No abstract

Adenosine formation and release were studied in 48-h-old cultured ciliary ganglia and confluent peripheral and CNS glial cultures from embryonic chicks. Metabolic poisoning induced by 30 mM 2-deoxyglucose and 2 micrograms/ml oligomycin reduced ATP concentration by 90%. An increase in adenosine accounted for 15-40% of the fall in ATP. Dilazep (3 X 10(-6) M), a nucleoside transport inhibitor, decreased both incorporation of adenosine (an index of nucleoside transport) and release of adenosine by 80-90%. Dilazep trapped the newly formed adenosine intracellularly. A concentration of alpha, beta-methylene ADP that inhibited ecto-5'-nucleotidase by 80-90% did not alter the concentration of adenosine or AMP in neurons, glia, or medium. The results demonstrate that adenosine is formed intracellularly and exported out of the cell via the nucleoside transporter. The participation of ecto-5'-nucleotidase was excluded.

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.

1ATP, 2-chloro-ATP, 2-methylthio-ATP, and their unnatural L-enantiomers, were synthesized and their effects tested on the guinea-pig taenia coli and urinary bladder, and the stimulated frog ventricle. 2 The potent P2-purinoceptor agonists, 2-chloro-ATP and 2-methylthio-ATP were, respectively, 30 and 200 times more effective than ATP in relaxing the guinea-pig taenia, but approximately as effective as ATP in contracting the guinea-pig bladder and augmenting the force of contraction of the frog ventricle. 3 A high degree of stereoselectivity was observed for relaxations of the guinea-pig taenia coli produced by theP2-purinoceptoragonists, and 2-methylthio-ATP was over 700 times more effective than its L-enantiomer. In contrast, stereoselectivity for contraction of the guinea-pig bladder was observed only at low concentrations with each pair of enantiomers, and a similar low stereoselectivity was displayed by the frog ventricle. 4 These results show that P2-purinoceptors mediating inhibitory responses in the guinea-pig taenia coli can show a high degree of stereoselectivity, while P2-purinoceptors mediating excitatory responses in the guinea-pig bladder and in the frog ventricle show little stereoselectivity. 5 The partial stereoselectivity of the P2-purinoceptor in smooth muscle contrasts with the absolute stereospecificity of P, -purinoceptors for adenosine on smooth muscle and autonomic nerve terminals and the absolute stereospecificity of the receptor for ADP on the human platelet.

1 The effects of adenyl compounds were examined on the guinea-pig and frog heart in terms of the Pl/P2-purinoceptor hypothesis. 2 The effects of two slowly degradable adenosine 5'-triphosphate (ATP) analogues; ,-Ymethylene adenosine 5'-triphosphate (APPCP) and a,p-methylene adenosine 5'-triphosphate (APCPP) were also examined. 3 Adenosine, adenosine 5 '-monophosphate (AMP), adenosine 5 '-diphosphate (ADP), ATP and APPCP produced inhibitory effects in guinea-pig atria. These inhibitory effects were antagonized competitively by theophylline and potentiated by dipyridamole. APCPP did not produce a similar inhibitory response. 4 Guinea-pig ventricles were insensitive to adenyl compounds. 5 ATP and ADP produced initial excitatory effects in frog atria which were followed by inhibitory effects. Adenosine and AMP produced inhibitory effects alone whereas APCPP produced excitatory effects only. The inhibitory effects were antagonized competitively by theophylline and potentiated by dipyridamole. 6 ATP, ADP, APPCP and APCPP evoked excitatory responses in frog ventricles. These responses were not affected by theophylline or dipyridamole. Adenosine and AMP were inactive on frog ventricles.