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

Isotonic and isometric properties of nine human bronchial smooth muscles we re studied under various loading and tone conditions. Freshly dissected bro nchial 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 optima l initial length (L-o) at baseline, was held constant. Compared with baseli ne, MCh decreased muscle length to 93 +/- 1% L-o (P < 0.001) before any ele ctrical stimulation, whereas Alb increased it to 111 +/- 3% L-o (P < 0.01). MCh significantly decreased maximum unloaded shortening velocity (0.045 +/ - 0.007 vs. 0.059 +/- 0.007 Lds), maximal extent of muscle shortening (8.4 +/- 1.2 vs. 13.9 +/- 2.4% L-0), and peak isometric tension (6.1 +/- 0.8 vs. 7.2 +/- 1.0 mN/mm(2)). Alb restored all these contractile indexes to basel ine values. These findings suggest that MCh reversibly increased the number of active actomyosin cross bridges under resting conditions, limiting furt her muscle shortening and active tension development. After the electricall y induced contraction, muscles showed a transient phase of decrease in tens ion below preload, This decrease in tension was unaffected by afterload lev els but was significantly increased by MCh and reduced by Alb. These findin gs suggest that the cross bridges activated before, but not during, the ele ctrically elicited contraction may modulate the phase of decrease in tensio n below preload, reflecting the active part of resting tension.