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The mechanisms underlying biphasic insulin secretion have not been completely elucidated. We compared the pattern of plasma insulin changes during hyperglycemic clamps in mice to that of glucose-induced insulin secretion and cytosolic calcium concentration ([Ca(2+)](c)) changes in perifused mouse islets. Anesthetized mice were infused with glucose to clamp blood glucose at 8.5 (baseline), 11.1, 16.7, or 30 mmol/l. A first-phase insulin response consistently peaked at 1 min, and a slowly ascending second phase occurred at 16.7 and 30 mmol/l glucose. Glucose-induced insulin secretion in vivo is thus biphasic, with a similarly increasing second phase in the mouse as in humans. In vitro, square-wave stimulation from a baseline of 3 mmol/l glucose induced similar biphasic insulin secretion and [Ca(2+)](c) increases, with sustained and flat second phases. The glucose dependency (3-30 mmol/l) of both changes was sigmoidal with, however, a shift to the right of the relation for insulin secretion compared with that for [Ca(2+)](c). The maximum [Ca(2+)](c) increase was achieved by glucose concentrations, causing half-maximum insulin secretion. Because this was true for both phases, we propose that contrary to current concepts, amplifying signals are also implicated in first-phase glucose-induced insulin secretion. To mimic in vivo conditions, islets were stimulated with high glucose after being initially perifused with 8.5 instead of 3.0 mmol/l glucose. First-phase insulin secretion induced by glucose at 11.1, 16.7, and 30 mmol/l was decreased by approximately 50%, an inhibition that could not be explained by commensurate decreases in [Ca(2+)](c) or in the pool of readily releasable granules. Also unexpected was the gradually ascending pattern of the second phase, now similar to that in vivo. These observations indicated that variations in prestimulatory glucose can secondarily affect the magnitude and pattern of subsequent glucose-induced insulin secretion. [ABSTRACT FROM AUTHOR]

Two sarcoendoplasmic reticulum Ca(2+)-ATPases, SERCA3 and SERCA2b, are expressed in pancreatic islets. Immunocytochemistry showed that SERCA3 is restricted to beta-cells in the mouse pancreas. Control and SERCA3-deficient mice were used to evaluate the role of SERCA3 in beta-cell cytosolic-free Ca(2+) concentration ([Ca(2+)](c)) regulation, insulin secretion, and glucose homeostasis. Basal [Ca(2+)](c) was not increased by SERCA3 ablation. Stimulation with glucose induced a transient drop in basal [Ca(2+)](c) that was suppressed by inhibition of all SERCAs with thapsigargin (TG) but unaffected by selective SERCA3 ablation. Ca(2+) mobilization by acetylcholine was normal in SERCA3-deficient beta-cells. In contrast, [Ca(2+)](c) oscillations resulting from intermittent glucose-stimulated Ca(2+) influx and [Ca(2+)](c) transients induced by pulses of high K(+) were similarly affected by SERCA3 ablation or TG pretreatment of control islets; their amplitude was increased and their slow descending phase suppressed. This suggests that, during the decay of each oscillation, the endoplasmic reticulum releases Ca(2+) that was pumped by SERCA3 during the upstroke phase. SERCA3 ablation increased the insulin response of islets to 15 mmol/l glucose. However, basal and postprandial plasma glucose and insulin concentrations in SERCA3-deficient mice were normal. In conclusion, SERCA2b, but not SERCA3, is involved in basal [Ca(2+)](c) regulation in beta-cells. SERCA3 becomes operative when [Ca(2+)](c) rises and is required for normal [Ca(2+)](c) oscillations in response to glucose. However, a lack of SERCA3 is insufficient in itself to alter glucose homeostasis or impair insulin secretion in mice. [ABSTRACT FROM AUTHOR]