Isotonic relaxation of control and sensitized airway smooth muscle

Authors: Stephens, N L.; Fust, A; Jiang, H; Li, W; Ma, X

Source: Canadian Journal of Physiology and Pharmacology, Volume 83, Number 10, October 2005 , pp. 941-951(11)

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Abstract:

Smooth muscle relaxation has most often been studied in isometric mode. However, this only tells us about the stiffness properties of the bronchial wall and thus only about wall capacitative properties. It tells us little about airflow. To study the latter, which of course is the meaningful parameter in regulation of ventilation and in asthma, we studied isotonic shortening of bronchial smooth muscle (BSM) strips. Failure of BSM to relax could be another important factor in maintaining high airway resistance. To analyze relaxation curves, we developed an index of isotonic relaxation, t_1/2(P, l_CE), which is the half-time for relaxation that is independent of muscle load (P) and of initial contractile element length (l_CE). This index was measured in curves of relaxation initiated at 2 s (normally cycling crossbridges) and at 10 s (latch-bridges). At 10 s no difference was seen for adjusted t_1/2(P, l_CE) between curves obtained from control and sensitized BSM, (8.38 ± 0.92 s vs. 7.78 ± 0.93 s, respectively). At 2 s the half-time was almost doubled in the sensitized BSM (6.98 ± 0.01 s (control) vs. 12.74 ± 2.5 s (sensitized)). Thus, changes in isotonic relaxation are only seen during early contraction. Using zero load clamps, we monitored the time course of velocity during relaxation and noted that it varied according to 3 phases. The first phase (phase i) immediately followed cessation of electrical field stimulation (EFS) at 10 s and showed almost the same velocity as during the latter 1/3 of shortening; the second phase (phase ii) was linear in shape and is associated with zero load velocity, we speculate it could stem from elastic recoil of the cells' internal resistor; and the third phase (phase iii) was convex downwards. The zero load velocities in phase iii showed a surprising spontaneous increase suggesting reactivation of the muscle. Measurements of intracellular calcium (Fura-2 study) and of phosphorylation of the 20 kDa myosin light chain showed simultaneous increments, indicating phase iii represented an active process. Studies are under way to determine what changes occur in these 3 phases in a sensitized muscle. And of course, in the context of this conference, just what role the plastic properties of the muscle play in relaxation requires serious consideration.Key words: airway smooth muscle, sensitized smooth muscle, isotonic relaxation, intracellular calcium transients, myosin light chain (20 kDa) phosphorylation.

La relaxation du muscle lisse a le plus souvent été examinée en mode isométrique. Toutefois, ce type d'examen nous informe uniquement sur les propriétés de rigidité de la paroi bronchique et, par conséquent, sur ses propriétés capacitives. Il nous renseigne peu sur l'écoulement gazeux. Afin d'étudier ce paramètre, qui est probablement le paramètre majeur dans la régulation de la ventilation et dans l'asthme, nous avons examiné le raccourcissement isotonique de bandes de muscles lisses bronchiques (MLB). L'incapacité de relaxation du MLB pourrait être un autre facteur important dans le maintien d'une résistance bronchique élevée. Pour analyser les courbes de relaxation, nous avons mis au point un index de relaxation isotonique, t_1/2(P, l_CE), correspondant à la demi-vie de la relaxation, qui est indépendante de la charge musculaire (P) et de la longueur initiale de l'élément contractile (l_CE). Nous avons mesuré cet index dans des courbes de relaxation, démarrées à 2 s (ponts transversaux des cycles normaux) et à 10 s (ponts à l'état verrouillé). À 10 s, aucune différence dans la t_1/2 (P ,l_CE) ajustée n'a été observée entre les courbes obtenues des MLB sensibilisés et témoins (8,38±0,92 s vs. 7,78±0,93 s. À 2 s, la demi-vie a presque doublé dans les MLB sensibilisés (6,98 ±0,01 s témoin vs. 12,74±2,5 s sensibilisés). Ainsi des modifications dans la relaxation isotonique ne sont observées qu'au début de la contraction. Nous avons suivi l'évolution de la vitesse durant la relaxation en utilisant une méthode de relâchement à la tension zéro; nous avons noté une évolution en 3 phases. La première (phase i) a immédiatement suivi l'arrêt de la SEF à 10 s et a montré sensiblement la même vitesse que durant le dernier tiers du raccourcissement; la seconde phase (phase ii) avait une forme linéaire et a été associée à la vitesse nulle à tension zéro — nous supposons que cela pourrait ê

Document Type: Research article

Publication date: 2005-10-01

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Isotonic relaxation of control and sensitized airway smooth muscle

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