Prostaglandin E2 acts via the hypothalamus to stimulate ACTH secretion in the fetal sheep.

Prostaglandin (PG) E2 is secreted into the fetal circulation during late gestation in the sheep. Exogenous infusion of PGE2 is associated with robust increases in the circulating concentrations of ACTH and cortisol and it has therefore been proposed that endogenous PGE2 modulates the activity of the fetal HPA axis, which is the primary determinant of parturition and essential for the maturation of vital organ systems. The sites of action of PGE2 within the HPA axis have not been clearly established. We have compared the effects of PGE2 infusion on ACTH and cortisol concentrations in intact fetuses and in those whose pituitaries were surgically disconnected from hypothalamic control (hypothalamo-pituitary disconnected; HPD fetuses operated at 111-113 days gestational age). The effect of advancing gestational age on these responses was investigated by infusing PGE2 at 121, 131, 141 and 148 days of gestation. The functional integrity of the pituitary corticotrophs was tested by injecting CRH (1 microgram) into intact and HPD fetuses at day 125. This test was repeated for the HPD fetuses at 160 days. The responsiveness of the adrenal glands was also tested by injecting 2.5 micrograms/kg synthetic ACTH1-24 (Synacthen) into both groups of fetuses at day 135, with the HPD group retested at day 155. PGE2 infusion was associated with a robust increase (P < 0.001) in plasma immunoreactive (ir) ACTH in intact fetuses at all gestational ages while HPD fetuses did not respond, except at day 148 when the response was small. Similarly, cortisol concentrations were increased (P < 0.001) during PGE2 infusion in intact fetuses but not in HPD fetuses, except for a minor increase at 148 days. The response of irACTH to exogenous CRH was similar in intact and HPD fetuses at 125 days and this response was maintained in HPD fetuses at day 160. The cortisol response of intact fetuses to ACTH1-24 exceeded that of HPD fetuses at day 135 and, in HPD fetuses, this response was unchanged when they were retested at 155 days, indicating that their adrenal responsiveness did not wane with time. We conclude that the effects of PGE2 on the fetal HPA axis are exerted predominantly or exclusively at a level above the pituitary gland.

Biological activity of adrenocorticotropic hormone precursors on ovine adrenal cells.

Adrenocorticotropic hormone (ACTH) is synthesized in the corticotrophs as a precursor, pro-opiomelanocortin (POMC), which is processed via proACTH to ACTH. Both precursors and ACTH are secreted. Although the steroidogenic activity of ACTH is well characterized, that of the precursors is not. This study assessed the capacity of POMC and proACTH to alter cortisol synthesis. POMC and proACTH were prepared by subjecting medium, conditioned by exposure to DMS-79 cells, to Sephadex chromatography, and the bioactivity was assessed in cultured-dissociated ovine adrenal cells. Alone neither POMC (< or = 2.6 nM) nor proACTH (< or = 0.7 nM) showed any consistent acute (6 h) stimulatory or inhibitory action on cortisol in either fetal or adult cells. In contrast, in fetal cells the precursors inhibited steroidogenic response to ACTH-(1-24). POMC at 2.6 nM, but not lower concentrations, decreased the cortisol responses to 0.01, 0.1, and 1 nM ACTH by at least 50%. ProACTH (0.70 and 0.23 nM) decreased the responses to ACTH at 0.01 nM by 89 and 67%, respectively, and at 0.1 nM by 49 and 34%, respectively. At 1 nM ACTH only 0.7 nM proACTH decreased the response to ACTH (by 69%). In contrast, in adult adrenal cells, the precursors did not significantly reduce the response to ACTH (range 0.01-1 nM). Therefore, these data indicate that POMC and proACTH can inhibit the cortisol response to ACTH in fetal adrenal cells, an effect that is concentration dependent. The data suggest that precursors may play a physiological role, possibly regulating fetal plasma cortisol concentrations.

Characterization of extracellular matrix macromolecules from bovine synovial capsule.

Synovial capsule from the metacarpophalangeal joints of cattle was shown to be a highly collagenous tissue, with a hydroxyproline content of 100 +/- 1 micrograms/mg dry weight and a water content of 70 +/- 3.6%. Type-I collagen made up 83% of the collagen present, and the remainder was type III. When incubated in explant culture, synovial capsule incorporated [3H]acetate into both glycoproteins and hyaluronan and [3H]acetate and [35S]sulfate into proteoglycans. The rate of synthesis of proteoglycans by synovial tissue was shown to be similar to that measured for collateral ligament from the same joint. Two populations of proteoglycans were observed to be synthesized by synovial capsule. More than 90% of the 35S-labelled proteoglycans eluted with a K(av) of 0.7 on Sepharose CL-4B, and the remainder of the radiolabelled macromolecules eluted from the column with a K(av) of less than 0.5. Analysis of the major population of proteoglycans showed it to consist of a dermatan sulfate-containing proteoglycan with a core protein of 45,000 Da that had the same N-terminal amino acid sequence as decorin.