IGF-I and bFGF differentially influence atrial natriuretic factor and alpha-smooth muscle actin expression in cultured atrial compared to ventricular adult rat cardiomyocytes.

In the present study, we compare expression, storage and secretion of the atrial natriuretic factor (ANF) in atrial and ventricular adult rat cardiomyocytes (aARC and vARC) in long-term culture. The influence of insulin-like growth factor-I (IGF-I) and of basic fibroblast growth factor (bFGF) on ANF production and secretion, as well as on the expression of a structural component, alpha-smooth muscle actin (alpha-sm actin), was studied in the two cell types. Antibodies against alpha-ANF were used for immunocytochemical localization of ANF. aARC contained more ANF-granules than vARC, and they were distributed throughout the cell bodies. Quantitative determination of ANF storage and secretion was done by radioimmunoassay (RIA; 125I), and it was demonstrated that aARC stored and secreted ANF 18- and 16-times more, respectively, when compared to vARC. Immuno-electron microscopy confirmed that ANF storing secretory granules were present in both types of cardiomyocytes. Expression of ANF and alpha-sm actin in aARC and vARC responded differently to treatment with either IGF-I or bFGF. In aARC, neither IGF-I nor bFGF had an influence on expression of ANF. In vARC, expression of ANF was downregulated by IGF-I and upregulated by bFGF with regard to both immunoreactivity and message. In contrast to vARC, expression of alpha-sm actin was not affected by IGF-I in aARC, whereas bFGF produced a strong upregulation similar to that found in vARC. Mitogen-activated protein kinases (MAPK) 42 and 44, though, were equally activated by bFGF and IGF-I in both aARC and vARC.