Adrenal growth and ontogeny of adrenal substance P, enkephalins and catecholamines in the ovine fetus

Biologically active peptides have been identified in the adrenal glands of several adult mammalian species. Some of these peptides appear to modulate the nicotine-induced catecholamine release from the adrenal medulla. The present study was carried out to investigate the presence and ontogeny of the peptides substance P, met-enkephalin and leu-enkephalin in the ovine fetal adrenal gland from 70 to 140 days gestation (term = 145-150 days). Concurrently, the growth of the fetal adrenal as well as the gestational changes in catecholamine content were determined. The maternal adrenal glands were also studied for comparison. The ovine fetal adrenal gland increased in weight with advancing gestation at a single exponential rate. Total adrenal substance P content correlated with gestational age, while met-enkephalin, leu-enkephalin and total catecholamine contents correlated with adrenal weight. The adrenal content (normalized as per unit protein) of substance P was highest in the young fetuses at 70 days gestation, decreased progressively towards term and, in the adult levels were significantly lower than those measured in the fetuses. The contents of met-enkephalin and leu-enkephalin were low in the young fetuses at 70 days gestation, but reached high levels at 130 to 140 days gestation. Maternal adrenal contents of the two enkephalins were significantly lower than those measured in the near-term fetal adrenal. Total catecholamine content in the fetal adrenal medulla increased as the fetus matured. Norpinephrine was the primary catecholamine present in the medulla of fetuses at 70 and 80 days gestation, while epinephrine was the major one in the adult. These results demonstrated that the fetal adrenal gland grew at a single exponential rate, instead of a biphasic rate. In addition, the ontogeny of substance P, met- and leu-enkephalin in the fetal adrenal followed different developmental patterns, and were different from that of the catecholamines.