ADRENERGIC RECEPTOR (ADR) GENOTYPE INFLUENCES THE EFFECTS OF STRENGTH TRAINING ON MID-THIGH INTERMUSCULAR FAT
Postgraduate
INTRODUCTION
Sarcopenia
is the age-associated loss of muscle mass and strength. These losses are also
associated with a deterioration in health status and with adverse effects on
functional abilities in the elderly (1). Aging adversely affects the quality,
as well as the quantity, of skeletal muscle (SM). For example, in African
American women, fat infiltration of muscle increases with age (2), leading to
the accumulation of intermuscular fat
(IMF) and the development of low density muscle (LDM). Elevated levels of thigh IMF have been linked to insulin
resistance in muscle and to the development of type 2 diabetes (3, 4). In
addition, higher LDM is associated with
lower muscle strength (5), with poorer leg function (6) and with greater
incidence of mobility limitations in the elderly (7).
Despite
this relationship of limb IMF and LDM to health and functional status and that
ST is now commonly prescribed for the prevention and treatment of sarcopenia
(8), little or no information is available on the effects of ST on limb IMF or
LDM. In this regard, Sipila et al. (9)
reported a reduced percent of thigh IMF in response to ST, but no information
on absolute IMF change was provided. The reduced percentage of fat may have just
been due to the increase in thigh muscle mass, which lowers the percent of fat
tissue, even if the total mass of fat doesn’t change. One study showed that the combination of ST
with aerobic exercise training decreased LDM (10) and another study showed that
ST together with a low-calorie diet reduced thigh IMF
(11). Unfortunately, neither of these two studies can
tell us the independent effects of ST on IMF or LDM. Thus, the first purpose of this study was to
examine the effects of ST on IMF and LDM.
ST has been
showed to increase sympathetic nerve activity (12). Norepinephrine (NE) derived
from sympathetic nerves regulates lipolysis by binding to stimulatory
adrenergic receptors (ADR) (β1, 2, or 3), mainly ADRβ2 in skeletal muscle (13),
and inhibitory ADRα2b receptors. The
balance between ADRβ2 and ADRα2b can thus determine the relative efficacy of NE
as a lipolytic hormone. ADRβ2 Gln27Glu polymorphisms have been associated with
fat adaptation to exercise training in some
(14, 15), but not all studies (16). The combined effects of ADRβ2 Gln27Glu and
ADRα2b Glu12/Glu9 polymorphisms on the total body fat
reduction induced by aerobic exercise training (15). However, studies
investigating genotype influences of ST effects on fat phenotypes are
unavailable. Thus, the second purpose of this study was to investigate ADR
genotype influences on IMF and LDM responses to ST. We hypothesized that ST
will significantly reduce IMF and LDM, and ADRβ2 and ADRα2b gene variants will
significantly influence these effects.