The relevance of resting tension to responsiveness and inherent tone of human bronchial smooth muscle

Abstract

1. In the present study the effects of resting tension on isometric responses of human airway smooth muscle to contractile and relaxant stimuli were investigated. Also, its effects on inherent smooth muscle tone were examined, in an effort to determine a pre-defined resting tension which can be considered optimal for in vitro studies.

2. Bronchial ring preparations (2–4 mm internal diameter) were suspended in tissue baths at a range of levels of resting tension (200–1600 mg). The responses to electrical field stimulation (EFS, 30 V, 0.2 ms, 8–30 Hz), carbachol (3 μM), isoprenaline (1 μM) and the 5-lipoxygenase inhibitor, zileuton (10 μM) were investigated along with the ability of the preparations to recover stable resting tension after 60 min of washing post challenge.

3. EFS induced monophasic contractions at low stimulation frequency (8 Hz). However, with increasing frequency and tension (15 and 30 Hz; > 400 mg) contractile responses were accompanied by a relaxant component. The magnitude of contractile responses increased with increasing resting tension to a plateau at between 500 and 700 mg. Relaxant responses when present, increased in magnitude with increasing resting tension.

4. The level of resting tension did not significantly alter responses to carbachol, but tissues at tensions ⩾ 1000 mg showed poor tension recovery after washing. Isoprenaline-induced relaxations increased with increasing resting tension (P<0.05) and tension recovery after washing was complete. The 5-lipoxygenase sensitive portion of the tension (33±4%) was not altered by the level of resting tension.

5. These results suggest that in human bronchial ring preparations of 2–4 mm internal diameter, resting tensions within the range 400–1000 mg could be considered optimal for isometric tension recordings of protocols involving both contraction and relaxation procedures.

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