Multiple mechanisms of manganese-induced quenching of fura-2 fluorescence in rat mast cells.

Whole-cell patch-clamp recordings of membrane currents and fura-2 measurements of free intracellular calcium concentration ([Ca2+]i) were used to study Mn2+ influx in rat peritoneal mast cells. The calcium-selective current, activated by depletion of intracellular calcium stores (ICRAC for calcium release-activated calcium current), supports a small but measurable Mn2+ current. In the presence of intracellular BAPTA, a Mn2+ current through ICRAC was recorded in isotonic MnCl2 (100 mM) without a significant quenching of fura-2 fluorescence. Its amplitude was 10% of that measured in physiological solution containing 10 mM Ca2+. However, following store depletion, a significant quenching of fura-2 fluorescence could be measured only when intracellular BAPTA was omitted, so that all the incoming Mn2+ could be captured by the fluorescent dye. Two other ionic currents activated by receptor stimulation also induced Mn2+ quenching of fura-2 fluorescence: a small current through non-specific cation channels of 50-pS unitary conductance and a distinct cationic current of large amplitude. In addition to these influx mechanisms, Mn2+ was taken up into calcium stores and was subsequently co-released with Ca2+ by Ca(2+)-mobilizing agonists.