Cardiomyocytes differentiated in vitro from embryonic stem cells developmentally express cardiac-specific genes and ionic currents

Cardiomyocytes differentiated in vitro from pluripotent embryonic stem (ES) cells of line D3 via embryo-like aggregates (embryoid bodies) were characterized by the whole-cell patch-clamp technique during the entire differentiation period. Spontaneously contracting cardiomyocytes were enzymatically isolated by collagenase from embryoid body outgrowths of early, intermediate, and terminal differentiation stages. The early differentiated cardiomyocytes exhibited an outwardly rectifying, transient K⁺ current sensitive to 4-aminopyridine and an inward Ca²⁺ current but no Na⁺ current. The Ca²⁺ current showed all features of L-type Ca²⁺ current, being highly sensitive to 1,4-dihydropyridines but not to ω-conotoxin. Cardiomyocytes of intermediate stage were characterized by the additional expression of cardiac-specific Na⁺ current. The delayed K⁺ current, and I(f) current. Terminally differentiated cardiomyocytes expressed a Ca²⁺ channel density about three times higher than that of early stage. In addition, two types of inwardly rectifying K⁺ currents (I(K1) and Ig(K.Ach)) and the ATP-modulated K⁺ current were found. During cardiomyocyte differentiation, several distinct cell populations could be distinguished by their sets of ionic channels and typical action potentials presumably representing cardiac tissues with properties of sinus node, atrium, and ventricle. Reverse transcription polymerase chain reaction revealed the transcription of α- and β-cardiac myosin heavy chain (MHC) genes synchronously with the first spontaneous contractions. Transcription of embryonic skeletal MHC gene at intermediate and terminal differentiation stages correlated with the expression of Na⁺ channels. The selective expression of α-cardiac MHC gene in ES cell-derived cardiomyocytes was demonstrated after ES cell transfection of the LacZ construct driven by the α-cardiac MHC promotor region followed by ES cell differentiation and β- galactosidase staining. In conclusion, our data demonstrate that ES cell- derived cardiomyocytes represent a unique model to investigate the early cardiac development and permit pharmacological/toxicological studies in vitro.