The effects of Ca++ and its antagonists (verapamil and La+++) upon the spontaneous contraction and the contracture induced by 60 mM K-Tyrode solution were studied in the isolated uterine muscle. Longitudinal muscle strips were prepared from the rat uteri at estrous stage. All experiments were performed in tris-buffered Tyrode solution which was aerated with 100% O2 and kept at 35˚. The results obtained were as follows: 1) In the uterine strips contracting spontaneously, both the amplitude of peak tension and the area of contraction curve increased dose-dependently in the range of 0.5 ~ 8 mM Ca++. The frequency of contraction increased as the concentration of Ca++ increased up to 2 mM, but above this concentration the frequency decreased. In Ca++-free media, however, contraction did not develop. In the contracture induced by 60 mM K-Tyrode solution, the developed tension increased dose-dependently as the concentration of external Ca++ increased to 8 mM. In the absence of external Ca++ K-contracture appeared, but it was not sustained. 2) The spontaneous contraction of rat uterus was suppressed by verapamil in proportion to an increase of its concentration and totally abolished at the concentration of 3 X 10-4 g/l, but the spontaneous contraction re-appeared by addition of Ca++. The amplitude of peak tension recovered completely but the recovery of frequency was incomplete. K-contracture decreased in a dose-dependent manner after the treatment with verapamil and totally disappeared at its concentration of 3 X 10-4 g/l. Even in this case contracture developed again by extra Ca++. 3) The spontaneous contractile activity was inhibited by La+++. At the concentration of 10-4M La+++, fibrillation appeared. In the strip inhibited by 10-5M La+++, contractility recovered completely by extra Ca++ while in the 10-4M La+++ treated preparation, the rhythmic spontaneous contraction did not develop even at the concentration of 16 mM Ca++. After the initial transient depression of contracture tension by 10-3M of La+++, the strip stowed considerably large size of contracture, hardly influenced by external Ca++ or verapamil. The results obtained in this experiment suggest that in the rat uterine muscle there would be some competitive actions between Ca++ and its antagonists. It is speculated that Ca++ plays an important role in the conduction of excitation, and La+++ influences upon cellular Ca++ mobilization and re-uptake process as well as transmembrane Ca++ transport in a K-depolarized state.
