Effects of fiber type and training on β-adrenoceptor density in human skeletal muscle

The density and distribution of β-adrenergic receptors in type I and II fibers of human gastrocnemius and quadriceps muscles were characterized in ten healthy sedentary subjects and in a subgroup of six subjects before and after 12 wk of endurance exercise training. Total tissue content of β-receptors was measured in frozen sections of skeletal muscle biopsies incubated with ¹²⁵I-labelled cyanopindolol in the presence and absence of 10^-5 M L-propranolol. The relative β-receptor densities of type I and II fibers were delineated autoradiographically. Muscle fiber types were identified in adjacent serial sections by histochemical staining of myofibrillar adenosine-triphosphatase (ATPase) activity. Type I fibers had a threefold greater β-receptor density than type II fibers of the same muscle [P < 0.001; type I-to-type II fiber ratio of β-receptor density was 3.06 ± 0.43 (SD)]. Exercise training elicited a change in muscle fiber subtype composition (+34% type IIa and -42% type IIb; P < 0.05 and P = 0.066, respectively), a 40% increase in citrate synthase activity of skeletal muscle (P = 0.01), and a 23% rise in peak oxygen uptake (P < 0.001). However, no change in total tissue content of β-receptors was observed after exercise training, even when receptor density was adjusted for preconditioning fiber type composition. Thus β-receptor density of type I fibers of human skeletal muscle is threefold greater than that of type II fibers. Enhanced capacity for aerobic metabolism after endurance exercise training is not associated with upregulation of total β-receptor density.