The Journal of General Physiology, Vol 65, 329-344, Copyright © 1975 by The Rockefeller University Press
Mechanical control of the time-course of contraction of the frog heart
E Bozler
Changes in load during most phases of an isotonic contraction of the frog
and turtle heart increased or decreased the duration of the twitch. It was
abbreviated by a maintained increase or by a brief decrease in load. The
relaxing effect of these procedures developed with a delay lasting more
than a second under some conditions and will be called lengthening
deactivation. The reverse procedures, a maintained diminution or a brief
increase in load, increased the duration of the twitch. This effect will be
called shortening activation. Although the termination of relaxation may be
delayed or advanced by the mechanical interventions mentioned, the normal
time- course of isotonic relaxation was always resumed later, regardless of
the starting level of the load, making it possible to measure accurately
changes in the duration of the twitch. The responses to changes in load
produce positive feedback during the isotonic contraction and explain, at
least in part, the difference in the time- course of isotonic and isometric
contraction. The effects of changes in load were much smaller and briefer
in the atrium than the ventricle.
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