Regulation of adipocyte insulin receptor number and metabolism during late-pregnancy

The regulation of the rate of fatty acid synthesis of rat adipose tissue during late-pregnancy has been investigated. Rats at day 18 of pregnancy were injected over a 2-day period with either prostaglandin F2 alpha (PGF2 alpha), PGF2 alpha plus progesterone, PGF2 alpha plus bromocriptine or carrier solutions, and were then killed on day 20 of pregnancy. Injections of PGF2 alpha resulted in a decreased rate of fatty acid synthesis, a lower serum-insulin concentration, and a reduced number of insulin receptors of adipocytes. Concurrent injections of progesterone, but not of bromocriptine, along with the PGF2 alpha prevented these effects of the latter. The results are consistent with our previous suggestion that the fall in serum progesterone concentration prior to parturition results in a reduction in the number of insulin receptors of adipocytes, which, along with a fall in the serum-insulin concentration, leads to a decrease in the anabolic activity of the tissue.