Canine tracheal smooth muscle can support significantly greater loads during elongation than shortening, with lower energy requirements than isometric contraction, suggesting mechanical stability during airway distension.
Highlights energy-efficient load support during elongation in airway smooth muscle; leaves open translation to human airway disease.
For smooth muscle in general there appears only one report dealing with force-velocity (FV) relationships of active muscle subjected to forcible elongation by application of loads (P) greater than its maximum isometric tetanic tension (Po); for airway smooth muscle (ASM) there is none. Since ASM may be subjected to increasing stretch during inspiration, the relationship is important and was therefore studied with canine tracheal smooth muscle (TSM) as a model. FV data for P less than Po could be fitted by Hill's hyperbolic equation. For P greater than Po, lengthening velocity was greater than predicted by the equation. However at equivalent velocities, the muscle during elongation could support a load three times greater than during shortening; in this it resembled skeletal muscle. From this it may be speculated that distension of the airway during inspiration would not be associated with mechanical instability. With reference to energy requirements of the elongating TSM it was shown, as has been for skeletal muscle, that the net rate of energy liberation (assessed by measuring tissue levels of adenosine triphosphate and creatine phosphate) in an elongating active muscle is less than that of a muscle contracting isometrically.
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Hanks et al. (1981) studied this question.