Abstract

Heart muscle fibers extracted at -20°C in 50% glycerol with 0.1 M histidine buffer at pH 7.00 for a short duration (approximately 3 to 15 days) were found to contain a "relaxing factor." This "relaxing factor" was not present in fibers extracted or preincubated for a prolonged time (over 3 weeks).

Ouabain increased the ATP-induced contraction of these glycerol-extracted fibers containing the "relaxing factor." This action of ouabain occurred only after a 20-minute contact with the fiber. This effect of ouabain was not seen when the "relaxing factor" was absent from the fibers. Thus ouabain was effective only in those fibers which contracted in response to Ca⁺⁺.