STRONTIUM-INDUCED REPETITIVE CALCIUM SPIKES IN A UNICELLULAR GREEN-ALGA

The divalent cation Sr2+ induced repetitive transient spikes of the cy tosolic Ca2+ activity [Ca2+](cy) and parallel repetitive transient hyp erpolarizations of the plasma membrane in the unicellular green alga E remosphaera viridis. [Ca2+](cy) measurements, membrane potential measu rements, and cation analysis of the cells were used to elucidate the m echanism of Sr2+-induced [Ca2+](cy) oscillations. Sr2+ was effectively and rapidly compartmentalized within the cell, probably into the vacu ole. The [Ca2+](cy) oscillations cause membrane potential oscillations , and not the reverse. The endoplasmic reticulum (ER) Ca2+-ATPase bloc kers 2,5-di-tert-butylhydroquinone and cyclopiazonic acid inhibited Sr 2+-induced repetitive [Ca2+](cy) spikes, whereas the compartmentalizat ion of Sr2+ was not influenced. A repetitive Ca2+ release and Ca2+ re- uptake by the ER probably generated repetitive [Ca-2+](cy) spikes in E . viridis in the presence of Sr2+. The inhibitory effect of ruthenium red and ryanodine indicated that the Sr2+-induced Ca2+ release from th e ER was mediated by a ryanodine/cyclic ADP-ribose type of Ca2+ channe l. The blockage of Sr2+-induced repetitive [Ca2+](cy) spikes by La3+ o r Gd3+ indicated the necessity of a certain influx of divalent cations for sustained [Ca2+](cy) oscillations. Based on these data we present a mathematical model that describes the baseline spiking [Ca2+](cy) o scillations in E. viridis.