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Adenosine‐induced release of cyclic adenosine 3',5'‐monophosphate from the left ventricle in the anaesthetized intact dog.

1. The influence of adenosine on the release of cyclic adenosine 3',5'‐monophosphate (cyclic AMP) from the heart was examined in twenty‐two anaesthetized intact dogs. The animals were pre‐treated with propranolol and vagotomized. The rate of nucleotide release from the left ventricle was determined as the product of the left ventricular myocardial plasma flow and the coronary veno‐arterial difference in plasma nucleotide concentration. 2. Adenosine was infused into the left ventricle during three successive 15 min periods at rates of 25, 50 and 100 n‐mole/kg. min, respectively. The mean blood pressure in the ascending aorta was prevented from falling by inflating a balloon placed into the thoracic aorta. The measurements were performed before the infusion and at the 10th min of each infusion period. The values given are means +/‐ S.E. 3. During the 45 min infusion of adenosine at increasing rate, the cyclic AMP concentration in arterial plasma increased from 7.0 +/‐ 0.3 to 14.0 +/‐ 0.9 p‐mole/ml. The nucleotide was released from the left ventricle at rates increasing from 48.2 +/‐ 7.1 to 206.5 +/‐ 56.2 p‐mole/100 g. min while the left ventricular myocardial blood flow increased from 127 +/‐ 6 to 399 +/‐ 33 ml./100 g.min. The oxygen consumption of the left ventricle was not modified. 4. When adenosine was infused at a rate of 100 n‐mol/kg.min, the thorax was opened and the apex of the heart and the left atrial appendage were removed for nucleotide assay. The cardiac cyclic AMP concentration did not differ from that observed in control dogs. 5. The results suggest that cyclic AMP is likely to be involved in the membrane and cellular events underlying the relaxant effect of adenosine on the coronary smooth muscle. The lack of change in cardiac cyclic AMP concentration, as determined by whole tissue extractions, is consistent with a study by others showing that, under normoxic conditions, cyclic AMP is released from a small, compartmentalized fraction of the cyclic AMP content of the heart. The elevation of the plasma nucleotide concentration could result from adenosine effects on various cell systems or organs, in addition to the observed release from the heart.