THE EFFECT OF ALTERED MOTOR ACTIVITY PATTERN ON SARCOPLASMIC RETICULUM FUNCTION IN FAST AND SLOW SKELETAL MUSCLES
The sarcoplasmic reticulum (SR) is a membrane structure within muscle that controls muscle contraction. With activity the SR releases calcium ions triggering muscle contraction. Muscle relaxation is produced by an energy-dependent calcium reuptake by the SR ATPase. Fast muscle with a shorter contraction time (CT) and one-half relaxation time ( 1/2RT) has a greater SR content and a higher rate of Ca('2+) uptake per unit SR than a slow twitch muscle. It has been generally known that skeletal muscles undergo adaptive changes in response to different types of motor activity pattern. The adaptations have been shown to involve mitochondrial and glycolytic enzymes, the actomyosin ATPase, the pattern of substrate utilization, and changes in contractile properties. But changes in SR function in response to altered motor activity have not been fully evaluated. It was my intention to evaluate the alterations in SR function and determine the role of the SR in controlling muscle contractile properties. Exercise training and immobilization models were used to evaluate use and disuse effects. In addition, thyrotoxicosis was induced by thyroid hormone treatment to study specific hormonal effects on skeletal muscle SR. In general, the results of these studies show changes in motor activity to produce fiber type-specific alterations. The reduced neural activity associated with hindlimb immobilization caused SR changes generally opposite to those induced by exercise training. One hour of daily endurance running caused a significant decline in the maximal rate of calcium uptake (V(,max)) by SR isolated from the type IIB SVL (superficial vastus lateralis), while no change was observed in the V(,max) of SR isolated from the type IIA DVL (deep portion of vastus lateralis) or the type I soleus muscle. On the other hand, after 6 wk of immobilization, the isolated SR membrane (FSR) from the type I muscle showed a decrease in V(,max) while the SR from the type IIA muscle showed no significant changes, and the type IIB muscles showed a significant increase in V(,max) and K(,m). Ca('2+)-stimulated ATPase activity did not change significantly following 6 wk of immobilization in any of the muscles studied. In the type I soleus muscle, thyrotoxicosis caused more than a 2-fold increase in FSR yield and stimulated both the V(,max) and the total Ca('2+) uptake of a crude homogenate (CH) preparation. However, no change was observed in the V(,max) or K(,m) of the FSR. In contrast, the CH and FSR prepared from the fast DVL and SVL were unaffected by thyrotoxicosis. In short, thyrotoxicosis alters SR function in slow but not fast muscles. For the exercise training and thyrotoxic models, alterations in SR function were well correlated with changes in twitch durations. But in the disuse model, the relationship was uncoupled and this was due, at least in part, to the large decrease in twitch and tetanic tension. The changes in the rate of tension development (+dP/dt) and/or tension decline (-dP/dt) were better indicators of SR function in this model.
KIM, DO HAN, "THE EFFECT OF ALTERED MOTOR ACTIVITY PATTERN ON SARCOPLASMIC RETICULUM FUNCTION IN FAST AND SLOW SKELETAL MUSCLES" (1981). Dissertations (1962 - 2010) Access via Proquest Digital Dissertations. AAI8203769.