[Study of the intracellular contractile mechanism of the urinary bladder smooth muscle using skinned fiber technique].

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 X 10(-7) M Ca2+. The maximal tension was induced at 10(-5) M. This maximal tension was approximately the same as the K(+)-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 3 mM to 7 mM), indicating that the sensitivity of the skinned muscle was affected by MgATP. Mg2+ above 6 mM 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 (25 mM) 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(-6) M. It seems from this result that "Ca-induced-Ca release mechanism" also exists in the urinary bladder smooth muscle.