Ca²⁺-. Sr²⁺-and Ba²⁺- Activated Tension of Glycerinated Smooth Muscles of Guinea-pig Taenia Coli

Abstract

1. The effects of free Ca²⁺, Sr²⁺ or Ba²⁺ concentration on the tension of the glycerinated smooth muscles of guinea pig taenia coli induced by ATP were examined in the presence of chelating agent, Mg²⁺ and ATP, and the sensitivity of the preparation to Sr²⁺ and Ba²⁺ was compared with that to Ca²⁺. 2. In the presence of 5mM MgCl₂ and 5mM ATP, a contracted glycerinated smooth muscle was relaxed by the addition of 1mM EGTA and the tension decreased by 75%, which was reversed by the addition of 1mM CaCl₂. The result indicates that a minute amount of Ca²⁺ contributes to the contraction of the preparation. 3. After removal of Mg²⁺, the ATP-contraction decreased to almost zero level. The result indicates that Mg²⁺ is essential for the ATP-contraction of the preparation. 4. The tension of the preparation increased with the increase of ATP concentration up to 4mM in the presence of 5mM Mg²⁺ without adding Ca²⁺. However, in the concentration range of ATP from 4mM to 7mM no futher increase of the tension was observed. 5. The threshold concentration of free Ca²⁺ for the ATP contraction was measured in the Ca-buffer solution and it was pCa = 5.5; the maximum tension being produced at pCa = 3.8. 6. The pSr-tension curve shifted to the right compared with that of pCatension curve; the threshold concentration was pSr=4 and the almost maximal tension was recorded at pSr = 2.6. 7. The pBa-tension curve shifted to the right in comparison with that of pSr-tension; the threshold concentration was pBa=3.5 and only 76.2% of the maximum tension was obtained at pBa = 2.6. 8. The relative sensitivity of the preparation to Sr²⁺ and Ba²⁺ as compared with that to Ca²⁺ was Ca²⁺ : Sr²⁺ : Ba²⁺ =1 : 1/28 : 1/67.