Figure 1.

Changes in β₂-AR expression in hypertrophied and atrophied skeletal muscles. (a)  β₂-AR stimulation using anabolic drugs downregulates β₂-AR expression in hypertrophied fast-twitch muscles but not in slow-twitch muscles [4, 7, 8, 14–17]. (b) Exercise training such as endurance training upregulates β₂-AR expression in exercise-recruited slow-twitch muscles, whereas no changes or downregulations are observed in fast-twitch muscles [18, 19], although muscle mass is not altered. However, although exercise training such as isometric strength training induces muscle hypertrophy, there is no insight regarding the effects of such exercise on β₂-AR expression. The differential effects of types of exercise training on physiological responses such as β₂-AR expression and muscle hypertrophy should be clarified in more detailed and are currently being investigated by our group. (c) Catabolic hormones or drugs such as glucocorticoids downregulate β₂-AR expression in nonatrophied slow-twitch muscles but not fast-twitch muscles [16, 20, 21]. (d) Muscle disuse downregulates β₂-AR expression in atrophied slow-twitch muscle, whereas no changes or upregulation of receptor expression are observed in fast-twitch muscles [14, 22]. Up arrow (open arrow): upregulation of β₂-AR expression; down arrow (filled arrow): downregulation of β₂-AR expression; lateral arrow (shade arrow): no change.