Compartmentalization of the adenosine pool of dog and rat hearts.

Abstract

Studies in rat and dog hearts examined the hypothesis that the cardiac adenosine pool contains an intracellular compartment. Enzymatically dispersed rat cardiocytes contain 70 pmol adenosine/mg protein which is resistant to 10 U/ml adenosine deaminase (ADA). Incubating dog heart homogenates for 1 min at 37.degree. C with 10 U ADA/ml did not change adenosine levels perceptibly from the average control value of 1.28 nmol/g. Studies employing [3H]hypoxanthine arabinoside as an adenosine surrogate showed that this nucleoside penetrates into pericardial superfusates, attaining concentrations equal to those in blood plasma by 30 min. Since blood, cardiac interstitium and pericardial superfusate are 3 compartments in series, this validates the use of pericardial superfusates equilibrated for .gtoreq. 30 min as probes of cardiac interstitial composition. In 8 dogs, pericardial superfusate adenosine concentration averaged 0.24 .mu.M. Cardiac muscle adenosine content averaged 0.87 nmol/g, indicating that the interstitial compartment accounts for only 6% of the cardiac pool. Dog cardiac muscle contains a [3H]adenosine binding protein whose size, affinity for adenosine analogs, and ability to synthesize S-adenosylhomocysteine (AdoHcy) suggest that it is S-adenosylhomocysteine hydrolase (SAH). Studies employing a dog erythrocyte model show that adenosine is bound to a protein in this cell; treatment with L-homocysteine greatly reduces the amount of adenosine recovered. The half-time for the dissociation of [3H]adenosine from SAH at 37.degree. C is 2.5 h, and in the presence of ADA, > 6 h. Although adenosine bound to SAH can serve as a substrate for AdoHcy synthesis, this experiment does not support the idea that the dissociation of adenosine occurs to a physiologically significant extent. The functional role of the intracellular adenosine compartment is uncertain.