Effects of load and tone on the mechanics of isolated human bronchial smooth muscle.

Isotonic and isometric properties of nine human bronchial smooth muscles were studied under various loading and tone conditions. Freshly dissected bronchial strips were electrically stimulated successively at baseline, after precontraction with 10(-7) M methacholine (MCh), and after relaxation with 10(-5) M albuterol (Alb). Resting tension, i.e., preload determining optimal initial length (Lo) at baseline, was held constant. Compared with baseline, MCh decreased muscle length to 93 +/- 1% Lo (P < 0.001) before any electrical stimulation, whereas Alb increased it to 111 +/- 3% Lo (P < 0.01). MCh significantly decreased maximum unloaded shortening velocity (0.045 +/- 0.007 vs. 0.059 +/- 0.007 Lo/s), maximal extent of muscle shortening (8.4 +/- 1.2 vs. 13.9 +/- 2.4% Lo), and peak isometric tension (6.1 +/- 0.8 vs. 7.2 +/- 1.0 mN/mm2). Alb restored all these contractile indexes to baseline values. These findings suggest that MCh reversibly increased the number of active actomyosin cross bridges under resting conditions, limiting further muscle shortening and active tension development. After the electrically induced contraction, muscles showed a transient phase of decrease in tension below preload. This decrease in tension was unaffected by afterload levels but was significantly increased by MCh and reduced by Alb. These findings suggest that the cross bridges activated before, but not during, the electrically elicited contraction may modulate the phase of decrease in tension below preload, reflecting the active part of resting tension.