Tetracaine stimulates insulin secretion from the pancreatic β-cell by release of intracellular calcium

The role of intracellular calcium stores in stimulus-secretion coupling in the pancreatic β-cell is largely unknown. We report here that tetracaine stimulates insulin secretion from collagenase-isolated mouse islets of Langerhans in the absence of glucose or extracellular calcium. We also found that the anesthetic evokes a dose-dependent rise of the intracellular free-calcium concentration ([Ca²⁺](i)) in cultured rat and mouse B-cells. The tetracaine-specific [Ca²⁺](i) rise also occurs in the absence of glucose, or in β-cells depolarized by exposure to a Ca²⁺-deficient medium (< 1 μM) or elevated [K⁺]₀. Furthermore, tetracaine (≥ 300 μM) depolarized the β-cell membrane in mouse pancreatic islets, but inhibited Ca²⁺ entry through voltage-gated Ca²⁺ channels in HIT cells, an insulin-secreting cell line. From these data we conclude that tetracaine-enhancement of insulin release occurs by mechanisms that are independent of Ca²⁺ entry across the cell membrane. The tetracaine-induced [Ca²⁺](i) rise in cultured rat β-cells and insulin secretion from mouse islets is insensitive to dantrolene (20 μM), a drug that inhibits Ca²⁺ release evoked by cholinergic agonists in the pancreatic β-cell, and thapsigargin (3 μM), a blocker of the endoplasmic reticulum (ER) Ca²⁺ pump. We conclude that the Ca²⁺ required for tetracaine-potentiated insulin secretion is released from intracellular Ca²⁺ stores other than the ER. Furthermore, tetracaine-induced Ca²⁺ release was unaffected by the mitochondrial electron transfer inhibitors NaN₃ and rotenone. Taken together, these data show that a calcium source other than the ER and mitochondria can affect β-cell insulin secretion.