A novel receptor for diadenosine polyphosphates coupled to calcium increase in rat midbrain synaptosomes.
1. Diadenosine polyphosphates, Ap4A and Ap5A, as well as ATP, alpha,beta-MeATP and ADP-beta-S, were able to elicit variable intrasynaptosomal Ca2+ increases in rat midbrain synaptic terminals. The origin of the Ca2+ increment was the extra synaptosomal space since the elimination of extracellular Ca2+ abolished the effect of all the agonists. 2. The P2-purinoceptor antagonist, suramin, did not affect the Ca(2+)-increase evoked by diadenosine polyphosphates but dramatically blocked the Ca2+ entry induced by ATP and its synthetic analogues. 3. The actions of Ap5A and ATP on the intrasynaptosomal Ca2+ increase did not cross-desensitize. 4. Concentration-response studies for diadenosine polyphosphates showed pD2 values of 54.5 +/- 4.2 microM and 55.6 +/- 3.8 microM for Ap4A and Ap5A, respectively. 5. The entry of calcium induced by diadenosine polyphosphates could be separated into two components. The first represented a selective voltage-independent Ca2+ entry; the second, a sustained phase which was voltage-dependent. 6. Studies on the voltage-dependent Ca(2+)-channels involved in the effects of the diadenosine polyphosphates, demonstrated that omega-conotoxin G-VI-A inhibited the sustained Ca(2+)-entry, suggesting the participation of an N-type Ca(2+)-channel. This toxin was unable to abolish the initial cation entry induced by Ap4A or Ap5A. omega-Agatoxin IV-A, tetrodotoxin, or nifedipine did not inhibit the effects of the diadenosine polyphosphates. 7. The effect of ATP on Ca(2+)-entry was abolished by nifedipine and omega-conotoxin G-VI-A, suggesting the participation of L- and N-type Ca(2+)-channels in the response to ATP. 8. These data suggest that Ap4A, Ap5A and ATP activate the same intracellular Ca2+ signal through different receptors and different mechanisms. Ap4A and Ap5A induce a more selective Ca2+-entry in a voltage-independent process. This is the first time that a selective action of diadenosine polyphosphate through receptors other than P1 and P2-purinoceptors has been described.
Pintor
J
J
Departamento de Bioquimica, Fac. Veterinaria, Universidad Complutense, Madrid, Spain.
Miras-Portugal
M T
MT
D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015226:Dinucleoside Phosphates / Q000494:pharmacology*; D008297:Male; D008636:Mesencephalon / Q000187:drug effects*; D017729:Presynaptic Terminals / Q000187:drug effects; D051381:Rats; D017208:Rats, Wistar; D015290:Second Messenger Systems; D013498:Suramin / Q000494:pharmacology; D013574:Synaptosomes / Q000187:drug effects*; D013779:Tetrodotoxin / Q000494:pharmacology; D013997:Time Factors
D015226:Dinucleoside Phosphates; D013779:Tetrodotoxin; C012275:P(1),P(5)-di(adenosine-5'-)pentaphosphate; C020733:diadenosine tetraphosphate; D013498:Suramin; D000255:Adenosine Triphosphate; D002118:Calcium