title: ADP exerts a protective effect against rundown of the Ca2+ current in bovine chromaffin cells.
abstract: In isolated chromaffin cells, the high-voltage-activated Ca2+ current, recorded using 5 mM Ca2+ as the divalent charge carrier, exhibits rundown within 10 min, which is delayed for 1 h at least by the addition of 1 mM adenosine 5'-triphosphate (ATP) to the pipette medium. The mechanism of this stabilizing action of ATP has been examined. ATP action is dose dependent; the rundown process, which was delayed at concentrations below 0.4 mM, was totally abolished at higher concentrations. The requirement for ATP was shown to be quite strict: 2 mM inosine 5'-triphosphate (ITP) could not replace ATP, whereas guanosine 5'-triphosphate (GTP) could, but at higher concentrations. This effect of ATP was shown to require the presence of MgCl2 and the liberation of a phosphate group since the ATP analogue 5'-adenylyl-imidodiphosphate (AMP-PNP) could not act as a substitute for ATP, suggesting an action through either adenosine 5'-diphosphate (ADP) or a phosphorylation step. ADP, in the presence of Mg2+ only, could replace ATP in the same concentration range. This effect was shown to be specific to ADP; it was maintained after blocking the pathways which convert ADP into ATP, and could not be mimicked by guanosine 5'-diphosphate (GDP). Similarly, ATP and ADP effects were abolished at an increased internal Ca2+ concentration (pCa 6 instead of pCa 7.7, where pCa = -log10[Ca2+]). Nevertheless, the presence of 1 mM Mg-ADP in the bathing solution did not prevent the rundown of the Ca2+ channels when going to the inside-out patch recording configuration.(ABSTRACT TRUNCATED AT 250 WORDS)
mesh_terms: D000244:Adenosine Diphosphate / Q000378:metabolism / Q000494:pharmacology*; D000255:Adenosine Triphosphate / Q000378:metabolism / Q000494:pharmacology; D000818:Animals; D001665:Binding Sites / Q000187:drug effects; D001921:Brain / Q000201:enzymology; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism*; D002417:Cattle; D002838:Chromaffin System / Q000187:drug effects / Q000378:metabolism*; D004594:Electrophysiology; D006868:Hydrolysis; D009711:Nucleotides / Q000494:pharmacology; D011494:Protein Kinases / Q000378:metabolism
chemical_list: D015220:Calcium Channels; D009711:Nucleotides; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D011494:Protein Kinases; D002118:Calcium