Mitochondrial contact sites detected by creatine phosphokinase activity in the hearts of normal and diabetic rats: Is mitochondrial contact sites formation a calcium-dependent process?

Mitochondrial contact sites (MiCS) are structures in the mitochondrial membrane containing the structure-bound mitochondrial isoenzyme of creatine phosphokinase that participates in the transfer of energy into the cytoplasm. This explains the increased formation of MiCS found in hearts with high metabolic activity. Earlier we demonstrated that enhanced MiCS formation may also be induced by perfusing the heart with increased, but still not cardiodepressive concentrations of Ca2+ (2.2 mmol.l(-1)) in the perfusate. Nevertheless, neither the molecular mechanism by which Ca2+ ions may induce an increase in MiCS formation, nor the dependence of Ca2+-induced MiCS formation on the intracellular Ca2+ level have yet been elucidated. In the present study we investigated the effect of Langendorff-perfusion with 2.2 mmol.l(-1) Ca2+ on formation of MiCS in normal as well as in diabetic hearts. The latter, namely, are characterized by altered metabolism as well as Ca2+-handling, resulting in elevated [Ca2+](i). We have found that the amounts of MiCS in diabetic hearts outnumbered those in normal hearts. Our results showed that in comparison to perfusion with 1.6 mmol.l(-1) Ca2+ a perfusion with 2.2 mmol.l(-1) Ca2+ is capable of significantly increasing (p < 0.01) the formation of MiCS in control hearts. In both groups of diabetic hearts the numbers of MiCS were significantly increased in comparison to healthy controls (p < 0.01). Moreover, no significant differences in amounts of MiCS were found between healthy hearts perfused with 2.2 mmol.l(-1) Ca2+ and diabetic hearts of both groups (p > 0.05). In diabetic hearts, MiCS formation in response to [Ca2+](e) was little manifested. Our results also confirmed that elevated [Ca2+](i) in all cases represented a signal for increased formation of MiCS in the heart.