Effects of ageing and senescence on pancreatic ß-cell function.

Ageing is generally associated with deterioration of organ function and regenerative potential. In the case of pancreatic ß-cells, an age-related decline in proliferative potential is well documented, and was proposed to contribute to the increased prevalence of type 2 diabetes in the elderly. The effects of ageing on ß-cell function, namely glucose-stimulated insulin secretion (GSIS), have not been studied as extensively. Recent work revealed that, surprisingly, ß-cells of mature mice and humans secrete more insulin than young ß-cells in response to high glucose concentrations, potentially serving to counteract age-related peripheral insulin resistance. This functional change appears to be orchestrated by p16(Ink4A) -driven cellular senescence and downstream remodelling of chromatin structure and DNA methylation, enhancing the expression of genes controlling ß-cell function. We propose that activation of the cellular senescence program drives life-long functional maturation of ß-cells, due to ß-cell hypertrophy, enhanced glucose uptake and more efficient mitochondrial metabolism, in parallel to locking these cells in a non-replicative state. We speculate that the beneficial aspects of this process can be harnessed to enhance GSIS. Other age-related mechanisms, which are currently poorly understood, act to increase basal insulin secretion levels also in low glucose conditions. This leads to an overall reduction in the amplitude of insulin secretion between low and high glucose at old age, which may contribute to a deterioration in metabolic control.

Reciprocal gastropancreatic modulations for the release of somatostatin-like immunoreactivity, glucagon, and insulin in the rat.

In order to assess the interrelationships between stomach and pancreas regarding the secretions of somatostatin-like immunoreactivity (SLI), glucagon (IRG), and insulin (IRI), concentrations of the three hormones were assayed in portal plasma and portal blood flow was measured in enterectomized rats before and after the selective removal of stomach or pancreas. Portal plasma SLI, IRG, and IRI concentrations were significantly increased by i.v. arginine in control rats (pancreas + stomach present). After gastrectomy, SLI, IRG, and IRI concentrations were, respectively, 52 +/- 13% (N = 15; P less than 0.005), 234 +/- 40% (P less than 0.001), and 119 +/- 15% (NS) of the pregastrectomy values. A decreased SLI secretion, an increased IRG release, and an unmodified basal IRI release were estimated by portal flow measurement. The A- and B-cell responses to arginine in the gastrectomized rats were significantly higher than in the control rats, while the D-cell response was no longer detectable. After pancreatectomy, by contrast, SLI concentrations were 360 +/- 75% of the prepancreatectomy values (N = 12; P less than 0.001). This reflected an actual increment of SLI release, taking into account the concomitant measurement of portal blood flow. The concentrations of IRG declined by 51 +/- 5% (P less than 0.001) and IRI was no longer measurable. A- and B-cell responses to arginine also were no longer detectable. These results suggest that in these experimental conditions (1) the stomach restrained pancreatic A- and B-cell responses to arginine, perhaps through the SLI released from the stomach and (2) the pancreas restrained gastric SLI secretion, perhaps through insulin.

Arbor Clinical Nutrition Updates: evidence-based clinical nutrition education using the Internet.

The Arbor Clinical Nutrition Updates (ACNU) is a weekly electronic nutrition journal for health professionals. Each issue summarises several recent clinical research papers appearing in the general medical and nutrition literature and which deal with a common nutrition topic. A commentary is added on how this research fits in with previous work, and what it all means for the practising clinician. ACNU is the world's most widely read electronic nutrition publication, with over 100,000 largely health-professional readers in 186 countries. It is published in nine languages and distributed by email without charge in both plain text and Acrobat formats. ACNU utilises a number of the Internet's unique characteristics to facilitate broad reach, currency and active reader feedback. This, together with its brevity and summarising format, helps to maintain its relevance to the nutrition education needs of health professionals, particularly those in clinical practice, and to overcome the factors most commonly reported by health professionals as obstacles to their greater adoption of evidence-based medicine. ACNU is intended to be a collaboration with the primary research journals to extend the reach of new nutrition research findings to a wider community of researchers, academics and clinicians than each journal might otherwise reach individually. As such, ACNU utilises the Internet to promote the goals of open-access publishing and evidence-based medicine.

Glucagon and insulin secretion and their biological activities in hypothermic rats.

To clarify the impact of hypothermia on the hormonal control of glucose metabolism, rats were rendered hypothermic (25 C) after catheterization of the portal vein. Glucose, insulin, glucagon, and catecholamine concentrations were serially monitored, and the regional blood flows were measured, allowing the estimation of hormone outputs. Hypothermia reduced the portal blood flow by 50% without changing arterial blood pressure, blood gases, or pH. Portal plasma insulin secretion dropped (0.05 +/- 0.01 vs. 0.23 +/- 0.04 mU/min), and glucagon secretion increased (0.81 +/- 0.18 vs. 0.38 +/- 0.10 ng/min). The B cell responses to glucose, arginine, and glucagon were abolished, while the A cell response to arginine was not significantly affected. Glucose intolerance was apparent after iv glucose or arginine loads. Haloperidol and to a lesser extent phentolamine suppressed the cold-induced glucagon rise. Phentolamine and to a lesser extent haloperidol alleviated the cold-induced suppression of insulin release. Propranolol, naloxone, and atropine were relatively inactive. The cold-induced glucose intolerance was not corrected by phentolamine treatment. A marked resistance to iv insulin was apparent in these rats, which is in contrast to a normal sensitivity to iv glucagon.

Glucagon release after stimulation of the lateral hypothalamic area in rats: predominant beta-adrenergic transmission and involvement of endorphin pathways.

Catheterization of the portal vein and stereotaxic implantation of electrodes in the lateral hypothalamic area (LHA) were performed in normal rats after thiopental anesthesia. Immunoreactive glucagon (IRG) and insulin (IRI), glucose, catecholamines, and beta-endorphin were monitored in portal and peripheral plasma, before and during electrical stimulation of the LHA. The influences on glucose and hormone concentrations of propranolol, phentolamine, atropine, and naloxone infusions were also investigated in similar rats. A basal portoperipheral concentration gradient was found for IRG, IRI, and catecholamines, but not for beta-endorphin. The LHA stimulation induced a significant rise in portal catecholamine, IRG, and glucose concentrations; IRI remained unchanged; the portoperipheral catecholamine gradient was augmented. These alterations were not observed after bilateral splanchnicectomy. Propranolol infusion abolished the LHA-dependent IRG and glucose rises. Naloxone reduced the IRG rise significantly. Phentolamine and atropine did not modify the LHA-induced reactions. These results suggest that the glucagon release which follows LHA electrical stimulation depends mainly on beta-adrenergic transmission by the splanchnic nerves. Opioid peptide receptors may modulate this effect.

Glucagon release induced by ventrolateral hypothalamic stimulation in the rat.

Catheterization of the portal vein and stereotaxic implantation of electrodes in the ventrolateral hypothalamus (VLH) were performed in normal rats after thiopental anesthesia. Immunoreactive glucagon (IRG), insulin (IRI), And glucose were monitored in portal plasma before and during electrical stimulation of the VLH (6 micro A, 50 Hz, 2 msec each, for 15 min). This stimulation induced a significant and reproducible IRG rise, followed by hyperglycemia. IRI remained unchanged. These alterations were not observed in control rats, i.e. in the absence of implantation; after VLH implantation without stimulation; or after implantation in the hippocampus or in the nucleus lenticularis. Bilateral splanchnicectomy abolished the IRG rise and hyperglycemia which followed VLH stimulation, while IRI was elevated both before and during electrical stimulation. Bilateral vagotomy did not suppress the A cell response to VLH stimulation, and it significantly reduced the IRI concentration in both basal and stimulatory periods. This resulted in sustained hyperglycemia. Attempts at total denervation of the pancreas induced patterns similar to that observed after splanchnicectomy alone. These results suggest that stimulation of the VLH can influence the endocrine pancreas and blood glucose levels by sympathetic nervous inputs which stimulate A cells and inhibit B cells.

Nutrition and general practice: an Australian perspective.

Australia has a government-subsidized, private medical system in which general practitioners (GPs) form the core component of primary care. There are approximately 20,000 active GPs and 80% of the population consults a GP each year. A new vocational register of GPs has been set up that requires training in general practice, followed by formal continuing education. I briefly review sources of information about Australian GPs' practices and knowledge of and attitudes toward nutrition. About 15-17% of GPs say they have a special interest in nutrition (20% of female GPs and 13% of male GPs). The main conditions for which advice is given are heart disease, hyperlipidemia, obesity, and diabetes. The extent of nutrition counseling by GPs is considerably less than might be expected from the strength of their statements about the importance of nutrition and long-term health. Obstacles to nutrition counseling are lack of time, lack of confidence, and inadequate nutrition knowledge, the last documented by objective testing. GPs express interest in learning more about nutrition (which may be partly driven by consumer pressure) but there is still little coherent teaching on the subject, specifically tailored for GPs. When asked their preferences for nutrition education, GPs tend to prefer educational material (such as diet charts) to give to patients.

Vitamin and iron status in new vegetarians.

This study assessed the biochemical status of a number of vitamins and iron in a group of new vegetarians. Values were compared with a group of omnivores of similar age. Satisfactory to high levels of serum folate, vitamin E, and riboflavin were found, and all were significantly higher in vegetarians than omnivores. Thiamin status was satisfactory in both groups although a small but statistically significant difference in favor of the omnivores was found. Serum vitamin B-12 was significantly lower in vegetarians, and iron status as measured by serum ferritin was very significantly lower in vegetarians. Pyridoxine status was similar in both groups. A number of sex differences were found in the vegetarian sample. New vegetarian women appear to be at particular risk of developing low iron stores.

The nervous control of rat glucagon secretion in vivo.

In order to study the efferent pathways of the nervous regulation of rat A and B cells, portal blood samples were obtained in vivo without interruption of the blood flow. Glucagon, insulin and catecholamines were determined and hepatic blood flow (EHBF) was estimated by a Brome-Sulfone-Phtaleine extraction method. Carotid blood pressure was monitored and a normal volaemia was maintained. Stimulation of the right vagus nerve increased EHBF and the releases of glucagon and insulin. Stimulation of splanchnic nerve increased the glucagon and catecholamine secretions and decreased that of insulin. Acute hypovolaemia as induced by blood withdrawal, caused hormonal consequences similar to those of splanchnic stimulation. It is suggested that the nervous control of pancreatic islets plays an important role in the rat species. Assessment of the haemodynamic status is critical for the valid interpretation of pancreatic hormone concentrations in experimental conditions. A sympathetic stimulation can account for the high glucagon and relatively low insulin secretions which characterize the hormonal pattern of stress.

A new technique for hepatic portal vein catheterization in freely moving rats.

To perform kinetic studies of pancreatic hormones in freely moving rats, we have designed a new technique for hepatic portal vein catheterization through the left branch of the portal vein. Rats were followed up for 14-38 days. Catheter patency allowed blood sampling in 86% of rats on postoperative day 8 and in 55% on day 12. Rat growth, liver functions, and regional blood flows (measured with radiolabeled microspheres) were normal. In freely moving rats fasted for 16 h, portal venous concentrations in the basal state were: glucose, 112 +/- 4 mg/dl; insulin, 67 +/- 10 microU/ml; and glucagon, 480 +/- 74 pg/ml. These values remained stable for a 1-h period. Intravenous glucose infusion induced a biphasic insulin release, significant from the 1st min of infusion on, and a significant decline in glucagon level. Intravenous arginine stimulated both A and B cells. The portoperipheral hormone concentration gradient, which was detected in the basal state, increased during arginine infusion, particularly in regard to insulin. Ingestion of a mixed meal induced an early glucagon release and a sustained rise in insulin and glucose concentrations.

The brain-islet axis: the nervous control of the endocrine pancreas.

The central nervous system exerts a control on the endocrine pancreas and can modulate the basic feed-back loop linking the concentration of the main energy substrates in blood with islet cell functions. Thus, the elementary glucose-insulin system can be modulated under physiological conditions by both the long-recognized entero-insular axis and by a brain-islet axis, particularly when insulin release occurs in anticipation of meals. Experimental stimulation or section of afferent nerves to the pancreas have demonstrated the existence of this nervous control. Changes in islet cell secretion during stress illustrate this influence under clinical conditions. A variety of experimental data suggest intervention of the brain-islet axis under the physiological circumstances: 1) Manipulation of certain hypothalamic centres followed by modification of feeding behaviour and of islet secretion. 2) Input to the brain is both humoral (carried by arterial carotid blood or the cerebrospinal fluid) and nervous, of sensory and visceral origins. Changes in these afferent pathways may influence islet secretion via the efferent pathways of the vagus and splanchnic nerves. 3) Besides acetylcholine and catecholamines, peptide neurotransmitters are likely to be involved in the transmission of these nervous inputs to islet cells. 4) Furthermore, hypothalamic factor (s) may also modify the endocrine pancreatic secretions.