Effect of Contractions in Vitroon Glycogen Content and Glycogen Synthetase Activity in Muscle

The influence of contraction on the content of glycogen in muscle as well as on the activity of the glycogen synthetase enzyme (EC 2.4. 1. 11.)—rate‐limiting for glycogen synthesis—has previously been investigated in vivo. In the present study this influence was studied in vitro , where interference of epinephrine and insulin on the the effects of contraction is more easily excluded. Electrically stimulated intact levator ani muscle preparations from prepubertal rats were used (Arvill, Acta endocr. 1967. 56. suppl. 122). Glycogen content decreased during 15 min of intermittent tetanization (10 slmin). There was no change in synthetase activity during the contractions. For the period following contractions an increase in the per cent active I‐form of the enzyme was found after 1, 2, 10 and 180 min. Resynthesis of glycogen was shown after 30–180 min. An inverse relation between % I‐form and glycogen content was found when data from 1–10 min after contractions were plotted together with the data from unstirnulated muscles. During repeated single twitches (1/s) for 60 min there was an enhanced penetration of xylose‐ 14 C and a higher rate of incorporation of glucose‐ 14 C into glycogen compared to control muscles. The present results are in good agreement with those previously obtained in vivo. Thus, the influence of contractions in vivo could be demonstrated also in vitro.