Abstract 3565: Regulation of Myocardial Creatine Uptake by Intra- and Extracellular Creatine Levels
Creatine plays an important role in energy metabolism in the heart. Cardiomyocytes accumulate creatine via a specific creatine transporter (CrT), the capacity of which is reduced in the failing heart, resulting in lower myocardial creatine concentration. To gain insight into how the CrT is regulated by intra- and extracellular creatine concentrations, we studied two mouse models of severely altered myocardial creatine levels, guanidinoacetate-N-methyl transferase knock out (GAMT−/−) mice and mice overexpressing the myocardial CrT (CrT-OE). We measured cardiac creatine uptake level in isolated creatine free GAMT−/− mouse hearts from using 14C-radiolabeled creatine, and found that creatine uptake capacity was massively elevated, with a 7-fold increase in Vmax. Cardiac creatine uptake levels in CrT-OE mice were increased on average 2.7-fold, showing a considerable variation, in line with a similar variation in creatine content as measured with HPLC. To understand whether the elevated CrT uptake was due to transcriptional regulation, we measured CrT mRNA levels in the GAMT−/− mouse hearts, but found only a 1.4-fold increase relative to WT values, and no significant difference relative to CrT mRNA levels in the CrT-OE mice. However, the increase in Cr uptake and in CrT mRNA levels in the GAMT−/− mice was prevented when mice were fed a creatine-supplemented diet, indicating that creatine uptake is subject to negative feedback regulation. To determine whether high intracellular creatine concentrations could downregulate CrT transcription, we correlated the expression of total and endogenous CrT mRNA with myocardial creatine levels in the CrT-OE mouse hearts. Total CrT mRNA (transgenic CrT + endogenous CrT) levels correlated well with myocardial creatine content (r=0.67; p<0.0001), but endogenous CrT mRNA levels did not correlate at all with myocardial creatine content (r=0.01;p=0.96), suggesting that intracellular creatine levels did not influence CrT transcription. In summary, creatine uptake can be massively upregulated in the heart and this upregulation is subject to feedback inhibition. In addition, our results strongly suggest that CrT activity is predominantly regulated by mechanisms other than alterations in gene expression.