Different effects of gradual vs. acute adenine nucleotide depletion on ATP cost of muscle contraction.

ATP and phosphocreatine (PCr) levels were chronically reduced in rat fast-twitch muscle by 40 and 87%, respectively, after rats were fed a diet containing 1% beta-guanidinopropionate (beta-GPA) for 9 wk. Myosin heavy chain distribution of superficial gastrocnemius muscle changed from 22:78% type IIA/type IIB in control to 39:61% in GPA-treated muscles. ATP cost of contractions was estimated from the PCr changes measured by gated 31P-nuclear magnetic resonance after brief (< 6 s) bursts of contractions. There was no significant change in the ATP cost of either twitch (0.3 mumol.g-1.twitch-1) or tetanic (2.5 mumol.g-1.100 ms tetanus-1) contractions in gastrocnemius muscle after chronic phosphagen depletion due to beta-GPA feeding. In two other groups of rats, ATP of gastrocnemius muscles was acutely depleted to inosine 5'-monophosphate (IMP) by tetanic stimulation. IMP reanimation was inhibited during a subsequent 75-min recovery period in one group by hadacidin treatment (100 mg/kg), resulting in 46% ATP depletion compared with the similarly stimulated and recovered control group. In contrast to gradual ATP depletion due to beta-GPA feeding, this acute ATP depletion treatment was associated with a 39% decrease in ATP cost of twitch contractions.