Cross-reinnervation of rabbit soleus muscle by the peroneal nerve induces a 90% transformation of slow into fast fibres. These changes are reflected in corresponding transformations of the enzyme activity pattern of energy metabolism, the isozyme pattern of lactate dehydrogenase and, in confirmation of previous results (Srihari et al. 1981), transitions from a slow to a fast type myosin light chain pattern. The transformation process appears to be complete after 6 months. Similar changes, although less extensive are also found in the soleus muscle of the contralateral leg. Fibre type transitions in the contralateral muscle are not accompanied by fibre type grouping, as seen in the cross-reinnervated muscle and therefore these changes appear to result from a transformation of the motor units themselves. This phenomenon is interpreted as a compensatory process in maintaining symmetry within the neuromotor system.

6‐Phosphogluconolactonase was purified to apparent homogeneity in a four‐step procedure from bovine erythrocytes. The extent of purification and the kinetic properties of the enzyme were evaluated with an optical test that was based on the hydrolysis of synthetic 6‐phosphogluconolactone. The active enzyme from bovine erythrocytes is a monomer with an approximate molecular weight of 30000. It exhibits Michaelis‐Menten kinetics and cofactors are not required for activity. The enzyme was found in a number of tissues. Its activity, when compared to the activity of the glycolytic enzymes, illustrates the importance of glucose oxidation via the pentose phosphate pathway relative to glycolysis.

Fast rabbit skeletal muscles (tibialis anterior and extensor digitorum longus) were stimulated for 2-28 days by electrodes implanted in the vicinity of the peroneal nerve to produce maximal contractions at two different frequency patterns: that occurring naturally in nerves to slow muscles (10 Hz continuously) or three bursts of tetani (40 Hz) per minute, each 5s in duration. Both types of frequency produced muscles more resistant to fatigue during isometric twitch contractions, and led to a prolongation of contraction time greater and more consistent with 10 Hz than with 40 Hz. The twitch/tetanus ration was significantly higher in muscles stimulated at 10 Hz for 3-4 weeks but was not different from controls in muscles stimulated at 40 Hz. Both types of stimulation led to the appearance of myosin light chains characteristic of slow muscles. Muscles stimulated for 4 weeks at 40 Hz developed greater twitch tension per gram, and had significantly smaller cross-sectional area of myofibrils than control muscles. It is concluded that long-term electrical stimulation of fast muscles can affect some muscle contractile properties to resemble those of slow muscles irrespective of frequency of stimulation, provided the total number of stimuli is comparable, the duration of stimulation is long enough (minimum 2 weeks) and all motor units are activated.

The differentiation of neuromuscular junctions of multiply innervated, slow anterior latissimus dorsi (ALD) and focally innervated, fast, posterior latissimus dorsi (PLD) muscles was studied in normal and curarized chick embryos. At 16 days of incubation, fibres of both muscles are contacted by several axon profiles, the number of which falls with age. In 18-day-old embryos individual endplates in ALD are usually contacted by three axon profiles, whereas in PLD, endplates are contacted only by a single large terminal profile. At this time, there is already a significant accumulation of cell organelles in the postsynaptic area. Treatment of embryos with curare during the 7th and 12th day of incubation delays the differentiation of the neuromuscular junction in both muscles. The paralysis dramatically affects the decrease of the number of axon profiles at individual endplates in both muscles. At 16 days the number of axon profiles was greater in embryos treated with curare than in the untreated controls. At 18 days when the number of axon profiles normally decreases, the endplates of both types of curarized muscles have an even greater number of axon profiles than at 16 days. Endplates in curarized PLD had up to 13 and in curarized ALD up to 12 axon profiles. The effects of curare gradually wore off and when the movements of the embryos again became more vigorous, the normal differentiation of neuromuscular junctions continued. At 21 days of incubation many embryos recover from curare and show endplates of normal appearance in both muscles. These results suggest that activity of the muscle is essential for the maturation of the neuromuscular junctions.