Studien über die Regulation der de novo-Synthese von Adenin-Nucleotiden im Herzen. In: Thauer R, Pleschka K, editors. Das Arterielle System. Heidelberg: Steinkopff; 1974. pp. 348-52. [DOI: 10.1007/978-3-642-85290-9_59]

Beta-adrenergic agonists stimulate the oxidative pentose phosphate pathway in the rat heart

The oxidative pentose phosphate pathway is poorly developed in the rat heart compared with other organs, since the activity of glucose-6-phosphate dehydrogenase (G-6-PDH), the first and rate-limiting enzyme of the oxidative pentose phosphate pathway, is low. As a consequence, the available pool of 5-phosphoribosyl-1-pyrophosphate and the rate of adenine nucleotide biosynthesis are limited. Isoproterenol, 24 hours after subcutaneous administration at 0.1, 1, and 25 mg/kg, stimulated the activity of G-6-PDH in whole hearts dose-dependently from 4.3 +/- 0.16 (control) to 6.6 +/- 0.35, 10.3 +/- 0.82, and 11.5 +/- 0.56 units/g protein, respectively. The activity of 6-phosphogluconate dehydrogenase, another of the enzymes in the oxidative pentose phosphate pathway, remained unchanged. G-6-PDH activity started to increase 12 hours after isoproterenol application, when the glycogenolytic and functional response was over, and reached a peak value between 24 and 48 hours. This stimulating effect was also demonstrated in cardiac myocytes that were isolated 28 hours after isoproterenol application. beta-receptor blockade with atenolol reduced the isoproterenol-induced increase in cardiac G-6-PDH activity by 90%. Cycloheximide, which inhibits translation, and actinomycin D, which interferes with transcription, attenuated it by 83% and 78%, respectively. These results indicate that cardiac beta-adrenergic receptors and enzyme protein synthesis are involved in this effect. Other beta-sympathomimetic agents such as dopamine, dobutamine, fenoterol, salbutamol, and terbutaline also stimulated myocardial G-6-PDH activity in a time- and dose-related manner. The calcium antagonist D 600 (gallopamil) reduced the isoproterenol-elicited stimulation by 65%, and verapamil blunted the fenoterol-induced increase by 50%. This suggests that Ca2+ ions also contribute to the stimulation of the cardiac oxidative pentose phosphate pathway.

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.

Studies on the regulation of the biosynthesis of myocardial adenine nucleotides

1) Changes in the rates of biosynthesis of adenine nucleotides in rat hearts under various experimental conditions are paralleled by corresponding alterations in the concentration of cyclic AMP. 2) Pentoses and pentitols cause an acceleration of the de novo synthesis of adenine nucleotides in the normal heart and a further amplification of the increase of adenine nucleotide synthesis in isoproterenol-stimulated hearts. 3) The enhancement of de novo synthesis of adenine and nucleotides induced by isoproterenol as well as by pentoses and pentitols appears to be causally related to a greater availability of 5-phos-phoribosyl-1-pyrophosphate.

[Kinetics of alpha phosphate turnover in the free nucleotides of isolated rabbit hearts during the incorporation of adenosine or uridine]

The isolated rabbit heart was perfused with tritium-labelled adenosine or uridine at constant concentration (respectively 10 and 2 micrometer) and specific activity. The transfer rates of the precursors into free nucleotides were obtained by calculating the uptake of isotope into the products. The rates of phosphorylation of the nucleotides were estimated by following the incorporation of 32PO4 3- into the alpha phosphate groups. The results obtained with the two isotopes show that the rate of labelling of the alpha phosphate groups is a good estimate of the turnover of nucleotides when the mechanism of synthesis involves phosphorylation of a riboside.