DIFFERENT EFFECTS OF GRADUAL VS ACUTE ADENINE-NUCLEOTIDE DEPLETION ONATP COST OF MUSCLE-CONTRACTION

ATP and phosphocreatine (PCr) levels were chronically reduced in rat f ast-twitch muscle by 40 and 87%, respectively, after rats were fed a d iet 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 musc les. ATP cost of contractions was estimated from the PCr changes measu red by gated P-31-nuclear magnetic resonance after brief (<6 s) bursts of contractions. There was no significant change in the ATP cost of e ither 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 chron ic phosphagen depletion due to beta-GPA feeding. In two other groups o f rats, ATP of gastrocnemius muscles was acutely depleted to inosine 5 '-monophosphate (IMP) by tetanic stimulation. IMP reamination was inhi bited during a subsequent 75-min recovery period in one group by hadac idin treatment (100 mg/kg), resulting in 46% ATP depletion compared wi th the similarly stimulated and recovered control group. In contrast t o gradual ATP depletion due to beta-GPA feeding, this acute ATP deplet ion treatment was associated with a 39% decrease in ATP cost of twitch contractions.