Ontogeny of immunoreactive insulin in the fetal bovine pancreas.

The aim of this study was to characterize the development of immunoreactive insulin (IRI) in the fetal bovine pancreas. Pancreatic IRI was acid extracted, and both pancreatic and serum IRI were quantitated by RIA. The amount of pancreatic IRI per wet tissue wt in first trimester fetuses was similar to that in the adult animal (8.2 +/- 0.7 and 5.9 +/- 1.7 U/g pancreas, respectively). IRI increased progressively during gestation, attaining 39.2 +/- 6.5 U/g pancreas in the third trimester, 7-fold higher than that in the adult. When pancreatic IRI concentrations were standardized for protein content of the extracts, a decrease was noted between the midsecond and third trimesters. This is most likely the result of dilution of the endocrine portion of the pancreas by the rapidly growing exocrine pancreas. IRI was also detectable in fetal sera from all three trimesters. In contrast to the profile for pancreatic concentrations of IRI, serum concentrations remained constant throughout gestation at approximately 20 microU/ml. Poly(A+)RNA was isolated from adult and fetal pancreata, and the relative levels of preproinsulin mRNA were assessed by DNA/RNA filter hybridization. There was a 2- to 3-fold increase in the relative level of preproinsulin mRNA in fetal pancreata between the first and second trimesters which was maintained through the third trimester. In the adult pancreas, preproinsulin mRNA levels were similar to those in the first trimester fetus. This profile for the ontogeny of pancreatic preproinsulin mRNA was similar to that for pancreatic IRI (units per pancreas) during fetal maturation. We conclude that in the bovine fetus: the endocrine pancreas synthesizes IRI during all three trimesters of development; pancreatic (units per g pancreas), but not serum, concentrations of IRI increase progressively as development proceeds; and the ontogeny of preproinsulin mRNA is paralleled by that of pancreatic IRI (units per pancreas).