Forearm muscle metabolism studied using 31P-MRS during progressive exercise to fatigue after Acz administration

The effects of acetazolamide (Acz)-induced carbonic anhydrase inhibition (CAI) on muscle intracellular thresholds (T) for intracellular pH (pH(i)) and inorganic phosphate-to-phosphate creatine ratio (P-i/PCr) and the plasma lactate (La-) threshold were examined in nine adult male subjects performing forearm wrist flexion exercise to fatigue. Exercise consisted of raising and lowering (1-s contraction, 1-s relaxation) a cylinder whose volume increased at a rate of 200 ml/min. The protocol was performed during control (Con) and after 45 min of CAI with Act (10 mg/kg body wt iv). T-PHi and T-Pj/PCr, determined using P-31- labeled magnetic resonance spectroscopy (MRS), were similar in Act (722 +/- 50 and 796 +/- 75 mW, respectively) and Con (855 +/- 211 and 835 +/- 235 mW, respectively). The pH(i) was similar at end-exercise (6.38 +/- 0.10 Act and 6.43 +/- 0.22 Con), but pH(i) recovery was slowed in Acz. In a separate experi ment, blood was sampled from a deep arm vein at the elbow for determination of plasma lactate concentration ([La-](pl)) and T-La-. [La-](pl) was lower (P < 0.05) in Act than Con (3.7 +/- 1.7 vs. 5.0 +/- 1.7 mnnol/l) at end-exercise and in early recovery, but T-La- was higher(1,433 +/- 243 vs. 1,041 +/- 414 mW, respectively). These data suggest that the lower [La-](pl) seen with CAI was not due to a delayed onset or rate ofmusde La- accumulation but may be related to impaired La- removal from muscle.