American Journal of Respiratory Cell and Molecular Biology

Bronchial hyperresponsiveness—excessive airway narrowing caused by stimuli that normally elicit limited or no response—is a cardinal feature of asthma. Ample evidence implicates airway smooth muscle as the key effector of acute airway narrowing during spontaneous asthma attacks and during bronchial provocation in the laboratory setting. For over a decade, investigators have sought to identify a functional abnormality of asthmatic smooth muscle that could explain the bronchial hyperresponsiveness seen in asthma. However, what might have seemed to be a certain outcome of that search—that the hyperresponsiveness so obvious in vivo would be reflected in excessive smooth muscle sensitivity or force generation in vitro—has not emerged. Of course, it is difficult enough to obtain smooth muscle from asthmatic airways for in vitro study. But perhaps more frustrating and enigmatic has been the substantial functional normality of those few asthmatic airway smooth muscle tissues that have been studied. Although some workers have reported increased constrictor sensitivity of tracheal or bronchial smooth muscle from asthmatic subjects (1-3), normal contractile function (1, 3, 4) or even constrictor hyporesponsiveness (5, 6) has been found as frequently. As seen below, it is worth noting that these studies almost exclusively employed isometric force generation as the principal index of contractile function.

In the absence of convincing functional abnormalities, researchers have begun to examine other potential explanations for asthmatic bronchial hyperresponsiveness. Airway biologists have put forth the alternate hypotheses that: (1) asthmatic muscle shortens excessively because its load is reduced as a result of decreased airway elastance (7, 8) or reduced mechanical interaction with lung parenchyma (9, 10); (2) the possibly greater airway smooth muscle mass in asthmatic airways (11, 12) generates more total force for airway narrowing, resulting in greater lumenal narrowing for a given degree of muscle stimulation (13); or (3) airway wall thickening caused by airway remodeling, airway edema, and/or cellular infiltration amplifies lumenal narrowing, even for a normal degree of airway smooth muscle shortening (14, 15). Evidence in support of a role for each of these potential “extra-muscular” contributors to bronchoconstrictor hyperresponsiveness has been documented. So, is it really the case that in vivo constrictor hyperresponsiveness in asthma has nothing at all to do with airway smooth muscle dysfunction?

The report by Fan and associates (16) in this issue of AJRCMB returns our attention to the possibility that more subtle abnormalities of airway smooth muscle function than those sought earlier could after all underlie asthmatic constrictor hyperresponsiveness. These investigators report that tracheal smooth muscle strips isolated from ovalbumin sensitized SJL mice, which exhibit hyperresponsiveness, also exhibit increased maximal shortening velocity and increased extent of total unloaded shortening, when compared with non-sensitized SJL mice or with sensitized or non-sensitized mice of the ASW strain. Importantly, no differences in isometric force generation were found among sensitized or control mice of either strain. These findings extend Dr. Stephens' initial observation that trachealis or bronchial smooth muscle from antigen sensitized dogs also shortens with elevated maximal velocity and to a greater total extent than do corresponding muscles from control animals, even though no differences in isometric force generation are evident (17, 18). Indeed, passive sensitization of normal human trachealis muscle by incubation in serum from atopic human subjects increases its maximal shortening velocity and capacity, without altering its isometric force generating capability (19).

The molecular mechanism underlying the increased maximal shortening velocity that accompanies antigen sensitization has been partially elucidated in dog trachealis. Sensitization increases the quantity and total activity of myosin light chain kinase (MLCK) within trachealis (though enzyme specific activity remains constant) (20). Reflecting the increased MLCK activity, there is greater phosphorylation of the 20 kD regulatory myosin light chain in sensitized trachealis, which in turn is believed to increase the actin-myosin crossbridge cycling rate (21-23). Phosphorylation of the 20 kD regulatory light chain is a key determinant of the maximal velocity of shortening, and no doubt explains the excessive velocity found in sensitized tissues. Importantly, isometric force generation reflects the number of actin-myosin bridges, whereas the shortening velocity reflects the rate of cycling of those bridges, independent of the number (21-23). So, the observation that isometric force generation is unchanged by antigen sensitization is not at odds with its clear-cut effect on shortening velocity. As noted above, prior studies of airway smooth muscle from asthma patients primarily have employed isometric force measurements to evaluate contractile function, and these have not disclosed any consistent abnormalities. However, in the one study that did evaluate the extent of unloaded shortening, airway smooth muscle strips from a single asthmatic patient did shorten excessively (7). Though it remains unknown whether asthmatic airway smooth muscle contracts with elevated maximal shortening velocity, a recent preliminary report indicates that MLCK content is increased in sensitized human bronchial smooth muscle (24). Together, these data suggest to us the as yet untested possibility that velocity of shortening is elevated in asthmatic airway smooth muscle.

If asthmatic airway smooth muscle does contract with excessive velocity, could that functional abnormality contribute to constrictor hyperresponsiveness in vivo? We believe so. To explain how, we must first recognize that the traditional understanding of airway lumen narrowing in asthma has emphasized that muscle length and airway caliber are set by a balance of static forces (10, 25). Force generated by airway smooth muscle statically is taken to be in mechanical equilibrium with the passive reaction force developed by the elastic load against which that muscle has shortened. Since both forces depend upon muscle length, the airway is presumed to accommodate itself to the muscle length at which these opposing forces have come into a static balance. In the case of a static mechanical equilibrium, the muscle shortening has been completed, the isometric force generated is all important, and the velocity of shortening is irrelevant.

In contrast with the concept of an equilibrium of static forces, we must recognize that tidal breathing is a dynamic event, and with each tidal breath the smooth muscle encircling each airway stretches. There is considerable evidence that such tidal stretching of smooth muscle occurs. First, airway resistance decreases at elevated lung volumes, even when airway smooth muscle is maximally contracted (26– 29). Second, the deadspace volume enclosed by the conducting airways increases with lung volume in both normal and asthmatic subjects, implying that central airway diameter increases (30). Finally, during breathhold after cessation of tidal breathing, the diameter of the central bronchi decreases in both normal and asthmatic subjects (31). It thus seems extremely likely that airway smooth muscle must stretch during each tidal breath and especially during deep inhalations. An important consequence of stretching, then releasing, constricted smooth muscle is that this length cycling can reduce muscle force generation substantially (32, 33), probably for two reasons: (1) there may be intracellular remodeling of the contractile apparatus itself (34) or of its insertion within the cytoskeleton (35) during inspiration, and (2) during the stretch, the actin and myosin filaments may slip relative to each other, in a fashion as to reverse the filament sliding that effected contraction and muscle shortening before the stretch (23). For the dynamic circumstances of muscle lengthening and shortening during breathing, it might be expected that times and rates would emerge as crucial considerations (33).

We suggest that the velocity of shortening during the expiratory phase or pause of each tidal cycle may be a major determinant of airway caliber, and could strongly influence the apparent in vivo bronchoconstrictor responsiveness. After release of a tidal breath-induced muscle stretch, the smooth muscle encircling the airway may shorten again, as illustrated by the fall in airway diameter during breathhold cited previously (31). If the airway smooth muscle can shorten quickly (relative to the inter-breath period) to take up the “slack” introduced by the tidal stretch, then the airway narrowing caused by muscle contraction and present prior to the tidal breath may be restored through most of exhalation. In this case, it would appear that the tidal inspiration and smooth muscle stretch had little net effect on airway tone or caliber. In contrast, if the airway smooth muscle shortens slowly relative to the time scale of the breathing cycle, then the lengthening (and bronchodilation) induced by each tidal breath could persist through much of the expiratory phase. Note also that deep inspiration, with an attendant large muscle stretch, might particularly accentuate this apparent bronchodilating effect for slowly contracting muscle (as suggested by Gunst [33]), but might have little net impact on bronchoconstriction generated by high velocity muscle. This might explain why forced deflations from total lung capacity in normal subjects seem to represent flow through airways in which smooth muscle is almost fully relaxed (36, 37). Deep breathing during bronchoconstrictor responsiveness testing would also lower the apparent bronchoconstrictor responsiveness in subjects with slowly contracting muscle.

By extension of this reasoning, one might anticipate that by preventing deep inspiration and profound airway smooth muscle stretch during bronchoconstrictor responsiveness testing, one could increase the apparent constrictor responsiveness, and that this increase would be most marked in individuals with slowly contracting airway smooth muscle. Skloot and colleagues (38) recently reported findings that could be consistent with this construct. They found that when normal human subjects avoided deep inhalations throughout methacholine responsiveness testing, they exhibited bronchoconstrictor responsiveness that resembled that of asthmatic subjects studied in a similar fashion. In marked contrast, though, the apparent responsiveness to methacholine fell substantially in normal subjects, but not in asthmatic subjects, when each inhaled to TLC during airflow obstruction assessment. We hypothesize that the differential effect of deep inhalation on induced bronchoconstriction in asthmatic versus normal subjects (38) might be caused by disparate velocities of smooth muscle shortening in asthmatic (? fast) versus normal (? slow) airways.

At present, no studies have addressed specifically whether airway smooth muscle from asthmatic individuals contracts with increased velocity. Furthermore, it is perilous to interpret the results of Fan and associates (16) or others as directly applicable to human asthma, for modestly increased shortening velocity can be induced in animal (16-18) or human (19) tissues by sensitization alone, an intervention that does not necessarily induce constrictor hyperresponsiveness in vivo. Still, these results and the potential physiologic consequences discussed here suggest a need for characterization of the contraction dynamics of asthmatic airway smooth muscle. If airway smooth muscle from asthmatic subjects does indeed contract with excessive velocity, then further studies might identify useful ways to slow contraction velocity (e.g., by inhibiting myosin light chain kinase activity [39]). Then, we would have the tools with which to assess the importance of abnormal airway smooth muscle contraction dynamics in hyperresponsiveness in vivo and, if a role is confirmed, a new therapeutic approach to controlling bronchoconstrictor hyperresponsiveness in asthma might be revealed.

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Supported by NIH Grants HL56399, AI34566 and HL33009.
Address correspondence to: Julian Solway, M.D., Professor of Medicine, University of Chicago, MC6026, 5841 S. Maryland Avenue, Chicago, IL 60637. E-mail:

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