Alteration of tension response to stretch with spontaneous contraction frequency in the guinea pig taenia coli and stiffness-work characteristics.

Previously we developed a data acquisition procedure to measure the tension responses of smooth muscles during their spontaneous contraction. This study was undertaken to determine whether the time course of the tension responses to stretches was altered with the ongoing spontaneous contractions. In addition, the stiffness, phase and work characteristics were evaluated from the tension responses in order to see how these characteristics were affected by the changes in the tension responses. The tension responses of the spontaneously contracting taenia coli to stretches had four phases. However, the latencies of the phases changed with the type of the spontaneous contractions during which the measurement was carried out. In the responses recorded during the slow spontaneous contractions (40-70 s), the second phase reached the minimum at 2.26 +/- 1.12 s and the third phase had the peak tension at 9.56 +/- 3.08 s. These latencies were 0.89 +/- 0.30 s and 3.91 +/- 1.10 s, respectively, when the experiments were undertaken during high frequency contractions (10-30 s). Besides, the maxima and minima in the stiffness, phase and work characteristics were also shifted to higher frequencies as the spontaneous contraction became faster. Work had a positive peak around 0.03 Hz for the slowly contracting taenia coli. The peak shifted to 0.1 Hz when the taenia coli contracted at higher frequency. The results show that the time course of the tension responses is determined by the contractile state of the taenia coli.