Differential modulation of voltage-dependent Ca2+ currents by EGTA and BAPTA in bovine adrenal chromaffin cells

Whole-cell patch-clamp recordings were made to examine the effects of the C a2+ chelators EGTA and BAPTA on the biophysical properties of voltage-opera ted Ca2+ currents in bovine adrenal chromaffin cells. Ca2+ currents in the presence of either EGTA or BAPTA over a concentration range of 0.1-60 mM we re recorded under otherwise identical conditions. Analysis of current-volta ge relationships yielded unexpected differences in several important parame ters such as the voltage dependence of activation, kinetics, slope, and rev ersal potential, which seemed to be unrelated to the Ca2+-binding propertie s of these chelators. Increasing concentrations of BAPTA augmented the peak Ca2+ current amplitude while current:amplitudes in the presence of EGTA re mained constant over the entire concentration range tested. Increasing conc entrations of BAPTA shifted the voltage sensitivity of Ca2+ currents by abo ut 15 mV towards positive voltages. EGTA, over the same concentration ran,a e, did not affect the voltage sensitivity. The shift in voltage sensitivity observed with BAPTA was unrelated to its faster Ca2+-binding kinetics, as it was also observed when substituting Ca2+ with Ba2+ as the charge carrier . The mechanism by which BAPTA affects Ca2+ channel voltage dependence also seems unrelated to kinase-mediated modulation of Ca2+ channels, since the protein-kinase-C- (PKC-) specific drugs bisindolylmaleimide and phorbol est er (PMA) neither mimicked nor prevented the action of BAPTA. The less speci fic kinase inhibitor staurosporine, however, augmented Ca2+ currents simila rly to BAPTA, but without affecting the voltage sensitivity. The BAPTA-medi ated shift in voltage sensitivity was partially suppressed by non-hydrolysa ble analogs of GTP (GDP[beta-S] and GTP[gamma-S]). Lowering [Mg2+](i) mimic ked the BAPTA-induced shift in voltage sensitivity and,prevented further sh ifts in voltage sensitivity by BAPTA. The results demonstrate that BAPTA an d EGTA, despite their similarities in terms of Ca2+ buffering, have dispara te effects on the voltage dependence of Ca2+ channels and careful selection of the chelator is required to quantitatively assess Ca2+ currents.