Regional modulations in tegumental glucose transporter kinetics in the rat tapeworm

Comparisons of glucose transporter kinetics in 8-day (Km = 0.34 mM, Vmax = 14 nmole.min-1.g-1), 10-day (Km = 0.46 mM, Vmax = 18 nmole.min-1.g-1), the first quartile of 17-day (Km = 0.51 mM, Vmax = 21 nmole.min-1.g-1), and the first quartile of 32-day (Km = 0.33 mM, Vmax = 39 nmole.min-1.g-1) rat tapeworms (Hymenolepis diminuta) suggest maximal velocities may vary with age. A gradient in glucose transporter density is suggested in the rat tapeworm by changes in the estimated transporter Vmax in the first through fourth quartiles. Alterations in the physiological efficiency (as indicated by the Vmax/Km ratio) and permeability (indicated by the unsaturated permeability-area product) of the glucose transporter were determined to be significantly greater in the first quartile than in other quartiles of 17-day hymenolepids. A similar trend was apparent in older (32-day) worms. In tapeworms maintained for 30 min in glucose-free medium, maximal velocities were highest in the anterior (first) quartile, and reductions were seen in successive second, third, and fourth quartiles. When worms were maintained in a medium containing 11 mM glucose, maximal velocities were about twofold greater, but the Vmax increased in each successive quartile. The apparent half-saturation constants, which indicate that concentration of external glucose at which half of the glucose transporter proteins are occupied, are reduced approximately 50% in tapeworms maintained in glucose-free medium. These studies demonstrate that regional differences exist in the glucose transporter of the rat tapeworm, analogous to the intestinal glucose gradient. Furthermore, substrate-induced modulations in the transporter may also exhibit independent regional variability.