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J. Biol. Chem., Vol. 279, Issue 18, 18407-18414, April 30, 2004

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Cytosolic [Ca²⁺] Transients in Dictyostelium discoideum Depend on the Filling State of Internal Stores and on an Active Sarco/Endoplasmic Reticulum Calcium ATPase (SERCA) Ca²⁺ Pump^*

Christina Schlatterer, Kathrin Happle, Daniel F. Lusche, and Jürgen Sonnemann

From the Faculty for Biology, University of Konstanz, 78457 Konstanz, Germany and the Institute of Pharmacology, Pediatric Oncology, and Hematology, University of Greifswald, 17487 Greifswald, Germany

Stimulation of Dictyostelium discoideum with cAMP evokes a change of the cytosolic free Ca²⁺ concentration ([Ca²⁺]_i). We analyzed the role of the filling state of Ca²⁺ stores for the [Ca²⁺] transient. Parameters tested were the height of the [Ca²⁺]_i elevation and the percentage of responding amoebae. After loading stores with Ca²⁺, cAMP induced a [Ca²⁺]_i transient in many cells. Without prior loading, cAMP evoked a [Ca²⁺]_i change in a few cells only. This indicates that the [Ca²⁺]_i elevation is not mediated exclusively by Ca²⁺ influx but also by Ca²⁺ release from stores. Reducing the Ca²⁺ content of the stores by EGTA preincubation led to a cAMP-activated [Ca²⁺]_i increase even at low extracellular [Ca²⁺]. Moreover, the addition of Ca²⁺ itself elicited a capacitative [Ca²⁺]_i elevation. This effect was not observed when stores were emptied by the standard technique of inhibiting internal Ca²⁺ pumps with 2,5-di-(t-butyl)-1,4-hydroquinone. Therefore, in Dictyostelium, an active internal Ca²⁺-ATPase is absolutely required to allow for Ca²⁺ entry. No influence of the filling state of stores on Ca²⁺ influx characteristics was found by the Mn²⁺-quenching technique, which monitors the rate of Ca²⁺ entry. Both basal and cAMP-activated Mn²⁺ influx rates were similar in control cells and cells with empty stores. By contrast, determination of extracellular free Ca²⁺ concentration ([Ca²⁺]_e) changes, which represent the sum of Ca²⁺ influx and efflux, revealed a higher rate of [Ca²⁺]_e decrease in EGTA-treated than in control amoebae. We conclude that emptying of Ca²⁺ stores does not change the rate of Ca²⁺ entry but results in inhibition of the plasma membrane Ca²⁺-ATPase. Furthermore, the activities of the Ca²⁺ transport ATPases of the stores are of crucial importance for the regulation of [Ca²⁺]_i changes.

Received for publication, July 3, 2003 , and in revised form, January 22, 2004.

^* This work was supported by the Deutsche Forschungsgemeinschaft. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

To whom correspondence should be addressed. Fax: 49-7531-882966; E-mail: Christina.Schlatterer{at}uni-konstanz.de.

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