Shortening induced deactivation in the guinea-pig urinary bladder

Shortening induced deactivation, the depressant effect of active muscle shortening on the subsequently measured isometric force, has been shown in smooth muscle strips and rings. The guinea-pig bladder permits the investigation of this phenomenon in a whole organ preparation. Previous work in our laboratory showed that shortening of the in vitro guinea-pig detrusor muscle had a depressant effect on the isovolumetric pressure that could be generated immediately afterwards. To test the hypothesis that this was caused by deactivation, the effects of active and passive detrusor shortening on the subsequently measured isovolumetric pressure were compared. The isovolumetric pressures measured after 5 min periods of recovery were taken as control values. It was found that the isovolumetric pressure after passive shortening was 7% smaller than the isovolumetric pressure without preceding shortening. This difference was ascribed to viscoelastic relaxation during shortening. Active shortening had an additional 8% depressant effect on isovolumetric pressure compared with passive shortening. The effects of active and passive shortening differed significantly. It was concluded that shortening induced deactivation in the guinea-pig urinary bladder smooth muscle in toto can be considered proven. The fact that deactivation is shown both by striated and smooth muscle preparations is in line with the assumption that it is caused by reduced actin-myosin interaction. The hypothesis that (in striated muscle) the latter is effected by a decrease in troponin-calcium binding, however, needs reconsideration.