Muscle length, shortening, myoplasmic [Ca2+], and activation of arterial smooth muscle.

The effect of muscle length on smooth muscle contraction was evaluated by measuring myoplasmic [Ca2+] (with aequorin), myosin light chain phosphorylation, length, and isometric stress in histamine-stimulated swine carotid media preparations. Tissues were equilibrated at the optimal length for stress development (Lo). Isometric contractions at short tissue lengths (0.7 Lo) were associated with a decrease in maximal stress development. Isometric contraction at 0.7 Lo also reduced the sensitivity to histamine as measured by steady-state increases in [Ca2+], phosphorylation, or stress. This suggests that decreased agonist sensitivity at shorter lengths is caused by reduced Ca2+ mobilization. Isotonic shortening also led to decreases in histamine sensitivity. Isometric contractions at 1.2 Lo were not associated with significant changes in histamine-induced increases in [Ca2+]. The [Ca2+] dependence of phosphorylation was not altered at 0.7 or 1.2 Lo. Sinusoidal length changes from 0.95 to 1.05 Lo at 1 Hz were not associated with significant changes in the resting or histamine-stimulated [Ca2+]. These results suggest that Ca2+ mobilization and the resulting contraction is relatively independent of length changes near Lo. Inactivation occurs at lengths substantially below Lo where Ca2+ mobilization by agonists is impaired.