Volume change and pressure development in muscle during contraction.

Volume change and pressure development in muscle during contraction. 1967.-Volume changes recorded in isometric and isotonic twitches of gastrocnemius muscle are similar to those recorded in sartorius muscle. The major differences are the greater difficulty in approaching isometric recording conditions in the gastrocnemius muscle and the possibility that a portion of the volume change could be due to the compression resulting from internal pressure development. The volume change has been shown to be a function of the length of the muscle and of the amount of tension developed at a given length. Since these factors influence (and are influenced by) the state of the contractile component, it is fair to conclude that at least a portion of the recorded volume changes are a function of the state of the contractile component. There is not sufficient agreement at present concerning the molecular changes accompanying a single twitch of striated muscle to ascribe the volume changes to particular molecular events. gastrocnemius muscle; contractile component; striated muscle I N .4 RECENT PASER, Baskin and Paolini (4) described the application of a new method of measuring the small volume changes in active muscle to the frog sartorius muscle. This method has now been applied to the more complex case of gastrocnemius muscle, in which the nonparallel arrangement of fibers results in the development of internal pressure during contraction. Such a pressure causes a compression of fluid within the muscle. Hill (6) has estimated this pressure to be approximately 200 mm Hg in muscles of l-2 g weight; the resulting compression would introduce a volume decrease of about 1 X 1 O-" ml. Previous experiments (l-4) using frog sartorius muscle have established the existence of a rapid volume increase followed by a slower decrease during an isometric twitch, as well as a volume increase accompanying shortening during an isotonic twitch. It was not known whether the volume change during contraction of gastrocnemius muscle would be characteristically different from sartorius muscle; it was expected that an additional volume decrease component corresponding to internal pressure development would be observed. Meyerhof (10) f ound the ratio p (the ratio of the area under an isometric twitch length-tension diagram to the product of maximum tension and reference length, an empirical value which estimates the percent of theoretical maximum work attained by the muscle) to be quite different for whole gastrocnemius and sartorius muscles. But Mashimo (9) has measured p …