STUDY OF THE INTRACELLULAR CONTRACTILE MECHANISM OF THE URINARY BLADDER SMOOTH MUSCLE USING SKINNED FIBER THECHNIQUE

Abstract

The intracellular contractile mechanism of the urinary bladder smooth muscle was studied using the saponin-treated skinned fiber in which cell membrane was chemically removed.
The chemically skinned bladder muscle showed a tension development which was dependent on Ca²⁺-concentration. The minimal Ca²⁺-concentration for the tension development was 2×10^-7M Ca²⁺. The maximal tension was induced at 10^-5M. This maximal tension was approximately the same as the K²⁺-induced tension development observed in the intact muscle. In addition, SDS-polyacrylamide gel electrophoresis showed that the contractile proteins were still preserved in the saponin-treated bladder smooth muscle. The Ca²⁺-concentration-tension response curve shifted to the left with an increase in MgATP concentration (from 3mM to 7mM), indicating that the sensitivity of the skinned muscle was affected by MgATP.
Mg²⁺ above 6mM caused a slow tension development by itself in the absence of Ca²⁺.
Ca²⁺-induced tension development was blocked by the addition of W-7 (calmodulin antagonist).
This result suggested that calmodulin (Ca²⁺-binding protein) regulates the actin-myosin interaction in the urinary bladder smooth muscle.
Caffeine solution (25mM) caused a rapid tension development in the skinned bladder smooth muscle which was loaded with Ca²⁺-concentration, however, this tension development decreased when the loaded Ca²⁺-concentration exceeded 10^-6M.
It seems from this result that “Ca-induced-Ca release mechanism” also exists in the urinary bladder smooth muscle.