Ca2+ uptake by the endoplasmic reticulum Ca2+-ATPase in rat microvascular endothelial cells

Author:

MOCCIA Francesco1,BERRA-ROMANI Roberto1,BARUFFI Silvana2,SPAGGIARI Santina2,SIGNORELLI Silvia1,CASTELLI Loretta1,MAGISTRETTI Jacopo1,TAGLIETTI Vanni1,TANZI Franco1

Affiliation:

1. Department of Physiological and Pharmacological Sciences, University of Pavia, Via Forlanini 6, 27100 Pavia, Italy

2. Department of Evolutionary and Functional Biology, University of Parma, Parco Area delle Scienze 11A, 43100 Parma, Italy

Abstract

In non-excitable cells, many agonists increase the intracellular Ca2+ concentration ([Ca2+]i) by inducing an inositol 1,4,5-trisphosphate (IP3)-mediated Ca2+ release from the intracellular stores. Ca2+ influx from the extracellular medium may then sustain the Ca2+ signal. [Ca2+]i recovers its resting level as a consequence of Ca2+-removing mechanisms, i.e. plasma-membrane Ca2+-ATPase (PMCA) pump, Na+/Ca2+ exchanger (NCX) and sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) pump. In a study performed in pancreatic acinar cells, evidence has been provided suggesting that, during the decay phase of the agonist-evoked Ca2+ transients, the Ca2+ concentration within the intracellular stores remains essentially constant [Mogami, Tepikin and Petersen (1998) EMBO J. 17, 435–442]. It was therefore hypothesized that, in such a situation, intracellular Ca2+ is not only picked up by the SERCA pump, but is also newly released through IP3-sensitive Ca2+ channels, with the balance between these two processes being approximately null. The main aim of the present work was to test this hypothesis by a different experimental approach. Using cardiac microvascular endothelial cells, we found that inhibition of the SERCA pump has no effect on the time course of agonist-evoked Ca2+ transients. This result was not due to a low capacity of the SERCA pump since, after agonist removal, this pump proved to be very powerful in clearing the excess of intracellular Ca2+. We showed further that: (i) in order to avoid a rapid removal of Ca2+ by the SERCA pump, continuous IP3 production appears to be required throughout all of the decay phase of the Ca2+ transient; and (ii) Ca2+ picked up by the SERCA pump can be fully and immediately released by agonist application. All these results support the model of Mogami, Tepikin and Petersen [(1998) EMBO J. 17, 435–442]. Since the SERCA pump did not appear to be involved in shaping the decay phase of the agonist-evoked Ca2+ transient, we inhibited the PMCA pump with carboxyeosin, and NCX with benzamil and by removing extracellular Na+. The results indicate that, during the decay phase of the agonist-evoked Ca2+ transient, the intracellular Ca2+ is removed by both the PMCA pump and NCX. Finally, we provide evidence indicating that mitochondria have no role in clearing intracellular Ca2+ during agonist-evoked Ca2+ transients.

Publisher

Portland Press Ltd.

Subject

Cell Biology,Molecular Biology,Biochemistry

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