Transport of exogenous 5-hydroxytryptamine across the outer plasma membrane of the syncytial tegument of Hymenolepis diminuta is by simple diffusion

The uptake of 5-hydroxytryptamine (5HT) from a 10 μM solution of exogenous [3H]5HT into the tegument of Hymenolepis diminuta was linear for the first 20 min of incubation. The rate of transport was 0.04 ± 0.01 pmol∙mg wet mass−1∙min−1, and there were no significant differences in the rate of uptake by the anterior, middle, and posterior regions of the body. The initial uptake was not Na+-dependent, was not saturable at up to 100 μM, was not highly temperature-dependent (Q10 ~ 1.2), and displayed activation energy of 11.8 kJ∙mol−1. Furthermore, uptake was not inhibited by p-chloromercuriphenyl sulphonic acid, imipramine, amiloride, or 5HT analogues, which collectively support a non-carrier-mediated uptake mechanism. Washing of the tissues with 10 mM 5HT after incubation in 10 μM [3H]5HT displaced less than 10% of the remaining [3H]5HT associated with the tissues, and little radioactivity was extracted by washing in acetone or chloroform. The uptake of [3H]5HT, however, was pH-dependent, the rate of uptake being closely correlated with the proportion of unprotonated 5HT. Only a small portion of the transported [3H]5HT was metabolized to a product associated with 5-hydroxyindoleacetic acid, and metabolism was significantly inhibited by the monoamine oxidase inhibitors iproniazid phosphate, deprenyl, and clorgyline. The present study showed that small amounts of [3H]5HT were taken up by H. diminuta by simple diffusion, little of the [3H]5HT was adsorbed to the surface of the worms or dissolved in the lipid phase of the plasma membrane, and some of the [3H]5HT taken up was metabolized by a monoamine oxidase-like enzyme.