Evidence that the presynaptic A2a-adenosine receptor of the rat motor nerve endings is positively coupled to adenylate cyclase.

The action of the A2a-adenosine analogue, CGS 21680C, on electrically evoked [3H]acetylcholine ([3H]-ACh) release, and its interaction with forskolin (an activator of adenylate cyclase), MDL 12,330A (an irreversible inhibitor of adenylate cyclase), rolipram (an inhibitor of cyclic AMP specific phosphodiesterase), dibutyryl- (db-cAMP) and 8-bromo- (8-Br-cAMP) cyclic AMP analogues (substances that mimic intracellular actions of cyclic AMP), were investigated using rat phrenic nerve-hemidiaphragm preparations. CGS 21680C facilitated [3H]-ACh release. Forskolin (but not 1,9-dideoxy forskolin), rolipram, db-cAMP and 8-Br-cAMP also increased evoked neurotransmitter release in a concentration-dependent manner. When the evoked [3H]-ACh release that is dependent on stimulation of the adenylate cyclase/cyclic AMP transduction system was supramaximally stimulated by these compounds, CGS 21680C (3 nmol/l) could not further increase [3H]-ACh release. Phosphodiesterase inhibition with low concentrations (< or = 30 mumol/l) of rolipram significantly potentiated the augmenting effect of CGS 21680C (1 nmol/l) on evoked [3H]-ACh release. MDL 12,330A (an irreversible inhibitor of adenylate cyclase) decreased evoked [3H]-ACh release. The irreversible blocking action of MDL 12,330A on [3H]-ACh release was overcome by by-passing cyclase activation with db-cAMP and 8-Br-cAMP, but could not be overcome with FSK or CGS 21680C. The inhibitory effect of MDL 12,330A on evoked [3H]-ACh release was not mimicked by nifedipine. It is concluded that the increase in [3H]-ACh release caused by CGS 21680C results from activation of an A2a-adenosine receptor positively linked to the adenylate cyclase/cyclic AMP system.