The effects of beta-adrenergic receptor blockers on heart mitochondrial metabolism

Impact of Aldosterone on the Failing Myocardium: Insights from Mitochondria and Adrenergic Receptors Signaling and Function

The mineralocorticoid aldosterone regulates electrolyte and blood volume homeostasis, but it also adversely modulates the structure and function of the chronically failing heart, through its elevated production in chronic human post-myocardial infarction (MI) heart failure (HF). By activating the mineralocorticoid receptor (MR), a ligand-regulated transcription factor, aldosterone promotes inflammation and fibrosis of the heart, while increasing oxidative stress, ultimately induding mitochondrial dysfunction in the failing myocardium. To reduce morbidity and mortality in advanced stage HF, MR antagonist drugs, such as spironolactone and eplerenone, are used. In addition to the MR, aldosterone can bind and stimulate other receptors, such as the plasma membrane-residing G protein-coupled estrogen receptor (GPER), further complicating it signaling properties in the myocardium. Given the salient role that adrenergic receptor (ARs)—particularly βARs—play in cardiac physiology and pathology, unsurprisingly, that part of the impact of aldosterone on the failing heart is mediated by its effects on the signaling and function of these receptors. Aldosterone can significantly precipitate the well-documented derangement of cardiac AR signaling and impairment of AR function, critically underlying chronic human HF. One of the main consequences of HF in mammalian models at the cellular level is the presence of mitochondrial dysfunction. As such, preventing mitochondrial dysfunction could be a valid pharmacological target in this condition. This review summarizes the current experimental evidence for this aldosterone/AR crosstalk in both the healthy and failing heart, and the impact of mitochondrial dysfunction in HF. Recent findings from signaling studies focusing on MR and AR crosstalk via non-conventional signaling of molecules that normally terminate the signaling of ARs in the heart, i.e., the G protein-coupled receptor-kinases (GRKs), are also highlighted.

Targeting Adrenergic Receptors in Metabolic Therapies for Heart Failure

The heart has a reduced capacity to generate sufficient energy when failing, resulting in an energy-starved condition with diminished functions. Studies have identified numerous changes in metabolic pathways in the failing heart that result in reduced oxidation of both glucose and fatty acid substrates, defects in mitochondrial functions and oxidative phosphorylation, and inefficient substrate utilization for the ATP that is produced. Recent early-phase clinical studies indicate that inhibitors of fatty acid oxidation and antioxidants that target the mitochondria may improve heart function during failure by increasing compensatory glucose oxidation. Adrenergic receptors (α1 and β) are a key sympathetic nervous system regulator that controls cardiac function. β-AR blockers are an established treatment for heart failure and α1A-AR agonists have potential therapeutic benefit. Besides regulating inotropy and chronotropy, α1- and β-adrenergic receptors also regulate metabolic functions in the heart that underlie many cardiac benefits. This review will highlight recent studies that describe how adrenergic receptor-mediated metabolic pathways may be able to restore cardiac energetics to non-failing levels that may offer promising therapeutic strategies.

Altered energy coupling in rat heart mitochondria following in vivo treatment with propranolol

Effects of acute and chronic treatment with propranolol on oxidative phosphorylation in rat heart mitochondria were examined. Acute propranolol treatment resulted in inhibition of coupled respiration with pyruvate + malate and succinate as substrates. Chronic treatment resulted in decreased state 3 respiration rates with all the substrates employed. The net effect of propranolol treatment was decreased ATP-phosphorylation rates suggesting that this was possibly one of the modes of its cardiodepressant activity. Additionally, chronic propranolol treatment brought about a decrease in the content of cytochrome c + c1 in heart mitochondria. Estimation of propranolol concentrations in serum, whole tissue homogenate and heart mitochondria indicated that although the mitochondria accumulated the highest amount of the drug, the intramitochondrial concentration of the drug was one or two orders of magnitude lower than that which is required to bring about inhibition of respiration under in vitro conditions. Besides, the concentrations reached under acute and chronic treatment conditions were almost comparable. The results, therefore, suggest that the action of the drug in vivo may involve more intricate mechanisms than those observed under in vitro conditions.

Normothermic ischemic cardiac arrest in the isolated working rabbit heart: effects of dl-nebivolol and atenolol

The effect of pretreatment with selective beta 1-adrenoceptor blockers (dl-nebivolol or atenolol) on myocardial mechanical activity, mitochondrial function, morphology, and calcium cytochemistry was studied during normothermic ischemic arrest and reperfusion of isolated working rabbit hearts. The hearts subjected to 25 min of ischemia followed by 30 min of post-ischemic reperfusion showed typical signs of severe myocardial ischemic damage. The ultrastructural changes showed a good relation with the changes in mechanical activity and mitochondrial function. To determine whether these changes could be prevented or reduced by beta 1-adrenoceptor blockade, dl-nebivolol or atenolol (0.62 mg/liter) was added to the perfusate 30 min before the induction of ischemia. The results showed that dl-nebivolol exerted a protective effect on recovery of mechanical activity, on mitochondrial function during reperfusion as well as on the ultrastructure as examined at the end of the reperfusion period. On the other hand, atenolol failed to protect the myocardium against ischemia-reperfusion damage in the isolated working rabbit heart.

Comparative study of the effects of acebutolol, atenolol, d-propranolol and dl,-propranolol on the alterations in energy metabolism caused by ischemia and reperfusion: a 31P NMR study on the isolated rat heart

31-P NMR spectroscopy data recorded for the isolated heart were analyzed, in conjunction with functional and biochemical variables, in order to investigate the effect observed for several different beta-adrenoceptor antagonists or the alterations provoked by global partial ischemia (37 degrees C, 24 minutes, 1% residual coronary flow) and reperfusion in the metabolism of the myocardium. During ischemia: intracellular acidosis, adenosine triphosphate (ATP) degradation, and inorganic phosphate (Pi) accumulation were found to be reduced whether the perfusion fluid contained: acebutolol 2.7 x 10(-5) M, atenolol 10(-5) M, d-propranolol 10(-5) M, or dl-propranolol 10(-5) M. On reperfusion metabolic and functional variables were variously affected by the different drugs, except the Pi level which was, in all series, significantly lower compared with control hearts. The adenylate charge and the glycogen stores were protected in the acebutolol, dl-propranolol, and d-propranolol groups. The ATP level was higher than in controls only in the acebutolol and atenolol groups. The intracellular pH recovered to values nonsignificantly different from preischemic values in the acebutolol and dl-propranolol-treated hearts only. The mechanical performance, expressed as the rate-pressure product, was unaltered by the ischemia-reperfusion sequence in the acebutolol and d-propranolol series, while decreasing significantly in controls and in the atenolol group. In dl-propranolol-treated hearts the mechanical activity, which in normoxic conditions was already halved during the effect of the drug, remained at this same level after ischemia. From these observations, it appears that the nonspecific properties of the drugs, as distinct from beta-blockade, play an important part in attenuating the ischemia-induced alteration in myocardial metabolism. Thus, it can be postulated that (1) the metabolic effects of dl-propranolol probably result largely from the reduction of heart work induced by this drug; (2) the maintenance of energy metabolism associated with the preservation of the myocardial activity, as observed in the case of acebutolol and d-propranolol, is possibly a consequence of the existence of a membrane-stabilizing activity.