Measurement of tracer efflux during cardiac cycle

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Abstract
A phasic efflux of K42 during the cardiac cycle, first described by Wilde in the turtle heart and later observed by us in perfused strips of frog ventricle, has been re-examined. With an improved technique, studies have been carried out on preparations of amphibian atrium and ventricle and mammalian [rabbit] papillary muscle using K42 and other radioactive ions, as well as labeled substances known to be largely restricted to the extracellular space. It is concluded that the efflux peaks seen in these studies are an artefact of the mechanical systole, and do not represent a primary increase in the unidirectional transmembrane movement of the tracer, as previously supposed.