Variables controlling the secretion of a somatomedin-like peptide by cultured porcine smooth muscle cells.

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Abstract
Somatomedin-C, a peptide growth factor that is also termed insulin-like growth factor I, stimulates smooth muscle cell replication; however, these cells proliferate in somatomedin-C-deficient medium. We postulated, therefore, that these cells might release a somatomedin-like molecule into the surrounding culture medium. Porcine aortic smooth muscle cells that were exposed to serum-free Dulbecco's modified minimum essential medium for 24 hours were found to release a somatomedin-like molecule that could be detected by a specific radioimmunoassay for somatomedin-C. Several variables in the design of tissue culture conditions were found to regulate the secretion of the somatomedin-like peptide. There was an inverse relationship between somatomedin-like peptide secretion and culture density. Cultures grown to a density of 110,000 cells/well secreted 0.18 +/- 0.02 U/ml per 10(5) cells, whereas cultures grown to 38,000 cells/well secreted 0.59 +/- 0.06 U/ml per 10(5) cells (P less than 0.001). Serum deprivation and days of preincubation before initiation of an experiment were found to influence somatomedin-like peptide secretion significantly. Platelet-derived growth factor, which stimulates smooth muscle cell replication, was a potent stimulant of somatomedin-like peptide secretion. Exposure to a concentration of 500 ng/ml platelet-derived growth factor induced a 2.8-fold increase in somatomedin-like peptide secretion over basal levels, (i.e., 0.37 +/- 0.05 to 0.98 +/- 0.7 U/ml, P less than 0.001). The effect of platelet-derived growth factor was enhanced (41% increase) by concomitant incubation with 1% somatomedin-C-deficient platelet poor plasma.(ABSTRACT TRUNCATED AT 250 WORDS)