HIGH ENDOCYTOTIC ACTIVITY OCCURS PERIODICALLY IN THE END-PLATE REGIONOF DENERVATED MOUSE STRIATED-MUSCLE FIBERS

High endocytotic activity after denervation of skeletal muscle occurs in a proportion of muscle fibers (both slow and fast fiber types) in t he endplate region. The present study was performed in order to examin e if a periodicity in the endocytotic activity could explain why the p rocess is not observed in all fibers at a given time. Three markers, h orseradish peroxidase (HRP), rhodamine B isothiocyanate-labeled dextra n, and fluorescein isothiocyanate-labeled dextran were used to demonst rate endocytotic activity of muscle fibers of the denervated mouse hem idiaphragm in vivo. Acetylcholine esterase staining was used in conjun ction with HRP uptake to determine the proportion of denervated muscle fibers with endocytotic activity in the endplate region at any one ti me. The results show that 25-50% of the muscle fibers display high end ocytotic activity in the endplate region at a given time 10 days after denervation. The existence of a periodicity in this endocytotic activ ity is suggested by results obtained using two different endocytotic m arkers administered at time intervals of 0-7 days, We conclude that lo ss of contact with the innervating motorneuron induces a high endocyto tic activity which occurs periodically in the perisynaptic region of s keletal muscle fibers. (C) 1995 Academic Press, Inc.