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1 Department of Pediatrics, Duke University Medical Center, Durham, North Carolina 27710; and 2 Department of Physiology, University of Manitoba, Winnipeg, Manitoba, Canada R3E 3J7
Juveniles of many species, including
humans, display greater airway responsiveness than do
adults. This may involve changes in airway smooth
muscle function. In the present work we studied force production and
shortening velocity in trachealis from 1-wk-old (1 wk), 3-wk-old (3 wk), and 3-mo-old (adult) guinea pigs. Strips were
electrically stimulated (60 Hz, 18 V) at their optimal length (lo) to obtain maximum active stress
(Po) and rate of stress generation. Then, force-velocity
curves were elicited at 2.5 s from the onset of the stimulus. By
applying a recently developed modification of Hill's equation for
airway smooth muscle, the maximum shortening velocity at zero load
(Vo) and the value 
· 
/
,
an index of internal resistance to shortening (Rsi), were calculated
(, , and are the constants of the equation). Po
increased little with maturation, whereas the rate of stress generation
increased significantly (0.40 ± 0.03, 0.45 ± 0.03, 0.51 ± 0.03 Po/s for 1 wk, 3 wk, and adult animals).
Vo slightly increased early with maturation to
decrease significantly later (1.79 ± 0.67, 2.45 ± 0.92, and 0.55 ± 0.09 lo/s for 1 wk, 3 wk, and adult
animals), whereas the Rsi showed an opposite trend (14.98 ± 5.19, 8.99 ± 3.01, and 32.07 ± 5.54 mN · mm
2 · lo1 · s
for 1 wk, 3 wk, and adult animals). This early increase
of force generation in combination with late increase of Rsi may explain the changes of Vo with age. An elevated
Vo may contribute to the incidence of airway
hyperresponsiveness in healthy juveniles.
airway reactivity; airway smooth muscle; contractility; force-velocity relationship; maturation
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