title: Glucagon-like peptide-1 increases cAMP but fails to augment contraction in adult rat cardiac myocytes.
abstract: The gut hormone, glucagon-like peptide-1 (GLP-1), which is secreted in nanomolar amounts in response to nutrients in the intestinal lumen, exerts cAMP/protein kinase A-mediated insulinotropic actions in target endocrine tissues, but its actions in heart cells are unknown. GLP-1 (10 nmol/L) increased intracellular cAMP (from 5.7+/-0.5 to 13.1+/-0.12 pmol/mg protein) in rat cardiac myocytes. The effects of cAMP-doubling concentrations of both GLP-1 and isoproterenol (ISO, 10 nmol/L) on contraction amplitude, intracellular Ca(2+) transient (CaT), and pH(i) in indo-1 and seminaphthorhodafluor (SNARF)-1 loaded myocytes were compared. Whereas ISO caused a characteristic increase (above baseline) in contraction amplitude (160+/-34%) and CaT (70+/-5%), GLP-1 induced a significant decrease in contraction amplitude (-27+/-5%) with no change in the CaT after 20 minutes. Neither pertussis toxin treatment nor exposure to the cGMP-stimulated phosphodiesterase (PDE2) inhibitor erythro-9-(2-hydroxy-3-nonyl)adenine or the nonselective PDE inhibitor 3-isobutyl-1-methylxanthine nor the phosphatase inhibitors okadaic acid or calyculin A unmasked an ISO-mimicking response of GLP-1. In SNARF-1-loaded myocytes, however, both ISO and GLP-1 caused an intracellular acidosis (DeltapH(i) -0.09+/-0.02 and -0.08+/-0.03, respectively). The specific GLP-1 antagonist exendin 9-39 and the cAMP inhibitory analog Rp-8CPT-cAMPS inhibited both the GLP-1-induced intracellular acidosis and the negative contractile effect. We conclude that in contrast to beta-adrenergic signaling, GLP-1 increases cAMP but fails to augment contraction, suggesting the existence of functionally distinct adenylyl cyclase/cAMP/protein kinase A compartments, possibly determined by unique receptor signaling microdomains that are not controlled by pertussis toxin-sensitive G proteins or by enhanced local PDE or phosphatase activation. Furthermore, GLP-1 elicits a cAMP-dependent modest negative inotropic effect produced by a decrease in myofilament-Ca(2+) responsiveness probably resulting from intracellular acidification.
authors.affiliation: Centro de Investigaciones Cardiovasculares, Facultad de Ciencias Médicas, Universidad Nacional de La Plata, La Plata, Argentina.
authors.forename: M G
authors.initials: MG
authors.lastname: Vila Petroff
authors.forename: J M
authors.initials: JM
authors.lastname: Egan
authors.forename: X
authors.initials: X
authors.lastname: Wang
authors.forename: S J
authors.initials: SJ
authors.lastname: Sollott
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D002316:Cardiotonic Agents / Q000494:pharmacology; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000031:analogs & derivatives / Q000378:metabolism* / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004558:Electric Stimulation; D005934:Glucagon / Q000494:pharmacology*; D052216:Glucagon-Like Peptide 1; D006321:Heart / Q000187:drug effects* / Q000502:physiology; D006863:Hydrogen-Ion Concentration; D007545:Isoproterenol / Q000494:pharmacology; D008297:Male; D009200:Myocardial Contraction / Q000187:drug effects; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D010446:Peptide Fragments / Q000494:pharmacology*; D011498:Protein Precursors / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D013873:Thionucleotides / Q000494:pharmacology; D013997:Time Factors
chemical_list: D002316:Cardiotonic Agents; D010446:Peptide Fragments; D011498:Protein Precursors; D013873:Thionucleotides; C015929:8-((4-chlorophenyl)thio)cyclic-3',5'-AMP; D052216:Glucagon-Like Peptide 1; D005934:Glucagon; D000242:Cyclic AMP; D007545:Isoproterenol; D002118:Calcium