The dynamin-related GTPase protein OPA1, localized in the intermembrane space 
and tethered to the inner membrane of mitochondria, participates in the fusion 
of these organelles. Its mutation is the most prevalent cause of Autosomal 
Dominant Optic Atrophy. OPA1 controls the diameter of the junctions between the 
boundary part of the inner membrane and the membrane of cristae and reduces the 
diffusibility of cytochrome c through these junctions. We postulated that if 
significant Ca²⁺ uptake into the matrix occurs from the lumen of the cristae, 
reduced expression of OPA1 would increase the access of Ca²⁺ to the transporters 
in the crista membrane and thus would enhance Ca²⁺ uptake. In intact H295R 
adrenocortical and HeLa cells cytosolic Ca²⁺ signals evoked with K⁺ and 
histamine, respectively, were transferred into the mitochondria. The rate and 
amplitude of mitochondrial [Ca²⁺] rise (followed with confocal laser scanning 
microscopy and FRET measurements with fluorescent wide-field microscopy) were 
increased after knockdown of OPA1, as compared with cells transfected with 
control RNA or mitofusin1 siRNA. Ca²⁺ uptake was enhanced despite reduced 
mitochondrial membrane potential. In permeabilized cells the rate of Ca²⁺ uptake 
by depolarized mitochondria was also increased in OPA1-silenced cells. The 
participation of Na⁺/Ca²⁺ and Ca²⁺/H⁺ antiporters in this transport process is 
indicated by pharmacological data. Altogether, our observations reveal the 
significance of OPA1 in the control of mitochondrial Ca²⁺ metabolism.