[Mechanism of the contracting actions of K, acetylcholine and Ba on the isolated rat rectum, with special reference to its relationship to Ca].

The contractile mechanism of K, ACh and Ba were investigated in relation to the mobilization of Ca in the rat's isolated rectum. Contractions by K, ACh and Ba consisted of the phasic contraction and the subsequent tonic contraction. From the effects of metabolic inhibition (anoxia, DNP) and Ca removal from bath solution on the contractions by these three agents, the following assumptions have been made: the phasic contractions by K and ACh are iduced by the release of Ca from the store sites and the passive influx of Ca, whereas that by Ba is induced by the release of Ca. Tht tonic contractions by K and ACh are maintained mainly by the active influx of Ca, whereas that by Ba is maintained by the active influx of Ca and the release of Ca. In addition, the direct stimulation to the contractile elements of muscle is partly concerned with the Ba-induced contraction. In the muscle cell membrane, three divisions of Ca store; the first is the store where Ca is most easily released, the second is the store where Ca is comparatively easily released and the third is the store where Ca is not easily released. K releases Ca from the first store, ACh does so from the first and second stores, and Ba does so from these three stores for producing contraction. These assumption were further supported by the analysis of the concentration action curves of the influence of Ca removal from bath solution on the phasic contractions and the tonic contractions induced by the three agents.