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 P-31-nuclear magnetic resonance after brief (<6 s) bursts of contractions. There was no significant change in the ATP cost of either twitch (0.3 mu mol.g(-1).twitch(-1)) or tetanic (2.5 mu mol.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 reamination 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.