A diurnal pattern in the uptake of uridine was displayed by the rat cestode Hymenolepis diminuta. No periodicity in the uptake of uracil was observed over a 48-hr period. A high level of uridine uptake occurred at 6 PM. when 10-day-old worms were in a posterior location in the intestine of rats maintained on a 6 PM.-6 AM. dark cycle-feeding regime, while low levels of uptake were correlated with an anteriad location at 6 AM. The lowest levels of uridine uptake were recorded at noon. Coincubation with thymine caused a stimulation of uridine uptake at midnight, 6 AM., and noon when uridine's transport rate in the absence of thymine was low. Stimulation was not demonstrable when uridine's transport rate was at its highest at 6 PM. Preincubation with uridine did not alter the diurnal uridine uptake pattern. This diurnal phenomenon is an important consideration essential to future studies on transport in parasitic and other organisms.
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