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 inotr opic effect of glucagon (Sauvadet, A., Rohn, T., Pecker, F. and Pavoin e, C. (1996) Circ. Res. 78, 102-109). We now show that mini-glucagon t riggers arachidonic acid (AA) release from [H-3]AA-loaded embryonic ch ick ventricular myocytes via the activation of a phospholipase A(2) se nsitive to submicromolar Ca2+ concentrations. The phospholipase A(2) i nhibitor, AACOCF3, prevented mini-glucagon-induced [Ca-45(2+)] accumul ation into the sarcoplasmic reticulum, but inhibitors of lipoxygenase, cyclooxygenase, or epoxygenase pathways were ineffective. AA applied exogenously, at 0.3 mu M, reproduced the effects of mini-glucagon on C a2+ homeostasis and contraction. Thus AA: (i) caused [Ca-45(2+)] accum ulation into a sarcoplasmic reticulum compartment sensitive to caffein e; 2) potentiated caffeine induced Ca2+ mobilization from cells loaded with Fura-a; 3) acted synergistically with glucagon or cAMP to increa se both the amplitude of Ca2+ transients and contraction of electrical ly stimulated cells. AA action was dose-dependent and specific since i t was mimicked by its non-hydrolyzable analog 5,8,11,14-eicosatetrayno ic acid but not reproduced by other lipids such as, arachidic acid, li nolenic 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 r ange. We conclude that AA drives mini-glucagon action in the heart and that the positive inotropic effect of glucagon on heart contraction r elies on both second messengers, cAMP and AA.