title: Dual sensitization of the adrenergic system in early myocardial ischemia: independent regulation of the beta-adrenergic receptors and the adenylyl cyclase.
abstract: Acute myocardial ischemia provokes sensitization of the adenylyl cyclase system. This sensitization can be differentiated in a receptor-specific and an enzyme-specific sensitization. The receptor-linked sensitization is characterized by an increase of beta-adrenergic receptors in the plasma membranes after 15 mins of global ischemia (49.8 +/- 3.6 to 67 +/- 6 fmol/mg protein) followed by a further increase (89 +/- 4 fmol/mg protein) after 50 min of ischemia in isolated perfused hearts. Concomitantly functionally coupled receptors which are able to bind the beta-agonist with high affinity, increased by 32% after 15 min and by 57% after 50 min of ischemia. The affinities of the receptors for their agonists or their antagonists remain unchanged. Maximally isoproterenol-stimulated adenylyl cyclase activity rose from 66 +/- 7 to 101 +/- 10 pmol cAMP/min/mg protein after 15 min of global ischemia indicating the beta-receptor-specific sensitization of the beta-adrenergic system. This sensitization was followed by a gradual decline of the adenylyl cyclase activity after 30 and 50 min of global ischemia. Additionally, 15 min of myocardial ischemia induced an enzyme-linked sensitization of the adenylyl cyclase activity as indicated by an increase of the forskolin-stimulated activity by about 25% (300 +/- 20 vs 378 +/- 25 pmol cAMP/min/mg protein). In contrast after 50 min of ischemia the total adenylyl cyclase activity declined (232 +/- 24 pmol cAMP/min/mg protein) despite the persistent increase of beta-adrenergic receptors in the plasma membranes. These data demonstrate that the enzyme-specific sensitization is only transient. The early sensitization and late inactivation of the adenylyl cyclase activity occurred independently of receptor activation and could not be prevented by beta-blockade (10(-6) M alprenolol). Cyanide perfusion (1 mM), used to block energy metabolism, lead to energy depletion similar to acute myocardial ischemia. This resulted in an increase of functionally coupled receptors with a time course comparable to that of global ischemia. Additional perfusion with desensitizing concentrations of the beta-agonist isoproterenol did not induce uncoupling or internalization of beta-adrenergic receptors in cyanide treated hearts, suggesting that the rise in functionally coupled receptors is due to a redistribution in part caused by the abolition of continuous receptor internalization. In contrast, the enzyme-linked sensitization is independent of cellular localization of the beta-adrenergic receptors. The increased activity was carried by the enzyme even after partial purification with solubilization and wheat germ affinity chromatography. These data suggest an ischemia-induced, covalent modification of the adenylyl cyclase.(ABSTRACT TRUNCATED AT 400 WORDS)
authors.affiliation: Department of Cardiology, University of Heidelberg, FRG.
authors.forename: R H
authors.initials: RH
authors.lastname: Strasser
authors.forename: J
authors.initials: J
authors.lastname: Krimmer
authors.forename: R
authors.initials: R
authors.lastname: Braun-Dullaeus
authors.forename: R
authors.initials: R
authors.lastname: Marquetant
authors.forename: W
authors.initials: W
authors.lastname: Kübler
mesh_terms: D000262:Adenylyl Cyclases / Q000378:metabolism*; D000818:Animals; D003327:Coronary Disease / Q000201:enzymology / Q000378:metabolism*; D019204:GTP-Binding Proteins / Q000378:metabolism; D006321:Heart / Q000294:innervation*; D066298:In Vitro Techniques; D008297:Male; D009206:Myocardium / Q000201:enzymology / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D011943:Receptors, Adrenergic, beta / Q000378:metabolism*; D013997:Time Factors
chemical_list: D011943:Receptors, Adrenergic, beta; D019204:GTP-Binding Proteins; D000262:Adenylyl Cyclases