SUMMARY1. Passive length-tension curves were established for cat soleus muscles that had been immobilized in different positions. Muscles that had been immobilized in the lengthened position showed no difference in their length-tension properties to those of normal muscles. However, those immobilized in the shortened position showed a considerable decrease in extensibility.2. Muscle fibre length, sarcomere length and the total number of sarcomeres along single teased fibres were also determined for muscles immobilized in different positions. Soleus muscles immobilized in the lengthened position were found to have 20 % more sarcomeres in series than normal muscles whilst those immobilized in the shortened position had 40 % less than normal muscles.3. When the plaster casts were removed from muscles that had been immobilized in the shortened position, the length-tension curves and sarcomere number returned to normal within 4 weeks. Muscles that were immobilized in a shortened position and then immobilized in a second position were found to rapidly adjust to the second position with respect to their passive length-tension properties and sarcomere number.4. A change in the number of sarcomere in series seems to be the way in which the sarcomere length of the muscle is adjusted to its new functional length. The change in the length-tension properties which accompanies a decrease in sarcomere number appears to be the mechanism which prevents the muscle from being overstretched. * All the experiments described in this paper were carried out in France.
The effects of overload on the connective tissue component of the soleus muscle of the rat have been investigated. Three weeks after tenotomy of its synergistic muscles the soleus underwent considerable increase in weight. This was shown to have resulted from an increase in size of the predominant fibre type. Whilst occasional groups of fibres appeared to have resulted from the splitting of large single fibres, there was not significant increase in the number of fibres in cross-section of the muscle belly. The connective tissue content of the overloaded muscles was investigated using both histological and biochemical techniques. It was found that muscle fibre hypertrophy was accompanied by an increase in the connective tissue component. Furthermore, there was an increase in the proportion of collagen to muscle fibre tissue.
These experiments were performed to determine how ectothermal animals maintain their locomotory ability during acute changes in muscle temperature, despite the large thermal dependence of the mechanical properties of their muscle. The electrical activity of the red (slow-twitch) and white (fast-twitch) muscle fibers of carp was monitored while the carp swam at various speeds at 10 and 20 degrees C. The patterns of recruitment of different fiber types were similar at both temperatures. At low speeds only the red muscle was active, whereas at high speeds the white muscle was active as well. The swimming velocity at which white muscle was initially recruited increased from 26 cm/s at 10 degrees C to 46 cm/s at 20 degrees C. These results suggest that the order of recruitment of motor units was the same at 10 and 20 degrees C but that the recruitment occurred over a narrower range of speeds at the low temperature. Hence, to generate the muscle power required to swim at a certain velocity, fish recruit more muscle fibers, which include faster fiber types when their muscle is cold than when their muscle is warm.
Insulin-like growth factor binding protein-4 (IGFBP-4) is an important member of the insulin-like growth factor (IGF) system. The IGFBP-4 has three domains of which the N-terminal sequence is important for the binding of IGF. It acts as a transport protein for IGF-I and IGF-II and modulates their biological effects. There is increasing evidence that IGFBP-4 inhibits IGF-induced cellular growth both in vitro and in vivo. IGFBP-4 can also mediate its actions through a mechanism independent of IGFs. IGFBP-4 level and expression in various tissues are influenced by IGFBP protease, nutrition, several growth factors and hormones. Overexpression of IGFBP-4 in transgenic animal models causes reduced growth of organs containing smooth muscle. Most cancers express IGFBP-4 at levels which correlate with their state of differentiation. However, the effects of IGFBP-4 on tumor growth are uncertain. In vitro studies have shown that overexpression of IGFBP-4 inhibit the growth of some colon cancer cells. Overexpression of IGFBP-4 in vivo has been reported to decrease the growth of prostate cancer. The effect of altered expression of IGFBP-4 in vivo in colon and other cancers needs to be explored as locally available IGFs appear to stimulate mitogenesis. Contents 1. Introduction 2. Structure and binding characteristics of IGFBP-4 3. Biological actions of IGFBP-4 4. Factors controlling IGFBP-4 expression 5.
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