[Adenosine: physiological and pharmacological actions].

Adenosine exerts numerous effects in the central and autonomic nervous systems, most of which seem to be receptor mediated. Several studies have revealed two distinct receptors, based upon effects of adenosine on adenylate cyclase activity, designed A1 or A2 according to whether the cyclase is inhibited or activated. However, since not all adenosine receptors are linked to adenylate cyclase some authors base their classification on the rank orders of potencies of adenosine analogues in eliciting responses. The purine seems to function as a modulatory substance in the heart, blood, ileum, vas deferens, and adipose tissue. In addition, important responses to exogenously added adenosine are also induced in the bronchi, urinary bladder, taenia coli, parietal cells of the stomach and renin secretion. Adenosine and its analogues elicit anticonvulsant responses, sedation and hypothermia through their actions in the central nervous system. The mechanisms by which adenosine elicits its responses have not been clearly established. The activation of A1 receptors depresses the release of neurotransmitters and inhibit the influx of Ca into nerve terminals. Whether this effect is induced by interaction with Ca channels or by impairment of Ca dependent processes associated with neurotransmitter release is unknown. In the rat heart adenosine inhibits adenylate cyclase and subsequently the phosphorylation of L-type Ca channels, resulting in a decrease of calcium influx in the muscle cell. The responses to activation of A2 receptors in smooth muscle consist in relaxation presumptively by an increase of K current which would hyperpolarize the cell.