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.