DEVELOPMENT OF A MUSCLE CELL CULTURE BIOASSAY FOR MEASURING PROLIFERATION-PROMOTING ACTIVITY IN SWINE SERU

A bioassay using L6 muscle cells in culture was developed and standardized for use in assessing the proliferation-promoting activity of swine serum. A method of standardization among experiments was based on the observed linear relationship between controls (2.5% swine serum) and controls plus 10('-7)M insulin. The regression equation is used to adjust data obtained on different experiments so that inter-assay comparisons can be made. This is the first reported standardization method for myogenic cell culture systems. Increases in proliferation-promoting activity were observed when blood serum was obtained from restrained, catheterized swine or via jugular veinpuncture of restrained swine compared to sera from un-restrained, catheterized swine. Because of these differences, blood samples were obtained from un-restrained catheterized animals in all studies reported here. Significant differences in proliferation-promoting activity ranging from 12% to 30% were observed in sera obtained from swine every 6 hours for 4 consecutive days. There was no correlation with blood levels of cortisol, insulin, somatomedin C (SMC) or pGH. Pooled serum samples taken over a 24 hour period each day for 3 consecutive days exhibited much less variability (6.3%). Consequently, 24 hour pools of sera were used to assess differences in serum proliferation-promoting activity in swine subjected to an imposed treatment. Pituitary pGH injected into 3 barrows at 0.027U/kg/day for 3 days had no effect on serum proliferation-promoting activity or SMC levels in the blood when compared to pre-injection levels. A higher dose of 0.21U/kg/day injected into 5 barrows for 3 days resulted in significant increases in serum proliferation-promoting activity in 4 out of 5 pigs (12% to 24% over pre-injection levels). In most instances, these increases paralleled increases in serum levels of SMC which ranged from 1.7 to 4 times pre-injection concentrations. Sera from the pig which exhibited no increase in proliferation-promoting activity had elevated levels of SMC (3 times pre-injection levels). It was concluded that pGH has an indirect effect on serum proliferation-promoting activity via increasing serum SMC levels, however, other growth factors or inhibitors must also be influencing the bioassay responses.