Ascaris suum: Characterization of Transmural and Hypodermal Potentials

Abstract

Electrophysiological measurements demonstrate that an electrochemical potential exists across the body wall of parasitic nematodes. The ionic dependence of this transmural electrical potential (Etm) in the gastrointestinal nematode Ascaris suum was investigated using conventional electrophysiological techniques. Etm recorded from intact A. suum maintained in artificial pseudocoelomic fluid (APF) was −40 ± 12 mV. This potential was more sensitive to external acetate than to Na+, K+, or Cl−, although elimination of most Na+ from the medium significantly hyperpolarized the body wall. An Ussing chamber and isolated segments of A. suum body wall were used to delineate the barrier characteristics of the individual components of the body wall: the cuticle and the inward- and outward-facing membranes of the hypodermis. The cuticle (i.e., muscle and hypodermis scraped away) is highly permeable to both inorganic and organic ions, with the rank-order of permeability among ions tested being K+ > Na+ = Cl− > acetate− > gluconate−. The inward- and outward-facing membranes of the hypodermis were more polarized than the body wall complex, exhibiting potentials in APF of −47.6 ± 6 mV (Ei) and −74.9 ± 7 mV (Eo) versus −26 ± 8 mV (Etm for isolated body wall segments), respectively. The electrical potential across the hypodermal membranes became depolarized when high K+ medium or low acetate medium was added to the muscle side, but not when added to the cuticle side of isolated body wall segments. Etm, Eo, and Ei were unaffected by reduction of Na+, K+, or Cl− concentrations in the recording medium. All three potentials, however, became markedly depolarized when the temperature of the incubation medium was reduced. These results indicate that the cuticle/hypodermis complex of A. suum is differentially permeable to both inorganic and organic ions and suggest that active transport of ions or outward diffusion of metabolic end-products contributes extensively to the maintenance of transmural electrochemical gradients.