Mechanisms of the contracting action of K, acetylcholine and Ba on the isolated rat rectum, especially in relation to Ca

Abstract

The contractile mechanisms of K, ACh [acetylcholine] 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 and Ca removal from bath solution on the contractions by these 3 agents, the following assumptions were made: the phasic contractions by K and ACh are induced 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. The 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. The direct stimulation to the contractile elements of muscle is partly concerned with the Ba-induced contraction. In the muscle cell membrane, 3 divisions of Ca stores; the 1st is the store where Ca is most easily released, the 2nd is the store where Ca is comparatively easily released and the 3rd is the store where Ca is not easily released. K releases Ca from the 1st store, ACh does so from the 1st and 2nd stores, and Ba does so from these 3 stores for producing contraction. These assumptions 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 3 agents.