Using several electrophoretic techniques, this study examined the effects of 3 weeks hindlimb suspension on the patterns of isomyosins, myosin heavy chain (HC) isoforms and myosin light chain (LC) isoforms in the soleus muscle of the rat. The suspended soleus showed a shift in the HC isoform distribution with a marked increase in fast HC isoforms and a commensurate decrease in HCI. In addition, the change in the fast HC isoforms consisted of the expression of HCIId and HC IIb absent in the normal soleus. In contrast to HC isoforms, suspension did not lead to appreciable changes in LC isoform distribution. Analyses of electrophoresis under nondenaturing conditions demonstrated that the normal soleus expressing HCI and HCIIa isoforms contained two isomyooins. Although, of the two isomyosins observed in the normal soleus, the faster migrating band most likely represented the HCIIa-based one (FMas), its mobility was not identical with that of the HCIIa-based isomyosin (FMaf) found in fast-twitch muscles, migrating in the order FMaf>FMas. FMas was designated as intermediate isomyosin (IM) . Some of the suspended soleus contained slow isomyosin (SM) and IM whereas the others comprised FM 3 and/or FM 2 as well as SM and IM. In spite of the expression of HCIIb and HCIId in the suspended soleus, FM 3 and FM 2 observed in these muscles exhibited distinct mobilities from either HCIId-based or HCIIb-based isomyosins comprised in fast-twitch muscles. These results suggest that some of newly expressed HCIId and/or HCIIb isoforms in the suspended soleus are associated with not only fast but also slow LC isoforms and function as a constitutive element of the myosin molecule.

This study examined the impact of acute high-intensity exercise on the rate of Ca²⁺uptake and release and Ca²⁺-stimulated adenosine triphosphatase (ATPase) activity of the sarcoplasmic reticulum (SR) in the soleus muscle (SOL) and the superficial region of the vastus lateralis muscle (VS) of rats. The animals were run on a 10% grade at 50 m/min of a motorized treadmill until fatigued (The average time to exhaustion was 306 sec.) . At exhaustion, glycogen concentrations were 65% and 85% less in the SOL and VS, respectively. The rate of Ca²⁺release induced by 4-chloro-m-cresol was un-changed in fatigued SOL and VS. The rate of Ca²⁺uptake stimulated by adenosine triphosphate (ATP) was significantly lower following exercise in VS but not in SOL. This lower rate observed in VS was paralleled by decreased catalytic activity of SR Ca²⁺-ATPase. The rate of Ca2+ uptake measured using adenosine diphosphate and phosphocreatine, as substrate was lower than that of ATP in fatigued VS. These findings suggest that, in fast-twitch muscles, high-intensity exercise not only reduces SR Ca²⁺-ATPase activity but also elicits a decrease in creatine kinase activity, probably resulting from nitric oxide that is produced during exercise.

Maximal oxygen uptake (Vo₂max) and lactate threshold were measured during an incremental bicycle ergometer test in 40 healthy middle-aged and older runners between 43 and 79 years of age. Although the 10-km run time slowed with increasing age, there were no significant differences in recent training habits or relative amount of body fat between four age groups. However, our cross-sectional data revealed an annual decrement of -0.74 ml/kg/ min/yr, which was significantly greater than that reported in previous studies. Vo₂max values for the runners were greater than those for sedentary men of similar ages by about 50% in each age group. Significant correlations were found between the age at the onset of running training and Vo₂max (r=-0.600, p<0.05) . Vo₂@LT declined significantly but less rapidly with age (r=0.686, p<0.05) than Vo₂max. Both the mean maximal heart rate (HRmax) and HR@LT also declined with age. No significant differences in HRmax were observed between the runners and sedentary men of the respective age groups. Significant correlations were also found between the estimated HRmax and directly measured HRmax (r=0.600) . Neither systolic blood pressure nor diastolic blood pressure during submaximal-maximal exercise were found to increase with age. We suggest that maintenance of a higher lactate threshold in older runners when expressed as a percentage of Vo₂max is attributable to a greater age-dependent decline in Vo₂max with a smaller change in Vo₂@LT.