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The effects of a stretch-release cycle (approximately 25% of the resting muscle fibre length, Lo) on both tension and [Ca2+]i in small, unstimulated, intact muscle fibre bundles isolated from adult and neonatal rats were investigated at 20 degrees C. The results show that the effects of the length change depended on the age of the rats. Thus, the length change produced three effects in the neonatal rat muscle fibre bundles, but only a single effect in the adult ones. In the neonatal fibre bundles, the length change led to an increase in resting muscle tension and to a transient increase in [Ca2+]i. The stretch-release cycle was then followed by a twitch-like tension response. In the adult fibre bundles, only the increase in resting tension was seen and both the transient increase in [Ca2+]i and the stretch-induced twitch-like tension response were absent. The amplitude of the twitch-like tension response was affected by both 2,3-butanedione monoxime and sarcomere length in the same manner as active twitch tension, suggesting that it arose from actively cycling crossbridges. It was also reversibly abolished by 25 mM K+, 1 microM tetrodotoxin and 1.5 mM lidocaine (lignocaine), and was significantly depressed (P &lt; 0.001) by lowering [Ca2+]o. These findings suggest that a rapid stretch in neonatal rats induces a propagated impulse that leads to an increase in [Ca2+]i, and that abolishing the action potential abolishes the stretch-induced twitch-like tension response. In 5- to 7-day-old rats, the twitch-like tension response was approximately 50 % of the isometric twitch. It then decreased progressively with age and was virtually absent by the time the rats were 21 days old. Interestingly, this is the same period over which rat muscles differentiate from their neonatal to their adult types.

A mechanical stretch induces contractile activation in unstimulated developing rat skeletal muscle in vitro