Decline of isometric force and fatigue resistance in skeletal muscles from spontaneously hypertensive rats.

Skeletal muscles from 6- to 7-month-old male spontaneously hypertensive rats showed a decrease in functional capacity compared with muscles from age-matched normotensive Wistar-Kyoto rats. Predominantly slow-twitch, oxidative soleus muscles developed less force (normalized to muscle wet weight) and were less resistant to fatigue. Heterogeneous, but largely fast-twitch, oxidative-glycolytic medial gastrocnemius muscles generated less force, were smaller in size (normalized to body weight), and did not demonstrate the same degree of stimulation-associated potentiation (staircase effect) as did the medial gastrocnemius of the normotensive rats. The reduced endurance of the soleus of hypertensive rats was not associated with either fiber redifferentiation or capillary "rarification," and the majority of the decline in force with time could not be attributed to impaired neuromuscular transmission. The decrease in muscle capacity in the spontaneously hypertensive rat, thus, appears to be the result of adaptive changes localized in the muscle cells themselves. The adaptive changes, regardless of site, have a pronounced negative effect on muscle function, and could significantly influence motor performance and antihypertensive therapy.