SERCA3 Ablation Does Not Impair Insulin Secretion but Suggests Distinct Roles of Different Sarcoendoplasmic Reticulum Ca2+ Pumps for Ca2+ Homeostasis in Pancreatic β-cells

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Abstract
Two sarcoendoplasmic reticulum Ca2+-ATPases, SERCA3 and SERCA2b, are expressed in pancreatic islets. Immunocytochemistry showed that SERCA3 is restricted to β-cells in the mouse pancreas. Control and SERCA3-deficient mice were used to evaluate the role of SERCA3 in β-cell cytosolic-free Ca2+ concentration ([Ca2+]c) regulation, insulin secretion, and glucose homeostasis. Basal [Ca2+]c was not increased by SERCA3 ablation. Stimulation with glucose induced a transient drop in basal [Ca2+]c that was suppressed by inhibition of all SERCAs with thapsigargin (TG) but unaffected by selective SERCA3 ablation. Ca2+ mobilization by acetylcholine was normal in SERCA3-deficient β-cells. In contrast, [Ca2+]c oscillations resulting from intermittent glucose-stimulated Ca2+ influx and [Ca2+]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 Ca2+ 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 [Ca2+]c regulation in β-cells. SERCA3 becomes operative when [Ca2+]c rises and is required for normal [Ca2+]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.