Abstract
Actions of cardiac steroids (CS) such as bufadienolides and cardenolides, on guinea pig taenia coli were studied using the double sucrose-gap method. When an appropriate dose of CS was applied, the taenia coli first contracted, then relaxed. After removal of CS, the relaxation was enhanced and continued for 20-40 min. In decreasing order of the relaxant action were bufalin, ouabain, cinobufagin and resibufogenin. Further application of CS after occurrence of the relaxation induced a secondary gradual contraction. The contraction occurred with the membrane depolarization and increase in spike discharge, and the relaxation corresponded fairly well with decrease in spike discharge and the membrane repolarization. Membrane resistance was decreased during the contraction as well as the relaxation. The decrease in membrane resistance continued during the relaxation after removal of CS. Na, K and Ca conductances were increased by CS application, respectively. The increase of Na conductance was relatively high during the contraction, and the increase of K conductance was remarkable during the relaxation and after removal of CS. From the foregoing results it is considered that the contracting action is due to inhibition of the Na pump and that the relaxant action is due to the change in electrical properties of membrane produced by a marked increase in intracellular Na and the increase of K permeability. The relaxation after removal of CS is considered to be due to the activated electrogenic Na pump as indicated from the changes in electrical properties of membrane.
