FAST AND SLOW SKELETAL-MUSCLES - CONTRACTILITY EVALUATED BY PAIRED STIMULI IN MICE

Abstract

A technique for in vitro study of mouse skeletal muscle was evaluated to determine if paired stimuli would allow estimation of new time parameters of contraction. Conventional tension measurements were obtained, plus the time measurements from the paired stimulus studies. The muscles were also subjected to fatigue by repetitive tetanization. A fast muscle (extensor digitorum longus or plantaris) and a slow muscle (soleus) were studied in each preparation. Studies were conducted at physiologic (35.degree. C) and at nonphysiologic (20.degree. C) temperatures. The isometric in vitro method allows subdivision of results into 3 categories of effect due to treatment related to the functional muscle compartments: chronotropic-excitation contraction coupling mechanisms, inotropic-contractile elements, and metabolic (fatigue)-energy supply mechanisms. When results at 20.degree. C were compared with physiologic temperature, a negative chronotropic effect was observed for all parameters in all 3 muscles. A positive inotropic effect was observed for twitch with no change or a negative effect for all other tension parameters. The lower temperature produced slight protection from fatigue. Models were developed to demonstrate how data from isometric studies fit the original concepts developed using isotonic methods. The model and method should prove of value in detecting the acute effect of drugs or of other therapy imposed on skeletal muscles.