Strontium-Induced Repetitive Calcium Spikes in a Unicellular Green Alga

The divalent cation Sr(2+) induced repetitive transient spikes of the cytosolic Ca(2+) activity [Ca(2+)](cy) and parallel repetitive transient hyperpolarizations of the plasma membrane in the unicellular green alga Eremosphaera viridis. [Ca(2+)](cy) measurements, membrane potential measurements, and cation analysis of the cells were used to elucidate the mechanism of Sr(2+)-induced [Ca(2+)](cy) oscillations. Sr(2+) was effectively and rapidly compartmentalized within the cell, probably into the vacuole. The [Ca(2+)](cy) oscillations cause membrane potential oscillations, and not the reverse. The endoplasmic reticulum (ER) Ca(2+)-ATPase blockers 2,5-di-tert-butylhydroquinone and cyclopiazonic acid inhibited Sr(2+)-induced repetitive [Ca(2+)](cy) spikes, whereas the compartmentalization of Sr(2+) was not influenced. A repetitive Ca(2+) release and Ca(2+) re-uptake by the ER probably generated repetitive [Ca(2+)](cy) spikes in E. viridis in the presence of Sr(2+). The inhibitory effect of ruthenium red and ryanodine indicated that the Sr(2+)-induced Ca(2+) release from the ER was mediated by a ryanodine/cyclic ADP-ribose type of Ca(2+) channel. The blockage of Sr(2+)-induced repetitive [Ca(2+)](cy) spikes by La(3+) or Gd(3+) indicated the necessity of a certain influx of divalent cations for sustained [Ca(2+)](cy) oscillations. Based on these data we present a mathematical model that describes the baseline spiking [Ca(2+)](cy) oscillations in E. viridis.