Characterization in vitro of H⁺ secretion and H⁺:Na⁺ exchange by an organism normally inhabiting a CO₂-rich environment: Hymenolepis diminuta (Cestoda) in the rat intestine

Abstract

H+ and Na+ transport by the intestinal parasite Hymenolepis diminuta were studied in vitro. The flatworms acidified the ambient fluid by secreting H+ and the acidification could not be correlated with organic acid excretion. Ambient CO2-independent H+ secretion was attributed to protons of metabolic origin: dephosphorylation reactions and ionization of organic acids within the tissues. Ambient CO2-dependent H+ secretion was attributed to protons produced as a result of the hydration of CO2 within the tissue and to the stimulation of anaerobic metabolic pathways by CO2 acting as a cosubstrate in energy metabolism. Studies in which Na+ uptake was stimulated by CO2 or glucose and inhibited by ouabain, amiloride, or Na+ replacement suggested a partial direct coupling of Na+ absorption and H+ secretion but the different activation energies and the effect of buffer anions other than HCO3− suggested an indirect interaction. Various interactions were considered, including the effect of CO2 and intracellular ion concentrations on metabolic reactions leading to the supply of protons for H+ secretion and energy for ion transport.

Properties of a Mg2+-dependent, and Ca2+-inhibited ATPase localized in the brush border of the surface epithelial syncytium of a parasitic flatworm

“…Considering the high partial pressures of C02 in lumen of the rat small intestine (15), and the resulting problems concerning tissue acidification (19) and active H+ secretion by H. diminuta (15,17,18), we anticipated the presence of a HC03--stimulated ATPase in the brush border of this parasite similar to that reported from acidifying vertebrate epithelia (26). However, the absence of an effect on ATPase activity by H C 0 3 over a wide range of concentrations and pH indicated the absence of an ATPase with proton transporting properties in the brush border of H. diminuta.…”

“…Since the intestinal lumen may contain relatively high concentrations of C02 resulting in the parasite's active secretion of H+ (15), we anticipated the presence of a HC03-stimulated ATPase in the brush border. Since the intestinal lumen may contain relatively high concentrations of C02 resulting in the parasite's active secretion of H+ (15), we anticipated the presence of a HC03-stimulated ATPase in the brush border.…”

Aspects of helminth metabolism

“…Eventually, when the concentration of carboxylic acids in H. diminuta had increased sufficiently, the ionization of carbonic acid would be repressed and this in turn should inhibit CO 2 fixation by PEPCK since bicarbonate ions, not C0 2 gas, are the source of C0 2 fixation. Podesta (1978) went on to show that hydrogen ion secretion by H. diminuta in vitro was not correlated to organic acid excretion. He considered that hydrogen ion secretion was the result of the hydration of CO 2 in the formation of bicarbonate ions for energy metabolism.…”