REGULATION OF RAT CARDIAC MYOCYTE GROWTH BY A NEURONAL FACTOR SECRETED BY PC12 CELLS

Sympathetic innervation of cardiac myocytes in vitro induces growth in dependent of anatomic contact between the neurons and myocytes and is not mediated by alpha- or beta-adrenergic receptor stimulation. To est ablish a model system that will allow purification and identification of the neuronal factor(s) responsible for mediating this regulation, w e have initiated studies utilizing conditioned medium from the PC12 ce ll line, PC12 cells acquire a cholinergic sympathetic neuronal phenoty pe when exposed to nerve growth factor. Culture medium conditioned by neuronal PC12 cells, but not nonneuronal PC12 cells, induces growth in newborn rat cardiac myocytes as measured by surface area and [S-35]me thionine incorporation into protein and increases expres sion of atrio natriuretic peptide, a marker for myocyte hypertrophy. The magnitude o f the growth response is dose-dependent and mimics the response to sym pathetic innervation. The myocyte response to conditioned medium is no t detectable after 24 h of exposure; maximal rate of protein synthesis is obtained within 48 h. Neuronally differentiated PC12 cell-conditio ned medium stimulation of growth could not be mimicked by alpha- or be ta-adrenergic agonists or muscarinic agonists, nor inhibited by alpha- or beta-adrenergic antagonists, nor by muscarinic antagonists. Neurop eptide Y and somatostatin, peptides known to be present in PC12 cells and sympathetic neurons, were also ineffective at reproducing the effe ct of neuronally differentiated PC12 cell-conditioned medium, These da ta indicate that neuronal cells release a soluble factor, different fr om neurotransmitter, which stimulates myocyte growth. They further ide ntify the PC12 cell line as providing a convenient and abundant supply of this molecule, thus facilitating its further characterization.