Phenylisopropyl-adenosine

Table III. Comparison of the Affinity Ratios for R- and S-Phenylisopropyl adenosine (PIA) Derived from Both Radioligand Binding Assays and Functional Measures of Aj Adenosine Receptor Activation...

The A1 receptor apparently mediates an inhibitory effect of adenosine, N°-L-phenylisopropyl-adenosine (L-PIA) and 2-Chloroadenosine on adenylate cyclase (19). Low concentrations of these nucleosides also block the cAMP accumulation induced by agonists of adenylate cyclase, such as isoproterenol and PGE2 (21). Adenosine and its derivatives have an high affinity for this site and their effects are also blocked by xanthines such as caffeine and theophylline (22). 

Other calorimetric studies [22] on human blood platelets were performed in order to clarify the mechanism of action of xanthine derivatives in relation to adenosine receptor. In addition, there were investigations on the effect of adenosine and adenosine agonists on platelet metabolism. The results showed that the two xanthine derivatives tested, enprofylline and theophylline, at therapeutic concentrations, did not modify the heat production induced by adenosine. Both adenosine and one adenosine agonist, NECA (5-N-ethyl carboxamide adenosine) increased platelet metabolism. The other adenosine agonist tested, PIA (L-N -phenylisopropyl-adenosine) induced a reduction of heat production. The adenosine uptake inhibitor, dipyridamol, was without effect. Thus, by microcalorimetry, useful information could be obtained concerning the complex mechanisms behind the effects on human cell metabolism of adenosine and adenosine agonists. The same applies to substances used in the treatment of asthma, xanthine derivatives they are thought to act by inhibition of adenosine receptors. 

---