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Using fluorescence-activated cell sorting (FACS), we have studied the effect of the differentiation-inducing factor (DIF) on cellular Ca 2+ in Dictyostelium discoideum. We have shown previously that freshly starved or postaggregation amoebae are heterogenous with respect to the amounts of cellular Ca 2+ that they contain; the L or “low Ca 2+ ” class exhibits a prespore tendency and the H or “high Ca 2+ ” class exhibits a prestalk tendency. Upon adding DIF, within 2 min there is an approximately twofold increase in the relative fraction of amoebae falling in the H class. A major part of the increase is caused by Ca 2+ influx from the extracellular medium. Therefore a rise in the level of cellular Ca 2+ is an early step in the signal transduction pathway following stimulation by DIF. Also, in parallel with the cellular heterogeneity in respect of Ca 2+ content, there is a heterogeneity in the response to DIF, which appears to be restricted to L cells. |
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