During the development of the murine heart, a variety of electrophysiological changes takes place: both the levels of expression of ion channels and their regulatory properties are altered (Kojima et al. 1990;Maki et al. 1996). During early murine embryogenesis, it has been shown that the L_type Ca¥ channel current (ICa) plays a dominant role in excitation, whereas the slow component of the delayed rectifier K¤ channel current (IK,s) is not so apparent until the later stages of embryonic development, or even after birth (Honore et al. 1991;Davies et al. 1995). Also, during development of the mouse heart, the â_adrenergic signalling pathway has been found to be functionally incomplete until

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