We have sought to determine whether the reported age-related increase in in vivo and in vitro PRL secretion in intact and/or postpubertally ovariectomized female rats results from increased number and/or function of lactotropic cells. Immunocytochemical staining with antirat-PRL serum, goat antirabbit serum, and peroxidase-antiperoxidase using H2O2 and 4-chloro-l-naphthol or 3′3′-diaminobenzidine hydrochloride as substrates was used to compare the percentages of lactotropes in primary suspensions of heterogeneous adenohypophyseal cells from groups of intact and 3-day ovariectomized mature (6 to 7 month) Wistar rats at estrus (E) or diestrusl-2 (D) with those from the corresponding old (22 to 24 month) rats in constant estrus (CE) or constant diestrus (CD). For both intact and ovariectomized old vs. mature rats, there were small (30–35%) but significant (P < 0.005) increases in lactotrope number. Lactotrope number did not differ (P > 0.005) between E vs. D rats, or CE vs. CD rats, either before or after ovariectomy; moreoever, ovariectomy per se did not alter (P > 0.005) lactotrope number in any of the groups. Electron microscopic examination of lactotropic cells derived from ovariectomized CE rats revealed ultrastructural changes compatible with estrogenic hyperstimulation; such alterations were not observed in cells from the corresponding E rats. Basal and 17β-estradiol (10−10 M)-stimulated in vitro PRL secretion was measured for 4 days in primary cultures from each of the above groups of rats. After correction for lactotrope number, PRL secretion was greater (P < 0.01) from cells of intact and ovariectomized CD vs. D rats, whereas 17β-estradiolstimulated (P < 0.001) but not basal (P > 0.005) PRL secretion was greater from the corresponding CE vs. E rats. There was no effect (P > 0.005) of reproductive status or of short term ovariectomy on in vitro PRL secretion. These data suggest that aging in the female rat is associated with alterations in both the number and function of pituitary lactotropic cells.
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