Effect of 5-hydroxytryptamine (5-HT; serotonin) on in vitro glucose uptake and glycogen reserves in Hymenolepis diminuta.

Glucose uptake by Hymenolepis diminuta was linear for up to 60 min following in vitro incubation in 10 mM glucose. Following the addition of 1 mM 5-HT, with a 95% O2/5% CO2, gas phase, glucose uptake by H. diminuta was significantly (P less than 0.001) enhanced, remaining linear and parallel to that of control groups between 15 and 60 min incubation. Under air, the rates of glucose uptake were higher, but were only significantly increased by 5-HT during the first 30 min of incubation. In further experiments, in the absence of glucose in the incubation media worm glycogen reserves decreased by over 50% after 60 min. With the addition of 1 mM 5-HT, the reduction in glycogen content was significantly (P less than 0.025) greater, exceeding 65% after 60min. When glucose was added to the incubation media, worm glycogen reserves were not significantly depleted irrespective of the presence or absence of 5-HT. Incubations under a 95% O2/5% CO2 gas phase did not significantly influence glycogen content compared to corresponding groups incubated in air. The results suggest that 5-HT stimulates glycolysis in H. diminuta through increased glucose uptake by the worm or by a reduction in glycogen reserves in the absence of external glucose.