An unexpected effect of risperidone reveals a nonlinear relationship between cytosolic Ca and mitochondrial Ca uptake.

Mitochondria actively contribute to cellular Ca homeostasis. The molecular mechanisms of mitochondrial Ca uptake and release are well characterized and are attributed to the multi-protein assembly of the mitochondrial Ca uniporter complex (MCUC) and the mitochondrial sodium-calcium exchanger (NCLX), respectively. Hence, Ca transfer from the endoplasmic reticulum (ER) and store-operated Ca entry (SOCE) into the mitochondrial matrix has been quantitatively visualized on the subcellular level using targeted fluorescent biosensors. However, a correlation between the amplitude of cytosolic Ca elevation with that in the mitochondrial matrix has not been investigated in detail so far. In the present study, we combined the Ca-mobilizing agonist histamine with the H-receptor antagonist risperidone to establish a well-tunable experimental approach allowing the correlation between low, slow, high, and fast cytosolic and mitochondrial Ca signals in response to inositol 1,4,5-trisphosphate (IP)-triggered ER Ca release. Our present data confirm a defined threshold in cytosolic Ca, which is necessary for the activation of mitochondrial Ca uptake. Moreover, our data support the hypothesis of different modes of mitochondrial Ca uptake depending on the source of the ion (i.e., ER vs SOCE).

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