Skeletal muscle Pi transport and cellular [Pi] studied in L6 myoblasts and rabbit muscle-membrane vesicles

In the rat skeletal myoblast line L6 and in a rabbit skeletal muscle sarcolemma/t-tubule vesicle preparation, [32P]Pi uptake was largely dependent on the transmembrane Na gradient. Na-dependent [32P]Pi uptake had a hyperbolic relationship to [Pi] and [Na], being half-maximal at 0.2-0.3 mM [Pi] and at 25-40 mM [Na]. In vesicles the Na-dependence suggests that approx. two Na are transported with each Pi, but the inhibition of [32P]Pi uptake at high pH suggests that the Pi monoanion is the transported form. Together these imply electrogenic transport and this is confirmed by the results of manipulating the vesicle membrane potential. Thus, electrogenic Na-Pi co-transport exploits both the sodium gradient and the cell membrane potential to maintain muscle cellular [Pi] against an unfavourable electrochemical gradient. The low [Pi] for half-maximal flux may partly explain the small effect of altered extracellular [Pi] on cellular [Pi]. In L6 myoblasts most 32P was first detectable in an organic phosphate pool rather than cellular Pi, while the specific activity of cell Pi rapidly reached 40% of that of extracellular Pi and was stable for at least 3 h. These results are discussed in terms of the organisation of cellular phosphate metabolism.