The release of adenosine and inosine from canine subcutaneous adipose tissue by nerve stimulation and noradrenaline.

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Abstract
Plasma and adipose tissue purine nucleosides were assayed by reversed phase high-performance liquid chromatography after purification of the samples. The adenosine content of unstimulated subcutaneous adipose tissue was close to 1 nmol/g. The concentrations of adenosine and inosine in canine arterial plasma were 0.26 .+-. 0.03 and 0.16 .+-. 0.03 .mu.M, respectively. In venous plasma from the canine subcutaneous adipose tissue the corresponding values were 0.32 .+-. 0.04 and 0.28 .+-. 0.06 .mu.M under basal conditions. The arterio-venous concentration difference of adenosine was linearly dependent upon the arterial adenosine concentration. At arterial concentrations below 0.3 .mu.M there was a net production of adenosine; above 0.3 .mu.M there was a net extraction of approximately 77% of the adenosine. Adenosine was extensively eliminated in blood. The major part of this elimination could be accounted for by metabolism to inosine, hypoxanthine and uric acid. Following sympathetic nerve stimulation (4 Hz for 20 min) the rate of adenosine outflow from adipose tissue increased from 0.33 .+-. 0.22 to a peak value of 1.2 .+-. 0.26 nmol/min. This corresponds to a net release of 8.7 .+-. 3.0 nmol/100 g tissue. Inosine outflow rose from 0.64 .+-. 0.37 to 5.3 .+-. 1.4 nmol/min, corresponding to a net release of 24.6 .+-. 8.7 nmol/100 g. Nerve stimulation also increased the release of [3H]purines from [3H]adenine pre-labeled adipose tissue. The fractional release increased 15-fold after stimulation. The radioactivity was mainly in the form of hypoxanthine, inosine and uric acid while adenosine was a minor component. When metabolism in blood was inhibited by dipyridamole and an adenosine deaminase inhibitor [erythro-9-(2-hydroxy-3-nanyl) adenine hydrochloride], nerve-stimulation-induced release of [3H]purines was mainly in the form of adenosine. Noradrenaline [norepinephrine] injection also induced a release of radioactive purines and of inosine. The outflow of endogenous adenosine was very small. Under basal conditions adenosine is present in arterial and venous canine plasma. The free extracellular tissue level may be similar to the basal arterial adenosine concentration. Sympathetic nerve stimulation and noradrenaline induces a marked release of adenosine, which is rapidly metabolized in the tissue and blood stream to inosine, hypoxanthine and uric acid. In adipose tissue the levels of adenosine reached after adrenergic stimulation appear high enough to be of physiological relevance.