Adenosine stimulates adenosine 3',5'-monophosphate and guanosine 3',5'-monophosphate accumulation in rat pinealocytes: evidence for a role for adenosine in pineal neurotransmission.

Adenosine produces a concentration-dependent increase in pinealocyte cAMP (EC50, approximately 0.3 nM) and cGMP accumulation (EC50, approximately 0.7 nM). Maximal increases in both nucleotides are evident 10 min after treatment; 1 h later values return to pretreatment levels. Concentration-dependent effects on cAMP are also observed with N6-(L-2-phenylisopropyl)adenosine (EC50, approximately 0.75 nM), 5'-N-ethylcarboxy aminoadenosine (EC50, approximately 0.75 nM), and 2-chloroadenosine (EC50, approximately 2.0 nM); the EC50 values for stimulation of cGMP with these agents are higher by a factor of 2-10. In the case of 5'-N-ethylcarboxy amidoadenosine, the concentration-response curve is biphasic, with a significant effect evident within the range of 1-100 pM. The stimulatory nature of this response and the relative potency of the agonists tested are consistent with the involvement of an A2-like adenosine receptor. Comparison of adenosine and the selective beta-adrenergic agonist isoproterenol indicated that their maximal EC50 values were generally similar. Studies with antagonists revealed that both 8-(p-sulfophenyl)theophylline (1 microM) and the xanthine amine congener (8-[4-[[[(2-aminoethyl)carbonyl]methyl]oxy]phenyl]1,3- dipropylxanthine (1 microM) inhibited the effects of adenosine (1 nM to 1 microM), but xanthine amine congener was more potent; the latter was markedly effective at 0.1 nM, whereas 8-(p-sulfophenyl)theophylline was nearly ineffective at this concentration. It was also determined that pineal cells generate extracellular adenosine from extracellular ATP. ATP is thought to be released along with catecholamines during neurotransmission. Hence, these studies support the view that adenosine could participate in the transsynaptic regulation of pineal function.