ROLE OF TRANSIENTLY ALTERED SARCOLEMMAL MEMBRANE-PERMEABILITY AND BASIC FIBROBLAST GROWTH-FACTOR RELEASE IN THE HYPERTROPHIC RESPONSE OF ADULT-RAT VENTRICULAR MYOCYTES TO INCREASED MECHANICAL-ACTIVITY IN-VITRO

One of the trophic factors that has been implicated in initiating or f acilitating growth in response to increased mechanical stress in sever al tissues and cell types is basic fibroblast growth factor (bFGF; FGF -2), Although mammalian cardiac muscle cells express bFGF, it is not k nown whether it plays a role in mediating cardiac adaptation to increa sed load, nor how release of the cytosolic 18-kD isoform of bFGF would be regulated in response to increased mechanical stress, To test the hypothesis that increased mechanical activity induces transient altera tions in sarcolemmal permeability that allow cytosolic bFGF to be rele ased and subsequently to act as an autocrine and paracrine growth stim ulus, we examined primary isolates of adult rat ventricular myocytes m aintained in serum-free, defined medium that were continually paced at 3 Hz for up to 5 d. Paced myocytes, but not nonpaced control cells, e xhibited a ''hypertrophic'' response, which was characterized by incre ases in the rate of phenylalanine incorporation, total cellular protei n content, and cell size. These changes could be mimicked in control c ells by exogenous recombinant bFGF and could be blocked in continually paced cells by a specific neutralizing anti-bFGF antibody. In additio n, medium conditioned by continually paced myocytes contained Signific antly more bFGF measured by ELISA and more mitogenic activity for 3T3 cells, activity that could be reduced by a neutralizing anti-bFGF anti body, The hypothesis that transient membrane disruptions sufficient to allow release of cytosolic bFGF occur in paced myocytes was examined by monitoring the rate of uptake into myocytes from the medium of 10-k D dextran linked to fluorescein, Paced myocytes exhibited a significan tly higher rate of fluorescein-labeled dextran uptake, These data are consistent with the hypothesis that nonlethal, transient alterations i n sarcolemmal membrane permeability with release of cytosolic bFGF is one mechanism by which increased mechanical activity could lead to a h ypertrophic response in cardiac myocytes.