Cell Interactions in Cardiac Development: (induction/local cues/heart development/heartrate/ion channels)

During early heart formation, the pre-cardiac mesoderm becomes regionally differentiated into segments destined to form ventricle, atria and sinoatrial tissue. Each region develops a characteristic beatrate and form of action potential, shaped by current through specific ion channels and membrane pumps. Fragments of pre-sinoatrial mesoderm that would normally have a rapid intrinsic beatrate, develop into beating heart tissue with a slow beatrate, characteristic of the ventricle, when transplanted into the prospective conoventricular region at stage 6-7. These transplants also express the ventricular isoform of myosin heavy chain, suggesting that regional commintment of the precardiac mesoderm is influenced by local cues. Application of the patch-clamp technique to single cells isolated from the ventricle of hearts at different ages during the first week of embryonic development has revealed changes in four currents that underlie the shaping of the ventricular action potential: the excitatory sodium current, the inward rectified K current, the delayed rectifier K current, and the T-type Ca current.