Acceleration of adenine nucleotide synthesis de novo during development of cardiac hypertrophy

Integrated Functions of Cardiac Energetics, Mechanics, and Purine Nucleotide Metabolism

Purine nucleotides play central roles in energy metabolism in the heart. Most fundamentally, the free energy of hydrolysis of the adenine nucleotide adenosine triphosphate (ATP) provides the thermodynamic driving force for numerous cellular processes including the actin-myosin crossbridge cycle. Perturbations to ATP supply and/or demand in the myocardium lead to changes in the homeostatic balance between purine nucleotide synthesis, degradation, and salvage, potentially affecting myocardial energetics and, consequently, myocardial mechanics. Indeed, both acute myocardial ischemia and decompensatory remodeling of the myocardium in heart failure are associated with depletion of myocardial adenine nucleotides and with impaired myocardial mechanical function. Yet there remain gaps in the understanding of mechanistic links between adenine nucleotide degradation and contractile dysfunction in heart disease. The scope of this article is to: (i) review current knowledge of the pathways of purine nucleotide depletion and salvage in acute ischemia and in chronic heart disease; (ii) review hypothesized mechanisms linking myocardial mechanics and energetics with myocardial adenine nucleotide regulation; and (iii) highlight potential targets for treating myocardial metabolic and mechanical dysfunction associated with these pathways. It is hypothesized that an imbalance in the degradation, salvage, and synthesis of adenine nucleotides leads to a net loss of adenine nucleotides in both acute ischemia and under chronic high-demand conditions associated with the development of heart failure. This reduction in adenine nucleotide levels results in reduced myocardial ATP and increased myocardial inorganic phosphate. Both of these changes have the potential to directly impact tension development and mechanical work at the cellular level. © 2024 American Physiological Society. Compr Physiol 14:5345-5369, 2024.

Mitochondrial-cell cycle cross-talk drives endoreplication in heart disease

[Figure: see text].

The adenylosuccinate synthetase-1 gene is activated in the hypertrophied heart

Adenylosuccinate synthetase 1 (ADSS1) functions as an important component in adenine nucleotide biosynthesis and is abundant in the heart. Here we report that the Adss1 gene is up-regulated in two in vivo rodent models of surgically induced cardiac hypertrophy. In addition, we examined an in vitro hypertrophy system of rat neonatal cardiomyocytes treated with angiotensin II to study Adss1 gene regulation. We show that this stimulus triggers a signaling cascade that results in the activation of the Adss1 gene. The induction of Adss1 gene expression was blocked by cyclosporin A in vitro, suggesting that calcineurin, a calmodulin activated phosphatase, is involved in this signaling pathway. Consistent with this view we provide evidence that the induction of Adss1 by angiotension II requires the presence of an NFAT binding site located 556 base pairs upstream of the Adss1 transcription start site. We propose that ADSS1 plays a role in the development of cardiac hypertrophy through its function in adenine nucleotide biosynthesis.

[Synthesis of adenine nucleotides from exogenous adenosine in the perfused rat heart under normoxic conditions and after ischemia]

The rate of synthesis of myocardial adenine nucleotides from exogenous adenosine was studied in the isolated rat heart perfused under normoxic conditions and following ischaemia. The rate of incorporation of adenosine depended on the extracellular concentration of the precursor, following Michaelis-Menten kinetics with a apparent Km of 51.3 microM and a maximal rate of incorporation of about 1 100 nmol g-1 (wet wt.) 30 min-1. The adenosine uptake induced an increase in ATP concentration (+ 20%) when the exogenous concentration of precursor exceeded 10 microM. Following low-flow ischaemia (0.5 ml/min, 30 min), the rate of incorporation of 5 microM adenosine was diminished (-23%), but adenine nucleotide level restoration was favoured by the nucleoside administration. After total ischaemia (24 min), the extent of the decrease in adenosine incorporation was the same as in the case of moderate ischaemia but adenine nucleotide content was not restored.

Metabolic intervention to affect myocardial recovery following ischemia

Myocardial recovery during reperfusion following ischemia is critical to patient survival in a broad spectrum of clinical settings. Myocardial functional recovery following ischemia correlates well with recovery of myocardial adenosine triphosphate (ATP). Adenosine triphosphate recovery is uniformly incomplete during reperfusion following moderate ischemic injury and is therefore subject to manipulation by metabolic intervention. By definition ATP recovery is limited either by (1) energy availability and application in the phosphorylation of adenosine monophosphate (AMP) to ATP or (2) availability of AMP for this conversion. Experimental data suggest that substrate energy and the mechanisms required for its application in the creation of high energy phosphate bonds (AMP conversion to ATP) are more than adequate during reperfusion following moderate ischemic injury. Adenosine monophosphate availability, however, is inadequate following ischemia due to loss of diffusable adenine nucleotide purine metabolites. These purine precursors are necessary to fuel adenine nucleotide salvage pathways. Metabolic interventions that enhance AMP recovery rather than those that improve substrate energy availability during reperfusion are therefore recommended. The mechanisms of various metabolic interventions are discussed in this framework along with the rationale for or against their clinical application.

Significance of the hexose monophosphate shunt in experimentally induced cardiac hypertrophy

1. In three models of cardiac hypertrophy in rats (aortic constriction, application of a single dose of isoproterenol and daily injections of triiodothyronine) the biosynthesis of myocardial adenine nucleotides was enhanced. 2. In hypertrophying hearts due to aortic constriction and isoproterenol application, the activity of glucose-6-phosphate dehydrogenase and the available pool of 5-phosphoribosyl-1-pyrophosphate were increased indicating a stimulation of the hexose monophosphate shunt. In triiodothyronine-treated animals only the cardiac pool of 5-phosphoribosyl-1-pyrophosphate turned out to be elevated. 3. In all three models of cardiac hypertrophy, the enhancement of myocardial adenine nucleotide biosynthesis was exaggerated by ribose. It thus appears that the 5-phosphoribosyl-1-pyrophosphate pool is the limiting factor for the increase of adenine nucleotide biosynthesis under these conditions. 4. Long-term i.v. infusion of ribose (200 mg/kg/h) in isoproterenol-treated rats prevented the decrease of the cardiac ATP concentration induced by isoproterenol. However, the isoproterenol-induced stimulation of total cardiac protein synthesis was not altered, suggesting that the ATP decline may not be the trigger for stimulating protein synthesis in this model of myocardial hypertrophy.

[The regulation of protein synthesis in heart muscle. Biochemical data, stimulative and inhibitory factors and their clinical significance (author's transl)]

The regulation of protein synthesis in heart muscle has been investigated by many authors under both normal and pathological conditions. This review summarizes the evidence for the dependence of normal heart protein synthesis from normal serum levels of insulin, amino acids, fatty acids and glucose. A decreased serum concentration of these substances causes an inhibition of heart muscle protein synthesis by 30--60%. Various drugs and other chemical lead to similar impairments of heat muscle protein synthesis. The resulting imbalance between synthesis and degradation of myocardial proteins with their half-times of 5--12 days gradually leads to a decrease in their myocellular concentration with a consequent impairment of myocardial function. Finally, the biochemial sequences are described which represent the important pathogenetic mechanisms in the development of heart muscle hypertrophy and in the adriamycin-induced cardiomyopathy.

Stimulation of myocardial adenine nucleotide biosynthesis by pentoses and pentitols

In rats, pentoses and pentitols, intravenously injected in a single dose of 100 mg/kg, induced a considerable enhancement of the available pool of 5-phosphoribosyl-1-pyrophosphate and of the rate of adenine nucleotide biosynthesis in the heart, but not in liver and kidney. De novo synthesis of adenine nucleotides not detectable in skeletal muscle of normal rats became measurable after application of ribose. The stimulatory effect of isoproterenol on myocardial adenine nucleotide biosynthesis could be further potentiated by ribose and xylitol, but not by glucose. The isoproterenol-induced decrease of cardiac adenine nucleotide concentrations could be almost completely prevented by repeated administrations of ribose. Thus, pentoses and pentitols in combination with beta-receptor stimulation markedly and quite specifically enhance adenine nucleotide biosynthesis in the rat heart. The results indicate that the increase in the available pool of 5-phosphoribosyl-1-pyrophosphate is an important factor for the enhancement of cardiac adenine nucleotide biosynthesis. Moreover, the availability of 5-phosphoribosyl-1-pyrophosphate and the rate of de novo synthesis of adenine nucleotides in the heart seem to be limited by the flow through the hexose monophosphate shunt.

Changes of myocardial adenine nucleotide and protein synthesis during development of cardiac hypertrophy

1. Studies on three models of cardiac hypertrophy (aortic constriction, application of isoproterenol, daily administrations of 3,3'5-triiodo-L-thyronine, respectively) reveal that the enhancement of de novo synthesis of adenine nucleotides occurs very early during the development of cardiac hypertorphy and always precedes the increase of protein synthesis. Therefore, it seems likely that the accelerated synthesis of adenine nucleotides is an important factor among those metabolic processes involved in the stimulation of protein synthesis in the hypertrophying heart. 2. As far as the mechanism for the observed enhancement of adenine nucleotide synthesis is concerned, it has been demonstrated that the available pool of 5-phosphoribosyl-l-pyrophosphate, an essential precursor substance of de novo synthesis, is increased in the hypertrophying heart due to isoproterenol and 3,3'5-triiodo-L-thyronine, respectively.

Effect of triiodothyronine on the biosynthesis of adenine nucleotides and proteins in the rat heart

1) During the development of myocardial hypertrophy induced by 3,3',5-triiodo-L-thyronine the enhancement of de novo synthesis of adenine nucleotides occurs very early and precedes the increase of protein synthesis. In this respect there is a striking parallelism with other types of cardiac hypertrophy. 2) The acceleration of adenine nucleotide synthesis under these experimental conditions seems to be due to a greater availability of cardiac 5-phosphoribosyl-1-pyrophosphate. 3) The triiodothyronine-induced enhancement of adenine nucleotide synthesis can be attenuated by beta-receptor-blocking agents.

Postbypass treatment

Ischemia induced by cross-clamping the aorta during open-heart operations initiates progressive metabolic derangement. If the duration of ischemia is short, these derangements are easily reversed by restoring the flow of blood containing oxygen and substrate. If ischemia is prolonged, treatment designed to ameliorate ischemic damage may be necessary. Three problems are discussed: (1) loss of adenine nucleotides, particularly adenosine triphosphate, (2) impairment of calcium sequestration, and (3) formation of microemboli in coronary vessels. The rationale for postbypass treatment is presented.

Effect of beta-adrenergic stimulation on myocardial adenine nucleotide metabolism

The effects of isoproterenol, propranolol, and compound D600 (α-isopropyl-α-[( N -methyl- N -homoveratryl)-γ-aminopropyl1-3, 4, 5-trimethoxyphenylacetonitrile) on myocardial adenine nucleotide metabolism were studied in rat hearts in situ. Isoproterenol in doses between 0.1 and 25 mg/kg induced an increase in heart rate concomitant with a significant acceleration in the de novo synthesis of adenine nucleotides (ATP, ADP, and AMP) and a diminution in their concentration. The effects of isoproterenol were antagonized by propranolol (1 and 50 mg/kg), which alone caused a reduction in the de novo synthesis of adenine nucleotides without inducing a change in their concentration. Compound D600 (10 mg/kg) brought about a slight elevation in the concentration of adenine nucleotides but did not influence the rate of de novo synthesis. The isoproterenol-induced diminution in adenine nucleotide concentration was prevented by D600; under these conditions, the acceleration of de novo synthesis was attenuated. These findings indicate that de novo synthesis of myocardial adenine nucleotides in the normal and the isoproterenol-stimulated heart is regulated not only by a feedback mechanism dependent on the concentration of adenine nucleotides but also by β-receptor-mediated alterations in carbohydrate metabolism which can cause changes in the size of the available pool of 5-phosphoribosyl-1-pyrophosphate.

De novo synthesis of myocardial adenine nucleotides in the rat. Acceleration during recovery from oxygen deficiency

De novo synthesis of adenine nucleotides was measured in rat hearts in situ and in isolated perfused rat hearts under normal conditions and during recovery from asphyxia or ischemia. Using l- 14 C-glycine as the precursor substrate, rates of de novo synthesis were determined from the total radioactivity of adenine nucleotides and from the mean specific activity of intracellular glycine. The rate of de novo synthesis of adenine nucleotides was 8.4±1.42 nmoles/g hour -1 in the heart in situ and 1.3±0.12 nmoles/g hour -1 in the isolated perfused heart. De novo synthesis of adenine nucleotides increased almost 100% in the heart in situ and about 580% in the isolated perfused heart during the first hour of recovery from asphyxia or ischemia. This acceleration is regarded as an adaptive process contributing to the postanoxic restoration of normal adenine nucleotide levels. Possible biochemical mechanisms that might be involved in the stimulation of the de novo pathway are a release of feedback inhibition of 5-phosphoribosyl-1-pyrophosphate amidotransferase, an enhanced synthesis of 5-phosphoribosyl-1-pyrophosphate, and an alternate way of 5-phosphoribosyl-amine formation.