Excitatory effect of the A2A adenosine receptor agonist CGS-21680 on spontaneous and K+-evoked acetylcholine release at the mouse neuromuscular junction.
The mechanism of action of the A2A adenosine receptor agonist 2-p-(2-carboxyethyl) phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride (CGS-21680) in the facilitation of spontaneous (isotonic and hypertonic condition) and K+-evoked acetylcholine (ACh) release was investigated in the mouse diaphragm muscles. At isotonic condition, the CGS-21680-induced excitatory effect on miniature end-plate potential (MEPP) frequency was not modified in the presence of CdCl2 and in a medium free of Ca2+ (0Ca2+-EGTA), but it was abolished after buffering the rise of intracellular Ca2+ with 1,2-bis-(2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid tetra(acetoxy-methyl) (BAPTA-AM) and when the Ca2+-ATPase inhibitor thapsigargin was used to deplete intracellular Ca2+ stores. CGS-21680 did not have a direct effect on the Ca2+-independent neurotransmitter-releasing machinery, since the modulatory effect on the hypertonic response was also occluded by BAPTA-AM and thapsigargin. CGS-21680 facilitation on K+-evoked ACh release was not altered by the P/Q-type voltage-dependent calcium channel (VDCC) blocker ω-Agatoxin IVA, but it was completely prevented by both, the L-type VDCC blocker nitrendipine (which is known to immobilize their gating charges), or thapsigargin, suggesting that the effects of CGS-21680 on L-type VDCC and thapsigargin-sensitive internal stores are associated. We found that the VDCC pore blocker Cd2+ (2 mM Ca2+ or 0Ca2+-EGTA) failed to affect the CGS-21680 effect in high K+ whereas nitrendipine in 0Ca2+-EGTA+Cd2+ occluded its action. The blockade of Ca2+ release from endoplasmic reticulum with ryanodine antagonized the facilitating effect of CGS-21680 in control and high K+ concentration. It is concluded that, at the mouse neuromuscular junction, activation of A2A receptors facilitates spontaneous and K+-evoked ACh release by an external Ca2+-independent mechanism but that involves mobilization of Ca2+ from internal stores: during spontaneous ACh release stimulating directly the ryanodine-sensitive stores and, at high K+, probably modulating the L-type VDCCs which may cause the opening of the ryanodine receptors that would be directly coupled to the channels. In both cases, Ca2+ released from the endoplasmic reticulum would be capable of activating the exocytotic machinery, thus producing facilitation of ACh release.
Laboratorio de Neurofisiología, Instituto de Investigaciones Médicas Alfredo Lanari, Universidad de Buenos Aires, CONICET, Combatientes de Malvinas 3150, 1427—Buenos Aires, Argentina.
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D000109:Acetylcholine / Q000378:metabolism*; D000241:Adenosine / Q000031:analogs & derivatives* / Q000494:pharmacology; D058908:Adenosine A2 Receptor Agonists / Q000494:pharmacology*; D000818:Animals; D005260:Female; D008297:Male; D051379:Mice; D009469:Neuromuscular Junction / Q000187:drug effects* / Q000378:metabolism*; D009924:Organ Culture Techniques; D010627:Phenethylamines / Q000494:pharmacology*; D011188:Potassium / Q000502:physiology*; D017729:Presynaptic Terminals / Q000187:drug effects* / Q000378:metabolism*; D043705:Receptor, Adenosine A2A / Q000187:drug effects / Q000502:physiology; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology
D058908:Adenosine A2 Receptor Agonists; D010627:Phenethylamines; D043705:Receptor, Adenosine A2A; C061282:2-(4-(2-carboxyethyl)phenethylamino)-5'-N-ethylcarboxamidoadenosine; D000241:Adenosine; D000109:Acetylcholine; D011188:Potassium