Fatigue and metabolism of frog muscle fibers during stimulation and in response to caffeine.
Maneno muhimu
Kikemikali
Tension and metabolite concentrations were measured in single frog muscle fibers at 15 degrees C in vitro, in response to electrical stimulation or to immersion in caffeine- or potassium chloride-Ringer. Sarcomere length equaled 2.3 micrometers. Interrupted stimulation for 150 s at 20 Hz or stimulation for 7.5 min at 1 Hz was followed by at least 20 min of fatigue, evidenced by a reduced 200-ms test contraction. Fatigued fibers contracted maximally in potassium chloride- or caffeine-Ringer. They had high lactate and glucose 6-phosphate concentrations and a reduced phosphocreatine (PCr) concentration. Adenosine 5'-triphosphate (ATP) concentration was approximately normal but was markedly reduced by a caffeine contracture. A plot of PCr consumption against the tension-time integral at different stimulation frequencies (25, 35, or 50 Hz) and durations had an intercept of 25.5 nmol PCr/mg protein at time zero and a corrected slope of 0.65 nmol approximately P/mg protein per kg . s . cm-2. Prolonged fatigue is not due to energy exhaustion or to the inability of muscle fibers to consume residual ATP but probably arises from long-lasting interference in excitation-contraction coupling, which can be reversed by KCl- or caffeine-induced release of Ca2+ from intracellular stores.