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Relationship between intracellular PO₂ recovery kinetics and fatigability in isolated single frog myocytes

University of California-San Diego, Department of Medicine, Physiology Division, La Jolla, California

Submitted 1 April 2004 ; accepted in final form 25 January 2005

ABSTRACT

In single frog skeletal myocytes, a linear relationship exists between "fatigability" and oxidative capacity. The purpose of this investigation was to study the relationship between the intracellular PO₂ (PI_O2) offset kinetics and fatigability in single Xenopus laevis myocytes to test the hypothesis that PI_O2 offset kinetics would be related linearly with myocyte fatigability and, by inference, oxidative capacity. Individual myocytes (n = 30) isolated from lumbrical muscle were subjected to a 2-min bout of isometric peak tetanic contractions at either 0.25- or 0.33-Hz frequency while PI_O2 was measured continuously via phosphorescence quenching techniques. The mean response time (MRT; time to 63% of the overall response) for PI_O2 recovery from contracting values to resting baseline was calculated. After the initial square-wave constant-frequency contraction trial, each cell performed an incremental contraction protocol [i.e., frequency increase every 2 min from 0.167, 0.25, 0.33, 0.5, 1.0, and 2.0 Hz until peak tension fell below 50% of initial values (TTF)]. TTF values ranged from 3.39 to 10.04 min for the myocytes. The PI_O2 recovery MRT ranged from 26 to 146 s. A significant (P < 0.05), negative relationship (MRT = –12.68TTF + 168.3, r² = 0.605) between TTF and PI_O2 recovery MRT existed. These data demonstrate a significant correlation between fatigability and oxidative phosphorylation recovery kinetics consistent with the notion that oxidative capacity determines, in part, the speed with which skeletal muscle can recover energetically to alterations in metabolic demand.

muscle fiber; fatigue; metabolic control; oxidative capacity

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