence that elevation of postprandial blood levels of the BCAAs alters the ultradian rhythm of GH secretion observed in the baboon during daylight hours. To mimic the postprandial rise in the BCAAs that occurs in diabetic patients, we infused either saline or a mixture of valine, leucine, and isoleucine into six conscious male baboons from 1530-1900 h daily for 4 days during and after the normal feeding time. On the last day of the infusions, blood samples were collected at 20-min intervals from 0800-1500 h and at 30-min intervals from 1500-2000 h. The amino acid infusions increased postprandial blood concentrations of the BCAAs 2- to 5-fold over control levels and lowered the blood concentrations of tyrosine, phenylalanine, and lysine compared to concentrations observed during control infusions. A significant elevation in GH levels occurred in association with BCAA treatment in each animal between 0800 and 1100 h, 13 h after the previous day's infusion. Average +/- SE maximum GH levels observed between 0800 and 1100 h were 11.6 +/- 2.9 ng/ml under experimental conditions compared to a control value of 3.8 +/- 1.2 (P less than 0.02). Whether the increased GH levels represented the generation of a new peak or a phase shift in a nocturnal peak was not determined. Combined with evidence that spontaneous release of GH is neurally regulated in the baboon, this study suggests that changes in the blood levels of the BCAAs modulate neural mechanisms that regulate GH rhythmicity.
Asunto(s)
Aminoácidos de Cadena Ramificada/farmacología , Hormona del Crecimiento/metabolismo , Animales , Ingestión de Alimentos , Isoleucina/farmacología , Leucina/farmacología , Lisina/farmacología , Masculino , Papio , Fenilalanina/farmacología , Factores de Tiempo , Tirosina/farmacología , Valina/farmacologíaRESUMEN
We have compared the effects of pulsatile and continuous glucagon administration on hepatocyte glucose production in order to clarify the physiological role of pulsatile hormone secretion. Two identical columns containing freshly isolated rat hepatocytes mixed with polyacrylamide gel beads were perifused with oxygenated tissue culture medium. A fixed total amount of glucagon was delivered to one column as a continuous 90-min infusion and to the other column as a series of six 3-min pulses. A 15-min interpulse interval was chosen in order to approximate the 10- to 12-min interval observed in primates while permitting the resolution of individual hepatocyte responses. With this protocol, the EC50 values for pulsatile and continuous glucagon administration were 186 +/- 41 and 884 +/- 190 (SD) pg/ml, respectively. For glucagon concentrations less than 1,000 pg/ml, pulsatile administration always led to greater hepatocyte glucose production than continuous administration (P = 0.008) and, in the dose range equivalent to concentrations in portal plasma, pulsed glucagon enhanced glucose production twofold. The data suggest that pulsatile secretion is the more efficient means for islet A cells to stimulate hepatic glucose production.
Asunto(s)
Glucagón/metabolismo , Glucosa/biosíntesis , Hígado/metabolismo , Animales , Glucagón/farmacología , Hígado/citología , Masculino , Métodos , Perfusión , Ratas , Ratas Endogámicas , Estimulación QuímicaRESUMEN
In 1980 we described an in vivo method for estimating the rate of glucose uptake among selected tissues during an acute insulin response. The method was based upon the same principles as Sokoloff's 2-deoxyglucose (2DG) method. We now report further examination of the basic assumptions of the model and validation of its general applicability by comparing the response of brain and other tissues to prolonged insulin infusion (while glucose is held constant) with their response to a single injection of insulin. The method provides a reproducible estimate of relative insulin response in any tissue that can be anatomically separated at death. Tissues that are minimally sensitive to insulin such as spleen, lung, skin, and gut do not display increments in the calculated value for net rate of tissue uptake of 2DG. Insulin-sensitive tissues display increased rates of uptake that are characteristic for each specific tissue, ranging in magnitude from 1.7- to 17.9-fold over basal among an array of insulin-sensitive tissues. The duration of a unit response to a sub-maximal dose of insulin also varied among the tissues, persisting for 20-30 min after plasma insulin had returned to basal in heart and for 10-20 min in the other insulin-sensitive tissues. The method provides a reproducible measure of glucose metabolism in vivo and has been validated as a means of quantifying relative insulin sensitivity among the peripheral tissues. During steady-state conditions with plasma glucose held constant, brain glucose metabolism was unaffected by a 60-min infusion of insulin.
Asunto(s)
Encéfalo/efectos de los fármacos , Desoxiazúcares/metabolismo , Desoxiglucosa/metabolismo , Glucosa/metabolismo , Insulina/farmacología , Tejido Adiposo/metabolismo , Animales , Glucemia/análisis , Encéfalo/metabolismo , Hipoglucemia/metabolismo , Insulina/administración & dosificación , Pulmón/metabolismo , Masculino , Matemática , Músculos/metabolismo , Miocardio/metabolismo , Hipófisis/metabolismo , Ratas , Ratas Endogámicas , Bazo/metabolismoRESUMEN
The dose-response characteristics of three skeletal muscles, three adipose tissue beds, and heart muscle to single i.v. injection of insulin were compared in vivo. Comparisons were made at 8 dose levels spanning the entire range for response by all tissues and for the integrated whole body response as reflected in the rate of disappearance of 3H-2-deoxyglucose from plasma. The insulin-sensitive tissues varied widely with respect to the magnitude of the maximal response and the sensitivity to insulin as judged by the effective dose 50% (ED 50). Among the muscles, a slow-twitch oxidative muscle, soleus, was more sensitive than the fast-twitch glycolytic muscle, extensor digitorum longus (EDL), while a mixed muscle, quadriceps femoris, displayed even lower sensitivity. Heart muscle sensitivity was comparable to EDL. Among the adipose sites, the rank order of sensitivity was subcutaneous greater than epididymal much greater than omental. The threshold for a detectable response to insulin was 0.013 U/kg rat.