ENDURANCE TRAINING EFFECTS ON THE CONTRACTILE ACTIVATION CHARACTERISTICS OF SINGLE MUSCLE-FIBERS FROM THE RAT DIAPHRAGM

1. Considerable debate exists as to whether the properties of diaphrag m muscles can be modified by training. As the diaphragm is chronically activated during normal respiration, it is of interest to determine w hether this muscle is resistant to further modification by exercise. T he aim of this study was to investigate the contractile activation cha racteristics of single skinned muscle fibres from tbe diaphragm of bot h CONTROL and TRAINED rats. 2. Male rats were subjected to a 20 week h igh-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 wer e 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 muscl e fibres were attached to a sensitive force transducer and activated i n Ca2+- and Sr2+-buffered solutions in order to determine relative for ce-pCa and force-pSr characteristics. Fibres were allocated into discr ete groups (population I, population II, intermediate, mixed) on the b asis of their physiological (contractile) properties, 4. Population I (slow-twitch) fibres from the diaphragm of TRAINED rats exhibited a re duced sensitivity to Ca2+ (indicating a rightward shirt of the force-p Ca relationship) compared to those diaphragm fibres from CONTROL anima ls, An increased number of population II (fast-twitch) fibres were sam pled from TRAINED rats, however, training did not affect the activatio n properties of these fibres, 5. The fact that training-induced altera tions in the contractile characteristics did occur in one group of fib res indicates that the intensity of training was sufficient to induce some modifications to the costal fibres of the diaphragm muscle, Howev er, the lack of alterations to the contractile characteristics of the population II (fast-twitch) fibres may also suggest that the overall p roperties of the diaphragm are relatively resistant to modification de spite high-intensity training.