ER calcium stores contribute to glucose-induced Ca ²⁺ waves and intercellular connectivity in mouse pancreatic islets

Defective insulin secretion is a hallmark of diabetes mellitus. Glucose-induced Ca ²⁺ 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) Ca ²⁺ mobilization in response to submaximal (11mM) and hyperglycemic (25mM) glucose concentrations. Inhibition of inositol 1,4,5 trisphosphate (IP ₃ ) receptors, and other ion channels, with 2-aminoethoxydiphenyl borate (2-APB) had minimal effects on the initial peak or intercellular connectivity provoked by 11mM glucose. However, 2-APB lowered subsequent glucose-induced cytosolic Ca ²⁺ increases and connectivity at both 11 and 25mM glucose. Unexpectedly, the activation of IP ₃ receptors with the muscarinic acetylcholine receptor agonist carbachol had minimal impact on the initial peak elicited by 11 mM glucose, but Ca ²⁺ waves at 11 and 25 mM glucose were more poorly coordinated. To determine whether ER calcium mobilization was sufficient to initiate Ca ²⁺ waves we next blocked sarco(endo)plasmic Ca ²⁺ ATPase (SERCA) pumps with thapsigargin, whilst preventing plasma membrane depolarization with the K _ATP -channel opener, diazoxide. Under these conditions, an initial cytosolic Ca ²⁺ increase was followed by secondary Ca ²⁺ waves that slowly subsided. The application of carbachol alongside diazoxide still enhanced Ca ²⁺ dynamics, though this activity was uncoordinated and beta cells were poorly connected. Our results show that ER Ca ²⁺ mobilization plays a relatively minor role in the initiation and propagation of Ca ²⁺ waves in response to glucose. On the other hand, ER stores are required to transition to sustained Ca ²⁺ waves.