Protein kinase A- and C-induced insulin release from Ca²⁺-insensitive pools

Insulin secretion is known to depend on an increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i). However, recent studies have suggested that insulin secretion can also be evoked in a Ca²⁺-independent manner. In the present study we show that treatment of intact mouse islets and RINm5F cells with protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) or protein kinase A (PKA) activator forskolin promoted insulin secretion with no changes of [Ca²⁺]_i. Moreover, insulin secretion mediated by PMA or forskolin was maintained even when extracellular or cytosolic Ca²⁺ was deprived by treatment of cells with ethylene glycol bis(β-amino ethyl ether)-N,N,N′,N′-tetraacetic acid (EGTA) or 1,2-bis(2-amino phenoxy)ethane-N,N,N′,N′-tetraacetic acid tetrakis(acetoxy methyl ester) (BAPTA/AM) in RINm5F cells. The secretagogue actions of PMA and forskolin were blocked by GF109203X and H89, selective inhibitors for PKC and PKA, respectively. PMA treatment caused translocation of PKC-α and PKC-ε from cytosol to membrane, implying that selectively PKC-α and PKC-ε isoforms might be important for insulin secretion. Co-treatment with high K⁺ and PMA showed a comparable level of insulin secretion to that of PMA alone. In addition, PMA and forskolin evoked insulin secretion in cells where Ca²⁺-dependent insulin secretion was completed. Our data suggest that PKC and PKA can elicit insulin secretion not only in a Ca²⁺-sensitive manner but also in a Ca²⁺-independent manner from separate releasable pools.