Reflections on what makes the heart grow.

Under conditions of experimental cardiac overload and hypertrophy in rats, a digoxinlike immunoreactivity appears in their serum which is correlated with cardiac growth. It is hypothesized that this is caused by the presence of an endogenous cardiotropic factor displaying cross immunoreactivity with digoxin. Additional evidence of the existence of the putative cardiotropic factor is provided by the finding that the sera of rats with cardiac overload displaying digoxinlike immunoreactivity stimulate the multiplication of rat cardiac myocytes in the tissue culture. This factor may be an adrenal steroid different from corticosterone and aldosterone. The name endocardin or endocardiotonin for this substance is suggested.