Rat and mouse cardiomyocytes show subtle differences in creatine kinase expression and compartmentalization.

Creatine kinase (CK) and adenylate kinase (AK) are energy transfer systems. Different studies on permeabilized cardiomyocytes suggest that ADP-channelling from mitochondrial CK alone stimulates respiration to its maximum, VO2_max, in rat but not mouse cardiomyocytes. Results are ambiguous on ADP-channelling from AK to mitochondria. This study was undertaken to directly compare the CK and AK systems in rat and mouse hearts. In homogenates, we assessed CK- and AK-activities, and the CK isoform distribution. In permeabilized cardiomyocytes, we assessed mitochondrial respiration stimulated by ADP from CK and AK, VO2_CK and VO2_AK, respectively. The ADP-channelling from CK or AK to mitochondria was assessed by adding PEP and PK to competitively inhibit the respiration rate. We found that rat compared to mouse hearts had a lower aerobic capacity, higher VO2_CK/VO2_max, and different CK-isoform distribution. Although rat hearts had a larger fraction of mitochondrial CK, less ADP was channeled from CK to the mitochondria. This suggests different intracellular compartmentalization in rat and mouse cardiomyocytes. VO2_AK/VO2_max was similar in mouse and rat cardiomyocytes, and AK did not channel ADP to the mitochondria. In the absence of intracellular compartmentalization, the AK- and CK-activities in homogenate should have been similar to the ADP-phosphorylation rates estimated from VO2_AK and VO2_CK in permeabilized cardiomyocytes. Instead, we found that the ADP-phosphorylation rates estimated from permeabilized cardiomyocytes were 2 and 9 times lower than the activities recorded in homogenate for CK and AK, respectively. Our results highlight the importance of energetic compartmentalization in cardiac metabolic regulation and signalling.