Aldosterone suppresses expression of an avian colonic sodium-glucose cotransporter

Pathophysiology of avian intestinal ion transport

The gut has great importance for the commercial success of poultry production. Numerous ion transporters, exchangers, and channels are present on both the apical and the basolateral membrane of intestinal epithelial cells, and their differential expression along the crypt-villus axis within the various intestinal segments ensures efficient intestinal absorption and effective barrier function. Recent studies have shown that intensive production systems, microbial exposure, and nutritional management significantly affect intestinal physiology and intestinal ion transport. Dysregulation of normal intestinal ion transport is manifested as diarrhoea, malabsorption, and intestinal inflammation resulting into poor production efficiency. This review discusses the basic mechanisms involved in avian intestinal ion transport and the impact of development during growth, nutritional and environmental alterations, and intestinal microbial infections on it. The effect of intestinal microbial infections on avian intestinal ion transport depends on factors such as host immunity, pathogen virulence, and the mucosal organisation of the particular intestinal segment.

Role of aldosterone and angiotensin II in insulin resistance: an update

The role of the Renin-Angiotensin-Aldosterone system (RAAS) on the development of insulin resistance and cardiovascular disease is an area of growing interest. Most of the deleterious actions of the RAAS on insulin sensitivity appear to be mediated through activation of the Angiotensin II (Ang II) Receptor type 1 (AT(1)R) and increased production of mineralocorticoids. The underlying mechanisms leading to impaired insulin sensitivity remain to be fully elucidated, but involve increased production of reactive oxygen species and oxidative stress. Both experimental and clinical studies also implicate aldosterone in the development of insulin resistance, hypertension, endothelial dysfunction, cardiovascular tissue fibrosis, remodelling, inflammation and oxidative stress. There is abundant evidence linking aldosterone, through non-genomic actions, to defective intracellular insulin signalling, impaired glucose homeostasis and systemic insulin resistance not only in skeletal muscle and liver but also in cardiovascular tissue. Blockade of the different components of the RAAS, in particular Ang II and AT(1)R, results in attenuation of insulin resistance, glucose homeostasis, as well as decreased cardiovascular disease morbidity and mortality. These beneficial effects go beyond to those expected with isolated control of hypertension. This review focuses on the role of Ang II and aldosterone in the pathogenesis of insulin resistance, as well as in clinical relevance of RAAS blockade in the prevention and treatment of the metabolic syndrome and cardiovascular disease.

Glucose regulation in birds

Birds maintain higher plasma glucose concentrations (P(Glu)) than other vertebrates of similar body mass and, in most cases, appear to store comparatively very little glucose intracellularly as glycogen. In general, birds are insensitive to the regulation of P(Glu) by insulin. However, there appears to be no phylogenetic or dietary pattern in the avian response to exogenous insulin. Moreover, the high levels of P(Glu) do not appear to lead to significant oxidative stress as birds are longer-lived compared to mammals. Glucose is absorbed by the avian gastrointestinal tract by sodium-glucose co-transporters (SGLTs; apical side of cells) and glucose transport proteins (GLUTs; basolateral side of cells). In the kidney, both types of glucose transporters appear to be upregulated as no glucose appears in the urine. Data also indicate that the avian nervous system utilizes glucose as a metabolic substrate. In this review, we have attempted to bring together information from a variety of sources to portray how glucose serves as a metabolic substrate for birds by considering each organ system involved in glucose homeostasis.

The metabolic syndrome in primary aldosteronism

Patients with hypertension have a high prevalence of concurrent metabolic abnormalities (eg, obesity, dyslipidemia, and hyperglycemia). Clustering of these risk factors, defined as the metabolic syndrome, is associated with a high cardiovascular risk profile. This review summarizes current knowledge about the prevalence and characteristics of the metabolic syndrome in primary aldosteronism, and discusses the possible pathophysiological link between aldosterone and individual components of the metabolic syndrome, other than hypertension. Impaired glucose metabolism due to insulin resistance appears to be the major contributor to metabolic dysfunction in primary aldosteronism. Experimental observations support the possibility that aldosterone could act directly on insulin receptor function. The potential proadipogenic role of aldosterone and its negative effect on insulin sensitivity through production of cytokines remains to be investigated. Higher rates of cardiovascular events reported in primary aldosteronism could be due in part to the increased prevalence of the metabolic syndrome in this disorder.

The metabolic syndrome in primary aldosteronism

Patients with hypertension have a high prevalence of concurrent metabolic abnormalities (eg, obesity, dyslipidemia, and hyperglycemia). Clustering of these risk factors, defined as the metabolic syndrome, is associated with a high cardiovascular risk profile. This review summarizes current knowledge about the prevalence and characteristics of the metabolic syndrome in primary aldosteronism, and discusses the possible pathophysiological link between aldosterone and individual components of the metabolic syndrome, other than hypertension. Impaired glucose metabolism due to insulin resistance appears to be the major contributor to metabolic dysfunction in primary aldosteronism. Experimental observations support the possibility that aldosterone could act directly on insulin receptor function. The potential proadipogenic role of aldosterone and its negative effect on insulin sensitivity through production of cytokines remains to be investigated. Higher rates of cardiovascular events reported in primary aldosteronism could be due in part to the increased prevalence of the metabolic syndrome in this disorder.

Endocrine regulation of ion transport in the avian lower intestine

The lower intestine (colon and coprodeum) of the domestic fowl maintains a very active, transporting epithelium, with a microvillus brush border, columnar epithelial cells, and a variety of transport systems. The colon of normal or high salt-acclimated hens expresses sodium-linked glucose and amino acid cotransporters, while the coprodeum is relatively inactive. Following acclimation to low salt diets, however, both colon and coprodeum shift to a pattern of high expression of electrogenic sodium channels, and the colonic cotransporter activity is simultaneously downregulated. These changes in the transport patterns seem to be regulated, at least in part, by aldosterone. Our recent work with this tissue has focused on whether aldosterone alone can account for the low salt pattern of transport. Other work has looked at the changes in morphology and in proportions of cell types that occur during chronic acclimation to high or low salt diets, and on a cAMP-activated chloride secretion pathway. Recent findings suggesting effects of other hormones on lower intestinal transport are also presented.

The effect of aldosterone on glucose metabolism

There is an association of glucose intolerance and diabetes with primary aldosteronism, but the frequency and mechanisms are not clear. This paper reviews the possible mechanisms of impaired glucose metabolism in primary aldosteronism. Patients with primary aldosteronism can have impaired pancreatic insulin release and reduction in insulin sensitivity. These effects may be due to hypokalemia, but the evidence suggests other factors such as a direct impact of excess aldosterone on insulin action in contributing to the metabolic dysfunction. In general adrenal surgery in cases of aldosterone-producing adenoma will correct the metabolic abnormalities, but it is less sure if treatment with spironolactone in cases of idiopathic hyperplasia will correct impaired glucose tolerance.

New ideas about aldosterone signaling in epithelia

The systemic actions of aldosterone are well documented; however, in comparison, our understanding of the cellular and molecular mechanisms by which aldosterone orchestrates these actions is rudimentary. Aldosterone exerts most of its physiological actions by modifying gene expression. It is now apparent that aldosterone represses almost as many genes as it induces. Several aldosterone-sensitive genes, including serum and glucocorticoid-inducible kinase (sgk) and small, monomeric Kirsten Ras GTP-binding protein (Ki-ras) have recently been identified. The molecular mechanisms and elements bestowing corticosteroid sensitivity on these and many other genes are becoming clear. Induction of Ki-Ras and Sgk is necessary and sufficient for some portion of aldosterone action in epithelia. These two signaling factors are components of a converging pathway with phosphatidylinositol 3-kinase positioned between them that enables both stabilizing the epithelial Na(+) channel (ENaC) in the open state as well as increasing the number of ENaC in the apical membrane. This aldosterone-induced signaling pathway contains many potential sites for feedback regulation and cross talk from other cascades and potentially impinges directly on the activity of transport proteins and/or cellular differentiation to modify electrolyte transport.

Regulation of SGLT1 expression in response to Na(+) intake

In the chicken intestine, the reduction in Na(+) intake led to a decrease in the transport of alpha-methyl-D-glucoside in the ileum (reduction of 42%) and in the rectum (51%). These reductions were reversed within 24 h after resalination and were inversely correlated to the changes in aldosterone plasma concentration. The reduction in intestinal hexose transport in the low Na(+)-fed animals was due to a decrease in the number of Na(+)-dependent D-glucose cotransporters (SGLT1) in the rectum (46%) and in the ileum (38%). Northern blot analysis showed that specific SGLT1 mRNA was expressed in the jejunum, ileum, and rectum. The amount of SGLT1 mRNA was the same in all intestinal regions and was not affected by Na(+) intake, supporting the view that the effects of dietary Na(+) on intestinal hexose transport involve posttranscriptional regulation of SGLT1. This study suggests that changes in SGLT1 expression may be involved in the homeostasis of Na(+).