The role of
insulin as a possible mediator of the
beta-adrenergic agonist stimulation of muscle growth was investigated. To exclude possible action of the beta-agonist on the pancreatic release of
insulin, diabetes was induced in rats by a
streptozotocin injection (100 mg/kg).
Insulin levels were almost not detectable in these rats. Feeding either normal diet or diet containing the
beta-adrenergic agonist clenbuterol (10 parts/million) did not alter plasma
insulin concentrations. The effects of
clenbuterol on muscle and
weight gain were determined in diabetic rats given daily
insulin replacement (D + I) and fed either a normal diet or
clenbuterol-treated diet.
Clenbuterol, fed for 1 wk, increased the wet weight of the gastrocnemius, soleus, and extensor digitorum longus muscles (15-23%) in both normal and D + I rats. Although
clenbuterol increased
body weight gain, it did not alter feed consumption and, therefore, feed efficiency (g gain/g food) was improved. Activities of
cathepsin B and N-acetyl-beta-
glucosaminidase, but not
cathepsin D, were elevated in the soleus muscles of
clenbuterol-treated rats. The
clenbuterol-induced increase in muscle growth in the
insulin-replaced diabetic rats indicated that this
beta-adrenergic agonist effect was not mediated by an alteration of circulating levels of
insulin, secondary to beta-agonist action on pancreatic
insulin release.