Structural and functional heterogeneity of insulin receptors

State of the Science on Brain Insulin Resistance and Cognitive Decline Due to Alzheimer's Disease

Type 2 diabetes mellitus (T2DM) is common and increasing in prevalence worldwide, with devastating public health consequences. While peripheral insulin resistance is a key feature of most forms of T2DM and has been investigated for over a century, research on brain insulin resistance (BIR) has more recently been developed, including in the context of T2DM and non-diabetes states. Recent data support the presence of BIR in the aging brain, even in non-diabetes states, and found that BIR may be a feature in Alzheimer's disease (AD) and contributes to cognitive impairment. Further, therapies used to treat T2DM are now being investigated in the context of AD treatment and prevention, including insulin. In this review, we offer a definition of BIR, and present evidence for BIR in AD; we discuss the expression, function, and activation of the insulin receptor (INSR) in the brain; how BIR could develop; tools to study BIR; how BIR correlates with current AD hallmarks; and regional/cellular involvement of BIR. We close with a discussion on resilience to both BIR and AD, how current tools can be improved to better understand BIR, and future avenues for research. Overall, this review and position paper highlights BIR as a plausible therapeutic target for the prevention of cognitive decline and dementia due to AD.

A comprehensive review of the effects of resveratrol on glucose metabolism: unveiling the molecular pathways and therapeutic potential in diabetes management

Resveratrol, a naturally occurring polyphenolic compound predominantly found in red wine and grapes, has garnered attention for its potential role in regulating carbohydrate digestion, glucose absorption, and metabolism. This review aims to deliver a comprehensive analysis of the molecular mechanisms and therapeutic potential of resveratrol in influencing vital processes in glucose homeostasis. These processes include carbohydrate digestion, glucose absorption, glycogen storage, insulin secretion, glucose metabolism in muscle cells, and triglyceride synthesis in adipocytes.The goal of this review is to offer an in-depth understanding of the multifaceted effects of resveratrol on glucose metabolism. By doing so, it presents valuable insights into its potential applications for preventing and treating metabolic disorders. This comprehensive examination of resveratrol's impact on glucose management will contribute to the growing body of knowledge on this promising natural compound, which may benefit researchers, healthcare professionals, and individuals interested in metabolic disorder prevention and treatment.

The impact of insulin on the serotonergic system and consequences on diabetes-associated mood disorders

The idea that insulin could influence emotional behaviours has long been suggested. However, the underlying mechanisms have yet to be solved and there is no direct and clear-cut evidence demonstrating that such action involves brain serotonergic neurones. Indeed, initial arguments in favour of the association between insulin, serotonin and mood arise from clinical or animal studies showing that impaired insulin action in type 1 or type 2 diabetes causes anxiety- and depressive symptoms along with blunted plasma and brain serotonin levels. The present review synthesises the main mechanistic hypotheses that might explain the comorbidity between diabetes and depression. It also provides a state of knowledge of the direct and indirect experimental evidence that insulin modulates brain serotonergic neurones. Finally, it highlights the literature suggesting that antidiabetic drugs present antidepressant-like effects and, conversely, that serotonergic antidepressants impact glucose homeostasis. Overall, this review provides mechanistic insights into how insulin signalling alters serotonergic neurotransmission and related behaviours bringing new targets for therapeutic options.

Elevating Insulin Signaling Using a Constitutively Active Insulin Receptor Increases Glucose Metabolism and Expression of GLUT3 in Hippocampal Neurons

Insulin signaling is an integral component of healthy brain function, with evidence of positive insulin-mediated alterations in synaptic integrity, cerebral blood flow, inflammation, and memory. However, the specific pathways targeted by this peptide remain unclear. Previously, our lab used a molecular approach to characterize the impact of insulin signaling on voltage-gated calcium channels and has also shown that acute insulin administration reduces calcium-induced calcium release in hippocampal neurons. Here, we explore the relationship between insulin receptor signaling and glucose metabolism using similar methods. Mixed, primary hippocampal cultures were infected with either a control lentivirus or one containing a constitutively active human insulin receptor (IRβ). 2-NBDG imaging was used to obtain indirect measures of glucose uptake and utilization. Other outcome measures include Western immunoblots of GLUT3 and GLUT4 on total membrane and cytosolic subcellular fractions. Glucose imaging data indicate that neurons expressing IRβ show significant elevations in uptake and rates of utilization compared to controls. As expected, astrocytes did not respond to the IRβ treatment. Quantification of Western immunoblots show that IRβ is associated with significant elevations in GLUT3 expression, particularly in the total membrane subcellular fraction, but did not alter GLUT4 expression in either fraction. Our work suggests that insulin plays a significant role in mediating neuronal glucose metabolism, potentially through an upregulation in the expression of GLUT3. This provides further evidence for a potential therapeutic mechanism underlying the beneficial impact of intranasal insulin in the clinic.

Tau, Diabetes and Insulin

Tau protein which was discovered in 1975 [310] became of great interest when it was identified as the main component of neurofibrillary tangles (NFT), a pathological feature in the brain of patients with Alzheimer's disease (AD) [39, 110, 232]. Tau protein is expressed mainly in the brain as six isoforms generated by alternative splicing [46, 97]. Tau is a microtubule associated proteins (MAPs) and plays a role in microtubules assembly and stability, as well as diverse cellular processes such as cell morphogenesis, cell division, and intracellular trafficking [49]. Additionally, Tau is involved in much larger neuronal functions particularly at the level of synapses and nuclei [11, 133, 280]. Tau is also physiologically released by neurons [233] even if the natural function of extracellular Tau remains to be uncovered (see other chapters of the present book).

Spatial dynamics of the insulin receptor in living neurons

Insulin signaling through the insulin receptor has long been studied in classic target organs, such as adipose tissue and skeletal muscle, where one of its effects is to increase glucose uptake. Insulin and insulin receptor are present in many areas of the brain, but the functions of brain insulin signaling outside feeding circuits are not well defined. It has been proposed that hippocampal insulin signaling is important for memory, that brain insulin signaling is deficient in Alzheimer's disease, and that intranasal insulin treatment improves cognition, but the mechanisms remain unclear and do not seem to involve increased glucose uptake by neurons. The molecular behavior of the insulin receptor itself is not well known in living neurons; therefore, we investigated the spatial dynamics of the insulin receptor on somatodendritic membranes of live rat hippocampal neurons in culture. Using single-molecule tracking of quantum dot-tagged insulin receptors and single-particle tracking photoactivation localization microscopy, we show that the insulin receptor is distributed over both dendritic shafts and spines. Using colocalization with synaptic markers, we also show that in contrast to the glutamate receptor subunit glutamate receptor subunit A1, the dynamics of the insulin receptor are not affected by association with excitatory synapses; however, the insulin receptor is immobilized by components of inhibitory synapses. The mobility of the insulin receptor is reduced both by low concentrations of the pro-inflammatory cytokine tumor necrosis factor α and by cholesterol depletion, suggesting an association with sphingolipid-rich membrane domains. On the other hand, the insulin receptor dynamics in hippocampal neurons are not affected by increased excitatory signaling. Finally, using real-time single-event quantification, we find evidence of strong insulin receptor exocytosis on dendritic shafts. Our results suggest an association of the neuronal insulin receptor with specific elements of the dendritic shaft, rather than excitatory synapses.

Insulin Receptor Isoforms in Physiology and Disease: An Updated View

The insulin receptor (IR) gene undergoes differential splicing that generates two IR isoforms, IR-A and IR-B. The physiological roles of IR isoforms are incompletely understood and appear to be determined by their different binding affinities for insulin-like growth factors (IGFs), particularly for IGF-2. Predominant roles of IR-A in prenatal growth and development and of IR-B in metabolic regulation are well established. However, emerging evidence indicates that the differential expression of IR isoforms may also help explain the diversification of insulin and IGF signaling and actions in various organs and tissues by involving not only different ligand-binding affinities but also different membrane partitioning and trafficking and possibly different abilities to interact with a variety of molecular partners. Of note, dysregulation of the IR-A/IR-B ratio is associated with insulin resistance, aging, and increased proliferative activity of normal and neoplastic tissues and appears to sustain detrimental effects. This review discusses novel information that has generated remarkable progress in our understanding of the physiology of IR isoforms and their role in disease. We also focus on novel IR ligands and modulators that should now be considered as an important strategy for better and safer treatment of diabetes and cancer and possibly other IR-related diseases.

The neuronal insulin receptor in its environment

Insulin is known mainly for its effects in peripheral tissues, such as the liver, skeletal muscles and adipose tissue, where the activation of the insulin receptor (IR) has both short-term and long-term effects. Insulin and the IR are also present in the brain, and since there is evidence that neuronal insulin signaling regulates synaptic plasticity and that it is impaired in disease, this pathway might be the key to protection or reversal of symptoms, especially in Alzheimer's disease. However, there are controversies about the importance of the neuronal IR, partly because biophysical data on its activation and signaling are much less complete than for the peripheral IR. This review briefly summarizes the neuronal IR signaling in health and disease, and then focuses on known differences between the neuronal and peripheral IR with regard to alternative splicing and glycosylation, and lack of data with respect to phosphorylation and membrane subdomain localization. Particularities in the neuronal IR itself and its environment may have consequences for downstream signaling and impact synaptic plasticity. Furthermore, establishing the relative importance of insulin signaling through IR or through hybrids with its homolog, the insulin-like growth factor 1 receptor, is crucial for evaluating the consequences of brain IR activation. An improved biophysical understanding of the neuronal IR may help predict the consequences of insulin-targeted interventions.

Insulin in the brain: its pathophysiological implications for States related with central insulin resistance, type 2 diabetes and Alzheimer's disease

Although the brain has been considered an insulin-insensitive organ, recent reports on the location of insulin and its receptors in the brain have introduced new ways of considering this hormone responsible for several functions. The origin of insulin in the brain has been explained from peripheral or central sources, or both. Regardless of whether insulin is of peripheral origin or produced in the brain, this hormone may act through its own receptors present in the brain. The molecular events through which insulin functions in the brain are the same as those operating in the periphery. However, certain insulin actions are different in the central nervous system, such as hormone-induced glucose uptake due to a low insulin-sensitive GLUT-4 activity, and because of the predominant presence of GLUT-1 and GLUT-3. In addition, insulin in the brain contributes to the control of nutrient homeostasis, reproduction, cognition, and memory, as well as to neurotrophic, neuromodulatory, and neuroprotective effects. Alterations of these functional activities may contribute to the manifestation of several clinical entities, such as central insulin resistance, type 2 diabetes mellitus (T2DM), and Alzheimer's disease (AD). A close association between T2DM and AD has been reported, to the extent that AD is twice more frequent in diabetic patients, and some authors have proposed the name "type 3 diabetes" for this association. There are links between AD and T2DM through mitochondrial alterations and oxidative stress, altered energy and glucose metabolism, cholesterol modifications, dysfunctional protein O-GlcNAcylation, formation of amyloid plaques, altered Aβ metabolism, and tau hyperphosphorylation. Advances in the knowledge of preclinical AD and T2DM may be a major stimulus for the development of treatment for preventing the pathogenic events of these disorders, mainly those focused on reducing brain insulin resistance, which is seems to be a common ground for both pathological entities.

The central insulin system and energy balance

Insulin acts throughout the body to reduce circulating energy and to increase energy storage. Within the brain, insulin produces a net catabolic effect by reducing food intake and increasing energy expenditure; this is evidenced by the hypophagia and increased brown adipose tissue sympathetic nerve activity induced by central insulin infusion. Reducing the activity of the brain insulin system via administration of insulin antibodies, receptor antisense treatment, or receptor knockdown results in hyperphagia and increased adiposity. However, despite decades of research into the role of central insulin in food intake, many questions remain to be answered, including the underlying mechanism of action.

Isolation and characterization of a mRNA encoding a novel insulin receptor (IR) subtype, IR2, from rainbow trout (Oncorhynchus mykiss) and patterns of expression of the four IR subtypes, IR1-IR4, in tissues and during embryonic development

Insulin (INS) plays a critical role in the growth, development, and metabolism of vertebrates. In this study, a cDNA encoding a novel insulin receptor (IR) subtype was isolated, cloned, and sequenced from the liver of rainbow trout. A 1525-bp cDNA encoding a partial amino acid sequence of the β-subunit including the transmembrane domain, the tyrosine kinase domain, and the 3' untranslated region (UTR) was obtained and designated IR2 based on comparison with known IR subtypes, including the three previously reported IR subtypes of trout. Trout IR2 shares 90.0%, 82.8%, and 84.3% nucleotide identity with previously characterized trout IR1, IR3 and IR4, respectively. Quantitative real-time PCR revealed that the four IR mRNAs were differentially expressed, both in terms of distribution among tissues as well as in terms of abundance within selected tissues of juvenile trout. IR1 mRNA was most abundant in spleen, liver, kidney, and muscle (white, red and cardiac), but least abundant in adipose. IR3 mRNA was most abundant in liver, spleen, kidney, and pancreas; in other tissues, levels of IR3 mRNA were uniformly abundant. By contrast, levels of IR2 and IR4 mRNA were uniformly abundant in most tissues, except in spleen where levels of IR4 were significantly lower. All IR subtypes were detected over the course of embryonic development. In head and tail regions, levels of IR2 and IR3 mRNA declined from pre-hatch (29 days post-fertilization, dpf) to post-hatch (68-90 dpf), whereas levels of IR1 and IR4 remained relatively unchanged. These findings contribute to our understanding of the evolution, distribution, and function of insulin receptors.

Soy protein intake has sex-specific effects on the risk of metabolic syndrome in middle-aged and elderly Chinese

Soy protein intake has been postulated to improve lipid profiles, glucose homeostasis, and blood pressure. However, data linking soy protein intake and metabolic syndrome (MetS) are limited. We evaluated the association between soy protein intake and the risk of MetS and its components among middle-aged and elderly Chinese. A cross-sectional study was conducted among 2811 Chinese men and women aged 50-70 y, who were free of diagnosed cardiovascular diseases and cancers. Dietary data, including soy protein intake, was collected using a 74-item FFQ. MetS was defined using the updated National Cholesterol Education Program Adult Treatment Panel III criteria for Asian-Americans. We used multivariate logistical regression models to quantify these associations. The median level of soy protein intake was 7.82 g/d (7.64 g/d in men and 8.02 g/d in women). Overall, the association of soy protein intake and the risk of MetS differed between men and women (P for interaction = 0.008). In men, the adjusted odds ratio comparing the extreme quartiles was 1.64 (95% CI: 0.95-2.81; P-trend = 0.077), whereas for women, it was 0.66 (95% CI: 0.42-1.03; P-trend = 0.138). Soy protein intake was positively associated with hyperglycemia (P-trend = 0.005) in men, whereas it was inversely associated with elevated blood pressure (P-trend = 0.049). It was not associated with any component in women. In conclusion, habitual soy protein intake may have sex-dependent effects on risk of MetS in middle-aged and elderly Chinese.

An insulin-like peptide regulates egg maturation and metabolism in the mosquito Aedes aegypti

Ingestion of vertebrate blood is essential for egg maturation and transmission of disease-causing parasites by female mosquitoes. Prior studies with the yellow fever mosquito, Aedes aegypti, indicated blood feeding stimulates egg production by triggering the release of hormones from medial neurosecretory cells in the mosquito brain. The ability of bovine insulin to stimulate a similar response further suggested this trigger is an endogenous insulin-like peptide (ILP). A. aegypti encodes eight predicted ILPs. Here, we report that synthetic ILP3 dose-dependently stimulated yolk uptake by oocytes and ecdysteroid production by the ovaries at lower concentrations than bovine insulin. ILP3 also exhibited metabolic activity by elevating carbohydrate and lipid storage. Binding studies using ovary membranes indicated that ILP3 had an IC(50) value of 5.9 nM that was poorly competed by bovine insulin. Autoradiography and immunoblotting studies suggested that ILP3 binds the mosquito insulin receptor (MIR), whereas loss-of-function experiments showed that ILP3 activity requires MIR expression. Overall, our results identify ILP3 as a critical regulator of egg production by A. aegypti.

Insulin receptor exon 11+/− is expressed in Zucker (fa/fa) rats, and chlorogenic acid modifies their plasma insulin and liver protein and DNA

In vivo studies confirmed that chlorogenic acid (CGA) improved glucose tolerance and mineral pool distribution in obese Zucker (fa/fa) rats. We found a significant decrease (P<.05) in postprandial blood glucose concentrations, which may have been due to an improved sensitivity to insulin. Impaired glucose tolerance and insulin resistance have been associated with differences in the hepatic mRNA expression of the spliced variants of the insulin receptor at exon 11. Spliced variants of the insulin receptor have not been studied in obese Zucker (fa/fa) rats, and no information exists about the effects of CGA in vivo as a possible insulin sensitizer. Thus, we studied the in vivo effect of CGA on plasma insulin concentrations during a glucose tolerance test, liver protein and DNA concentrations, the hepatic activity of glucose-6-phosphatase (G-6-PASE) and the mRNA expression of the two variants of the insulin receptor at exon 11. Zucker (fa/fa) rats were implanted with jugular vein catheters. Chlorogenic acid was administered (5 mg/kg body weight per day) for 3 weeks via intravenous infusion. In the CGA-treated group, areas under the curve (AUC) for blood glucose and plasma insulin improved (P<.005), and the protein and DNA concentrations in the liver increased (P<.05). No significant differences (P>.05) were found between groups for the hepatic G-6-PASE activity. The insulin receptor exon 11(+) and the exon 11(-) variants were expressed in the liver of Zucker (fa/fa) rats without significant changes (P>.05). Chlorogenic acid improved some cellular mechanisms that are stimulated by insulin.

Misregulation of alternative splicing causes pathogenesis in myotonic dystrophy

Myotonic dystrophy (DM), the most common form of adult onset muscular dystrophy, affects skeletal muscle, heart, and the central nervous system (CNS). Mortality results primarily from muscle wasting and cardiac arrhythmias. There are two forms of the disease: DM1 and DM2. DM1, which constitutes 98% of cases, is caused by a CTG expansion in the 3' untranslated region (UTR) of the DMPK gene. DM2 is caused by a CCTG expansion in the first intron of the ZNF9 gene. RNA containing CUG- or CCUG-expanded repeats are transcribed but are retained in the nucleus in foci. Disease pathogenesis results primarily from a gain of function of the expanded RNAs, which alter developmentally regulated alternative splicing as well as pathways of muscle differentiation. The toxic RNA has been implicated in sequestration of splicing regulators and transcription factors thereby causing specific symptoms of the disease. Here we review the proposed mechanisms for the toxic effects of the expanded repeats and discuss the molecular mechanisms of splicing misregulation and disease pathogenesis.

Regulated membrane trafficking of the insulin-responsive glucose transporter 4 in adipocytes

Since the discovery of insulin roughly 80 yr ago, much has been learned about how target cells receive, interpret, and respond to this peptide hormone. For example, we now know that insulin activates the tyrosine kinase activity of its cell surface receptor, thereby triggering intracellular signaling cascades that regulate many cellular processes. With respect to glucose homeostasis, these include the function of insulin to suppress hepatic glucose production and to increase glucose uptake in muscle and adipose tissues, the latter resulting from the translocation of the glucose transporter 4 (GLUT4) to the cell surface membrane. Although simple in broad outline, elucidating the molecular intricacies of these receptor-signaling pathways and membrane-trafficking processes continues to challenge the creative ingenuity of scientists, and many questions remain unresolved, or even perhaps unasked. The identification and functional characterization of specific molecules required for both insulin signaling and GLUT4 vesicle trafficking remain key issues in our pursuit of developing specific therapeutic agents to treat and/or prevent this debilitating disease process. To this end, the combined efforts of numerous research groups employing a range of experimental approaches has led to a clearer molecular picture of how insulin regulates the membrane trafficking of GLUT4.

Purification and characterization of a feline hepatic insulin receptor

OBJECTIVE

To elucidate the functional characteristics of a highly purified soluble liver insulin receptor in cats.

SAMPLE POPULATION

Frozen livers from domestic cats were obtained commercially.

PROCEDURES

The feline hepatic insulin receptor was purified from Triton X-100 solubilized plasma membranes by the use of several chromatography matrices, including affinity chromatography on an insulin-Sepharose matrix.

RESULTS

The receptor, although not homogeneous, was purified 3,000-fold. Two silver-stained protein bands were identified following sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) with molecular weight of 134,000 and 97,000, which are similar to insulin receptors isolated from other animals. This isolated receptor had steady-state insulin binding by 40 minutes at 24 C. Optimal insulin binding occurred at pH 7.8 and with 150 mM NaCl. Under these conditions, a curvilinear Scatchard plot was obtained with the isolated receptor. Using a 2 binding-site model, the feline insulin receptor had a high-affinity low-capacity site with a dissociation constant (KD; nM) of 3 and a low-affinity high-capacity site with a K(D) of 1,180. The receptor also had tyrosine kinase activity toward an exogenous substrate that was stimulated by insulin and protamine.

CONCLUSIONS AND CLINICAL RELEVANCE

Many of the reported characteristics of the liver insulin receptor in cats are similar to those for the receptor isolated from other animals and tissues, although some differences exist. These similarities suggest that characterization of the feline insulin receptor is important to understanding insulin resistance in cats with diabetes as well as in humans with diabetes.

Insulin receptor and insulin receptor substrate-I in rat retinae

Insulin receptor substrate-I (IRS-I) is a major cytosolic substrate of the insulin receptor Expression of insulin receptor and IRS-I, and the distribution of these components of the insulin-signalling pathway, were investigated in rat retinae. Insulin receptor and IRS-I were located in retinal sections with anti-insulin receptor and anti-IRS-I antibodies. Reverse transcription-polymerase chain reaction (RT-PCR) of retinal mRNA was performed with primers specific for insulin receptor and IRS-I gene sequences. Immunohistochemistry demonstrated distinct but closely associated staining patterns for insulin receptor and IRS-I throughout rat retinae. The RT-PCR product from rat retinal insulin receptor mRNA corresponded to the high affinity insulin receptor isoform. The RT-PCR product for retinal IRS-I mRNA agreed with that predicted from the gene sequence. The expression of IRS- I and insulin receptors indicates a signalling mechanism by which insulin can influence retinal metabolism or function.

Gastric motor and cardiovascular effects of insulin in dorsal vagal complex of the rat

Insulin-binding sites exist in the lower brain stem of the rat, raising the possibility that the circulating hormone may have cardiovascular and gastric effects at this site. Therefore, we investigated the autonomic effects of applying rat insulin to the surface of the dorsal medulla (0.3 and 3 microU/rat) or microinjecting it into the dorsal vagal complex (DVC) (0.1-10 nU/site) in anesthetized rats. Application of rat insulin to the surface (3 microU/rat) and its microinjection into the DVC (1 and 10 nU/site) both evoked marked, albeit transient, increases in intragastric pressure, pyloric and greater curvature contractile activity, and blood pressure. Much higher doses of human (100 mU) and porcine insulin (3 mU) were needed to evoke modest changes in gastric motor and cardiovascular function when applied to the surface of the dorsal medulla. In addition, a 1,000-fold higher dose of porcine insulin (10 microU) in the DVC was not enough to mimic the autonomic effects of rat insulin microinjected into the same site. The excitatory gastric motor effects of rat insulin in the lower brain stem were abolished by vagotomy, whereas spinal cord transection blocked insulin-evoked increases in blood pressure. To test whether the gastric motor effects of rat insulin in the lower brain stem were caused by potential contamination with pancreatic polypeptide, we microinjected rat pancreatic polypeptide into the DVC at a single dose of 2 pmol. Only a modest increase in intragastric pressure in response to the hormone was observed. Thus it is likely that insulin, through its action in the lower brain stem, may be implicated in the pathogenesis of gastrointestinal and cardiovascular complications in hyperinsulinemia. In addition, species variations in the amino acid sequence of insulin may affect its biological activity in the brain of different species.

Insulin receptor exon 11+/- isoform mRNA in spontaneously hypertensive and adrenocorticotropin-hypertensive rats

OBJECTIVE

To test the hypothesis that insulin resistance of the spontaneously hypertensive rat (SHR) and adrenocorticotropin-hypertensive rat is related to a difference in the proportion of the functionally different, alternatively spliced exon 11 isoforms of the insulin receptor.

DESIGN

We determined the proportions of mRNA for the exon 11+ and exon 11- isoforms in various tissues of SHR and Wistar-Kyoto rats aged 3, 6, 9 and 12 weeks, which span the pre-hypertensive phase through to established hypertension, as well as in Sprague-Dawley rats with adrenocorticotropin-induced hypertension and Sprague-Dawley controls.

METHODS

Detection of mRNA involved a reverse-transcriptase polymerase chain reaction technique specific for each isoform and quantification was by slot and dot blot hybridization.

RESULTS

Mean proportions of exon 11+ mRNA in SHR, Wistar-Kyoto rats, adrenocorticotropin-hypertensive rats and Sprague-Dawley control rats at each age were 95% for liver, 82% for adipose tissue, 77% for kidney, 66% for adrenal, 53% for heart, 26% for cerebral cortex, 23% for hypothalamus, and 3% for skeletal muscle. There was also no difference in concentration of total insulin receptor mRNA.

CONCLUSIONS

The absence of any difference in proportions of insulin receptor mRNA isoforms argues against the hypothesis that an alteration of differential splicing plays a role in the models of hypertension studied.

Histologic distribution of insulin and glucagon receptors

Insulin and glucagon are the hormonal polypeptides secreted by the B and A cells of the endocrine pancreas, respectively. Their major physiologic effects are regulation of carbohydrate metabolism, but they have opposite effects. Insulin and glucagon have various physiologic roles, in addition to the regulation of carbohydrate metabolism. The physiologic effects of insulin and glucagon on the cell are initiated by the binding of each hormone to receptors on the target cells. Morphologic studies may be useful for relating biochemical, physiologic, and pharmacologic information on the receptors to an anatomic background. Receptor radioautography techniques using radioligands to label specific insulin and glucagon receptors have been successfully applied to many tissues and organs. In this review, current knowledge of the histologic distribution of insulin and glucagon receptors is presented with a brief description of receptor radioautography techniques.