Transport of neurotransmitter precursors in a syncytial epithelium

1. Tegumental transport of choline at concentrations ranging from 0.005–5.0 mM provided no evidence for saturable, carrier-mediated entry of this amine in the tegument of the rat tapeworm (Hymenolepis diminuta). 2. In contrast, the large neutral amino acid tryptophan appears to be taken up via a high-affinity transporter. In the 1st quartile of 17-day-old tapeworms (Km = 0.033 mM, Vmax = 0.7 nmol·min−1 · g−1), in the 2nd quartile (Km = 0.015 mM, Vmax = 0.3 nmol·min−1 · g−1), in the 3rd quartile (Km = 0.022 mM, Vmax = 0.5 nmol · min−1 · g−1) and in the 4th quartile (Km = 0.025 mM, Vmax = 0.5 nmol · min−1 · g−1) saturable tryptophan transport was kinetically characterized. 3. The non-saturable diffusion component (Kd) for tryptophan transport ranged from 3.8–10.2 μl · min−1 · g−1. 4. These studies suggest choline does not appear to be transported across the tapeworm tegument. Saturable transport of tryptophan via a high-affinity carrier is reported, and no regional variations in indole amino acid uptake were detected.