Creatine transporter activity and content in the rat heart supplemented by and depleted of creatine

The intracellular creatine concentration is an important bioenergetic parameter in cardiac muscle. Although creatine uptake is known to be via a NaCl-dependent creatine transporter (CrT), its localization and regulation are poorly understood. We investigated CrT kinetics in isolated perfused hearts and, by using cardiomyocytes, measured CrT content at the plasma membrane or in total lysates. Rats were fed control diet or diet supplemented with creatine or the creatine analog β-guanidinopropionic acid (β-GPA). Creatine transport in control hearts followed saturation kinetics with a K m of 70 ± 13 mM and a V max of 3.7 ± 0.07 nmol · min−1 · g wet wt−1. Creatine supplementation significantly decreased the V max of the CrT (2.7 ± 0.17 nmol · min−1 · g wet wt−1). This was matched by an ∼35% decrease in the plasma membrane CrT; the total CrT pool was unchanged. Rats fed β-GPA exhibited a >80% decrease in tissue creatine and increase in β-GPAtotal. The V max of the CrT was increased (6.0 ± 0.25 nmol · min−1 · g wet wt−1) and the K m decreased (39.8 ± 3.0 mM). The plasma membrane CrT increased about fivefold, whereas the total CrT pool remained unchanged. We conclude that, in heart, creatine transport is determined by the content of a plasma membrane isoform of the CrT but not by the total cellular CrT pool.