Protein deficiency in pregnant rats causes decreased levels of plasma somatomedin and its carrier protein associated with reduced plasma levels of placental lactogen and hepatic lactogenic receptor number.

Rats were fed either a 20% lactalbumin (control) or a 5% lactalbumin (low protein) diet for the last 2 weeks of pregnancy. At day 20 of gestation, rat serum placental lactogen levels, measured by radioreceptor assay, were significantly decreased by the low protein diet, thus confirming our earlier findings. The number of microsomal membrane lactogenic receptors, measured on the maternal livers at the end of pregnancy, was severely reduced in the livers of the low protein group, whereas protein deficiency did not affect binding affinity. Serum concentrations of somatomedin, measured by a competitive binding assay after acid treatment of the serum to remove endogenous carrier protein, were extensively reduced in the low protein group. The amounts of the somatomedin carrier proteins in the serum were assayed by separation on Sephacryl-S300 columns into higher- and lower-molecular-weight fractions peak 2 and peak 3, respectively. For the low protein diet group, both fractions showed a reduction in binding capacity, more marked in the case of peak 2. Since placental lactogen is known to influence output of somatomedin by the liver, we hypothesize that protein deficiency during pregnancy causes a fall in serum somatomedin level by reducing secretion of placental lactogen, which regulates its production by the liver.