Development of soleus muscles in SHR: relationship of muscle deficits to rise in blood pressure.

Skeletal muscles from 24- to 28-wk-old spontaneously hypertensive rats (SHR) exhibit decreased contractile capacity and resistance to fatigue. The present study was designed to determine the age at which these deficits first appear and their relationship to the development and progression of the rise in blood pressure. SHR soleus was significantly weaker than age-matched Wistar-Kyoto (WKY) soleus at all ages studied, but resistance to fatigue varied with age. Soleus muscles in 6- to 8-wk-old SHR were, on average, more fatigue resistant than age-matched WKY muscles. Fatigue resistance in 16- to 18-wk-old animals, however, was similar in the two strains. There were no significant differences in soleus growth or fiber type distributions in the strains between 6 and 18 wk of age. WKY soleus in 24- to 28-wk-old animals were hyperpolarized after the fatigue test. SHR fibers, in contrast, did not hyperpolarize after exercise, possibly reflecting an age-related reduction in sarcolemmal Na+ pump number or function. Soleus in younger SHR also provided an indication of a developing membrane dysfunction, since extracellularly recorded M waves showed greater changes in SHR than in age-matched WKY muscles during exercise. The rise of blood pressure in SHR is genetically based, but it is not clear that the genetic defects responsible for hypertension also produce the observed deficits in skeletal muscle function.