THE INTERMEDIATE MUSCLE FIBER OF RATS AND GUINEA PIGS

Abstract

The soleus, plantaris and gastrocnemius muscles of 60 Sprague-Dawley rats and 70 Hartley guinea pigs were studied histochemically. In the plantaris and gastrocnemius muscles, myosin adenosine triphosphatase activity differentiated intermediate fibers from red and white fibers as determined by malate dehydrogenase and succinate dehydrogenase and reduced nicotinamide adenine dinucleotide diaphorase activities. Contrary to what is commonly reported, red fibers could not be distinguished from white fibers on the basis of myosin adenosine triphosphatase activity as is commonly reported. The intermediate fiber was characterized by minimal menadione-mediated α-glycerophosphate dehydrogenase, phosphorylase and myosin adenosine triphosphatase activity and moderate malate dehydrogenase and succinate dehydrogenase and reduced nicotinamide adenine dinucleotide diaphorase activities. It is suggested that fibers with intermediate oxidative enzyme activity are physiologically slow, white fibers are fast and red fibers are moderately slow or even fast contracting fibers.