Post-tetanic responses in creatine-depleted rat EDL muscle

The total creatine content of rat extensor digitorum longus (EDL) muscles is reduced from 26.7 mumol X g-1 to 5.2 mumol X g-1 after 4 weeks on a diet containing beta-guanidinopropionate. The resting muscles of these animals contain only 5% of the normal creatine phosphate and about 30% of the normal adenosine triphosphate (ATP). Isometric twitch and brief tetanic responses are not significantly different from normal. However, following a 1-second tetanus, the normal twitch potentiation (+80%) is reversed in depleted muscles (-40%). The initial suppression lasts 1-2 seconds and is followed by a small delayed potentiation (+25%). Relaxation rate, which is normally increased following a tetanus, is considerably reduced in depleted muscles. Reversal of potentiation appears to be due to a suppression of twitch activation. The suppression may result from a transient fall in delta GATP, which is thought to be a critical parameter in the uptake of Ca++ by the sarcoplasmic reticulum (SR).