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      Pulsatility of insulin release – a clinically important phenomenon

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          The mechanisms and clinical importance of pulsatile insulin release are presented against the background of more than half a century of companionship with the islets of Langerhans. The insulin-secreting β-cells are oscillators with intrinsic variations of cytoplasmic ATP and Ca 2+. Within the islets the β-cells are mutually entrained into a common rhythm by gap junctions and diffusible factors (ATP). Synchronization of the different islets in the pancreas is supposed to be due to adjustment of the oscillations to the same phase by neural output of acetylcholine and ATP. Studies of hormone secretion from the perfused pancreas of rats and mice revealed that glucose induces pulses of glucagon anti-synchronous with pulses of insulin and somatostatin. The anti-synchrony may result from a paracrine action of somatostatin on the glucagon-producing α-cells. Purinoceptors have a key function for pulsatile release of islet hormones. It was possible to remove the glucagon and somatostatin pulses with maintenance of those of insulin with an inhibitor of the P2Y 1 receptors. Knock-out of the adenosine A 1 receptor prolonged the pulses of glucagon and somatostatin without affecting the duration of the insulin pulses. Studies of isolated human islets indicate similar relations between pulses of insulin, glucagon, and somatostatin as found during perfusion of the rodent pancreas. The observation of reversed cycles of insulin and glucagon adds to the understanding how the islets regulate hepatic glucose production. Current protocols for pulsatile intravenous infusion therapy (PIVIT) should be modified to mimic the anti-synchrony between insulin and glucagon normally seen in the portal blood.

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          Autonomic regulation of islet hormone secretion--implications for health and disease.

           B. Ahrén (2000)
          The pancreatic islets are richly innervated by parasympathetic, sympathetic and sensory nerves. Several different neurotransmitters are stored within the terminals of these nerves, both the classical neurotransmitters, acetylcholine and noradrenaline, and several neuropeptides. The neuropeptides, vasoactive intestinal polypeptide, pituitary adenlyate cyclase activating polypeptide and gastrin releasing peptide are constituents of the parasympathetic nerves, whereas the neuropeptides galanin and neuropeptide Y are localised to sympathetic nerve terminals. Furthermore, the neuropeptide calcitonin gene-related peptide is localised to sensory nerves and cholecystokinin is also an islet neuropeptide, although the nature of the cholecystokinin nerves is not established. Stimulation of the autonomic nerves and treatment with neurotransmitters affect islet hormone secretion. Thus, insulin secretion is stimulated by parasympathetic nerves or their neurotransmitters and inhibited by sympathetic nerves or their neurotransmitters. The islet autonomic nerves seem to be of physiological importance in mediating the cephalic phase of insulin secretion, in synchronising the islets to function as a unit allowing oscillations of islet hormone secretion, and in optimising islet hormone secretion during metabolic stress, e.g. hypoglycaemia and neuroglycopenia. The autonomic nerves could also be involved in the islet adaptation to insulin resistance with possible implication for the development of glucose intolerance and Type II (non-insulin-dependent) diabetes mellitus. It is concluded that islet innervation, through the contribution of all branches of the autonomic nerves and several different neurotransmitters is of importance both for the physiology and pathophysiology of the islets.
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            Downregulation of GLP-1 and GIP receptor expression by hyperglycemia: possible contribution to impaired incretin effects in diabetes.

            Stimulation of insulin secretion by the incretin hormones glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic peptide (GIP) has been found to be diminished in type 2 diabetes. We hypothesized that this impairment is due to a defect at the receptor level induced by the diabetic state, particularly hyperglycemia. Gene expression of incretin receptors, GLP-1R and GIPR, were significantly decreased in islets of 90% pancreatectomized (Px) hyperglycemic rats, with recovery when glucose levels were normalized by phlorizin. Perifused islets isolated from hyperglycemic Px rats showed reduced insulin responses to GLP-1 and GIP. To examine the acute effect of hyperglycemia on incretin receptor expression, a hyperglycemic clamp study was performed for 96 h with reduction of GLP-1 receptor expression but increase in GIP receptor expression. Similar findings were found when islets were cultured at high glucose concentrations for 48 h. The reduction of GLP-1 receptor expression by high glucose was prevented by dominant-negative protein kinase C (PKC)alpha overexpression, whereas GLP-1 receptor expression was reduced with wild-type PKCalpha overexpression. Taken together, GLP-1 and GIP receptor expression is decreased with chronic hyperglycemia, and this decrease likely contributes to the impaired incretin effects found in diabetes.
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              Glucose induces closure of single potassium channels in isolated rat pancreatic beta-cells.

              The major physiological stimulus for the secretion of insulin from the pancreatic beta-cell is an increase in the plasma glucose concentration. It is well established that glucose-stimulated insulin secretion is associated with the appearance of electrical activity in the beta-cell; glucose concentrations above the threshold level for insulin release produce a slow membrane depolarization followed by either oscillatory bursts of action potentials (5-15 mM glucose) or continuous spiking (greater than 16 mM glucose). Tracer flux studies and microelectrode measurements using intact islets of Langerhans have indicated that the initial depolarization induced by glucose is caused by a decrease in the resting membrane permeability to potassium. Evidence also suggests that the electrical, ionic and secretory responses to glucose are mediated by the metabolism of the sugar within the beta-cell. By using cell-attached membrane patches from isolated rat pancreatic beta-cells, we have now identified a potassium channel (G-channel) that is active at the resting potential and is inhibited by glucose. Closure of this channel requires glucose metabolism. This is the first report of a potassium channel whose activity is modulated by glucose, and which may couple metabolic and ionic events involved in the secretion of insulin.

                Author and article information

                Ups J Med Sci
                Upsala Journal of Medical Sciences
                Informa Healthcare
                December 2009
                08 December 2009
                : 114
                : 4
                : 193-205
                Department of Medical Cell Biology, Uppsala University, Uppsala Sweden
                Author notes
                Correspondence: Professor Bo Hellman, Department of Medical Cell Biology, Biomedicum Box 571, SE-75123 Uppsala, Sweden. Fax: +46 184714059. E-mail: Bo.Hellman@ 123456mcb.uu.se
                © Upsala Medical Society

                This is an open-access article distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the source is credited.

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