Functional effects of uridine triphosphate on the atrophied soleus muscle of rat after unloading

The purposes of the study were to determine the effects of a pyrimidine nucleotide, the uridine triphosphate (UTP), on the contractile and histochemical properties of the soleus (SOL) muscle following disuse atrophy due to hindlimb unloading (HU) hypokinesia. UTP was injected either during the HU period (2 weeks) or later during the recovery period. In this latter condition, contractile and histochemical properties were studied after 5, 8, 11, and 15 days of spontaneous recovery. HU induced decreases in the SOL weight, force output (twitch and tetanic tensions), time to peak tension during the twitch, and the percentage of type I fibers. The injection of UTP during the HU period did not counteract the modification in speed-related properties, but the decrease in force output was partly counteracted and the proportion of type II C fibers was increased. When UTP was injected during the recovery periods, force-related properties recovered more rapidly. These results suggest that UTP may reduce the loss of force induced by atrophy.