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Cytoplasmic Ca2+ oscillation in rat megakaryocytes evoked by a novel type of purinoceptor.
Author(s) -
Uneyama C,
Uneyama H,
Akaike N
Publication year - 1993
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.1993.sp019885
Subject(s) - biophysics , adenosine triphosphate , egta , chemistry , intracellular , reversal potential , patch clamp , bapta , quinidine , adenosine , electrophysiology , uridine triphosphate , membrane potential , channel blocker , biochemistry , medicine , nucleotide , calcium , biology , receptor , organic chemistry , gene
1. The responses of megakaryocytes isolated from rat bone marrow to externally applied adenosine triphosphate (ATP) were investigated in the whole‐cell mode by the use of nystatin perforated patch‐clamp technique. 2. ATP at 1‐100 microM evoked periodic outward currents at a holding potential of ‐40 mV. The reversal potential of the currents was close to K+ equilibrium potential (EK) and the K+ channel blockers such as quinine and quinidine suppressed the currents, indicating that the outward currents are predominantly carried by K+. 3. Since it has been reported that adenosine diphosphate (ADP) evoked monophasic K+ current using a conventional whole‐cell recording, we compared the results obtained by perforated and conventional patch‐clamp techniques. The crucial difference between our results and previous results was due to the intracellular perfusion with internal solution containing a high concentration of EGTA by which both current shape and concentration response were modified. 4. The membrane permeable Ca2+ chelator, 1,2‐bis(O‐aminophenoxy)ethane‐N,N,N',N'‐tetraacetic acid (acetoxy methyl ester; BAPTA AM), inhibited the K+ current concentration dependently, suggesting that ATP‐induced oscillatory K+ currents are caused by changes in cytoplasmic free Ca2+ concentration ([Ca2+]i). 5. With increasing ATP concentration, the frequency and the maximum amplitude of K+ current oscillation increased and the latency of current, which is the period required to activate the first K+ current after ATP application, decreased. 6. ADP, 2‐methylthio‐ATP and ATP‐gamma‐S could also evoke the periodic K+ currents, but adenosine, uridine triphosphate (UTP) and alpha‐beta‐methylene adenosine 5'‐triphosphate (AMP‐CPP) failed. 2‐Methylthio‐ATP was the most potent agonist; next was ADP which showed a 10‐30 times stronger effect than ATP. Cross‐desensitization was observed between ATP and ADP, but not between ATP or ADP and thrombin. 7. Extracellular Ca2+ was not required for the ATP‐induced K+ current activation, indicating that Ca2+ released from intracellular pools induced the oscillatory response. In addition, the agonist potency increased when extracellular Ca2+ concentration ([Ca2+]o) decreased, suggesting that the principal agonists might be ATP4‐ and ADP3‐. 8. The results suggest the presence of a novel subtype of purinoceptor in the megakaryocyte plasma membrane which induces cytoplasmic Ca2+ oscillation and evokes periodic K+ current flux.