As more than 95% of cellular ATP produced under aerobic condition is delivered by the mitochondrial oxidative phosphorylation, these organelles are first of all recognised as the major cellular power plant. However, data are accumulating that indicate that mitochondria play also an important role as targets, decoders, transmitters and integrators of many intracellular signals. These functions, which allow for coordinating energy metabolism with diverse cellular processes, result from mitochondrial properties, including their ability for generating inner membrane potential, production of reactive oxygen species (ROS), presence of proapoptotic factors in the mitochondrial intermembrane space, susceptibility of the outer mitochondrial membrane to proapoptotic Bcl-2 protein family members, formation of permeability transition pore, and presence of the mitochondrial DNA controlled by proteins encoded in the nuclear DNA. Among molecules, which may shape mitochondrial metabolism, coordinate their activity with other cellular processes and be involved in the retrograde communication between mitochondria and the nucleus, and therefore plying the role in intracellular signal transduction, are proteins (e.g. transcription factors, protein kinases, proapoptotic proteins) as well as non-protein low molecular weight substances, such as ROS, nucleotides and Ca²⁺.

Mitochondrial Ca²⁺ handling is based on a balance between calcium entry into and calcium release from these organelles. Moderate increase in mitochondrial Ca²⁺ concentration has stimulatory effect on oxidative phosphorylation, whereas mitochondrial Ca²⁺ overload may result in mitochondrial damage and cell death. Moreover, mitochondria may profile cytosolic Ca²⁺ signals and therefore influence a variety of cellular calcium-dependent processes.

The place of mitochondria in intracellular signalling pathways with special attention to intracellular Ca²⁺ signalling will be discussed.

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