Haematoloechus medioplexus: Uptake, localization, and fate of tritiated arginine

Abstract

The uptake, localization, and fate of tritiated arginine in Haematoloechus medioplexus was studied using autoradiography of paraffin and frozen tissue sections and liquid scintillation counting techniques. Autoradiographic and liquid scintillation studies on ligated and unligated parasites demonstrated that, during in vitro incubation, only the gut epithelium was active in arginine absorption. Autoradiographic studies also showed that arginine was not preferentially assimilated into any organ or tissue studied. Preincubation of parasites in cycloheximide inhibited the initial assimilation of arginine; autoradiography of frozen sections showed that cycloheximide did not inhibit absorption through the gut lining. Iodoacetate and dinitrophenol (DNP) were both effective inhibitors of arginine absorption. Liquid scintillation studies demonstrated that parasites must first be preincubated in iodoacetate for 5 min for this inhibitor to be totally effective. Dinitrophenol was instantaneous in inhibiting arginine absorption; i.e., preincubation was not a prerequisite for inhibition. Apparently the iodoacetate must first have been absorbed to inhibit absorption, while the DNP affected the permeability of cell membranes bordering the gut lumen. Of the arginine assimilated, 94% was incorporated into the protein fraction of these parasites; the remainder was found in the carbohydrate and lipid fractions. Using pulse labeled parasites, it was found that these worms could excrete, or secrete, radioactive compounds. The tegument and/or excretory pore was responsible for the excretion of a radioactive, ninhydrin positive compound, which was probably the original arginine, or a small peptide containing the original arginine, or an amine. These worms also regurgitated a radioactive compound from the gut which was not reactive with ninhydrin. No correlation between total uptake of labeled arginine and length of incubation could be found. This was likely due to the ability of these worms to absorb, as well as excrete, arginine and the end products of arginine metabolism during short-term incubations, or to the exhaustion of the amino acid pool in these worms resulting in cessation of protein synthesis.