Comparison between rat prolactin radioimmunoassay and bioassay values under different experimental and physiological conditions

Dietary fat affects plasma prolactin in female F344 rats under conditions of ether stress

The influence of amount and type of dietary fat on circulating concentrations of prolactin and estradiol-17 beta in female F344 rats from which blood was sampled by decapitation under ether anesthesia was compared with that in rats from which blood was collected without anesthesia. The animals were fed isonutrient (adjusted for differences in energy density) semipurified diets containing 5% or 20% (by weight) sunflower seed oil or lard. Blood was sampled by decapitation with or without standardized ether anesthesia during the afternoon of proestrus-estrus or the morning of metestrus-diestrus, as determined by examination of vaginal smears. Plasma hormone concentrations were measured by radioimmunoassay. Prolactin levels were lower during proestrus-estrus in rats fed a low-fat diet than in animals fed a high-fat diet, statistically independent of the type of dietary fat, but only when blood was sampled by decapitation under ether anesthesia [p = 0.0384, 2-way analysis of variance (ANOVA)]. No such difference was found in rats decapitated without anesthesia. This effect of amount of dietary fat on prolactin in proestrus-estrus animals anesthetized with ether was predominantly present in animals fed polyunsaturated fat (p < 0.05, 1-way ANOVA and Tukey's test) and was statistically not significant in rats fed saturated fat diets. During metestrus-diestrus, prolactin levels were significantly lower in animals fed a high-saturated fat diet than in those fed low-saturated fat, low-unsaturated fat, or high-unsaturated fat diets, independent of the blood sampling conditions (p < 0.05, 2-way ANOVA and Tukey's test). No consistent effects on estradiol-17 beta levels were found in type or amount of dietary fat or in presence or absence of ether anesthesia before decapitation. Growth, apparent digestibility of fat, and caloric intake were similar in all four dietary groups, but food consumption was higher and food conversion efficiency was lower in animals fed low-fat diets than in those fed high-fat diets. This study confirms the hypothesis that effects of dietary fat, particularly polyunsaturated fat, on circulating prolactin occur only during (ether) stress. Because stress is a frequent and normal phenomenon, this observation implies that the mammary glands of animals with a high dietary intake of polyunsaturated fat are frequently exposed to higher circulating prolactin concentrations than rats fed a low-fat diet, which may be a major mechanism by which dietary fat enhances rat mammary carcinogenesis.

Isolation of cleaved prolactin variants that stimulate DNA synthesis in specific cell types in rat pituitary cell aggregates in culture

Using an affinity column of monoclonal antibodies against rat prolactin (PRL), PRL immunoreactive material secreted by rat pituitary aggregate cell cultures was purified in milligram amounts from the culture medium. Further purification of the PRL immunoreactive molecules was achieved by anion-exchange and reversed-phase h.p.l.c. At least four variants could be distinguished which showed PRL-like bioactivity in the Nb2 lymphoma bioassay. On SDS/PAGE two variants migrated as single bands under both reducing and non-reducing conditions. The two other variants migrated as single bands under non-reducing conditions, but yielded under reducing conditions two fragments, the larger of which had molecular masses of approx. 16 and 17 kDa respectively. These variants were therefore considered to be cleaved PRL (clPRL) variants, and were designated clPRL-1 and clPRL-2 respectively. These variants were not artefactually produced at low pH during the h.p.l.c. purification step. Each of the clPRLs represented about 0.6-1.0% of total PRL. The cleavage site of clPRL-1, determined by N-terminal and C-terminal amino acid sequence analysis, was in the large disulphide loop between Tyr-145 and Leu-146. Purified clPRL-1 added to pituitary aggregate cell cultures from 14-day-old female rats in the presence of the dopamine agonist bromocriptine increased [3H]thymidine incorporation into DNA in gonadotrophs and thyrotrophs, but not in other pituitary cell types. Under the same experimental conditions the main forms of PRL and clPRL-2 were inactive. These data demonstrate that PRL is cleaved within the pituitary gland between specific amino acid residues, and that PRL cleaved between Tyr-145 and Leu-146 may have a specific biological role as a paracrine growth regulator in this tissue.

Effect of naltrexone on stress-induced bioactive prolactin release in aging male rats

Effects of the opiate receptor antagonist naltrexone (NALT, 10 mg/kg) on basal and novel environment stress-induced prolactin (PRL) release were examined in young (3-5 month) and old (22-24 month) male Copenhagen-Fischer 344 rats. Radioimmunoassay and Nb2 lymphoma bioassay were used to determine plasma immunoreactive (ir-) and bioactive (bio-) PRL levels, respectively. Although basal plasma irPRL levels were greater, bioPRL levels were significantly lower in old as compared to young rats. NALT induced significant decreases in basal plasma ir- and bioPRL concentrations in young rats, but had little or no effect on irPRL or bioPRL levels in old rats. Stress-induced elevations in irPRL levels were similar, but increases in bioPRL levels were attenuated in vehicle-treated old as compared to young rats. NALT pretreatment blocked stress-induced ir- and bioPRL release in both age groups. These results indicate that the role of endogenous opioid peptides (EOP) in regulating basal PRL secretion appears to be diminished, whereas EOP continue to be involved in stress-induced PRL release in the aging animal.

Biological activities of glycosylated and nonglycosylated porcine prolactin

Nonglycosylated and glycosylated porcine prolactin (PRL) were separated using concanavalin A-Sepharose CL-6B column chromatography and tested for mitogenic and lactogenic activities, as well as immunoaffinity and receptor binding characteristics compared to total (nonseparated) porcine PRL. Mitogenic activity, using Nb2 lymphoma cells, was 4- and 50-fold greater (P less than 0.01) for total PRL than nonglycosylated and glycosylated PRL, respectively. Glycosylated PRL had 64% higher (P less than 0.05) lactogenic activity than nonglycosylated or total PRL. In a homologous radioimmunoassay (RIA), displacement was greatest for total, followed by the nonglycosylated and glycosylated forms of PRL. Competitive inhibition of porcine [125I]-(total) PRL by radioinert total, nonglycosylated and glycosylated PRL in a homologous radioreceptor assay (RRA) indicated similar Ka values for total and nonglycosylated PRL, but different receptor numbers, while radioinert glycosylated PRL had a higher Ka, but bound fewer receptors. Therefore, glycosylated porcine PRL has greater lactogenic activity and higher binding affinity despite decreased mitogenicity, while nonglycosylated PRL had characteristics similar to total PRL. Results from the homologous RRA and the Nb2 assay suggest that both forms of PRL are necessary to achieve biological effects similar to those for total PRL. The two forms of PRL may have individual and collective effects, while changes in the ratio between these forms may influence physiologically diverse effects of PRL on target tissues.

Effects of sequential acute stress exposure on stress-induced pituitary luteinizing hormone and prolactin secretion

The present study was carried out to determine the effects of repetitive acute stress exposure on pituitary secretion of both luteinizing hormone (LH) and prolactin (PRL). Adult male rats were exposed to sequential episodes of acute novel environment stress separated by intervals of either 60 or 120 minutes. Serial blood samples were obtained from animals before, during and after each stress episode via indwelling intra-cardiac cannulas. The imposition of 10 minute episodes of novel environment stress on an hourly basis eventually rendered the hypothalamic-hypophyseal LH axis refractory to the stimulatory effect of stress. If sequential stress was imposed at 120 minute intervals, LH release was significantly enhanced during each exposure. A different pattern of PRL release was observed during the same sequential stress schedule. After an initial increase in hormone release in response to the first hourly stress episode, PRL levels were unaltered during the second and third hourly stress exposures. Thereafter, plasma PRL levels showed a trend toward a progressive increase in release during each successive episode, and were significantly elevated above preceding baseline levels during the fourth and fifth hourly stress exposures. In rats exposed to stress every two hours, a significant increase in PRL levels occurred following the first, but not the second stress episode. Hormone release was again enhanced in response to the third exposure to novel environment. The present results demonstrate that the repetitive exposure to acute novel environment stress results in differential alterations in pituitary LH and PRL secretion over time, and that the timing of repeated episodes is an important determinant of continued responsiveness to stress, particularly with regard to LH release. These findings suggest that the LH and PRL hormonal responses to at least this specific stressor are mediated by independent neural mechanisms.