On the down-regulation of cross-bridges occurring as changes in the cycling rate during tonic smooth muscle contractions.

As regards cross-bridge down-regulation, the following statements may summarize the results of our experiments: 1) The time course of cross-bridge down-regulation is the initial high turn-over rate followed by a continuous retardation without any distinct loss in force generation. These mechanisms allowed a rapid force development and an improvement in force maintenance. 2) Cross-bridge down-regulation was seen in the tracheal preparation of both the rat and the guinea pig. As indicated by the greater time constants of the post-vibration force recovery, the cross-bridge cycling rate seems to be distinctly lower in the guinea pig than in the rat trachea. 3) Cross-bridge down-regulation only occurs in smooth muscle preparations with intact cell membranes. This result would seem to hint that the sarcoplasmatic calcium concentration and/or the intactness of the cell membrane might be essential for producing cross-bridge down-regulation. 4) The recovery from cross-bridge down-regulation requires a rest period of more than 10s. The cross-bridges remain down-regulated during this time even after an increase in the sarcoplasmatic calcium concentration which was induced by a subsequent stimulus. This result indicates that changes in the sarcoplasmatic calcium cannot explain all the problems arising during cross-bridge down-regulation.