A BIOLOGICAL ASSAY METHOD FOR CARDIOTONIC STEROIDS , USING FROG RECTUS ABDOMINIS MUSCLE

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Abstract
Many years ago Katagi (1) and Freund (2) reported that some cardiotonic steroids and their glycosides could induce a sustained contracture in such skeletal muscle rich in slow muscle fibres as rectus abdominis muscle or gastrocnemius muscle of the frog after a rather long latent period. However, any due attention was not paid to this finding until very recently, when the same phenomenon was re-observed in this laboratory (3). Further study of this phenomenon in this laboratory (4) revealed one and the same structural requirements for both the contracture-inducing action and the cardiotonic action of these compounds, although it was demonstrated that this contracture was of nervous origin, somehow mediated by acetylcholine. In order to test the possibility whether this phenomenon can be used as an assay method for the cardiotonic action of the cardenolides and bufadienolides, the relative potencies of the eight representative cardiotonic steroids were determined, using the length of the latent period of these compounds to produce contracture in the frog rectus abdominis muscle as a measure of their contracture-inducing activity, and was compared with the relative potencies of these compounds to produce cardiotonic action.