[Contractile properties of the striated muscle fibres of esophageal muscle in comparison with skeletal muscle in rats].

The contractile properties of the striated oesophageal muscle of the rat were studied in vitro at 35 degrees C and 25 degrees C. They were compared with those of the slow-twitch soleus muscle and the fast-twitch extensor digitorum longus (EDL) muscle of the same animals. The contraction time of the oesophageal muscle is 30 msec (+/- 2.5 msec S.E.) at 35 degrees C. It is little but significantly longer than those of the soleus muscle. It is remarkable that the twitch-tetanus ratio of the oesophageal muscle fibres is approximately twice that of the skeletal muscles. Immediately after a tetanic stimulation the amplitude of a single twitch of the fast EDL is increased (posttetanic potentiation); in the slow soleus muscle it is unchanged or decreased, however. Also the muscle fibres of the oesophagus exhibit a post-tetanic-potentiation, but this is smaller than in EDL. A decrease of the temperature of the bathing solution causes in preparations of the fast EDL a higher tension developed by a single twitch (cold potentiation). This is not detectable in preparations of the slow soleus muscle. The oesophageal muscle shows also a remarkable cold potentiation, but it is smaller than in EDL. An increase of the extracellular K+-concentration evokes a short-lasting contracture in the investigated rat muscles. The mechanical thresholds of the oesophageal muscle fibres are clearly higher than those of the soleus muscle but lower than those of the EDL. The striated oesophageal muscle fibres of the rat in spite of their low speed of contraction show some properties of muscle fibres of the fast twitch type.