This invention involves a mouse cardiac cell line (HL-1) derived from a transplantable mouse cardiomyocyte lineage (AT-1) which can be serially passaged in culture greater than two hundres times and retain the characteristics of adult atrial cardiac muscle cells. This invention also relates to a cell culture system which can be used as an in vitro model of cardiac muscle cells to screen and test cardiac drugs. This invention also relates to the use of this mouse cardiac cell line to produce factors in cardiac cells.