TOXIC EFFECTS OF THE IONOPHORE A23187 ON CHARA

Internodal cells of Chara corallina were treated with the weakly acidi c ionophore A23187 in order to examine the effects of high calcium inf lux on cytoplasmic processes. A23187 at concentrations of 24-40 mmol m -3 increased the influx of Ca-45 by 10-20-fold, and caused large incre ases in the total cytoplasmic Ca2+ content. The effects of the ionopho re were similar at both pH 5 and at pH 8 although the various response s were considerably slower at high pH. In solutions containing Ca2+ at a concentration of 0.5 mol m-3, increased calcium influx was correlat ed with reductions in the rate of protoplasmic streaming, strong acidi fication of the cytoplasm and a lowering of the ATP concentration. How ever, the effects on ATP concentration and protoplasmic streaming were greater when A23187 was applied in the absence of Ca2+. This indicate s that, at least in Chara, the physiological effects of A23187 are not due to increased influx of Ca2+ across the plasma membrane. It was co ncluded that in view of the slow responses to A23187 these effects pro bably resulted from accumulation of the ionophore in organelle membran es resulting in the disruption of energy transduction and/or intracell ular Ca2+ homeostasis. In view of the widespread use of A23187, we fee l that there are good reasons to be circumspect in the interpretation of the physiological effects of this ionophore.