REGULATION OF MUSCLE GLYCOGENOLYTIC FLUX DURING INTENSE AEROBIC EXERCISE AFTER CAFFEINE INGESTION

This study examined the effects of caffeine (Caf) ingestion on muscle glycogen use and the regulation of muscle glycogen phosphorylase (Phos ) activity during intense aerobic exercise. In two separate trials, 12 untrained males ingested either placebo (P1) or Caf (9 mg/kg body wt) 1 h before cycling at 80% maximum O-2 consumption ((V) over dot o(2ma x)) for 15 min. Muscle biopsies were obtained from the vastus laterali s at 0, 3, and 15 min of exercise. In this study, glycogen ''sparing'' was defined as a 10% or greater reduction in muscle glycogen use duri ng exercise after Caf ingestion compared with P1. Muscle glycogen use decreased by 28% (P1 255 +/- 38 vs. Caf184 +/- 24 mmol/kg dry muscle) after Caf in six subjects [glycogen sparers (Sp)] but was unaffected b y Caf in six other subjects [nonsparers (NSp), P1 210 +/- 35 vs. Caf 2 14 +/- 37 mmol/kg dry muscle]. In both groups, Caf significantly incre ased resting free fatty acid concentration, significantly increased ep inephrine concentration by twofold duping exercise, and increased the Phos a mole fraction at 3 min of exercise compared with P1, although n ot significantly. Caf improved the energy status of the muscle during exercise in the Sp group: muscle phosphocreatine (PCr) degradation was significantly reduced (P1 47.9 +/- 3.6 vs. Caf 40.4 +/- 6.7 mmol/kg d ry muscle at 3 min) and the accumulations of free ADP and free AMP (P1 6.8 +/- 1.3 vs. Caf 3.1 +/- 1.4 mu mol/kg dry muscle at 3 min; P1 8.7 +/- 0.8 vs. Caf 4.7 +/- 1.1 mu mol/kg dry muscle at 15 min) were sign ificantly reduced. Caf had no effect on these measurements in the NSp group. It is concluded that the Caf-induced decrease in flux through P hos (glycogen-sparing effect) is mediated via an improved energy statu s of the muscle in the early stages of intense aerobic exercise. This may be related to an increased availability of fat and/or ability of m itochondria to oxidize fat during exercise preceded by Caf ingestion. It is presently unknown why the glycogen-sparing effect of Caf does no t occur in all untrained individuals during intense aerobic exercise.