Chemical energy usage during isometric twitches of frog sartorius muscle intoxicated with an isomer of creatine, beta-guanidinopropionate.

Frogs were injected for several weeks with beta-guanidinopropionate, an isomer of creatine. Their sartorius muscles were isolated, poisoned with iodoacetate and stimulated isometrically with 75 shocks/min in nitrogen until rigor. In comparison with sartorius muscles of untreated frogs, they contained more free creatine and less phosphocreatine, but the same content in total creatine and ATP. They also contained beta-guanidinopropionate both free and phosphorylated. However, muscles in rigor contained the same concentration of the phosphorylated form as resting muscles, i.e., phospho-beta-guanidinopropionate was not split during contraction. The number of twitches performed before rigor was decreased. There was no change in the chemical energy usage (sum of phosphocreatine breakdown and twice ATP breakdown) per twitch.