Water balance and its relation to fermentation acid production in the intestinal parasites Hymenolepis diminuta (Cestoda) and Moniliformis moniliformis (Acanthocephala).

Water balance and its relation to carbohydrate metabolism was examined in Hymenolepis diminuta in parallel with the putative osmoconformer Moniliformis moniliformis. Worms were removed from rat intestines, weighed, and incubated (37 C) 1 hr in rat serum and various salines, some with mannitol to vary osmotic concentration from 150 to 400 mOsm/L. Worms were removed at 15-min intervals, weighed, and returned to the test solution. Rat serum and a Ringer's saline (pH 7.4 and 300 mOsm/L) with or without 5 mM glucose were isotonic to M. moniliformis, which behaved like an osmometer, shrinking, or swelling in proportion to external osmotic changes. Hymenolepis diminuta rapidly lost 20-25% wet weight in these solutions and regained lost water when 5 mM glucose was added to the saline. Tapeworms maintained constant body weight between 210 and 335 mOsm/L, but they rapidly gained or lost water outside of this range. Glucose metabolism and uptake of [3H]glucose from the medium increased progressively between 210 and 310 mOsm/L, whereas uptake rates of [3H]leucine, 22Na+, and 36Cl- were not affected. Unbuffered saline (initial pH 6.5 and 300 mOsm/L) had a lower pH (5.0) and higher osmolality (307 mOsm/L) after a 1-hr incubation with tapeworms. Such saline was less hypertonic than unconditioned saline to freshly obtained worms. A Ringer's saline (300 mOsm/L) containing 50 mM acetate- was also hypertonic (greater than 20% weight loss) to tapeworms at pH 7.4, but it was hypotonic (greater than 20% weight gain) at pH 5.0. Isotonicity at 300 mOsm/L was achieved with pH 5.0 and 20 mM acetate-, the approximate pH and fermentation acid concentration in an infected rat intestine. Rats infected with tapeworms (12 days old) were fasted for 2 days. Starved worms were smaller but had the same percentage of body water and internal osmolality as controls. These results show that H. diminuta can regulate its body water content and that water balance is closely related to the fermentation acid concentration and pH of the bathing medium.