The action potential in Chara: Ca2+ release from internal stores visualized by Mn2+‐induced quenching of fura‐dextran

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Abstract
Summary: The action potential (AP) in Chara is associated with a transient elevation in the concentration of cytoplasmic‐free Ca2+ ([Ca2+]cyt). The quenching properties of the fluorescent Ca2+ indicator dye fura‐dextran, in combination with Mn2+, was used to investigate whether this [Ca2+]cyt transient is due to Ca2+ release from internal stores or to Ca2+ influx across the plasma membrane. Adding Mn2+ to the external medium or pre‐injection of Mn2+ into the vacuole caused no perceivable quenching of the fura fluorescence, during an AP. This makes it unlikely that Ca2+ influx across the plasma membrane or the tonoplast contributes significantly to the [Ca2+]cyt transient in an excited cell. When cells were pre‐incubated in external solutions containing Mn2+ from 25 to 30 mM APs evoked a transient quenching of fura fluorescence in Mn2+‐free solutions. Under these conditions, the quenching must be attributed to an AP‐associated release of Mn2+ from internal stores. Based on the finding that exposing cells to millimolar concentrations of Mn2+ caused a progressive quenching of the fura fluorescence in non‐excited cells, it can be assumed that some Mn2+ enters the cells during pre‐incubation and is loaded into internal stores. During excitation, this stored Mn2+ is released together with Ca2+.