ADP exerts a protective effect against rundown of the Ca2+ current in bovine chromaffin cells

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. In conclusion, the stabilizing effect of ATP may be interpreted by a Mg²⁺-ADP binding site present on high-voltage-activated Ca²⁺ channels. A localization of such an ADP regulatory site on the L-type Ca²⁺ channel itself cannot be excluded, though with an additional requirement since Mg-ADP alone is not able to maintain the corresponding activity on excised patches.