LOCALIZATION, CHARACTERIZATION, AND 2ND-MESSENGER COUPLING OF PITUITARY ADENYLATE CYCLASE-ACTIVATING POLYPEPTIDE RECEPTORS IN THE FETAL HUMAN ADRENAL-GLAND DURING THE 2ND-TRIMESTER OF GESTATION

The distribution and pharmacological properties of pituitary adenylate cyclase-activating polypeptide (PACAP) receptors were studied in the fetal human adrenal gland during the second trimester of gestation. Au toradiographic studies, using [I-125]PACAP27 as a radioligand, reveale d that PACAP-binding sites are exclusively located on chromaffin cells of adrenals from fetuses 14-20 weeks old. Biochemical characterizatio n of binding revealed the occurrence of a single class of PACAP-bindin g sites with a dissociation constant value of 0.32-0.74 nmol/L and a b inding capacity of 0.30-0.81 pmol/mg wet tissue. PACAP27 and PACAP38 w ere equipotent in competing for [I-125]PACAP27 binding (IC50 = 0.28-0. 64 nmol/L and 0.15-0.81 nmol/L, respectively), and the Hill coefficien ts were close to 1. In contrast, vasoactive intestinal polypeptide was much less efficient in displacing the tracer (IC50 = 4-362 nmol/L), a nd the Hill coefficients were less than 0.6. PACAP38 induced a dose-de pendent increase in cAMP production in fetal human adrenal cell suspen sion (ED50 = 0.07+/-0.02 nmol/L), as well as in cells maintained in cu lture for 5 days (5.4+/-1.8 nmol/L). In constrast, PACAP38 induced a m odest increase in inositol phosphate formation. These data indicate th at type I PACAP receptors are present in the early stages of the human medulla organization during the process of migration of chromaffin ce lls from the periphery to the central part of the gland. The present r esults suggest that PACAP could be involved in the regulation of the h uman adrenochromaffin cells during ontogenesis.