Abstract. A defined cultivation system was developed for the differentiation of pluripotent embryonic stem cells of the mouse into spontaneously beating cardiomyocytes, allowing investigations of chronotropic responses, as well as electrophysiological studies of different cardioactive drugs in vitro.

The β-adrenoceptor agonists (—)isoprenaline and clenbuterol, the mediators of cAMP metabolism, forsko-lin and isobutylmethylxanthine (IBMX), the α1-adreno-ceptor agonist (—)phenylephrine, and the heart glyco-side digitoxine induced a positive, the muscarinic cholin-oceptor agonist carbachol and L-type Ca2+ channel blockers nisoldipine, gallopamil and diltiazem induced a negative chronotropic response.

In early differentiated cardiomyocytes β1-, α1-, but not β2-adrenoceptors, cholinoceptors, as well as L-type Ca2+ channels participated in the chronotropic response. In terminally differentiated cardiomyocytes β2-adrenoceptors and digitoxine responses were also functionally expressed.

The contractions of spontaneously beating cardiomyocytes were concommitant with rhythmic action potentials very similar to those described for embryonic cardiomyocytes and sinusnode cells. We conclude that cardiomyocytes differentiating from pluripotent embryonic stem cells are able to develop adrenoceptors and cholinoceptors and signal transduction pathways as well as L-type Ca2+ channels as a consequence of cell-cell interactions during embryoid body formation in vitro, independent of the development in living organisms.

The cellular system described may be useful as in vitro assay for toxicological investigations of chronotropic drugs and a model system for studying commitment and cellular differentiation in vitro.