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POINT-COUNTERPOINT COMMENTS

Comment on Point:Counterpoint: "Airway smooth muscle is/is not useful"

Krannert Institute of Cardiology
Indianapolis, Indiana
e-mail: lieford{at}iupui.edu

The following letter is in response to Point:Counterpoint "Airway smooth muscle is/is not useful" that appeared in the April issue (vol. 102, pages 1708–1711).

To the Editor: Seow (6), Fredberg (3, 6), and Mitzner (5) proposed that airway smooth muscle is a useless, vestigial remnant of phylogeny, perhaps retained only because of its common ontogenetic origin with the gut. After reviewing 13 proposed reasons why it might be useful, they conclude that none is sufficient to justify its existence or the trouble it causes. Mead (4) responded with an elegant analysis showing how it is likely to aid mucous removal. Here are two additional reasons why it might have evolved and been retained.

First, it is part of an alarm system, akin to allergic pruritus, warning us of exposures to dangerous toxins and allergens to which we might otherwise be insensible. Although complete airway closure is fatal (5), uncomfortable partial closure is likely to provoke life-preserving action.

Second, while no single proposed raison d'etre contributes a large adaptive advantage, cumulatively they provide a distinct benefit. For example, inhalation of objects irremovable by cough and cilia but removable with the aid of peristalsis is probably so rare that separate peristaltic structures are unlikely to have evolved for the sole purpose of removing them. But the consequences of retained foreign bodies are sufficiently deadly (1, 2) that the rare salvation provided by peristalsis could add incrementally to its selective advantage. I agree that many foreign bodies seen wafting out of the bronchi are likely moved by ciliary action (5), but what about truly difficult objects, say, dense round solids of the correct size to occlude a bronchus?

REFERENCES

1. Davis JH, Saldana MJ. Pulmonary death and forensic pathology of the lung. In: Pathology of Pulmonary Diseases, edited by Saldana MJ. Philadelphia, PA: Lippincott, 1994, p. 187–188.
2. Fraser RS, Müller NL, Coleman N, Paré PD. Diagnosis of Diseases of the Lung. Philadelphia, PA: Saunders, 1999.
3. Fredberg JJ. Counterpoint: Airway smooth muscle is not useful. J Appl Physiol 102: 1709–1710, 2007.[Free Full Text]
4. Mead J. Airway smooth muscle is useful. J Appl Physiol 102:1708–1709, 2007.[Free Full Text]
5. Mitzner W. Airway smooth muscle: the appendix of the lung. Am J Respir Crit Care Med 169: 1–4, 2004.[Free Full Text]
6. Seow CY, Fredberg JJ. Historical perspective on airway smooth muscle: the saga of a frustrated cell. J Appl Physiol 91: 938–952, 2001.[Abstract/Free Full Text]

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