Cardiomyocytes from Creatine-Deficient Mice Lacking L-Arginine: Glycine Amidinotransferase (AGAT) Show No Changes in Mitochondrial Organization and Cellular Compartmentation

In cardiac muscle, the creatine kinase (CK) system temporally buffers ADP and ATP concentrations near sites of ATP production and consumption. It has also been suggested to be an important spatial buffer facilitating the transport of ADP and ATP. This was corroborated by a study of CK knockout mice. However, a recent study on cardiomyocytes from creatine-deicient guanidinoacetate methyltransferase (GAMT) knockout mice showed no effect on mitochondrial organization and compartmentation. It has been suggested that in GAMT knockout mice, accumulated guanidinoacetate can be used as a substrate instead of creatine. Therefore, we studied the same parameters in L-arginine:glycine amidinotransferase (AGAT) mice, which are also creatine-deficient, but do not accumulate guanidinoacetate. Three-dimensional mitochondrial organization at whole cell level was assessed by confocal microscopy. Kinetic measurements on permeabilized cardiomyocytes included the affinity of oxidative phosphorylation to exogenous ADP and ATP, competition between mitochondria and pyruvate kinase for ADP produced by ATPases, ADP-kinetics of endogenous pyruvate kinase and ATP-kinetics of ATPases. Our breeding records show that AGAT-/- mice are less viable than their wild-type congeners. Most died at four to five months of age. In cardiomyocytes, visual inspection of mitochondrial organization suggests no essential difference between AGAT knockout and wildtype mice. Also, there is no difference in the kinetics of respiration and ATPases. Thus, our data so far suggest no differences in cardiomyocyte mitochondrial organization and cellular compartmentation between AGAT knockout and wildtype mice. We conclude that the reduced viability of AGAT mice is most likely not due to cardiac failure. The lack of compensatory changes in mitochondrial organization and cellular compartmentation in AGAT as well as GAMT mice raise questions regarding the importance of the CK system as a spatial buffer.