Effects of muscle length and load on intracellular Ca2+ in tracheal smooth muscle.

Canine tracheal smooth muscle strips were loaded with the bioluminescent Ca2+ indicator aequorin, mounted in a tissue bath, and attached to an electromagnetic lever to investigate the effect of changes in muscle length and load on cytosolic free Ca2+. The intracellular Ca2+ concentration ([Ca2+]i) was inversely correlated with muscle length and active force developed during isometric contractions elicited by electrical field stimulation. In addition, quick release to either constant length or constant load at any time point during an active contraction resulted in an increase in [Ca2+]i. These observations are consistent with the possibility that the binding of Ca2+ to contractile or regulatory proteins is decreased when muscle length or active force development is decreased, resulting in the release of bound Ca2+ into the cytoplasm. However, the possibility that changes in muscle length or load affect other mechanisms that regulate [Ca2+]i cannot be excluded.