Sub-second calcium coupling between outside medium and subplasmalemmal stores during overstimulation/depolarisation-induced ciliary beat reversal in Paramecium cells

As amply documented by electrophysiology, depolarisation in Paramecium induces a Ca2+ influx selectively via ciliary voltage-dependent Ca2+-channels, thus inducing ciliary beat reversal. Subsequent downregulation of ciliary Ca2+ has remained enigmatic. We now analysed this aspect, eventually under overstimulation conditions, by quenched-flow/cryofixation, combined with electron microscope X-ray microanalysis which registers total calcium concentrations, [Ca]. This allows to follow Ca-signals within a time period (≥30 ms) smaller than one ciliary beat (∼50 ms) and beyond. Particularly under overstimulation conditions (∼10−5 M Ca2+ before, 0.5 mM Ca2+ during stimulation) we find in cilia a [Ca] peak at ∼80 ms and its decay to near-basal levels within 110 ms (90%) to 170 ms (100% decay). This [Ca] wave is followed, with little delay, by a [Ca] wave into subplasmalemmal Ca-stores (alveolar sacs), culminating at ∼100 ms, with a decay to original levels within 170 ms. Also with little delay [Ca] slightly increases in the cytoplasm below. This implies rapid dissipation of Ca2+ through the ciliary basis, paralleled by a rapid, transient uptake by, and release from cortical stores, suggesting fast exchange mechanisms to be analysed as yet. This novel type of coupling may be relevant for some phenomena described for other cells.