GLUCOSE AND SODIUM FLUXES ACROSS THE BRUSH BORDER OF HYMENOLEPIS DIMINUTA (CESTODA)

¹ Department of Biology, Rice University, Houston, Texas 77001

When Hymenolepis diminuta was preincubated in Na⁺-free KRT (tris-maleate buffered Krebs-Ringer saline) for varying time intervals, followed by incubation in ¹⁴C-glucose in Na⁺-fee KRT, the influx of glucose in worms was lowered significantly. This effect of Na⁺ deletion on glucose influx was totally reversible by incubating worms in KRT ([ Na⁺] = 154 meq/l).

When Na⁺ in the medium was replaced with K⁺, tris, or choline, a similar decrease in glucose influx in worms was noted; replacement of Na⁺ with Li⁺ resulted in a glucose influx rate significantly higher than that obtained with K⁺, tris, or choline as the replacement cation. In media with a suboptimal Na⁺ concentration (25 meq/l), influx of 0.5 mM glucose in worms was unaffected by varying concentrations of K⁺ (0-100 meq/l).

Apparent Michaelis-Menten kinetics were observed when glucose influx as a function of glucose concentration was determined in media with Na⁺ concentrations of 154, 50, 25, and 10 meq/l. There was a slight decrease in the apparent transport constant (K_t) for glucose influx and a marked decrease in maximal glucose influx (Jⁱ_G^max with decreasing Na⁺ concentration. Glucose influx in H. diminuta as a function of the Na⁺ concentration displayed apparent Michaelis-Menten kinetics, and the Na⁺ concentration necessary to attain Jⁱ_G^max/2 lowered significantly at lower glucose concentrations.

A large Na⁺ influx was associated with glucose influx in H. diminuta, and the influxes of Na⁺ and glucose exhibited first order kinetics for at least 2 min. In media with Na⁺ concentrations of 5, 25, and 50 meq/l, coupling coefficients (Jⁱ_Na/ Jⁱ_G) were 1.90, 1.60, and 0.87, respectively, or inversely related to the Na⁺ concentration. Coupling coefficients were independent of the glucose concentration over a 50-fold range. Glucose-coupled Na⁺ influx in worms was a hyperbolic function of glucose concentration. In the absence of glucose, Na⁺ influx in H. diminuta apparently occurred, in part, by a mediated process which was unaffected by phlorizin. There was a small mediated glucose influx in worms in the absence of external Na⁺, 70% of which was abolished by phlorizin.

The cotransport of Na⁺ and glucose in H. diminuta is (1) compared with other similar systems in both tapeworms and mammals, (2) presented in support of Crane's Na⁺-gradient hypothesis for transport and accumulation of sugars, (3) tentatively explained in the form of a hypothesized model, and (4) discussed briefly in relation to the evolution of parasitic flatworms.