Abstract
Isometric contractions of isolated rat papillary muscles driven at a constant rate have been recorded from paired muscles from hearts of rats 2–6 months of age. At 37°C and a rate of 30/min. tension reaches maximum in 30–60 minutes and then declines slowly during the succeeding 4–12 hours. The maximum tension and the rate of decline with time are related to the diameter of the muscle; however, it has not been possible to demonstrate a definite limit to diameter below which the contraction of the muscle is independent of variations in size. Most factors which increase the force of contraction do not prevent the slow decrease in tension with time. Two exceptions are low temperature (27°C) and high rate (180/min.); under either of these conditions a steady level of tension is maintained for several hours or more. The response of the isolated rat papillary muscle to various physiological alterations is consistent and reproducible if variations in size are considered.
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