Increase in resting tension of tracheal muscle due to rigor during metabolic inhibition.

Resting tension of canine tracheal smooth muscle increased when glucose and oxygen were withdrawn from the bathing medium. Similar treatment of muscle stimulated with carbachol caused first a relaxation and then a secondary increase in tension. The increase in tension due t0 metabolic inhibition, unlike normal tracheal contractions, was insensitive to calcium depletion, was not associated with an active state, and was accompanied by marked reduction of tissue adenosine triphosphate, creatine phosphate, and glycogen content. Because muscle stiffness was also increased we concluded that hypoxic glucose-free contracture is due to rigor and not to an increased tissue calcium level as has been previously suggested. Rigor shortening during lightly loaded isotonic conditions is better maintained than rigor tension during isometric conditions. Our results also indicate that rigor tension is reduced irreversibly on imposition of a load and, therefore, the load-extension relationship during rigor in smooth muscle should be studied by making only small load changes.