Anabolic effects of the beta 2-adrenoceptor agonist salmeterol are dependent on route of administration

It is reported that a long duration of action is required for beta 2-adrenoceptor agonists to evoke an anabolic response. In the present study, we compare the potency of clenbuterol with that of the new long-acting compound salmeterol, when given at equimolar doses to female Wistar rats by different routes of administration. Given orally for 10 days, salmeterol had no effect on growth at a dose of 120 micrograms/day, whereas at 2.4 mg/day the drug caused significant increases in body and carcass weight and in the mass of the mixed-fiber gastrocnemius/plantaris and tibialis anterior muscles, but there were no increases in the slow-twitch soleus muscles. A similar growth response was seen when clenbuterol was given orally at a dose of only 97 micrograms/day, with an additional response seen in soleus muscle at 1.9 mg/day. Thus clenbuterol was more potent than salmeterol when given by this route of administration. When the drugs were infused by osmotic minipump, both salmeterol (130 micrograms/day) and clenbuterol (100 micrograms/day) caused increases in body weight gain and in the weights of the mixed-fiber muscles, with the most dramatic effect of infusion being to greatly increase the anabolic effect of salmeterol in soleus muscle. A single intraperitoneal injection of salmeterol (53 micrograms) or clenbuterol (40 micrograms) caused a similar rapid increase in the concentration of adenosine 3',5'-cyclic monophosphate in gastrocnemius muscle. These results indicate that the potency of salmeterol in vivo is dependent on its route of administration and that slow-twitch muscles are less sensitive than mixed-fiber muscles to the anabolic effects of beta 2-adrenoceptor agonists.