EFFECTS OF CLENBUTEROL AND SALBUTAMOL ON TISSUE RUBIDIUM UPTAKE IN-VIVO

In anesthetized rats, injection of the beta(2)-adrenoceptor (beta(2)-A R) agonist clenbuterol (0.45 mu mol/kg) caused a marked stimulation of (RbCl)-Rb-86 (Rb) uptake by skeletal muscle, but had no effect on oth er tissues; soleus muscle showed the largest (144% increase) response. Injection of another beta(2)-AR agonist (salbutamol 0.45 mu mol/kg) h ad no effect on Rb uptake by any tissue except soleus muscle (83%). Bo th agonists increased body (colonic) temperature to the same extent. A 3-day treatment with salbutamol as a dietary admixture had no effect on body weight, muscle mass, or tissue Rb uptake, whereas the same tre atment using clenbuterol produced significant increases in body weight and muscle mass and significant decreases in Rb uptake in three of th e four muscle groups studied; Rb uptake in soleus was not affected. In another experiment, the short-term effect of clenbuterol injection on muscle Rb uptake was found to be resistant to a high dose (20 mg/kg) of the selective beta(2)-AR antagonist ICI 118551. It was concluded th at the selective effects of short-term administration of clenbuterol o n muscle Rb uptake, coupled with its effects over 3 days on Rb uptake and muscle hypertrophy, implicate beta-AR modulation of cation transpo rt (possibly via Na,K-adenosine triphosphatase [ATPase] activity) in t he anabolic effects of clenbuterol on muscle protein deposition. Since the stimulation of Rb uptake by clenbuterol was resistant to high dos es of a selective beta(2)-AR antagonist and since salbutamol had littl e or no effect on muscle hypertrophy or Rb uptake, it is suggested tha t clenbuterol may exert its effects via an atypical beta-AR. Copyright (C) 1995 by W.B. Saunders Company