Reduction of L-[14C]cystine to L-[14C]cysteine in the presence of 1mM cysteinylglycine increases the uptake rate in HEK(GLT1), HEK(GLAST) and HEK(EAAC1) cells, but only a small proportion (<10%) of L-[14C]cysteine uptake in HEK(GLT1) and HEK(GLAST) cells occurs by the high affinity glutamate transporters