Diphenylbarbituric acid: its effects on neuromuscular and spinal cord function in the cat

In studies of neuromuscular function in the cat soleus muscle, diphenylbarbituric acid (DPB; 10-80 mg/kg i.v.) depressed both the degree and the duration of posttetanic potentiation (PTP) in a dose-dependent manner. At doses of greater than 20 mg/kg, twitch strength was increased in both indirectly stimulated and directly stimulated chronically denervated preparations, indicating a direct effect of DPB on muscle. In the spinal cord, DPB (10-60 mg/kg) depressed both monosynaptic (2N) and polysynaptic discharges, with flexor and extensor reflexes being similarly affected. In addition, DPB was not selective for isolated 2N as compared with posttetanically potentiated monosynaptic responses. Thus, in its actions on neuromuscular function DPB resembles phenytoin; in the spinal cord the drug resembles phenobarbital. The data suggest that the capacity of a drug to curb high-frequency repetitive discharges is more important than curbing recruitment as exhibited by PTP of the 2N reflex.