ABSTRACT
Glucose enters eukaryotic cells via two types of membrane-associated carrier proteins, the Na(+)/glucose cotransporters (SGLT) and the facilitative glucose transporters (GLUT). The SGLT family consists of six members. Among them, the SGLT1 and SGLT2 proteins, encoded by the solute carrier genes SLC5A1 and SLC5A2, respectively, are believed to be the most important ones and have been extensively explored in studies focusing on glucose fluxes under both physiological and pathological conditions. This review considers the regulation of the expression of the SGLT promoted by protein kinases and transcription factors, as well as the alterations determined by diets of different compositions and by pathologies such as diabetes. It also considers congenital defects of sugar metabolism caused by aberrant expression of the SGLT1 in glucose-galactose malabsorption and the SGLT2 in familial renal glycosuria. Finally, it covers some pharmacological compounds that are being currently studied focusing on the interest of controlling glycemia by antagonizing SGLT in renal and intestinal tissues.
Subject(s)
Gene Expression Regulation/genetics , Signal Transduction/genetics , Sodium-Glucose Transporter 1/genetics , Sodium-Glucose Transporter 2/genetics , Transcription, Genetic/genetics , Animals , Diabetes Mellitus/genetics , Diabetes Mellitus/physiopathology , Gene Expression Regulation/physiology , Humans , Signal Transduction/physiology , Sodium-Glucose Transporter 1/physiology , Sodium-Glucose Transporter 2/physiology , Transcription, Genetic/physiologyABSTRACT
Glucose enters eukaryotic cells via two types of membrane-associated carrier proteins, the Na+/glucose cotransporters (SGLT) and the facilitative glucose transporters (GLUT). The SGLT family consists of six members. Among them, the SGLT1 and SGLT2 proteins, encoded by the solute carrier genes SLC5A1 and SLC5A2, respectively, are believed to be the most important ones and have been extensively explored in studies focusing on glucose fluxes under both physiological and pathological conditions. This review considers the regulation of the expression of the SGLT promoted by protein kinases and transcription factors, as well as the alterations determined by diets of different compositions and by pathologies such as diabetes. It also considers congenital defects of sugar metabolism caused by aberrant expression of the SGLT1 in glucose-galactose malabsorption and the SGLT2 in familial renal glycosuria. Finally, it covers some pharmacological compounds that are being currently studied focusing on the interest of controlling glycemia by antagonizing SGLT in renal and intestinal tissues.
Subject(s)
Animals , Humans , Gene Expression Regulation/genetics , Signal Transduction/genetics , Sodium-Glucose Transporter 1/genetics , /genetics , Transcription, Genetic/genetics , Diabetes Mellitus/genetics , Diabetes Mellitus/physiopathology , Gene Expression Regulation/physiology , Signal Transduction/physiology , Sodium-Glucose Transporter 1/physiology , /physiology , Transcription, Genetic/physiologyABSTRACT
We hypothesize that, in kidney of diabetic rats, hepatocyte nuclear factors (HNF-1alpha and HNF-3beta) play a critical role in the overexpression of solute carrier 2A2 (SLC2A2) gene. Diabetic rats submitted or not to rapid (up to 12h) and short-term (1, 4 and 6 days) insulin treatment were investigated. Twofold increase in GLUT2 mRNA was observed in diabetic, accompanied by significant increases in HNF-1alpha and HNF-3beta expression and binding activity. Additional 2-fold increase in GLUT2 mRNA and HNF-3beta expression/activity was observed in 12-h insulin-treated rats. Six-day insulin treatment decreased GLUT2 mRNA and HNF-1alpha expression and activity to levels of non-diabetic rats, whereas HNF-3beta decreased to levels of non-insulin-treated diabetic rats. Our results provide evidence for a link between the overexpression of SLC2A2 gene and the transcriptional activity of HNF-1alpha and HNF-3beta in kidney of diabetic rats. Furthermore, recovery of SLC2A2 gene after 6-day insulin treatment also involves HNF-1alpha and HNF-3beta activity.
Subject(s)
Glucose Transporter Type 2/physiology , Hepatocyte Nuclear Factor 1-alpha/physiology , Hepatocyte Nuclear Factor 3-beta/physiology , Kidney/metabolism , Animals , Diabetes Mellitus/genetics , Diabetes Mellitus/metabolism , Electrophoretic Mobility Shift Assay , Gene Expression Regulation/drug effects , Glucose Transporter Type 2/genetics , Glucose Transporter Type 2/metabolism , Insulin/pharmacology , Kidney/drug effects , Male , RNA, Messenger/metabolism , Rats , Rats, WistarABSTRACT
Oral health complications in diabetes include decreased salivary secretion. The SLC5A1 gene encodes the Na(+)-glucose cotransporter SGLT1 protein, which not only transports glucose, but also acts as a water channel. Since SLC5A1 expression is altered in kidneys of diabetic subjects, we hypothesize that it could also be altered in salivary glands, contributing to diabetic dysfunction. The present study shows a diabetes-induced decrease (p < 0.001) in salivary secretion, which was accompanied by enhanced (p < 0.05) SGLT1 mRNA expression in parotid (50%) and submandibular (30%) glands. Immunohistochemical analysis of parotid gland of diabetic rats revealed that SGLT1 protein expression increased in the luminal membrane of ductal cells, which can stimulate water reabsorption from primary saliva. Furthermore, SGLT1 protein was reduced in myoepithelial cells of the parotid from diabetic animals, and that, by reducing cellular contractile activity, might also be related to reduced salivary flux. Six-day insulin-treated diabetic rats reversed all alterations. In conclusion, diabetes increases SLC5A1 gene expression in salivary glands, increasing the SGLT1 protein content in the luminal membrane of ductal cells, which, by increasing water reabsorption, might explain the diabetes-induced decrease in salivary secretion.
Subject(s)
Diabetes Mellitus, Experimental/physiopathology , Salivary Glands/metabolism , Salivary Glands/pathology , Sodium-Glucose Transporter 1/physiology , Animals , Blotting, Northern , Blotting, Western , Immunohistochemistry , Male , Rats , Rats, Wistar , Sodium-Glucose Transporter 1/genetics , Sodium-Glucose Transporter 1/metabolismABSTRACT
AIM: Glimepiride, a low-potency insulin secretagogue, is as efficient on glycaemic control as other sulphonylureas, suggesting an additional insulin-sensitizer role. The aim of the present study was to confirm the insulin-sensitizer role of glimepiride and to show extra-pancreatic effects of the drug. METHODS: Three-month-old monosodium glutamate (MSG)-induced obese insulin-resistant rats were treated (OG) or not treated (O) with glimepiride for 4 weeks and compared with age-matched non-obese rats (C). Insulin sensitivity in whole body, glucose transporter 4 (GLUT4) protein content, glucose uptake and glycogen synthesis in oxidative skeletal muscle and phospho-glycogen synthase kinase (p-GSK3) and glycogen content in liver were analysed. RESULTS: Insulin sensitivity, analysed by the insulin tolerance test, was 30% lower in O than in C rats (p < 0.05), and OG rats recovered this parameter (p < 0.05). In oxidative muscle, glimepiride increased the GLUT4 protein content (50%, p < 0.001) and recovered the obesity-induced reduction ( approximately 20%) of the in vitro insulin-stimulated glucose uptake and incorporation into glycogen. In liver, glimepiride increased p-GSK3 (p < 0.01) and glycogen (p < 0.05) contents. CONCLUSION: The increased GLUT4 protein expression and glucose utilization in oxidative muscle and the increased insulin sensitivity and glycogen storage in liver evidence the insulin-sensitizer effect of glimepiride, which must be important to enable the glimepiride drug to promote an efficient glycaemic control.
Subject(s)
Hypoglycemic Agents/pharmacology , Insulin Resistance , Sulfonylurea Compounds/pharmacology , Animals , Glucose Transporter Type 4 , Liver/metabolism , Muscle, Skeletal/metabolism , Rats , Rats, WistarABSTRACT
Mutations in Na(+)-glucose transporters (SGLT)-2 and hepatocyte nuclear factor (HNF)-1alpha genes have been related to renal glycosuria and maturity-onset diabetes of the young 3, respectively. However, the expression of these genes have not been investigated in type 1 and type 2 diabetes. Here in kidney of diabetic rats, we tested the hypotheses that SGLT2 mRNA expression is altered; HNF-1alpha is involved in this regulation; and glycemic homeostasis is a related mechanism. The in vivo binding of HNF-1alpha into the SGLT2 promoter region in renal cortex was confirmed by chromatin immunoprecipitation assay. SGLT2 and HNF-1alpha mRNA expression (by Northern and RT-PCR analysis) and HNF-1 binding activity of nuclear proteins (by EMSA) were investigated in diabetic rats and treated or not with insulin or phlorizin (an inhibitor of SGLT2). Results showed that diabetes increases SGLT2 and HNF-1alpha mRNA expression (~50%) and binding of nuclear proteins to a HNF-1 consensus motif (~100%). Six days of insulin or phlorizin treatment restores these parameters to nondiabetic-rat levels. Moreover, both treatments similarly reduced glycemia, despite the differences in plasma insulin and urinary glucose concentrations, highlighting the plasma glucose levels as involved in the observed modulations. This study shows that SGLT2 mRNA expression and HNF-1alpha expression and activity correlate positively in kidney of diabetic rats. It also shows that diabetes-induced changes are reversed by lowering glycemia, independently of insulinemia. Our demonstration that HNF-1alpha binds DNA that encodes SGLT2 supports the hypothesis that HNF-1alpha, as a modulator of SGLT2 expression, may be involved in diabetic kidney disease.
Subject(s)
Diabetic Nephropathies/metabolism , Diabetic Nephropathies/physiopathology , Hepatocyte Nuclear Factor 1-alpha/genetics , Kidney/physiology , Sodium-Glucose Transporter 2/genetics , Animals , Blood Glucose/drug effects , Diabetes Mellitus, Experimental/drug therapy , Diabetes Mellitus, Experimental/metabolism , Diabetes Mellitus, Experimental/physiopathology , Diabetic Nephropathies/drug therapy , Electrophoretic Mobility Shift Assay , Hepatocyte Nuclear Factor 1-alpha/metabolism , Hyperglycemia/drug therapy , Hyperglycemia/metabolism , Hyperglycemia/physiopathology , Hypoglycemic Agents/pharmacology , Immunoprecipitation , Insulin/pharmacology , Male , Phlorhizin/pharmacology , RNA, Messenger/metabolism , Rats , Rats, Wistar , Sodium-Glucose Transporter 2/metabolismABSTRACT
The GLUT4 transporter plays a key role in insulin-induced glucose uptake, which is impaired in insulin resistance. The objective of the present study was to investigate the tissue content and the subcellular distribution of GLUT4 protein in 4- to 12-year-old control, obese and insulin-treated diabetic mongrel female dogs (4 animals per group). The parametrial white adipose tissue was sampled and processed to obtain both plasma membrane and microsome subcellular fractions for GLUT4 analysis by Western blotting. There was no significant difference in glycemia and insulinemia between control and obese animals. Diabetic dogs showed hyperglycemia (369.9 +/- 89.9 mg/dl). Compared to control, the plasma membrane GLUT4, reported per g tissue, was reduced by 55% (P < 0.01) in obese dogs, and increased by 30% (P < 0.05) in diabetic dogs, and the microsomal GLUT4 was increased by approximately 45% (P < 0.001) in both obese and diabetic animals. Considering the sum of GLUT4 measured in plasma membrane and microsome as total cellular GLUT4, percent GLUT4 present in plasma membrane was reduced by approximately 65% (P < 0.001) in obese compared to control and diabetic animals. Since insulin stimulates GLUT4 translocation to the plasma membrane, percent GLUT4 in plasma membrane was divided by the insulinemia at the time of tissue removal and was found to be reduced by 75% (P < 0.01) in obese compared to control dogs. We conclude that the insulin-stimulated translocation of GLUT4 to the cell surface is reduced in obese female dogs. This probably contributes to insulin resistance, which plays an important role in glucose homeostasis in dogs.
Subject(s)
Adipocytes/metabolism , Diabetes Mellitus/veterinary , Dog Diseases/metabolism , Monosaccharide Transport Proteins/metabolism , Muscle Proteins/metabolism , Obesity/veterinary , Animals , Biological Transport , Blotting, Western , Cell Membrane/metabolism , Diabetes Mellitus/drug therapy , Diabetes Mellitus/metabolism , Disease Models, Animal , Dog Diseases/drug therapy , Dogs , Female , Glucose Transporter Type 4 , Hypoglycemic Agents/administration & dosage , Hypoglycemic Agents/metabolism , Insulin/administration & dosage , Insulin/metabolism , Microsomes/metabolism , Monosaccharide Transport Proteins/analysis , Muscle Proteins/analysis , Obesity/metabolism , Ovariectomy/veterinaryABSTRACT
The GLUT4 transporter plays a key role in insulin-induced glucose uptake, which is impaired in insulin resistance. The objective of the present study was to investigate the tissue content and the subcellular distribution of GLUT4 protein in 4-to 12-year-old control, obese and insulin-treated diabetic mongrel female dogs (4 animals per group). The parametrial white adipose tissue was sampled and processed to obtain both plasma membrane and microsome subcellular fractions for GLUT4 analysis by Western blotting. There was no significant difference in glycemia and insulinemia between control and obese animals. Diabetic dogs showed hyperglycemia (369.9 ± 89.9 mg/dl). Compared to control, the plasma membrane GLUT4, reported per g tissue, was reduced by 55 percent (P < 0.01) in obese dogs, and increased by 30 percent (P < 0.05) in diabetic dogs, and the microsomal GLUT4 was increased by approximately 45 percent (P < 0.001) in both obese and diabetic animals. Considering the sum of GLUT4 measured in plasma membrane and microsome as total cellular GLUT4, percent GLUT4 present in plasma membrane was reduced by approximately 65 percent (P < 0.001) in obese compared to control and diabetic animals. Since insulin stimulates GLUT4 translocation to the plasma membrane, percent GLUT4 in plasma membrane was divided by the insulinemia at the time of tissue removal and was found to be reduced by 75 percent (P < 0.01) in obese compared to control dogs. We conclude that the insulin-stimulated translocation of GLUT4 to the cell surface is reduced in obese female dogs. This probably contributes to insulin resistance, which plays an important role in glucose homeostasis in dogs.
Subject(s)
Animals , Female , Dogs , Adipocytes , Diabetes Mellitus, Experimental , Insulin , Obesity , Biological Transport , Blotting, Western , Cell Membrane , Disease Models, Animal , MicrosomesABSTRACT
Renal glucose reabsorption is mediated by luminal sodium-glucose cotransporters (SGLTs) and basolateral facilitative glucose transporters (GLUTs). The modulators of these transporters are not known, and their substrates glucose and Na+ are potential candidates. In this study we examined the role of glucose and Na+ filtration rate on gene expression of glucose transporters in renal proximal tubule. SGLT1, SGLT2, GLUT1 and GLUT2 mRNAs were assessed by Northern blotting; and GLUT1 and GLUT2 proteins were assessed by Western blotting. Renal cortex and medulla samples from control rats (C), diabetic rats (D) with glycosuria, and insulin-resistant 15-month old rats (I) without glycosuria; and from normal (NS), low (LS), and high (HS) Na+-diet fed rats were studied. Compared to C and I rats, D rats increased (P < 0.05) gene expression of SGLT2 by approximately 36%, SGLT1 by approximately 20%, and GLUT2 by approximately 100%, and reduced (P < 0.05) gene expression of GLUT1 by more than 50%. Compared to NS rats, HS rats increased (P < 0.05) SGLT2, GLUT2, and GLUT1 expression by approximately 100%, with no change in SGLT1 mRNA expression, and LS rats increased (P < 0.05) GLUT1 gene expression by approximately 150%, with no changes in other transporters. In summary, the results showed that changes in glucose or Na+ filtrated rate modulate the glucose transporters gene expression in epithelial cells of the renal proximal tubule.
Subject(s)
Gene Expression Regulation , Glucose/metabolism , Kidney Tubules, Proximal/metabolism , Monosaccharide Transport Proteins/genetics , Sodium/metabolism , Animals , Blotting, Northern , Blotting, Western , Diabetes Mellitus, Experimental/genetics , Diabetes Mellitus, Experimental/metabolism , Diet , Glomerular Filtration Rate , Glucose Transporter Type 1 , Glucose Transporter Type 2 , Male , Membrane Glycoproteins/genetics , Membrane Glycoproteins/metabolism , Monosaccharide Transport Proteins/metabolism , RNA, Messenger , Rats , Rats, Wistar , Sodium-Glucose Transporter 1 , Sodium-Glucose Transporter 2ABSTRACT
Because of conflicting results in the literature, further studies are needed to confirm an association between the degree of salt consumption and insulin sensitivity. The aim of this study was to measure insulin sensitivity in rats fed from weaning to adulthood with a low (LSD), normal (NSD), or high (HSD) salt diet. Body weight, carcass lipid content, blood glucose, nonesterified fatty acids, plasma insulin, plasma renin activity, and a glucose transporter (GLUT4) were measured. A euglycemic hyperinsulinemic clamp was used in 52 anesthetized rats. Body weight was higher in rats on LSD than in those on NSD (P<0.05) or HSD (P<0.001). Percentage fat carcass content was higher (P<0.05) in rats on LSD than in those on NSD. Basal plasma insulin and glucose levels were not altered (P>0.05) by salt consumption. Nonesterified fatty acids were lower in rats on HSD than in those on LSD (P<0.05) or NSD (P<0.01). Glucose uptake was lower in rats on LSD than in those on NSD (P<0.05) or HSD (P<0. 001). When a euglycemic hyperinsulinemic clamp was used on pair-weight rats, similar results were obtained, which suggests that the effect of LSD on insulin sensitivity was not due to higher body weight. GLUT4 in insulin-sensitive tissues was increased in rats on HSD except in the cardiac muscle. Captopril treatment partially reversed low insulin sensitivity in LSD rats, whereas losartan did not change it, which indicates that the effect of LSD on insulin sensitivity is angiotensin independent. In conclusion, the present results demonstrate that chronic dietary salt restriction induces a decrease in insulin sensitivity not associated with renin-angiotensin system activity or body weight changes.
Subject(s)
Aging/physiology , Diet, Sodium-Restricted , Insulin Resistance/physiology , Muscle Proteins , Angiotensin-Converting Enzyme Inhibitors/pharmacology , Animals , Antihypertensive Agents/pharmacology , Biological Transport/drug effects , Body Weight , Captopril/pharmacology , Glucose/pharmacokinetics , Glucose Transporter Type 4 , Hyperinsulinism/physiopathology , Losartan/pharmacology , Male , Monosaccharide Transport Proteins/analysis , Rats , Rats, Wistar , Sodium Chloride, Dietary/pharmacology , WeaningABSTRACT
Although the pineal gland influences several physiological systems, only a few studies have investigated its role in the intermediary metabolism. In the present study, male Wistar rats, pinealectomized or sham-operated 6 wk before the experiment, were submitted to both intravenous glucose tolerance tests (IVGTT) and insulin binding as well as glucose transport assays in isolated adipocytes. The insulin receptor tyrosine kinase activity was assessed in liver and muscle. The insulin secretory response during the IVGTT was impaired, particularly in the afternoon, and the glucose transport responsiveness was 33% lower in pinealectomized rats. However, no difference was observed in the insulin receptor number of adipocytes between groups as well as in insulin-stimulated tyrosine kinase activity, indicating that the initial steps in the insulin signaling were well conserved. Conversely, a 40% reduction in adipose tissue GLUT-4 content was detected. In conclusion, pinealectomy is responsible for both impaired insulin secretion and action, emphasizing the influence of the pineal gland on glucose metabolism.
Subject(s)
Glucose Intolerance/etiology , Hypoglycemic Agents/pharmacology , Insulin/pharmacology , Muscle Proteins , Pineal Gland/physiology , Adipose Tissue/cytology , Adipose Tissue/drug effects , Animals , Biological Transport/physiology , Deoxyglucose/pharmacokinetics , Glucose/metabolism , Glucose Tolerance Test , Glucose Transporter Type 4 , Insulin/metabolism , Liver/enzymology , Male , Monosaccharide Transport Proteins/metabolism , Muscle, Skeletal/enzymology , Rats , Rats, Wistar , Receptor Protein-Tyrosine Kinases/metabolism , Receptor, Insulin/metabolismABSTRACT
The effect of sodium chloride salt restriction and overload on insulin sensitivity is still an open question. Some authors have shown that NaCl salt restriction increases insulin resistance, whereas others have reported the opposite. In the present study, the objective was to get some more insight on this issue by studying the influence of dietary salt content on glucose uptake in isolated adipocytes. Male Wistar rats were fed from weaning either low (0.15%) or high (7.94%) salt diets. On the 12th week of age, weight and tail-cuff blood pressure were measured, followed 10 days later by an intravenous glucose tolerance test with concomitant insulin determinations. One week later, the rats were killed by decapitation and epididymal adipocytes were obtained for glucose metabolism evaluation. No weight differences were observed between both groups of animals. Blood pressure was significantly higher (P < .001) on salt overloaded rats (146 +/- 11 mm Hg) than on salt restricted ones (115 +/- 5 mm Hg). Dietary salt content did not influence the area under the curve of plasma glucose. Area under the curve of insulin levels was lower (P = .023) on the high than on the low salt diet. A higher (P < .001) glucose uptake in the absence and in the presence of insulin was observed in adipocytes from rats on the high salt diet. The median effective concentration (EC50) from the dose-response curves of glucose uptake was the same on both groups of animals. Glucose oxidation and incorporation into lipids was also enhanced by salt overload. High salt increased insulin receptor density (P < .001). In conclusion, salt overload increased blood pressure, and high and low salt dietary content did not influence insulin sensitivity based on the unchanged EC50 from the in vitro studies. However, insulin-independent glucose uptake, oxidation, and incorporation into lipids were enhanced in adipocytes from rats on the high salt diet. The lower levels of insulin during the glucose tolerance test on salt-loaded animals may be a consequence of the higher insulin-independent glucose uptake in that group.
Subject(s)
Adipocytes/metabolism , Blood Pressure/drug effects , Glucose/metabolism , Insulin Resistance/physiology , Sodium Chloride, Dietary/pharmacology , Adipocytes/drug effects , Animals , Antimetabolites/metabolism , Body Weight/drug effects , Creatinine/blood , Deoxyglucose/metabolism , Epididymis/cytology , Epididymis/metabolism , Hypertrophy, Left Ventricular/chemically induced , In Vitro Techniques , Lipid Metabolism , Male , Oxidation-Reduction , Rats , Rats, WistarABSTRACT
Isolated adipocytes from rats submitted to four weeks of ad libitum feeding (AL) or meal feeding (MF, 2 h/22 h, feeding/fasting, meal time: 8:00-10:00 a.m.) schedules or pre-incubated with or without melatonin (0, 1 nM, 10 nM, 100 nM) for 5 h were submitted to insulin-stimulated [3H]-2-deoxyglucose (0.1 mM, 0.12 microCi) uptake rate measurements and insulin binding assays. Insulin sensitivity was defined as the hormone concentration capable of producing the half-maximal transport rate. Insulin sensitivity varied depending on the previous conditions of the adipocytes. In MF animals, adipose cells were more sensitive (EC50 = 0.175 ng/ml) just at the moment of the expected meal. In AL rats, sensitivity was lower (EC50 = 0.678 ng/ml) at 8:00 a.m. and increased (EC50 = 0.398 ng/ml) at 4:00 p.m. These data clearly implicate the expectation of food and period of the day with the regulation of insulin action. All these modifications in sensitivity occurred without any change in insulin receptor number or affinity. Melatonin, a secretory product of the pineal gland, induced an increase in cell sensitivity to insulin in adipocytes incubated with the highest hormone concentration (100 nM). We conclude that factors related to feeding training and circadian rhythmicity modulate cell sensitivity to insulin.
Subject(s)
Adipocytes/metabolism , Eating/physiology , Insulin/metabolism , Melatonin/pharmacology , Periodicity , Adipocytes/drug effects , Animals , Deoxyglucose/pharmacokinetics , Insulin Resistance/physiology , Rats , Receptor, Insulin/metabolism , Time FactorsABSTRACT
Isolated adipocytes from rats submitted to four weeks of ad libitum feeding (AL) or meal feeding (MF, 2 h/22 h, feeding/fasting, meal time: 8:00-10:00 a.m.) schedules or pre-incubated with or without melatonin (0, 1 nM, 10 nM, 100 nM) for 5 h were submitted to insulin-stimulated [3H]-2-deoxyglucose (0.1 mM, 0.12 microCi) uptake rate measurements and insulin binding assays. Insulin sensitivity was defined as the hormone concentration capable of producing the half-maximal transport rate. Insulin sensitivity varied depending on the previous conditions of the adipocytes. In MF animals, adipose cells were more sensitive (EC50 = 0.175 ng/ml) just at the moment of the expected meal. In AL rats, sensitivity was lower (EC50 = 0.678 ng/ml) at 8:00 a.m. and increased (EC50 = 0.398 ng/ml) at 4:00 p.m. These data clearly implicate the expectation of food and period of the day with the regulation of insulin action. All these modifications in sensitivity occurred without any change in insulin receptor number or affinity. Melatonin, a secretory product of the pineal gland, induced an increase in cell sensitivity to insulin in adipocytes incubated with the highest hormone concentration (100 nM). We conclude that factors related to feeding training and circadian rhythmicity modulate cell sensitivity to insulin.