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.
Palma
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Laboratorio de Neurofisiología, Instituto de Investigaciones Médicas Alfredo Lanari, Universidad de Buenos Aires, CONICET, Combatientes de Malvinas 3150, 1427—Buenos Aires, Argentina.
Muchnik
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S
Losavio
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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