Evidence for differential regulation of lactate metabolic properties in aged and unloaded rat skeletal muscle

Abstract

Skeletal muscles of elderly individuals show fatigue resistance and reduced lactate accumulation compared with those of young subjects during activities that recruit a small amount of muscle mass. To explore the mechanism underlying the functional changes in aged muscle, we focused on lactate metabolic properties, including monocarboxylate transporter (MCT) 1 and MCT4, in muscles from old and young control rats and hindlimb-suspended young rats. MCT1 expression was lower in soleus (SOL) of old rats than in SOL of young control rats, but was similar in young control and hindlimb-suspended rats. MCT4 expression was lower in extensor digitorum longus (EDL) of old rats than in that of young control rats, but did not differ between young control and hindlimb-suspended rats. The ratio of lactate dehydrogenase to citrate synthase activities was higher in SOL of hindlimb-suspended and old rats than in SOL of young control rats, and was lower in EDL of old rats than in those of young control and hindlimb-suspended rats. Our data suggest that aging causes metabolic changes that can reduce lactate accumulation during exercise and increase fatigue resistance in skeletal muscle, and that these changes result from aging rather than from inactivity.

Introduction

Both aging and muscle unloading induce morphological and metabolic changes in skeletal muscle and lead to attenuated measures of exercise performance, such as maximal workload and maximal oxygen uptake (Capelli et al., 2006, Convertino, 1997, Ferri et al., 2007, Fleg et al., 2005, Mattern et al., 2003). The reduced functional capacity of aged skeletal muscle may be a consequence not only of aging but also of age-related physical inactivity. However, in human studies, older individuals are more fatigue resistant than younger subjects during activities that recruit a relatively small amount of muscle mass (Bilodeau et al., 2001, Ditor and Hicks, 2000, Hunter et al., 2004, Hunter et al., 2005, Kent-Braun et al., 2002, Lanza et al., 2004). In humans, there is a greater reduction in intracellular pH in young adults than in older individuals during prolonged muscle contraction (Kent-Braun et al., 2002, Lanza et al., 2005). Furthermore, aging is associated with reduced lactate efflux during muscle contraction (Hepple et al., 2003), and this becomes even more severely impaired between late middle age and senescence than between young adulthood and late middle age (Hepple et al., 2004). In contrast, the intracellular lactate concentration increases more during prolonged muscle contraction in unloaded rat muscle than in control muscle (Grichko et al., 2000, Witzmann et al., 1983). These inconsistencies between aged and immobilized muscles suggest that aging, but not age-related inactivity, is associated with reduced lactate accumulation and therefore with increased fatigue resistance in aged skeletal muscle.

During contraction, even under the fully aerobic condition, lactate is produced continuously by the glycolytic system in muscle cells. The intracellular system, called intracellular lactate shuttle, transports the produced lactate into mitochondria to be oxidized to reproduce ATP (Brooks, 2000, Brooks, 2002, Gladden, 2004). Importantly, lactate can be transported across the plasma membrane from glycolytic to oxidative fibers within a working muscle, and from fast-type to slow-type muscles as well. This fiber-to-fiber and muscle-to-muscle transport system, i.e. cell–cell lactate shuttle, facilitates lactate utilization as a respiratory fuel (Brooks, 2000, Brooks, 2002, Gladden, 2004, Juel and Halestrap, 1999). Lactate flux across the plasma membrane is mediated by specific isoforms of monocarboxylate transporter (MCT), MCT1 and MCT4. MCT1, which is abundant in oxidative muscle fibers, facilitates uptake of lactate into the fibers (Baker et al., 1998, McCullagh et al., 1996, McCullagh et al., 1997), and in contrast, MCT4 is predominant in glycolytic muscle fibers and is relevant in lactate efflux from fibers (Bonen et al., 2000, Juel and Halestrap, 1999, Wilson et al., 1998). These MCTs have been implicated in enhanced lactate transport by exercise training in rat (Baker et al., 1998) and human (Bonen et al., 1998, Dubouchaud et al., 2000) skeletal muscles. In fact, MCTs possess high K_m and V_max values for sarcolemmal lactate transport under physiological lactate concentrations (Brooks, 2000).

Based on the increased fatigue resistance and reduced lactate accumulation in aged skeletal muscle, we hypothesized that aging, rather than muscle unloading, is associated with changes in the metabolic properties relevant to lactate shuttle in skeletal muscle, leading to a reduction in lactate accumulation during muscle contraction. To test this hypothesis, we quantified the expression of MCT1 and MCT4 proteins, pyruvate kinase (PK), lactate dehydrogenase (LDH), and citrate synthase (CS) activities in aged skeletal muscles, and compared them with those in young unloaded and young normal skeletal muscles. PK activity is an estimate of anaerobic capacity (Layman et al., 1981). LDH activity is also directly related to the glycolytic capacity in skeletal muscle (Powers et al., 1992). CS, the flux-generating enzyme of the tricarboxylic acid cycle, is a standard marker of oxidative capacity (Bass et al., 1969, Powers et al., 1992).