Coupling of exocytosis to depolarization in rat pancreatic islet β-cells: effects of Ca²⁺, Sr²⁺ and Ba²⁺-containing extracellular solutions

Summary

Using rat β-cells we present evidence that Sr²⁺ and Ba²⁺, like Ca²⁺, support depolarization-induced increases in membrane capacitance which reflect insulin granule exocytosis. Even with identical total charge entry, Sr²⁺ and Ba²⁺ are 3–5 and 20-fold less effective than Ca²⁺ in supporting release. While exocytosis supported by Sr²⁺ is graded with cation entry and complete within 250ms of depolarization, exocytosis supported by Ba²⁺ begins abrupty after a threshold of charge entry and continues for many seconds. Ba²⁺-supported release continues in the presence of greatly enhanced cytosolic Ca²⁺ buffering, arguing against release of Ca²⁺ from stores as its principal action. These results suggest that Sr²⁺ and Ba²⁺ support exocytosis largely by binding to Ca²⁺-dependent release-activating sites, though with less affinity than Ca²⁺.