Skeletal muscle glucose uptake requires delivery of glucose to the sarcolem
ma, transport across the sarcolemma, and the irreversible phosphorylation o
f glucose by hexokinase (HK) inside the cell. Here, a novel method was used
in the conscious rat to address the roles of these three steps in controll
ing the rate of glucose uptake in soleus, a muscle comprised of type I fibe
rs, and two muscles comprised of type II fibers. Experiments were performed
on conscious rats under basal conditions or during hyperinsulinemic euglyc
emic clamps. Rats received primed, constant infusions of 3-O-methyl-[H-3]gl
ucose (3-O-MG) and [1-C-14]mannitol. Total muscle glucose concentration and
the steady-state ratio of intracellular to extracellular 3-O-MG concentrat
ion, which distributes based on the transsarcolemmal glucose gradient (TSGG
), were used to calculate glucose concentrations at the inner and outer sar
colemmal surfaces ([G](im) and [G](om), respectively) in muscle. Muscle glu
cose uptake was much lower in muscle comprised of type II fibers than in so
leus under both basal and insulin-stimulated conditions. Under all conditio
ns, the TSGG in type II muscle exceeded that in soleus, indicating that glu
cose transport plays a more important role to limit glucose uptake in type
II muscle. Although hyperinsulinemia increased [G](im) in soleus, indicatin
g that phosphorylation was a limiting factor, type II muscle was limited pr
imarily by glucose delivery and glucose transport. In conclusion, the relat
ive importance of glucose delivery, transport, and phosphorylation in contr
olling the rate of insulin-stimulated muscle glucose uptake varies between
muscle fiber types, with glucose delivery and transport being the primary l
imiting factors in type II muscle.