in a method for determining the influence of a test substance on the heart activity of a vertebrate the following steps are performed : a ) preparation of a culture of spontaneously active heart cells of the vertebrate , b ) extracellular measurement of electrophysiological data of the heart cells from step a ), c ) addition of the test substance to the culture from step a ), d ) repetition of the measurement from step b ), and e ) comparison of data from step b ) and step d )