Temporal effects of estradiol and diethylstilbestrol on pituitary and plasma prolactin levels in ovariectomized Fischer 344 and Holtzman rats: a comparison of radioimmunoassay and Nb2 lymphoma cell bioassay.

An Nb2 lymphoma cell bioassay (Nb2BA) and a radioimmunoassay (RIA) were used to compare plasma and pituitary levels of prolactin in ovariectomized Fischer 344 (F344) and Holtzman rats treated with either diethylstilbestrol (DES) or estradiol for up to 8 weeks. The objectives were to determine whether there were temporal differences in prolactin responses in strains with different genetic predispositions to estrogen-induced pituitary tumor formation and to determine whether the results of the two assay methods were equivalent. All rats were ovariectomized for 7 days and all except controls received subcutaneous Silastic implants of DES or 17 beta-estradiol and were sacrificed at intervals from 2 days to 8 weeks later. Pituitary content and plasma levels of prolactin were determined by Nb2BA and RIA and the ratio of these measurements was calculated. DES induced a significant increase in pituitary prolactin in F344 rats by 2 days of treatment, as measured by RIA. Pituitary content increased to a peak by Day 4, after which a gradual decline occurred until the end of the experiment. Nb2BA measurements were similar to those obtained by RIA, except at 8 weeks, when the content determined by Nb2BA was significantly higher than the content determined by RIA. When estradiol was given to F344 rats a pattern of increase and subsequent decrease in pituitary content similar to that seen with DES was observed and levels measured by Nb2BA and RIA were essentially equivalent. Plasma levels of prolactin in DES-treated F344 rats increased exponentially through the 8 weeks, and the Nb2BA measurements were significantly greater than levels determined by RIA throughout the treatment period. Estradiol treatment produced a pattern of change in plasma levels of prolactin similar to that observed with DES, except that RIA and Nb2BA measurements were not different. Comparable results were obtained in Holtzman rats, except plasma levels were not increased to the same degree as seen in F344 rats. From these results, we conclude that DES, but not estradiol, can selectively increase the secretion of prolactin that is more bioactive than immunoreactive and that this effect of DES is observed in F344 and Holtzman rats, although F344 rats released more prolactin in response to estrogens than did Holtzman females.     

A comparison of plasma prolactin levels in young female Long-Evans and Holtzman rats as measured by Nb2 lymphoma bioassay and radioimmunoassay

This study was conducted to determine the plasma levels of prolactin in prepubertal and young, postpubertal, proestrus rats of mammary tumor-susceptible (Sprague-Dawley) and tumor-resistant (Long-Evans) strains using a sensitive bioassay-Nb2 lymphoma cell replication. Prepubertal Long-Evans rats had significantly higher levels of prolactin than did Holtzman Sprague-Dawley rats of the same age. Likewise, Long-Evans rats secreted significantly more prolactin into the blood on the afternoon and evening of proestrus than did Holtzman rats. Finally, ovariectomized Long-Evans rats released more prolactin into the blood at 1 day, but not at 8 or 15 days, of treatment with diethylstilbestrol. Prolactin levels determined by conventional radioimmunoassay and by bioassay were similar except on the afternoon of proestrus, when, in both strains of rats, the bioassay to radioimmunoassay ratio increased significantly above 1.0 during the late evening. In addition, the ratio was significantly less than 1.0 in the early and late afternoon in the Holtzman rats, but not Long-Evans rats. These data indicate that a strain of rats that is resistant to experimentally induced mammary cancer has higher prolactin levels in the blood than does a strain that is susceptible to mammary cancer at a time when mammary gland growth is rapid. Furthermore, there are times during the proestrus prolactin surge when the bioassay yielded higher and lower values of prolactin than radioimmunoassay of the same samples, suggesting functional heterogeneity of prolactin that may impact on mammary gland or other target tissue function.     

The influence of estrogen on plasma prolactin levels induced by thyrotrophin releasing hormone (IRH), clonidine and serotonin in ovariectomized rats.

Prolactin levels were determined in the plasma of ovariectomized and ovariectomized estrogen treated rats by RIA following intraarterial injection of TRH, (1 and 10 μg/rat), clonidine (5 mg/kg) and serotonin (10 mg/kg). In ovariectomized rats, TRH had no effect on plasma prolactin whereas serotonin and clonidine induced slight and moderate increases respectively. In contrast, TRH induced a significant increase in plasma prolactin in estrogen-treated rats while the effects of the other two agents were enhanced only slightly (clonidine) or very markedly (serotonin). These results indicate that the prolactin-releasing activity of TRH is dependent on estrogen and that estrogen differentially affects noradrenergic and serotonergic components of the neuroendocrine mechanism that controls prolactin. It is also suggested that clonidine and serotonin probably do not increase plasma prolactin by releasing endogenous TRH.     

Evidence for a rapid in vivo effect on estradiol-17 beta on prolactin secretion in ovariectomized rats.

Repeated intraarterial injections of synthetic thryrotropin releasing hormone (TRH, 1 microgram/rat) increased plasma prolactin levels 4 hours after a single subcutaneous injection of 10 micrograms estradiol-17 beta (E2-17 beta) in rats ovariectomized 1, 2 or 4 weeks and at 2 hours after E2-17 beta injection in rats ovariectomized for 6 weeks. The effect of TRH was still present at 24 but not 48 hours after estradiol treatment. TRH-induced increases in plasma prolactin were similar in groups of rats treated with 10 micrograms E2-17 beta (s.c.) or implanted with 0.5 cm Silastic capsules of crystalline E2-17 beta (s.c.) whereas smaller, yet significant, TRH-induced increases in plasma prolactin were observed in rats injected s.c. with 1.0 microgram E2-17 beta. Single intraarterial injections of TRH at 4 or 8 hours after E2-17 beta treatment induced increases in plasma prolactin similar in magnitude to those observed at the same times after E2-17 beta in rats given repeated TRH injections. No effect of TRH was observed in ovariectomized rats given sesame oil and E2-17 beta treatment did not influence plasma prolactin in rats given saline instead of TRH. Intraarterial administration of serotonin creatinine sulfate (5-HT, 10 mg/kg body weight) induced marked increases in plasma prolactin in rats ovariectomized for 4 weeks which were potentiated at 2 and 6 hours after E2-17 beta (10 micrograms) treatment. The data show that estradiol has a fairly rapid stimulatory effect on plasma levels of prolactin induced by two different secretagogues but the exact site and mechanism of action remain unresolved.     

The effects of estradiol on pituitary responsiveness to dopamine in vitro: a comparison of ovariectomized Fischer 344 and Holtzman rats.

The objective of this study was to determine if the effectiveness of dopamine as an inhibitor of prolactin is altered by estradiol in strains of rats which show marked differences in estrogen-induced pituitary hyperplasia. Groups of Fischer 344 and Holtzman Sprague-Dawley rats were ovariectomized and implanted with Silastic capsules of estradiol. Rats were sacrificed by rapid decapitation following a brief period of ether anesthesia at 2, 4, 6, 8 weeks (F-344) or at 2 and 8 weeks (Holtzman) of estradiol treatment. The pituitary was removed and cut into fragments which were either snap frozen for initial prolactin content measurements or incubated for 60 min in the presence or absence of dopamine (1 x 10(-6) M). Prolactin was measured in the plasma, in sonicates of the pituitary and in the incubation medium by double antibody radioimmunoassay. Pituitary weight and plasma levels of prolactin were significantly less in Holtzman rats compared to Fischer 344 females at 2 or 8 weeks of estradiol treatment but pituitary concentrations of prolactin were not different between the two strains. Pituitary fragments from Fischer 344 rats studied at 2 and 4 weeks of estradiol treatment did not respond to the removal of dopamine in vitro whereas pituitary fragments from Holtzman rats obtained at 2 weeks of estradiol treatment did release significantly more prolactin in the absence than in the presence of dopamine. Pituitary fragments taken from Fischer 344 rats at 6 and 8 weeks were responsive to dopamine whereas pituitary tissue from Holtzman rats was not responsive at 8 weeks. The data indicate that temporal differences in responsiveness to the inhibitory effects of dopamine occur in strains which are susceptible or resistant to the formation of pituitary tumors following prolonged estradiol treatment.     

Size homogeneity of plasma prolactin in the female rat following various natural, pharmacological and stressful stimuli

Gel filtration on Sephadex G-150 was performed on freshly drawn plasma from ovariectomized, estrogen-treated rats following 10 minutes of ether inhalation, intraperitoneal administration of TRH (1 μg/rat) or pimozide (500 μg/kg body weight) and at the peak of the estrogen-induced afternoon surge of prolactin (1700 h). Plasma from intact lactating rats 30 minutes after suckling was also subjected to gel filtration. For comparison, homogenates of fresh and frozen pituitaries from ovariectomized, estrogen-treated rats were chromatographed. Prolactin activity was determined by RIA in each fraction eluted between the void and total volumes of the column. Immunoreactive prolactin in plasma following all experimental procedures eluted as a single component with a K_av of approximately 0.6. Chromatography of the fresh pituitary homogenate showed prolactin immunoactivity at the void volume and at a K_av of 0.4 and 0.6. A homogenate of frozen pituitary contained a component with a K_av of 0.3 in addition to those seen in fresh pituitary. These studies demonstrate that although the rat pituitary contains multiple molecular forms of immuno-reactive prolactin, only one small component is found in plasma.     

Ultracytochemical localization of estrogen-stimulated guanylate cyclase in rat uterus

Estrogens are known to increase cyclic guanosine monophosphate (cGMP) levels in the uterus of rats by enhancing guanylate cyclase (GC) activity. In the present study, the cytochemical localization of GC activity was studied in the uteri of immature and ovariectomized rats after treatment with diethylstilbestrol (DES), progesterone, estrogen antagonist (CI628), and a combination of DES and CI628. Twenty-four hours after the first dose of DES, moderate to strong guanylate cyclase activity was indicated by lead phosphate precipitate on the luminal microvillar and basolateral surfaces of epithelial cells, whereas strong activity was found on the plasma membranes of fibroblasts, endothelial cells, and myometrial cells. The enzyme activity in the epithelial cells declined slightly 24 hr after the second daily dose of DES. Uterine tissues from DES-treated rats that were preheated at 60 degrees C for 30 min or preincubated with a GC inhibitor showed no reaction product. Guanylate cyclase activity was not observed cytochemically in the uterine tissues of the vehicle control (immature or ovariectomized) or progesterone-and CI628-treated animals. Weak guanylate cyclase activity was observed on the plasma membranes of epithelial cells and endothelial cells after doses of DES and CI628 were given simultaneously. The biochemical assays of the total homogenate in vitro indicated that uterine GC showed about a twofold increase after one dose of DES and a 1.3-fold increase following two doses (one dose per day) of DES when compared with their respective nontreated controls, or with progesterone-treated uteri. GC was found in particulate (09%) and cytosol (10%) fractions.These data demonstrated that DES stimulated uterine guanylate cyclase activity, while progesterone and CI628 were ineffective at the doses used. Estrogen antagonist CI628 doses not completely suppress the effect of DES.     

Ovariectomized Sprague-Dawley and Long-Evans rats release prolactin differently in response to estrogen

Mature female Sprague-Dawley (SD) and Long-Evans (LE) rats were ovariectomized (OVX), fitted with indwelling atrial catheters and given a single sc injection of either 25 or 100 μg polyestradiol phosphate (PEP); seven days later blood samples were withdrawn at two hour intervals from 1100 to 2100 hours to detect the presence of an afternoon surge of prolactin (PRL). Other groups of OVX rats of both strains also treated with PEP and catheterized as above were sampled before and at 2, 5, 10 and 30 min after iv administration of 1 μg synthetic thyrotropin releasing hormone (TRH). Pituitary (AP) and uterine weights were determined following sacrifice one day after TRH treatment. Separate groups of OVX rats of both strains treated with 100 μg PEP were decapitated 7 days later and each AP was removed and homogenized. The AP homogenates and plasma samples were assayed for PRL by radioimmunoassay. Rats of both strains had afternoon PRL surges and in both strains the magnitude and/or duration of the surges were enhanced by the higher dose of PEP. However, within each PEP dose LE rats released significantly more PRL during the surge than did SD rats. Rats of both strains also released PRL in response to TRH and this response was enhanced in both strains by the higher of the two doses of PEP. However, there were no differences between the strains at 25 μg PEP and at 100 μg PEP SD rats released significantly more PRL to TRH than did LE rats. Pituitary weight and PRL concentration were not different between the strains at either dose of PEP but LE rats had significantly heavier uteri at both doses of PEP compared to SD rats. These data not only show that strain differences exist in estrogen-induced or mediated PRL release in the rat but also indicate that the differences are not uniform. This latter observation suggests that the estrogen-induced mechanisms examined in this study are for the most part independent of each other.     

Changes in pituitary prolactin responsiveness to TRH during pregnancy

Prolactin plasma concentration during pregnancy was determined in rats treated with thyrotropin-releasing hormone (TRH). Day 0 of pregnancy was defined as the day sperm were first found in the vagina. All blood samples were obtained in unanesthetized rats which had previously received a cannula in the right common carotid. On Day 8 of pregnancy, plasma prolactin concentrations reached a peak between 2400 and 0800 hr (lights on from 0600 to 1800 hr). Injection of TRH (1 microgram/kg body wt) via the carotid artery increased plasma prolactin levels within 5 min. The largest increase occurred when TRH was given during the prolactin surge, whereas much smaller effects were found when TRH was given at the beginning or after the end of the surge period. Thus, the sensitivity of the prolactin cell to TRH appears to be the greatest when the secretory activity of the cell is high. It was then determined whether there was any change in the sensitivity of the prolactin cell to TRH after the prolactin surges had disappeared at midpregnancy. Injection of TRH between 1100 and 1200 hr increased prolactin less on Day 12 than on Day 8 of pregnancy. Since placental lactogen (PL) levels in the plasma are high on Day 12 compared to Day 8, and are inhibitory to prolactin secretion, it was reasoned that PL may be the factor which caused the reduced sensitivity to TRH. However, hysterectomy on Day 11 failed to increase the pituitary responsiveness to TRH the next day. In summary, these data indicate that the pituitary responsiveness to factors that stimulate prolactin, such as TRH, varies with relation to the time of pregnancy or presence of the nocturnal surge. What cellular mechanism is responsible for these sensitivity changes is not known.     

Prolactin does not mediate the suppressive effect of the suckling stimulus on luteinizing hormone secretion in ovariectomized lactating rats

The plasma LH concentration in ovariectomized lactating rats is low for 14 days postpartum, while the prolactin concentration is high during this period. We examined the effect of the inhibition of increased prolactin secretion with bromocriptine (CB-154) on the LH secretion in lactating rats ovariectomized on day 2 (day 0 = day of parturition). Blood samples were collected through an indwelling atrial cannula every day. LH levels were kept low until day 9 in lactating rats injected daily with CB-154 (0.6 mg/day, s.c.). The duration of the period during which LH secretion was suppressed was shorter in lactating rats treated with CB-154 than in saline-injected controls. The replacement with ovine prolactin by means of a mini-osmotic pump (0.3 mg/day, s.c.) in CB-154-treated lactating rats restored the duration of LH suppression. In rats deprived of their pups on day 2, the LH concentration rose immediately after removal of the pups and the LH level was not significantly different between rats treated with CB-154, ovine prolactin and saline, indicating that neither the CB-154 treatment nor the high level of prolactin alone has any effect on LH secretion in rats deprived of their pups. The present results clearly demonstrate that prolactin does not mediate the suppressing effect of the suckling stimulus on LH secretion in early lactation and support our theory that the suckling stimulus controls the LH and prolactin secretion independently at the hypothalamic level.     

Prolactin in the marsupial Macropus eugenii, during the estrous cycle, pregnancy and lactation

An heterologous double antibody radioimmunoassay (RIA) using a guinea-pig antiserum (33-9) raised against human prolactin and 125I-ovine prolactin has been developed to measure prolactin (Prl) in plasma and pituitary preparations of marsupials. In this system, purified tammar and kangaroo Prl preparations showed parallel dose-response curves as did serial dilutions of crude pituitary homogenates of tammar, possum and eastern grey kangaroo. Serial dilutions of plasma from ovariectomized and lactating female and castrate male tammars showed immunoreactivity, and plasma Prl levels increased after injection of TRH. The assay has been used to monitor changes in plasma Prl in female tammars in various reproductive states. Plasma Prl remained at basal concentrations of 20 to 30 ng/ml throughout the estrous cycle, at estrus and during pregnancy. However, just prior to parturition, there was a 2- to 3-fold increase in Prl concentrations which declined to basal levels after birth. During early lactation, Prl levels were low but increased to maximum concentration in the second half of lactation.     

Relationship between bioassay and radioimmunoassay measurement of prolactin in the IPL rat, a hypoprolactinemic rat strain

IPL (Institut Pasteur, Lyon) nude, hypoprolactinemic rats exhibit delayed puberty and a complete lack of lactation. To characterize the secretion of circulating forms of prolactin (PRL) of these rats, PRL concentrations were measured in serum and pituitaries of males and females under various physiological conditions. Two assay methods, a radioimmunoassay (RIA) and a sensitive bioassay (NB2BA) were employed. Normal rats of the Sprague-Dawley strain were tested simultaneously, as controls. The pituitary content of PRL, estimated either by RIA or by NB2BA, in IPL nude males and females was similar to that of normal male and female rats. On the contrary, serum PRL levels of IPL male rats, measured by RIA or NB2BA, were significantly reduced when compared to normal rats. In both groups, there was a close correlation between the results obtained by the two methods, the NB2BA estimates being higher. However, the NB2BA/RIA ratio was significantly decreased in serum from IPL nude rats compared to controls, indicating that the circulating form of PRL was less bioactive in this group. Castrated male rats injected with estradiol showed sharply increased PRL values as estimated by RIA or NB2BA. The increase was greater (35-fold) in IPL nude rats then in normal rats (9-fold), but these increases resulted in serum PRL levels being similar in the two groups. However, the NB2BA/RIA ratio remained significantly reduced in IPL nude rats. In female rats, PRL was measured during different physiological states: estrus, diestrus, proestrus at 1000, 1200, and 1600 h and Days 1 and 21 of gestation and 2 days postpartum.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of hypersecretion of growth hormone and prolactin on plasma levels of glucagon and insulin in GH3-cell-tumor-bearing rats, and the influence of bromocriptine treatment

Plasma levels of prolactin, growth hormone, glucagon insulin and glucose were measured in non-treated control rats, bromocriptine-treated control rats and GH3-cell-tumor-bearing rats with and without bromocriptine treatment. Bromocriptine treatment increased plasma levels of glucagon, insulin and glucose in control rats. Tumor-bearing rats had increased body weight and increased plasma levels of prolactin, growth hormone, glucagon, insulin and glucose. Bromocriptine treatment reduced body weight and decreased the plasma levels of prolactin, glucagon and insulin, as compared to non-treated tumor-bearing rats. The drug had no effect on plasma levels of growth hormone and glucose. These results indicate that, in GH3-cell-tumor-bearing rats, prolactin, glucagon and insulin are more sensitive to the action of bromocriptine than growth hormone.     

Effects of cysteamine on the hypothalamic-pituitary axis in the rat

The effects of cysteamine (CSH) on hypothalamic concentrations of neuropeptides were reviewed and correlated with available information on changes in pituitary hormone content and circulating pituitary hormone levels. In our study, we found notable changes in the morphology of lactotropes from female Long-Evans rats treated for 7 days with CSH (300 mg/(kg X day) per os). Forming granules increased in number, and crinophagy, which is the augmented incorporation of these granules into lysosomes, was evident. Storage granules were reduced in number. These changes were not suppressed by simultaneous administration of 17 beta-estradiol (50 micrograms/day s.c.) for 7 days. CSH administration failed to prevent estrogen-induced lactotrope hyperplasia. Serum prolactin levels were unaffected by CSH treatment. The morphological changes in the adenohypophysis did not resemble those observed when rats were treated with bromocriptine. The rough endoplasmic reticulum luminal density was reduced in gonadotropes from intact CSH-treated rats after 1 wk. CSH treatment suppressed the development of castration cells and significantly reduced serum luteinizing hormone levels in ovariectomized rats. The morphological effects of CSH appeared to be confined to lactotropes and gonadotropes.     

Bromocriptine is unable to suppress TRH-stimulated beta-endorphin/beta-lipotropin and cortisol secretion in normal pregnant women after delivery

The course of plasma beta-endorphin/beta-lipotropin, cortisol and prolactin (PRL) levels was followed from 0.5 till 5 h after normal delivery in 13 healthy women. Six subjects who did not want to breast-feed their child received 2.5 mg bromocriptine orally 1 h after delivery. After 3 h the effect of the intravenous administration of 200 micrograms thyrotropin-releasing hormone (TRH) was also measured. Elevated plasma beta-endorphin and cortisol levels decreased after delivery in a (log) linear fashion which was not influenced by bromocriptine. TRH elicited a significant short-lived identical increase in plasma beta-endorphin/beta-lipotropin concentrations in the control and the bromocriptine-treated subjects. TRH similarly delayed the rapid decline in plasma cortisol levels in both groups of women. Basal and TRH-induced PRL levels were rapidly suppressed by bromocriptine. These studies show the presence of a paradoxical increase of beta-endorphin/beta-lipotropin and cortisol levels in response to TRH occurring shortly after delivery in normal women. This response cannot be mediated by the placenta. The absence of an inhibiting effect of bromocriptine on basal and TRH-induced beta-endorphin and cortisol release does not lend support to the hypothesis of the presence of a functionally active intermediate pituitary lobe in man early in puerperium.     

Mechanism of estrogen-induced 17-beta-hydroxysteroid dehydrogenase in ovariectomized rat uterus

Estradiol (E2), progesterone or medroxyprogesterone acetate can induce biosynthesis of the 17-beta-hydroxysteroid dehydrogenase (17-beta-HSD) in the mammalian uterus. For further understanding the 17-beta-HSD induction which may be mediated by the conjugation of the E2 to its receptor, premature ovariectomized rats were treated with E2, or with a synthetic steroid, diethylstilbestrol (DES), an agonist for the E2 receptor but not a substrate for 17-beta-HSD. Histological observation and uterus weight were examined as parameters to evaluate uterine response to those hormones at different durations of treatment. The 17-beta-HSD in ovariectomized rat uterus of each group was also examined by histochemical and biochemical assays. The results showed that the 17-beta-HSD activity in the uterus can be induced by E2 or DES, after daily treatment for 1, 14 and 28 days, but much higher in DES treated animals. The uterus weight demonstrated a "negative linear correlation" to the enzyme activity in all E2 treated groups, but not in DES or control rats. Accordingly, it was indicated that the 17-beta-HSD induction was regulated by conjugation of E2 or DES to its receptor. Therefore, we believe that the 17-beta-HSD gene in the rat uterus is another estrogen responsive gene.     

Rat lymphoma cell bioassay for prolactin: Observations on its use and comparison with radioimmunoassay

The rat Nb₂ node lymphoma cell bioassay (BA) for prolactin (PRL) was validated for use in our laboratories. During the course of this validation we observed that rat prolactin (NIAMDD-RP-1) stimulated cell division by as much as 16.5 fold over the range of 0.04 to 40.0 ng/ml at the end of 72 hours of incubation. We also observed a dose related increase in the size of the lymphoma cells. Prolactin concentrations in rat plasma, serum, anterior pituitary (AP) homogenates and milk were measured by both radioimmunoassay (RIA) and BA. In individual BA's there was parallelism between samples and standard; but when several dilutions of the same plasma and pituitary homogenates were assayed repeatedly, higher PRL levels were consistently observed for the more concentrated samples. At low or moderate levels of plasma PRL there was excellent agreement between RIA and BA; however, at high levels plasma PRL bioactivity exceeded radioimmunoactivity by a small, but significant, amount. A comparison of pituitary PRL concentrations measured by RIA and BA were in good agreement when homogenization was done at pH 10.6. However, when homogenization was done at pH 7.6, slightly but significantly more PRL was extracted when assayed by BA than when assayed by RIA.     

Effects of arginine vasotocin on levels of plasma gonadotropins and prolactin in ovariectomized conscious rats

Arginine vasotocin was injected into the third ventricle or intravenously in conscious, ovariectomized rats and its effect on gonadotropin and prolactin release evaluated. The peptide lowered plasma levels of both LH and prolactin in doses of 40 or 100 ng given intraventricularly. The higher dose was slightly more effective than the lower dose. Intravenous injection of a 1-microgram dose of vasotocin failed to alter plasma LH in the ovariectomized animals; however, a 5-micrograms dose induced a slight depression apparent at only 60 min following injection. Intravenous injection of 1 microgram produced a significant lowering of plasma prolactin, whereas a dramatic lowering followed the injection of the higher dose. Plasma FSH was unaffected in these experiments. Incubation of dispersed anterior pituitary cells from ovariectomized rats with various doses of vasotocin revealed no effect of the peptide on the release of FSH, LH, or prolactin. It also did not alter the response to LHRH, but it partially blocked the action of dopamine to inhibit prolactin release. The data indicate that quite low doses of arginine vasotocin act within the brain to inhibit LH and prolactin secretion in ovariectomized, conscious animals.     

Size heterogeneity of plasma prolactin in the rat: TRH and serotonin-induced changes

Synthetic thyrotropin releasing hormone (TRH, 1 μg/rat) and serotonin (5-HT, 10 mg/kg body weight), when administered intraarterially to separate groups of ovariectomized, estrogen-treated rats, induced rapid and marked elevations in plasma prolactin which were essentially equal in magnitude and time of onset. However, gel filtration chromatography of the plasma on Sephadex G-100 indicated that different molecular forms of prolactin were induced by these two secretagogues. These findings suggest that these agents have different mechanisms of action and that there are secretory mechanisms in the pituitary for prolactin that operate independently of secretion rate.     

Size heterogeneity of pituitary and plasma prolactin: Effect of chronic estrogen treatment

Pituitary homogenates and plasma from untreated and estrogen treated ovariectomized rats were subjected to gel filtration chromatography and the prolactin in fractions collected between the void and total elution volumes of the columns was determined by radio- immunoassay. Three components of prolactin, identified as “void volume”, “big” and “little” according to increasing elution volumes, were observed in pituitary homogenates of ovariectomized rats. These three components accounted for 4, 11 and 85% of the total prolactin activity respectively. Estrogen treatment of ovariectomized rats increased the total prolactin in the pituitary and also selectively increased the “big” component to 21% of the prolactin activity on the column. A smaller increase was also observed in the “void volume” component. Gel filtration of the plasma obtained from estrogen-treated rats before and during the estrogen-induced afternoon surge of prolactin showed that “little” prolactin was the predominate form being secreted and that the “void volume” and “big” components were also released. The release of the components was not in proportion to that observed in the pituitary and the larger components were released in a nonuniform manner. The “void volume” component appeared in the plasma as the surge began but then disappeared as the “big” component appeared at the peak of the surge. The big component decreased as the surge waned leaving primarily “little” component in plasma. The data indicate (1) that estrogen stimulates the formation of the larger components of prolactin in the pituitary (2) that the types of prolactin released into plasma of estrogen-treated ovariectomized rats is not in proportion to that found in the pituitary and (3) that the heterogeneous forms of prolactin are selectively released into plasma during the prolonged secretory episode of the afternoon surge of prolactin induced by estrogen.