Mouse embryonic stem (ES) cells differentiate in vitro into a variety of cell types including spontaneously contracting cardiac myocytes. We have utilized the ES cell differentiation culture system to study the development of the cardiac contractile apparatus in vitro. Difficulties associated with the cellular and developmental heterogeneity of this system have been overcome by establishing attached cultures of differentiating ES cells, and by the micro-dissection of the contracting cardiac myocytes from culture. The time of onset and duration of continuous contractile activity of the individual contracting myocytes was determined by daily visual inspection of the cultures. A functional assay was used to directly measure force production in ES cell-derived cardiac myocyte preparations. The forces produced during spontaneous contractions in the membrane intact preparation, and during activation by Ca2+ subsequent to chemical permeabilization of the surface membranes were determined in the same preparation. Results showed a transition in contractile sensitivity to Ca2+ in ES cell-derived cardiac myocytes during development in vitro. Cardiac preparations isolated from culture following the initiation of spontaneous contractile activity showed marked sensitivity of the contractile apparatus to activation by Ca2+. However, the Ca2+ sensitivity of tension development was significantly decreased in preparations isolated from culture following prolonged continuous contractile activity in vitro. The alteration in Ca2+ sensitivity obtained in vitro paralleled that observed during murine cardiac myocyte development in vivo. This provides functional evidence that ES cell-derived cardiac myocytes recapitulate cardiogenesis in vitro. Alterations in Ca2+ sensitivity could be important in optimizing the cardiac contractile response to variations in the myoplasmic Ca2+ transient during embryogenesis. The potential to stably transfect ES cells with cardiac regulatory genes, together with the availability of a functional assay using control and genetically modified ES cell-derived cardiac myocytes, will permit determination of the functional significance of altered cardiac gene expression during cardiogenesis in vitro.

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