Physical activity changes the regulation of mitochondrial respiration in human skeletal muscle

Abstract

This study explores the importance of creatine kinase (CK) in the regulation of muscle mitochondrial respiration in human subjects depending on their level of physical activity. Volunteers were classified as sedentary, active or athletic according to the total activity index as determined by the Baecke questionnaire in combination with maximal oxygen uptake values (peak V(O2), expressed in ml min(-1) kg(-1)). All volunteers underwent a cyclo-ergometric incremental exercise test to estimate their peak V(O2) and V(O2) at the ventilatory threshold (VT). Muscle biopsy samples were taken from the vastus lateralis and mitochondrial respiration was evaluated in an oxygraph cell on saponin permeabilised muscle fibres in the absence (V(0)) or in the presence (V(max)) of saturating [ADP]. While V(0) was similar, V(max) differed among groups (sedentary, 3.7 +/- 0.3, active, 5.9 +/- 0.9 and athletic, 7.9 +/- 0.5 micromol O2 min(-1) (g dry weight)(-1)). V(max) was correlated with peak V(O2) (P < 0.01, r = 0.63) and with V(T) (P < 0.01, r = 0.57). There was a significantly greater degree of coupling between oxidation and phosphorylation (V(max)/V(0)) in the athletic individuals. The mitochondrial K(m) for ADP was significantly higher in athletic subjects (P < 0.01). Mitochondrial CK (mi-CK) activation by addition of creatine induced a marked decrease in K(m) in athletic individuals only, indicative of an efficient coupling of mi-CK to ADP rephosphorylation in the athletic subjects only. It is suggested that increasing aerobic performance requires an enhancement of both muscle oxidative capacity and mechanisms of respiratory control, attesting to the importance of temporal co-ordination of energy fluxes by CK for higher efficacy.

Figures

Figure 1. Oxygen consumption rates of skinned fibres plotted as a function of ADP concentration

Respiration of fibres taken from athletic (▪), active (•) and sedentary (▴) individuals was measured in the absence (A) or presence (B) of 20 mm creatine. Data obtained were fitted with a Michaelis-Menten equation.

Figure 2. Basal and maximal mitochondrial respiratory rate in saponin-treated fibres

Muscle fibre bundles were taken from the vastus lateralis muscle of sedentary (SED), active (ACT) and athletic (ATH) subjects. Basal (V̇0) and maximal (V̇max) ADP-stimulated respiration (expressed in μmol O2 min−1 (g dry weight)−1) was measured in saponin-skinned fibres (A); the acceptor control ratio (ACR, V̇max/V̇0) represents the coupling between oxidation and phosphorylation (B). Data are presented as means ± s.e.m. Significantly different from SED: * P < 0.05, *** P < 0.001; significantly different from ATH: § P < 0.05.

Figure 3. Relation between oxidative capacities of skeletal muscle and exercise oxygen uptake

Maximal mitochondrial respiration (V̇max) was significantly correlated with peak oxygen uptake (peak V̇O2) (A, P < 0.01, r = 0.63) and with oxygen uptake at the ventilatory threshold (V̇O2 at VT; B, P < 0.01, r = 0.57) in normal subjects. Symbols indicating data points from individuals in each group: sedentary, ▴; active, ♦; athletic, ▵.

Figure 4. ADP sensitivity of mitochondrial respiration in the presence or the absence of creatine in skeletal muscle

ADP-stimulated respiration was measured as a function of [ADP] with or without creatine (20 mm) in skinned fibres from the vastus lateralis muscle of sedentary (SED), active (ACT) and athletic subjects (ATH). The apparent Km for ADP was calculated using a non-linear fitting of the Michaelis-Menten equation. Data are means ± s.e.m. Significantly different from ATH, §§ P < 0.01; significantly different from Km without creatine for ATH, †P < 0.05.