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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)
AuthorsJ H Williams, C W Ward, G A Klug
JournalJournal of applied physiology (Bethesda, Md. : 1985) (J Appl Physiol (1985)) Vol. 75 Issue 2 Pg. 586-93 (Aug 1993) ISSN: 8750-7587 [Print] UNITED STATES
PMID8226456 (Publication Type: Journal Article, Research Support, U.S. Gov't, P.H.S.)
Chemical References
  • Caffeine
  • Calcium
Topics
  • Animals
  • Caffeine (pharmacology)
  • Calcium (metabolism, pharmacology)
  • In Vitro Techniques
  • Ion Exchange
  • Isometric Contraction (physiology)
  • Kinetics
  • Male
  • Muscles (cytology, drug effects, physiology)
  • Rana pipiens
  • Sarcoplasmic Reticulum (physiology)

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