137 DEVELOPMENT AND APPLICATION OF A HIGHLY SPECIFIC SOLID-PHASE RIA FOR ALPHA-MSH: LACK OF THE HORMONE IN PREGNANTS AND NEWBORNS

Alpha-MSH has been reported to occur in the plasma of pregnant women (Nature 273: 163, 1978). Since it appeared that the RIA used for those studies was not sufficiently sensitive and specific, we have developed a new solid-phase RIA for alpha-MSH with a sensitivity of 2-4 pg per ml of plasma and a negligible cross-reaction with other hormones (ACTH: <0.1%). Goats were immunized with chemically defined carrier protein-MSH complexes and the antibodies covalently coupled to a non-sedimenting acrylamide polymer. The new antibody-carrier complex allows an immune extraction from samples containing 30-50% plasma before addition of the tracer, so that any 'plasma effects' could be avoided. Applying this RIA to 136 samples from the 6th week of pregnancy till term and to cord blood samples showed clearly that they did not contain detectable alpha-MSH in the circulation. In contrast, alpha-MSH was shown to be present in fetal life, supporting the hypothesis of its trophic action on the adrenal. It seems therefore that the presence of MSH is restricted to the fetus and is independent from the mother.