HEXOSEMONOPHOSPHATE CHANGES IN MUSCLE IN RELATION TO RATE OF STIMULATION AND WORK PERFORMED

Abstract

Equal amts. of work and lactic acid (LA) were produced by 60 isometric contractions of isolated frog gastrocnemii at rates of 10-60 per min. but twice as much hexosephosphate (HP) was formed with the rapid rate. Single 10-sec. tetani produced approximately the same amt. of LA and still more HP than 60 twitches at 60 per min. In 60 twitches of an unloaded muscle 1/2 as much HP and 1/5 as much LA was formed as in contractions against the tension lever. Significant delayed formation of LA was not observed after 10-sec. tetani against a spring permitting considerable shortening of the muscle. After-formation of LA but not of HP was observed after short tetani against a weak spring and following 10-sec. tetani against a stiff tension lever. LA formation was proportional to the tension developed, but HP formation per mgm. LA was greatest when very little tension was developed (no load) and least with max. tension. Considering the effects of both rate and work, it is concluded that a muscle can produce the greatest amt. of tension with the least expenditure of glycogen (sum of LA and HP) when it its producing max. tension with single contractions at a slow rate.