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 (I_CRAC for calcium release-activated calcium current), supports a small but measurable Mn²⁺ current. In the presence of intracellular BAPTA, a Mn²⁺ current through I_CRAC 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 Ca²⁺-mobilizing agonists.