Release and effects of catecholamines in myocardial ischemia

Metabolic Considerations in Direct Procurement and Perfusion Protocols with DCD Heart Transplantation

Heart transplantation with donation after circulatory death (DCD) provides excellent patient outcomes and increases donor heart availability. However, unlike conventional grafts obtained through donation after brain death, DCD cardiac grafts are not only exposed to warm, unprotected ischemia, but also to a potentially damaging pre-ischemic phase after withdrawal of life-sustaining therapy (WLST). In this review, we aim to bring together knowledge about changes in cardiac energy metabolism and its regulation that occur in DCD donors during WLST, circulatory arrest, and following the onset of warm ischemia. Acute metabolic, hemodynamic, and biochemical changes in the DCD donor expose hearts to high circulating catecholamines, hypoxia, and warm ischemia, all of which can negatively impact the heart. Further metabolic changes and cellular damage occur with reperfusion. The altered energy substrate availability prior to organ procurement likely plays an important role in graft quality and post-ischemic cardiac recovery. These aspects should, therefore, be considered in clinical protocols, as well as in pre-clinical DCD models. Notably, interventions prior to graft procurement are limited for ethical reasons in DCD donors; thus, it is important to understand these mechanisms to optimize conditions during initial reperfusion in concert with graft evaluation and re-evaluation for the purpose of tailoring and adjusting therapies and ensuring optimal graft quality for transplantation.

Preconditioning of isolated rat heart is mediated by protein kinase C

Catecholamines have been implicated in the phenomenon of ischemic preconditioning. We have previously demonstrated that ischemic preconditioning against postischemic mechanical dysfunction in the isolated rat heart is mediated by the alpha 1-adrenergic receptor. The purpose of this study was to delineate the signal transduction of preconditioning distal to the alpha 1-adrenergic receptor. Our results suggest that (1) transient ischemia and alpha 1-adrenergic receptor-induced preconditioning is inhibited by protein kinase C (PKC) antagonists, (2) functional protection against global ischemia/reperfusion injury can be induced by infusion of diacylglycerol, the second messenger of the alpha 1-adrenergic pathway, and (3) transient ischemia and alpha 1-adrenergic preconditioning are both characterized by similar translocation of PKC-delta to the sarcolemma of myocardium. These findings suggest that PKC is an effector of preconditioning in the isolated rat heart.

Preconditioning against myocardial dysfunction after ischemia and reperfusion by an alpha 1-adrenergic mechanism

Preconditioning may find ready applicability in humans facing scheduled global cardiac ischemia-reperfusion (IR) during bypass or transplantation, where such a maneuver is feasible before arrest. Our objective was to delineate and exploit the endogenous preconditioning mechanism triggered by transient ischemia (TI) and thereby attenuate myocardial postischemic mechanical dysfunction by clinically acceptable means. Preconditioning by 2 minutes of TI followed by 10 minutes of normal perfusion protected isolated rat left ventricle function assessed after 20 minutes of global, 37 degrees C ischemia and 40 minutes of reperfusion. Final recovery of developed pressure (DP) was improved (91.5 +/- 1.9% of equilibration DP versus unconditioned IR control, 57.4 +/- 2.4%, P < .01) and was accompanied by increased contractility (+/- dP/dt). Norepinephrine release increased after TI, and reserpine pretreatment abolished TI preconditioning. This suggests that endogenous norepinephrine mediates functional preconditioning in rat. Brief pretreatment (2 minutes) with exogenous norepinephrine reproduced the protection (89.1 +/- 1.4%) of postischemic function. Functional protection persisted after the hemodynamic effects had resolved. Norepinephrine-induced preconditioning was simulated by phenylephrine and blocked by alpha 1-adrenergic receptor antagonist. TI preconditioning was similarly lost after selective alpha 1-adrenergic receptor blockade. We conclude that transient ischemic preconditioning is mediated by the sympathetic neurotransmitter release and alpha 1-adrenergic receptor stimulation. Although the postreceptor mechanism remains unclear, functional protection after IR does not seem related to the magnitude of ATP depletion and elevation of resting pressure during ischemia. Rather, the endogenous mechanisms facilitate both recovery of mechanical function and ATP repletion during reperfusion.

Alpha 1-receptor-independent activation of protein kinase C in acute myocardial ischemia. Mechanisms for sensitization of the adenylyl cyclase system

The activity of the adrenergic system plays an important role in the genesis of malignant arrhythmias and the spreading of the infarcted zone in acute myocardial ischemia. Acute myocardial ischemia induces an increased activity of adenylyl cyclase. This sensitization at the enzyme level as shown in the isolated perfused rat heart occurs rapidly after the onset of ischemia (5-15 minutes) and is rapidly reversible on reperfusion. With prolonged ischemia, it is only transient and is followed by a gradual loss of the adenylyl cyclase activity. The increased activity of adenylyl cyclase is even retained after partial purification, suggesting a covalent modification of the enzyme. Blockade of alpha 1-adrenergic receptors does not prevent this sensitization, demonstrating that it occurs independently of alpha 1-adrenergic receptor activation. Only blockade of protein kinase C by various inhibitors, such as polymyxin B or staurosporine, is able to completely prevent this sensitization process. Moreover, in acute myocardial ischemia an activation of protein kinase C could be identified using its translocation from the cytosol to the particulate fraction as an indicator. Blockade of alpha 1-adrenergic receptors using prazosin fails to prevent the activation of protein kinase C and consequently the sensitization of the adenylyl cyclase system, indicating that the ischemia-induced translocation of protein kinase C occurs independently of alpha 1-adrenergic receptors. These data characterize for the first time an important interaction of two effector enzymes of two distinct signal transduction pathways, i.e., the adenylyl cyclase system and the protein kinase C system in acute myocardial ischemia.(ABSTRACT TRUNCATED AT 250 WORDS)

Supersensitivity of the adenylyl cyclase system in acute myocardial ischemia: evaluation of three independent mechanisms

Malignant arrhythmias and the spreading of the infarcted zone in acute myocardial ischemia may be influenced by the sympathetic system. It has been known for quite some time that acute ischemia leads to an increased release of endogenous catecholamines. Adaptive mechanisms at the postsynaptic level such as receptor desensitization, which are operative under normoxic conditions, are abolished in acute myocardial ischemia. On the contrary, three newly characterized, distinct mechanisms lead to a transiently increased activity of the beta-adrenergic system in the early phase of acute ischemia: 1) Functionally coupled beta-adrenergic receptors are rapidly and persistently increased at the cell surface due to the impairment of beta-agonist-promoted uncoupling and internalization. 2) Despite the reversible increase of inhibitory, muscarinic M2 receptors, the inhibitory pathway of the adenylyl cyclase systems becomes ineffective since the coupling protein, Gi, is rapidly impaired. Both the Gi-linked GTPase-activity and the binding of [gamma-35S]GTP are reduced by 25-30% without any loss of the total protein. Stimulatory effects prevail at the G-protein level since in the early period of ischemia the stimulatory G-protein, Gs, remains intact. 3) The adenylyl cyclase is transiently sensitized by about 30%. This increased activity is closely associated with the partially purified enzyme and may be due to a rapidly reversible covalent modification. Prolonged ischemia, in contrast, results in a general decrease of the cyclase activity notwithstanding any changes at the receptor or G-protein level. The individual mechanisms may play distinct and/or complimentary roles in the early sensitization of the adenylyl cyclase system in acute myocardial ischemia.