Arachidonic acid drives mini-glucagon action in cardiac cells

Recent studies have shown that glucagon is processed by cardiac cells into its COOH-terminal (19-29) fragment, mini-glucagon, and that this metabolite is an essential component of the contractile positive inotropic effect of glucagon (Sauvadet, A., Rohn, T., Pecker, F. and Pavoine, C. (1996) Circ. Res. 78, 102-109). We now show that mini-glucagon triggers arachidonic acid (AA) release from [³H]AA-loaded embryonic chick ventricular myocytes via the activation of a phospholipase A₂ sensitive to submicromolar Ca²⁺ concentrations. The phospholipase A₂ inhibitor, AACOCF3, prevented mini- glucagon-induced [⁴⁵Ca²⁺] accumulation into the sarcoplasmic reticulum, but inhibitors of lipoxygenase, cyclooxygenase, or epoxygenase pathways were ineffective. AA applied exogenously, at 0.3 μM, reproduced the effects of mini-glucagon on Ca²⁺ homeostasis and contraction. Thus AA: (i) caused [⁴⁵Ca²⁺] accumulation into a sarcoplasmic reticulum compartment sensitive to caffeine; 2) potentiated caffeine-induced Ca²⁺ mobilization from cells loaded with Fura-2; 3) acted synergistically with glucagon or cAMP to increase both the amplitude of Ca²⁺ transients and contraction of electrically stimulated cells. AA action was dose-dependent and specific since it was mimicked by its non-hydrolyzable analog 5,8,11,14- eicosatetraynoic acid but not reproduced by other lipids such as, arachidic acid, linolenic acid, cis-5,8,11,14,17-eicosapentaenoic acid, cis- 4,7,10,13,16,19-docosahexaenoic acid, or arachidonyl-CoA, even in the micromolar range. We conclude that AA drives miniglucagon action in the heart and that the positive inotropic effect of glucagon on heart contraction relies on both second messengers, cAMP and AA.