Studies on the growth of the fetal guinea pig. Changes in the plasma concentration of sulphation-promoting activity and of insulin-like growth factors during gestation.

Plasma sulphation-promoting activity, insulin-like growth factors (IGF)-I and -II and insulin concentrations have been measured in the developing guinea. Sulphation-promoting activity fell progressively from midgestation to term in a manner that correlated fairly closely with the decline in the fractional rate of body and liver growth. The changes in plasma concentrations of IGF-I, & -II were quite different, both being substantially higher than fetal plasma insulin concentration. IGF-I, like insulin, plasma concentration rose between 35 and 50 days, then declined as term approached. These changes correlated with phases of lipid, DNA and RNA synthesis, but not of glycogen synthesis in the fetal liver. Fetal Plasma IGF-II concentration was comparatively low until about 55 days gestation then rose sharply, at the time that large quantities of glycogen are deposited in the fetal liver and myelation is initiated in the fetal brain, to achieve very high levels by 60-63 days then declining as term approaches. Plasma IGF-I and -II, measured by radioimmunoassay and radioreceptor assay respectively, showed the same cross-reactivity or binding characteristics as peptides in human or rat plasma. On separation by hplc IGF-I consisted of two major fractions and IGF-II of one. This picture was the same if the peptides were from fetal or adult plasma. There was no evidence of a distinctly fetal class of IGF. Plasma IGF-I and -II concentrations did not correlate with changes in fetal size or fractional rate of growth.