Ontogeny of Angiotensin-II-Induced Prolactin Release in vivo and in vitro in Female and Male Rats

The prolactin-releasing effect of angiotensin II (AII) was studied in the developing female and male rat in vivo and in vitro. AII (50 and 100 µg/100 g b.w.) was injected intraperitoneally to female and male rats aged 4, 12, 20 and 28 days and males aged 38 days. AII (10–6M) was also tested in pituitaries incubated in vitro from animals of both sexes aged 12, 20 and 28 days. In addition, as two subtypes of AII receptors have been characterized on the basis of displacement with specific AII antagonists, we used the nonpeptide AII receptor antagonists losartan (ATI subtype) and PD 123319 (AT2 subtype) to determine the AII receptor subtype functionally involved in AII-induced prolactin secretion in vivo in 25-day-old male rats. The efficiency of the prolactin-releasing effect of AII in vivo increased with age, and first responses were observed at 20 days of age in both sexes. No sexual differences were encountered. On the other hand, AII-induced prolactin release from pituitaries incubated in vitro was first demonstrated at 12 days in females and at 20 days in males. The effect increased with age in both sexes, and, at 28 days, pituitaries from females released more prolactin in response to AII than those from males. Losartan (3 mg/kg) completely abolished AII (50µg/100g b.w.)-induced prolactin release in vivo, while PD 123319 (3 mg/kg) did not. This suggests that pituitary ATI receptors are functionally involved in the prolactin release induced by AII in vivo. In vivo, the integrity of the hypothalamic-hypophyseal unit is maintained, and therefore the inhibitory influence of the dopaminergic system, which is stronger in the female developing rat, could mask the sexual differences encountered in vitro. Finally, increasing efficiency of AII-induced prolactin release may be related to increasing prolactin titers in juvenile and prepubertal rats.