Synergistic Actions of Glucagon and Miniglucagon on Ca 2+ Mobilization in Cardiac Cells

Abstract

Abstract It has been recently shown that the physiological processing of glucagon into its C-terminal (19-29) fragment, miniglucagon, by cardiac cells was essential for the contractile positive inotropic effect of the hormone. However, the mechanisms underlying the effects of miniglucagon remained undetermined. In the present study, we assessed the effects of miniglucagon on Ca 2+ homeostasis in embryonic chick ventricular myocytes. In quiescent cells, short-term applications of 0.1 nmol/L miniglucagon markedly increased the accumulation of 45 Ca into intracellular compartments resistant to digitonin lysis and sensitive to caffeine. Ca 2+ accumulation into the sarcoplasmic reticular (SR) store was further attested by fura 2 imaging studies on quiescent or prestimulated cells: miniglucagon potentiated Ca 2+ release from the SR compartment triggered by caffeine and evoked a rise in cytosolic Ca 2+ when applied on cells pretreated with 1 μmol/L thapsigargin, a specific inhibitor of the SR Ca 2+ pump. Glucagon alone produced a small cytosolic Ca 2+ signal that was considerably amplified by miniglucagon. The action of glucagon was mimicked by 8-bromo-cAMP and was blocked by isradipine, suggesting that it relied on the activation of L-type Ca 2+ channels, via phosphorylation. We conclude that the combined actions of miniglucagon and glucagon on Ca 2+ accumulation into SR stores and Ca 2+ release from the same stores are likely to support the positive inotropic effect elicited in vivo by glucagon on heart contraction.