ABSTRACT
Introduction In line with the metabolic roles of carnitine highlighted in earlier chapters, the concentration of carnitine within a tissue tends to be dependent upon the oxidative capacity of the tissue. Skeletal muscle has high oxidative capacity to cope with the energy demands of physical activity, and therefore has a high carnitine concentration. Coupled with the fact that skeletal muscle comprises a large proportion of body mass, more than 95% of the total body carnitine pool resides within skeletal muscle. The enzyme responsible for the final step of carnitine biosynthesis, butyrobetaine hydroxylase, is not expressed in skeletal muscle, and so carnitine must be imported from its sites of endogenous synthesis and dietary absorption via the circulation (see Chapters 1 and 2). However, the circulating plasma carnitine concentration is approximately 100-fold less than that of skeletal muscle (50 vs. 5000 μM), and so carnitine must be actively transported into the tissue. This process is therefore crucial to whole body carnitine homeostasis and will form the basis of this chapter.
