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 fatigu e. The present study was designed to determine the age at which these deficits first appear and their relationship to the development and pr ogression 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 W KY muscles. Fatigue resistance in 16- to 18-wk-old animals, however, w as similar in the two strains. There were no significant differences i n soleus growth or fiber type distributions in the strains between 6 a nd 18 wk of age. WKY soleus fibers in 24- to 28-wk-old animals were hy perpolarized after the fatigue test. SHR fibers, in contrast, did not hyperpolarize after exercise, possibly reflecting an age-related reduc tion in sarcolemmal Na+ pump number or function. Soleus in younger SHR also provided an indication of a developing membrane dysfunction, sin ce extracellularly recorded M waves showed greater changes in SHR than in age-matched WKY muscles during exercise. The rise of blood pressur e in SHR is genetically based, but it is not clear that the genetic de fects responsible for hypertension also produce the observed deficits in skeletal muscle function.