Small papillary muscles about 2 mm long and 0.2 mm thick were dissected from the right ventricles of 16-19-day-old rats. Resting (between twitches) and active (at twitch peaks) striation patterns were photographed in living muscles using a light microscope. External muscle length was varied from L max , the length at which peak twitch tension was maximum, to 0.75L ma x, the length at which peak twitch tension was about 10% of maximum. Resting and active tension versus muscle length curves were

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... imilar to those obtained from other papillary muscle preparations. Resting average sarcomere length at Lmax was about 2.23^; it decreased with decreasing muscle length in the range between L m ax and 0.75L max . Near 0.75L max , resting average sarcomere length was about 1.5-1.6/z. Considerable internal shortening occurred during contractions, and the active average sarcomere lengths measured at the twitch peaks were less than the resting values. At L max , the active average sarcomere length was 1.98/x-At 0.75L ma x, there was only about a 3-6% decrease in average sarcomere length at the twitch peaks. However, at external muscle lengths between L max and 0.75L ma x more internal shortening was present than there was at L max , since average sarcomere length decreases of about 15% were observed. The finding that peak active tension decreases as sarcomere length decreases below about 2.0/* suggests that some of the factors limiting force generation at short lengths in skeletal muscle may also limit it in mammalian cardiac muscle. Methods Male 16-19-day-old CD Wistar rats (Charles River Breeding Laboratories) weighing 35-43 g were anesthe-299 by guest on