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 meas urements of free intracellular calcium concentration ([Ca2+]i) were us ed to study Mn2+ influx in rat peritoneal mast cells. The calcium-sele ctive current, activated by depletion of intracellular calcium stores (I(CRAC) for calcium release-activated calcium current), supports a sm all but measurable Mn2+ current. In the presence of intracellular BAPT A, a Mn2+ current through I(CRAC) was recorded in isotonic MnCl2 (100 mM) without a significant quenching of fura-2 fluorescence. Its amplit ude was 10% of that measured in physiological solution containing 10 m M Ca2+. However, following store depletion, a significant quenching of fura-2 fluorescence could be measured only when intracellular BAPTA w as omitted, so that all the incoming Mn2+ could be captured by the flu orescent dye. Two other ionic currents activated by receptor stimulati on 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 subse quently co-released with Ca2+ by Ca2+-mobilizing agonists.