Ontogeny of shortening velocity in porcine trachealis
We examined the effect of maturation on force-velocity (F-V) parameters in porcine tracheal smooth muscle (TSM) to determine the relationship between maximal isometric contractile force (P(o)) and maximal velocity of shortening (V(max)). Strips of TSM excised from 1-day-old neonatal swine (neo; n = 8), 2-wk-old swine (2ws; n = 7), and 10-wk-old swine (10ws; n = 7) were tethered to an electromagnetic lever system for F-V analysis of contractility. TSM strips were activated by electrical field stimulation at optimal resting tension, voltage, and length (L(o)) so that maximal reproducible contractile force (P(o)) was elicited. Velocities were measured at the early phase of isometric contraction (3.1 ± 0.1 s for neo, 2.9 ± 0.1 s for 2ws, and 3.1 ± 0.1 s for 10ws; P = NS). Shortening velocity increased progressively with maturation; V(max) was 0.164 ± 0.011 L(o)/s for neo, 0.194 ± 0.013 L(o)/s for 2ws (P < 0.05 vs. neo), and 0.260 ± 0.024 L(o)/s for 10ws (P < 0.01 vs. neo; P < 0.05 vs. 2ws). Maximal isometric force generation increased substantially during the first 2 wk of postnatal life and thereafter returned to neonatal levels; P(o) was 71.5 ± 2.1 mN/mm2 for neo, 95.4 ± 7.0 mN/mm2 for 2ws, and 74.7 ± 6.2 mN/mm2 for 10ws (P < 0.05, 2ws vs. neo and 10ws). In separate studies, we also determined whether differences in V(max) occurred during the normal cycling phase of the cross bridge (3 s) or during the slowly cycling phase of the latch bridge (8 s) in tissue from 12 additional animals. No difference in V(max) was determined for TSM at 8 s among the three age groups. We demonstrate that shortening velocity increases during maturation of airway smooth muscle in young swine. Our data demonstrate ontogenic increases in responsiveness and maximal velocity of shortening in swine TSM that are not related directly to maximal force generation in the same tissues.
Ikeda, K; Mitchell, RW; Guest, KA; Seow, CY; Kirchhoff, CF; Murphy, TM; Leff, AR
American Journal of Physiology Lung Cellular and Molecular Physiology
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