DUAL SENSITIZATION OF THE ADRENERGIC SYSTEM IN EARLY MYOCARDIAL-ISCHEMIA - INDEPENDENT REGULATION OF THE BETA-ADRENERGIC RECEPTORS AND THE ADENYLYL CYCLASE

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 β-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 β-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 β-receptor-specific sensitization of the β-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 sesitization 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 β-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 β-blockade (10-6 m alprenolol). Cyanide perfusion (1 m m), 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 β-agonist isoproterenol did not induce uncoupling or internalization of β-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 β-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. This enzyme-specific sensitization or its late inactivation of the enzyme does not occur after cyanide perfusion demonstrating that energy depletion is not solely responsible for this sensitization process. The characterization of these distinct pathomechanisms and time courses of the ischemia-induced activation of the β-adrenergic system allowed to differentiate a receptor- versus an enzyme-specific sensitization of the adenylyl cyclase system in acute myocardial ischemia. Such dual sensitization of the β-adrenergic system may contribute to the increased sensitivity of the infarcted heart to catecholamines. © 1990.