title: Fatigue-induced alterations in Ca2+ and caffeine sensitivities of skinned muscle fibers.
abstract: The purpose of this investigation was to examine the Ca2+ and caffeine sensitivities of skinned skeletal muscle fibers after fatigue. Single frog semitendinosus fibers were chemically skinned in either a rested state or after tetanic contractions (80 Hz, 100 ms) evoked at 2 s-1 for 5 min. This protocol reduced tetanic force to 1.8 +/- 0.2% of control. Maximal Ca(2+)-activated force (F0, 20 degrees C) was not significantly different between rested and fatigued fibers. However, the concentration of Ca2+ required to evoke 50% of F0 was significantly lower in the fatigued fibers (1.80 +/- 0.18 vs. 1.33 +/- 0.16 microM; P < 0.05), an effect that persisted as the skinned fiber was allowed to incubate in the relaxing solution for > 90 min. The addition of caffeine (25 mM) after Ca2+ loading of the sarcoplasmic reticulum (SR) for periods of 5-30 s (0.25 microM free [Ca2+]) evoked smaller contractures in fatigued fibers than in rested fibers. However, when the loading period was prolonged (60-240 s), force developed after caffeine application was not significantly different between conditions. This suggests that the rate, but not the maximal capacity of Ca2+ loading by the SR, is reduced by fatigue. After Ca2+ loading (120 s), the minimal caffeine concentration required to evoke a contracture in fatigued fibers (5.7 +/- 0.3 mM) was significantly greater than that of control fibers (3.1 +/- 0.4 mM), an effect that persisted with prolonged incubation of the skinned fibers. In addition, the rate of force increase in response to 8 mM caffeine was reduced in fatigued fibers by 41%.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Muscular Function Laboratory, Virginia Polytechnic Institute and State University, Blacksburg 24061-0326.
authors.forename: J H
authors.initials: JH
authors.lastname: Williams
authors.forename: C W
authors.initials: CW
authors.lastname: Ward
authors.forename: G A
authors.initials: GA
authors.lastname: Klug
mesh_terms: D000818:Animals; D002110:Caffeine / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology*; D066298:In Vitro Techniques; D007474:Ion Exchange; D007537:Isometric Contraction / Q000502:physiology; D007700:Kinetics; D008297:Male; D009132:Muscles / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D011894:Rana pipiens; D012519:Sarcoplasmic Reticulum / Q000502:physiology
chemical_list: D002110:Caffeine; D002118:Calcium