The priming effect of glucose on insulin secretion from isolated islets of Langerhans

Abstract

Summary. Biphasic insulin secretion from perifused rat islets of Langerhans was enhanced if islets had previously been stimulated with glucose 16.6 mmol/1. The priming effect of glucose was reduced if mannoheptulose (16.6 mmol/1), deuterium oxide (D20; 98% v/v) or adrenaline (10 ~tmol/l) was included in the medium during the initial stimulation period, or if Calcium was omitted. Glyceraldehyde (16.6 mmol/1) but not theophylline (5 mmol/1) could substitute for glucose during the initial stimulation and make islets more responsive to subsequent stimulation. The results suggest that the priming effect of glucose on insulin secretion may be related to 1) glucose metabolism and 2) Ca fluxes in the B cell and the consequent activation of the microtubular system. Neither the generation of intracellular cyclic AMP nor the release of insulin per se appears to be involved in the priming process.Key words: Biphasic insulin secretion, perifused islets, glucose priming, mannoheptulose, glyceraldehyde, calcium, deuterium oxide, cyclic AMP, adrenaline.Acute exposure of the rat pancreatic B cell to glucose leads to biphasic secretion which is characterised by a short-lived burst of insulin release (phase 1) followed by a slower progressive rise in the secretory rate known as phase 2 [1][2][3][4]. Studies in vivo in man [5] and in vitro with the perfused rat pancreas [6] and perifused islets of Langerhans [7] have shown that a short period of glucose stimulation may also potentiate the secretory response of the B cell to a second pulse of glucose given shortly afterwards. Although it has been suggested that this effect occurs only if the concentration of glucose present during the initial stimulation period is in excess of approximately 14 mmol/1 in man [5] or 11 mmol/1 in perifused rat islets [7], the biochemical mechanisms mediating this phenomenon are poorly understood. In the present study the possible roles of 1) glucose metabolism 2) calcium and the microtubular system, and 3) cyclic AMP have been examined. The experiments were carried out using perifused rat islets of Langerhans. Materials and Methods Isolation and Perifusion of Islets of LangerhansMale Wister albino rats weighing approximately 200 g were fed ad libitum on a standard laboratory diet containing 48% carbohydrate, 21.3% protein and 3.4% fat. Islets were isolated by collagenase digestion [8]. The medium used for extraction and subsequent experiments was a bicarbonate-buffered salt solution with the following ionic composition: Na + 141; K + 5.9; Ca 2+ 2.5; Mg 2+ 1.2; PO~-1.2; CI-101 and HCO-3 24.9mmol/1; pH 7.4. The medium was supplemented with the sodium salts of glutamic, lactic and fumaric acids at a concentration of 5 rnmol/l [9]. The gas phase was O2:CO2 (95:5). The calcium concentration of 'calcium-free' media was 0.03mmol/1 as determined by atomic absorption spectrophotometry. When islets were exposed to deuterium oxide (D20) the ionic composition of the medium was unchanged but D20 replaced H20 to give a final concentration of 98% (v/v). The pH was ad...

Reduced early and late phase insulin response to glucose in isolated spiny mouse (Acomys cahirinus) islets: a defective link between glycolysis and adenylate cyclase

“…As would be expected from previous observations that second phase response to glucose was less deranged [17], late insulin release is markedly stimulated during prolonged exposure (60 min) of Acomys islets to glyceraldehyde. Such findings as well as other observations from our group [2,5,7] and others [3,4,8,9] suggest that the mechanisms that control first and second phase insulin release are distinct and dissociable.…”

“…Upon stimulation with glucose, normal islets exhibit biphasic dynamics of insulin release, representing the net expression of the interaction between several regulatory events, including time-dependent inhibitory and potentiating signals [1][2][3][4][5][6][7][8][9][10]. Limited information exists regarding the nature of these regulatory signals.…”

Time-dependent inhibition of insulin release: glucose-arginine interactions in the perfused rat pancreas

“…We [1-4, 8, 10, 11] and others [5][6][7]9] have shown that many agents which stimulate the acute release of insulin also modify the responsiveness of the islet for a considerable time, resulting in either potentiation or inhibition of subsequent insulin responses. We have termed these effects time-dependent potentiation and inhibition (TDI), respectively.…”

“…However, when glucose was added to the second arginine pulse only, the inhibition generated by the first arginine pulse did not express itself, insulin release remaining similar to control. We conclude that: (1) short stimulations of the pancreas by arginine or glucose generate long-lasting inhibition of the insulin response to subsequent stimulations; (2) synergistic amplification of the insulin response by addition of glucose to arginine obliterates the inhibition; (3) glucose does not suppress the induction of inhibition, it blocks the expression of the inhibitory signal on insulin secretion; (4) these in vitro findings are in keeping with observations in normal and hyperglycaemic man.Key words: Perfused pancreas, insulin secretion, arginine, synergism, time-dependent inhibition, time-dependent potentiation.Studies in vivo and in vitro have demonstrated that the insulin secretion rate is modified by prior sensitization of the pancreas to various insulin releasers [1][2][3][4][5][6][7][8][9][10]. While some stimulators like glucose may generate either a state of potentiation or inhibition in the islet according to the conditions of exposure [2,3,8], arginine induces only an inhibitory state [3,11].…”

“…Studies in vivo and in vitro have demonstrated that the insulin secretion rate is modified by prior sensitization of the pancreas to various insulin releasers [1][2][3][4][5][6][7][8][9][10]. While some stimulators like glucose may generate either a state of potentiation or inhibition in the islet according to the conditions of exposure [2,3,8], arginine induces only an inhibitory state [3,11].…”