THE EFFECTS OF EXERCISE TRAINING OF DIFFERENT INTENSITIES ON NEUROMUSCULAR-JUNCTION MORPHOLOGY

Little is known about the effects of exercise training on neuromuscula r junction morphology in skeletal muscle. The objectives of this inves tigation were: 1) to determine if exercise training would elicit chang es in neuromuscular junction morphology, 2) to determine if exercise t raining of different intensities would evoke specific changes in neuro muscular junction morphology, and 3) to determine whether changes in n euromuscular junction structure occur independently of changes in musc le fibre type and size. Twenty-four age and size matched male Sprague- Dawley rats were randomly assigned to three groups: high-intensity tra ined (HIT), low-intensity trained (LIT), or untrained. Neuromuscular j unction morphology of the soleus muscle was determined via immunofluor escent staining. Presynaptic acetylcholine vesicles were visualized wi th SV-2 antibody in conjunction with fluorescein isothiocyanate labell ed secondary antibody. Postsynaptic acetylcholine receptors were ident ified with rhodamine labelled alpha-bungarotoxin. Laser scanning micro scopy was used to produce images of synapses, which were used to quant itate the following: total area of SV-2 and alpha-bungarotoxin stainin g, density of acetylcholine vesicles and receptors, structural complex ity, and synaptic coupling. To visualize nerve terminal branching, a s maller number of neuromuscular junctions were stained with C-2 antibod y, which reacts with a neurofilament epitope, in conjunction with fluo rescein isothiocyanate labelled secondary antibody. Total length of br anching, number of branches, average length of branches, and ratio of secondary to primary branches per neuromuscular junction were determin ed. Citrate synthase activity, fibre type composition and fibre cross- sectional areas of the soleus muscle were assessed to determine the pr esence of a training effect in that muscle. Results indicate that trai ning did induce hypertrophy of the neuromuscular junction that was ind ependent of muscle hypertorphy. Although the HIT and LIT groups exhibi ted similar hypertrophic responses of the neuromuscular junction, the HIT group displayed more dispersed synapses than the LIT group. Neithe r exercise training program, however, resulted in altered densities of acetylcholine vesicles or receptors, nor did training significantly c hange synaptic coupling. Nerve terminal branching was also affected by exercise training. Neuromuscular junctions from the HIT group demonst rated a greater total length of branching, average length per branch, and number of finer, or secondary, branches than those of the LIT grou p.