Absolute rates of adenosine formation during ischaemia in rat and pigeon hearts

Abstract

1. The activities of ecto- and cytosolic 5'-nucleotidase (EC 3.1.3.5), adenosine kinase (EC 2.7.1.20), adenosine deaminase (EC 3.5.4.4) and AMP deaminase (EC 3.5.4.6) were compared in ventricular myocardium from man, rats, rabbits, guinea pigs, pigeons and turtles. The most striking variation was in the activity of the ecto-5'-nucleotidase, which was 20 times less active in rabbit heart and 300 times less active in pigeon heart than in rat heart. The cytochemical distribution of ecto-5'-nucleotidase was also highly variable between species. 2. Adenosine formation was quantified in pigeon and rat ventricular myocardium in the presence of inhibitors of adenosine kinase and adenosine deaminase. 3. Both adenosine formation rates and the proportion of ATP catabolized to adenosine were greatest during the first 2 min of total ischaemia at 37 degrees C. Adenosine formation rates were 410 +/- 40 nmol/min per g wet wt. in pigeon hearts and 470 +/- 60 nmol/min per g wet wt. in rat hearts. Formation of adenosine accounted for 46% of ATP plus ADP broken down in pigeon hearts and 88% in rat hearts. 4. The data show that, in both pigeon and rat hearts, adenosine is the major catabolite of ATP in the early stages of normothermic myocardial ischaemia. The activity of ecto-5'-nucleotidase in pigeon ventricle (16 +/- 4 nmol/min per g wet wt.) was insufficient to account for adenosine formation, indicating the existence of an alternative catabolic pathway.

Kinetics of adenylate metabolism in human and rat myocardium

“…In human heart the Vmax was about 30% of that in rat heart (Table 1); however, the A T m-va]ues were almost identical (Table 1). Our results on ADA activity in human and rat heart agree with those published by Meghji et al [5], although they made only one-point estimations. The kinetic data on rat heart are in line with Arch and Newsholme [26].…”

“…The /¡Tm-values for AMP-D were similar in human and rat heart, i.e., in the lower millimolar range (Table 1). We found lower activities of AMP-D in both Meghji et al [5]. This is presumably due to their overesti mation caused by EH N A's incomplete inhibition of ADA at 25 mM AMP.…”

“…For AMP-D determination Moss' method [17] modified by Meghji et al [5] was used. The reaction mixture contained 20 mM sodium dimethylglutarate (pH 7.0), 1 mM EDTA, 0.1 mM dithiothreitol, 0.15 M KC1, 1 /xM EHNA, 1 mM ATP, and AMP in the range 0.5-15 mM.…”

“…In contrast, the activity and regulation of enzymes involved in purine-nucleotide metabolism, like AK, ADA, the cellular process has been studied more thoroughly and is likely to be more important. ADA activity varies consid erably in hearts of different species [5]. Metabolic in hibitors for purine degradation, such as allopurinol and EHNA, are in use in cardioplegie solutions [6][7][8], as are adenosine catabolites like inosine to treat heart failure [9,10].…”

Understanding myocardial ischemic preconditioning, and the implications for a role of adenosine catabolism

“…Although adenosine can be rephosphorylated to AMP by adenosine kinase at the expense of ATP, it does not for obvious reasons seem to be the preferred pathway in the ischemic myocardium (155,156). Newlyformed adenosine crosses the myocyte membrane by simple diffusion and accumulates in the interstitial fluid.…”