ER calcium stores contribute to glucose-induced Ca²⁺ waves and intercellular connectivity in mouse pancreatic islets

Defective insulin secretion is a hallmark of diabetes mellitus. Glucose-induced Ca²⁺ oscillations are critical for the stimulation of insulin secretion, though the mechanisms through which these propagate across the islet are poorly understood. Here, we use beta cell-targeted GCaMP6f to explore the role of endoplasmic reticulum (ER) Ca²⁺ mobilization in response to submaximal (11 mM) or hyperglycemic (25 mM) glucose, mimicking diabetes. Inhibition of inositol 1,4,5-trisphosphate (IP₃) receptors, and other ion channels, with 2-aminoethoxydiphenyl borate (2-APB), had minimal effects on the initial peak or intercellular connectivity provoked by 11 mM glucose. However, 2-APB lowered subsequent glucose-induced cytosolic Ca²⁺increases and connectivity at both 11 and 25 mM glucose. Unexpectedly, the activation of IP₃ receptors with the muscarinic acetylcholine receptor agonist carbachol had minimal impact on the initial peak elicited by 11 mM glucose, but Ca²⁺ waves at 11 and 25 mM glucose were more poorly coordinated. To determine whether ER calcium mobilization was sufficient to initiate Ca²⁺ waves we next blocked sarco(endo)plasmic Ca²⁺ ATPase (SERCA) pumps with thapsigargin, whilst preventing plasma membrane depolarization with the K_ATP-channel opener, diazoxide. Under these conditions, an initial cytosolic Ca²⁺increase was followed by secondary Ca²⁺ waves that subsided slowly. The application of carbachol alongside diazoxide still enhanced Ca²⁺dynamics, though activity was uncoordinated. After genetic deletion of SERCA2 in beta cells, Ca²⁺wave frequency, but not connectivity, were lowered. Our results show that ER Ca²⁺ mobilization plays a relatively minor role in the initiation and propagation of Ca²⁺ waves in response to glucose but is needed for sustained Ca²⁺waves.