Absence of a plateau in length-tension relationship of rabbit papillary muscle when internal shortening is prevented.

Abstract

IN the length–tension relationship of the isolated papillary muscle there is a region in which increased muscle length is associated with an increased force of contraction. Several problems make it difficult to determine the physiological basis for this relationship. Thick and thin filament lengths and sarcomere lengths have not been determined with the same accuracy in heart muscle as in skeletal, though there is evidence that these lengths are nearly the same in both muscles1–3. Papillary muscle cannot normally be tetanised or made to contract at low temperatures, and there is reason to believe that it is not fully activated by a single stimulus in the usual bathing medium4,5. In at least some papillary muscles there is considerable internal shortening during a twitch contraction6,7. Such length changes during contraction could change the amount of force developed by affecting the amount of thick and thin filament overlap and by producing shortening deactivation8,9. We have developed a photoelectronic servo apparatus similar to that developed by Gordon et al. (see refs 10 and 11 for details of operation) for use with papillary muscles to hold constant the length of a segment of preparation during contraction.