Concerted activation of different voltage-gated Ca 2+ channel isoforms may determine the kinetics of insulin release from pancreatic islets. Here we have elucidated the role of R-type Ca V 2.3 channels in that process. A 20% reduction in glucose-evoked insulin secretion was observed in Ca V 2.3-knockout (Ca V 2.3 -/-) islets, close to the 17% inhibition by the R-type blocker SNX482 but much less than the 77% inhibition produced by the L-type Ca 2+ channel antagonist isradipine. Dynamic insulin-release measurements revealed that genetic or pharmacological Ca V 2.3 ablation strongly suppressed second-phase secretion, whereas first-phase secretion was unaffected, a result also observed in vivo. Suppression of the second phase coincided with an 18% reduction in oscillatory Ca 2+ signaling and a 25% reduction in granule recruitment after completion of the initial exocytotic burst in single Ca V 2.3 -/-β cells. Ca V 2.3 ablation also impaired glucose-mediated suppression of glucagon secretion in isolated islets (27% versus 58% in WT), an effect associated with coexpression of insulin and glucagon in a fraction of the islet cells in the Ca V 2.3 -/-mouse. We propose a specific role for Ca V 2.3 Ca 2+ channels in second-phase insulin release, that of mediating the Ca 2+ entry needed for replenishment of the releasable pool of granules as well as islet cell differentiation. IntroductionSystemic glucose tolerance is orchestrated by the regulated release of insulin and glucagon from the β and α cells of the pancreatic islets of Langerhans. The α and β cells are electrically excitable and use electrical signals to couple changes in blood glucose concentration to stimulation or inhibition of hormone release. In both cell types, influx of extracellular Ca 2+ through voltage-gated Ca 2+ channels with resultant elevation of intracellular Ca 2+ concentration ([Ca 2+ ] i ) triggers exocytosis of the hormone-containing secretory granules. Like other electrically excitable cells, both α and β cells contain several types of voltage-gated Ca 2+ channel (1, 2). Assigning physiological functions to the respective Ca 2+ channels is central to the understanding of electrical and secretory activities in these cells.Voltage-gated Ca 2+ channels are divided into 3 subfamilies: (a) L-type high voltage-activated (HVA) Ca 2+ channel family that comprises the Ca V 1.1, 1.2, 1.3, and 1.4 channels and is inhibited by dihydropyridines (DHPs) (1, 3, 4); (b) non-L-type HVA channels Ca V 2.1 (P/Q-type), 2.2 (N-type), and 2.3 (R-type) that are sensitive to ω-agatoxin IVA and ω-conotoxin GVIA and SNX482, respectively (1, 4, 5); and (c) the low voltage-activated (LVA) T-type Ca 2+ channel family (Ca V 3.1, 3.2, and 3.3). The latter subtype differs electrophysiologically from the HVA Ca 2+ channels in opening transiently already upon modest depolarization (6, 7) and fulfilling important roles in pacemaker cells (8).

Concerted activation of different voltage-gated Ca 2+ channel isoforms may determine the kinetics of insulin release from pancreatic islets. Here we have elucidated the role of R-type Ca V 2.3 channels in that process. A 20% reduction in glucose-evoked insulin secretion was observed in Ca V 2.3-knockout (Ca V 2.3 -/-) islets, close to the 17% inhibition by the R-type blocker SNX482 but much less than the 77% inhibition produced by the L-type Ca 2+ channel antagonist isradipine. Dynamic insulin-release measurements revealed that genetic or pharmacological Ca V 2.3 ablation strongly suppressed second-phase secretion, whereas first-phase secretion was unaffected, a result also observed in vivo. Suppression of the second phase coincided with an 18% reduction in oscillatory Ca 2+ signaling and a 25% reduction in granule recruitment after completion of the initial exocytotic burst in single Ca V 2.3 -/-β cells. Ca V 2.3 ablation also impaired glucose-mediated suppression of glucagon secretion in isolated islets (27% versus 58% in WT), an effect associated with coexpression of insulin and glucagon in a fraction of the islet cells in the Ca V 2.3 -/-mouse. We propose a specific role for Ca V 2.3 Ca 2+ channels in second-phase insulin release, that of mediating the Ca 2+ entry needed for replenishment of the releasable pool of granules as well as islet cell differentiation. IntroductionSystemic glucose tolerance is orchestrated by the regulated release of insulin and glucagon from the β and α cells of the pancreatic islets of Langerhans. The α and β cells are electrically excitable and use electrical signals to couple changes in blood glucose concentration to stimulation or inhibition of hormone release. In both cell types, influx of extracellular Ca 2+ through voltage-gated Ca 2+ channels with resultant elevation of intracellular Ca 2+ concentration ([Ca 2+ ] i ) triggers exocytosis of the hormone-containing secretory granules. Like other electrically excitable cells, both α and β cells contain several types of voltage-gated Ca 2+ channel (1, 2). Assigning physiological functions to the respective Ca 2+ channels is central to the understanding of electrical and secretory activities in these cells.Voltage-gated Ca 2+ channels are divided into 3 subfamilies: (a) L-type high voltage-activated (HVA) Ca 2+ channel family that comprises the Ca V 1.1, 1.2, 1.3, and 1.4 channels and is inhibited by dihydropyridines (DHPs) (1, 3, 4); (b) non-L-type HVA channels Ca V 2.1 (P/Q-type), 2.2 (N-type), and 2.3 (R-type) that are sensitive to ω-agatoxin IVA and ω-conotoxin GVIA and SNX482, respectively (1, 4, 5); and (c) the low voltage-activated (LVA) T-type Ca 2+ channel family (Ca V 3.1, 3.2, and 3.3). The latter subtype differs electrophysiologically from the HVA Ca 2+ channels in opening transiently already upon modest depolarization (6, 7) and fulfilling important roles in pacemaker cells (8).

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Background and aims: The gastric hormone ghrelin has been reported to stimulate food intake, increase weight gain, and cause obesity but its precise physiological role remains unclear. We investigated the long term effects of gastrectomy evoked ghrelin deficiency and of daily ghrelin injections on daily food intake, body weight, fat mass, lean body mass, and bone mass in mice. Methods: Ghrelin was given by subcutaneous injections (12 nmol/mouse once daily) for eight weeks to young female mice subjected to gastrectomy or sham operation one week previously. Results: Gastrectomy reduced plasma concentrations of total ghrelin (octanoylated and des-octanoylated) and active (octanoylated) ghrelin by ,80%. Immediately after injection of ghrelin, the plasma concentration was supraphysiological and was still elevated 16 hours later. Daily food intake was not affected by either gastrectomy or ghrelin treatment. The effect of ghrelin on meal initiation was not studied. At the end point of the study, mean body weight was 15% lower in gastrectomised mice than in sham operated mice (p,0.001); daily ghrelin injections for eight weeks partially prevented this weight loss. In sham operated mice, ghrelin had no effect on body weight. The weight of fat was reduced in gastrectomised mice (230%; p,0.01). This effect was reversed by ghrelin, enhancing the weight of fat in sham operated mice also (+20%; p,0.05). Gastrectomy reduced lean body mass (210%; p,0.01) and bone mass (220%; p,0.001) compared with sham operated mice. Ghrelin replacement prevented the gastrectomy induced decrease in lean body mass but did not affect bone. In sham operated mice, ghrelin affected neither of these two parameters. Conclusions: Ghrelin replacement partially reversed the gastrectomy induced reduction in body weight, lean body mass, and body fat but not in bone mass. In sham operated mice, ghrelin only increased fat mass. Our results suggest that ghrelin is mainly concerned with the control of fat metabolism and that ghrelin replacement therapy may alleviate the weight loss associated with gastrectomy.

Gx resulted in loss of calvarial, trabecular and cortical bone in the rat. AKG counteracted the effect of Gx on calvaria and trabecular bone but not on cortical bone.

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Both ovariectomy (Ovx) and gastrectomy (Gx) induce osteopaenia in rats and humans. While the effect of Ovx has been ascribed to oestrogen deficiency, the underlying mechanism behind Gx is poorly understood. Alendronate, oestrogen and parathyroid hormone (PTH) are known to prevent the osteopaenia induced by Ovx in rats. The purpose of the present study was to determine whether alendronate, oestrogen or PTH could also prevent Gxevoked osteopaenia. Rats were Ovx-, Gx-, or were sham-operated (Sham) and were then treated with alendronate (50 µg/kg/day), oestrogen (10 µg/kg/day) or PTH(1-84) (75 µg/kg/day) for eight weeks. At sacrifice, serum PTH was unaffected by surgery (Ovx, 64 8 pg/ ml; Gx, 75 13 pg/ml; Sham, 58 11 pg/ml). The bone mineral density (BMD) of the fifth lumbar vertebra (L5) was analysed. Ovx and Gx reduced the BMD (ash weight/volume) of the L5 by 15 4% and 22 3% respectively. Trabecular BMD and the cortical bone mineral content (BMC) of the femur were assessed using peripheral computed tomography. Both Ovx and Gx markedly reduced trabecular BMD in the metaphyseal area of the distal femur (Ovx, 37 7%; Gx, 49 7%). The cortical BMC of the femur was only slightly reduced. Alendronate prevented trabecular bone loss after both Ovx and Gx, while oestrogen and PTH prevented trabecular bone loss after Ovx but not after Gx.In conclusion, the bisphosphonate alendronate prevented both Ovx-and Gx-induced trabecular bone loss. In contrast, PTH and oestrogen prevented Ovx-induced but not Gx-induced trabecular bone loss, suggesting that the mechanism behind the trabecular bone loss in Ovx rats differs from that in Gx rats. The results support the notion that the mechanism of action for the bone-sparing effect of these drugs differs. The ability of alendronate, and probably also other bisphosphonates, to prevent Gx-evoked osteopaenia in the rat might be of potential clinical interest when dealing with post-Gx osteopaenia in humans.