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 microM, 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 mini-glucagon action in the heart and that the positive inotropic effect of glucagon on heart contraction relies on both second messengers, cAMP and AA.
Sauvadet
A
A
INSERM Unité 99, Hôpital Henri Mondor, 94010 Créteil, France.
Rohn
T
T
Pecker
F
F
Pavoine
C
C
D015117:5,8,11,14-Eicosatetraynoic Acid / Q000494:pharmacology; D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000494:pharmacology; D000818:Animals; D016718:Arachidonic Acid / Q000378:metabolism*; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D002478:Cells, Cultured; D000697:Central Nervous System Stimulants / Q000494:pharmacology; D002642:Chick Embryo; D004305:Dose-Response Relationship, Drug; D004357:Drug Synergism; D004558:Electric Stimulation; D005934:Glucagon / Q000494:pharmacology*; D006706:Homeostasis / Q000187:drug effects; D009200:Myocardial Contraction / Q000187:drug effects; D009206:Myocardium / Q000378:metabolism*; D010446:Peptide Fragments / Q000494:pharmacology*; D010741:Phospholipases A / Q000378:metabolism; D054467:Phospholipases A2; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism; D013268:Stimulation, Chemical
D000697:Central Nervous System Stimulants; D010446:Peptide Fragments; D015117:5,8,11,14-Eicosatetraynoic Acid; D015124:8-Bromo Cyclic Adenosine Monophosphate; D016718:Arachidonic Acid; D002110:Caffeine; C039265:glucagon (19-29); D005934:Glucagon; D010741:Phospholipases A; D054467:Phospholipases A2; D002118:Calcium