Clenbuterol, a beta 2-adrenoceptor agonist, has therapeutic potential for the treatment of muscle-wasting diseases, yet its effects, especially at the single-fiber level, have not been fully characterized. Male C57BL/10 mice were allocated to three groups: Control-Treated mice were administered clenbuterol (2 mg.kg-1. day-1) via their drinking water for 15 wk; Trained-Treated mice underwent low-intensity training (unweighted swimming, 5 days/wk, 1 h/day) in addition to receiving clenbuterol; and Control mice were sedentary and untreated. Contractile characteristics were determined on membrane-permeabilized fibers from the extensor digitorum longus (EDL) and soleus muscles. Fast fibers from the EDL and soleus muscles of Treated mice exhibited decreases in Ca2+ sensitivity. Endurance exercise offset clenbuterol's effects, demonstrated by similar Ca2+ sensitivities in the Trained-Treated and Control groups. Long-term clenbuterol treatment did not affect the normalized maximal tension of fast or slow fibers but increased the proportion of fast fibers in the soleus muscle. Training increased the proportion of fibers with high and intermediate succinate dehydrogenase activity in the EDL and soleus muscles, respectively. If clenbuterol is to be used for treating muscle-wasting disorders, some form of low-intensity exercise might be encouraged such that potentially deleterious slow-to-fast fiber type transformations are minimized. Indeed, in the mouse, low-intensity exercise appears to prevent these effects.

We investigated the contractile activation characteristics of single membrane‐permeabilized fibres from the following muscles from humans: the levator palpebrae superioris (LPS), an extraocular muscle; the orbicularis oculi (OO), a facial muscle; and the vastus lateralis (VL), a major muscle of the thigh. Single permeabilized muscle fibres were isolated from each of the different muscles, attached to a sensitive force transducer and activated by rapid immersion in buffered solutions of varying [Ca2+] and [Sr2+]. Fibres were allocated into discrete populations based on their contractile characteristics, including their differential force responses during Ca2+ and Sr2+ activation. With the exception of one fibre from the LPS, all 152 fibres sampled from the three different human muscles could be classified into either population I (slow, type I) or population II (fast, type II) based on their force‐pCa(pSr) relations. The LPS muscle fibre which was unable to be classified into the two major fibre populations displayed a combination of the typical force‐pCa(pSr) relations for mammalian fast and slow muscle fibres. Although fibres from the LPS, OO and VL muscles had similar differential sensitivities to Ca2+and Sr2+, the steepness of the force‐pCa(pSr) curves for fibres from the LPS and OO muscles were highly variable compared with those for fibres from the VL muscle. Specific forces (N cm−2) of the smaller diameter fibres from the LPS and OO muscles were significantly lower than those of fibres from the VL muscle. The differences in the contractile activation characteristics between fibres from the VL muscle and those of fibres from facial (OO) muscles and extraocular (LPS) muscles, reflect the differences in their fibre composition that are responsible for their functional specificity.

1. Considerable debate exists as to whether the properties of diaphragm muscles can be modified by training. As the diaphragm is chronically activated during normal respiration, it is of interest to determine whether this muscle is resistant to further modification by exercise. The aim of this study was to investigate the contractile activation characteristics of single skinned muscle fibres from the diaphragm of both CONTROL and TRAINED rats. 2. Male rats were subjected to a 20 week high-intensity endurance exercise training programme that consisted of running on a motorized treadmill, 5 days/week, 90-120 min/day, 27-30 m/min, up a 20 degrees incline. At the conclusion of training, rats were killed with an overdose of ether and costal regions of the diaphragm were removed and stored in a glycerol-based skinning solution at -20 degrees C. 3. Single skinned (membrane-permeabilized) diaphragm muscle fibres were attached to a sensitive force transducer and activated in Ca(2+)- and Sr(2+)-buffered solutions in order to determine relative force-pCa and force-pSr characteristics. Fibres were allocated into discrete groups (population I, population II, intermediate, mixed) on the basis of their physiological (contractile) properties. 4. Population I (slow-twitch) fibres from the diaphragm of TRAINED rats exhibited a reduced sensitivity to Ca2+ (indicating a rightward shift of the force-pCa relationship) compared to those diaphragm fibres from CONTROL animals. An increased number of population II (fast-twitch) fibres were sampled from TRAINED rats, however, training did not affect the activation properties of these fibres.(ABSTRACT TRUNCATED AT 250 WORDS)