Milk prolactin is biologically active

Fat-free milk from cow and goat was chromatographed on Sephadex G-100 and the prolactin (PRL) activity of the fractions determined by radioimmunoassay (RIA). A single prolactin component was observed in 3 cow and 3 goat milk samples with a Vf/Vt ratio of approximately 0.5. Fractions in which PRL was detected by RIA and fractions on either side of the PRL peak were combined, dialyzed and freeze dried. The fractions were assayed for biological activity using the pseudopregnant rabbit mammary gland in organ culture; the degree of secretory response was evaluated histologically. Milk prolactin was biologically active. In the RIA cow milk PRL and one of 2 samples of goat milk PRL gave dose response curves parallel with the bovine PRL standard. In the bioassay the dose response curves for cow milk PRL and ovine PRL were parallel while goat milk PRL was parallel when the results were compared on a weight basis but not on the basis of prolactin content of the preparations assayed by RIA.     

Heterogeneity of big-big hPRL in hyperprolactinemia

Sera from a patient with macroprolactinoma (case 1) and from a hyperprolactinemic woman with regular menstruation (case 2) were analyzed for prolactin activity by gel filtration using Sephadex G-100, Sephadex G-200 and TSK G3000SW columns. The chromatographic profile by Sephadex G-100 showed that the percentage of immunoreactive big-big hPRL was 10.7% in case 1 and 64.1% in case 2. On Sephadex G-200 and TSK G3000SW columns, the molecular weight of big-big hPRL was estimated to be more than 500,000 daltons (big-big1 hPRL) in case 1 and approximately 250,000-300,000 daltons (big-big2 hPRL) in case 2. Big-big1 hPRL in case 1 was converted to big and little hPRLs when the serum was treated with 2-mercaptoethanol (2-ME), but part of the big-big2 hPRL in case 2 was converted to a larger molecule. Radioactive big-big hPRL generated by mixing labeled hPRL with the serum from case 1 was eluted with the void volume on Sephadex G-100 column and was not converted to the other molecular forms after 2-ME treatment. There were two radioactive big-big hPRL on TSK G3000SW column and these estimated molecular weights were more than 300,000 daltons. The data demonstrated the existence of at least two forms of big-big hPRL in the serum and indicated that radioactive big-big hPRL may be different from these hPRLs in the serum.     

Heterogeneity of circulating prolactin in the bitch

Different molecular forms of circulating prolactin (PRL) are known to occur in several species. As no such information was available in dogs, we assessed the molecular profile of circulating PRL in bitches. Pooled sera from covertly (CTRL) and overtly pseudopregnant (PSPT) diestrous bitches with high or low (> 10 or < 10 ng x mL(-1), respectively) serum PRL (measured by ELISA) were analyzed by Sephadex G-100 and Concanavalin A-Sepharose column chromatography. Four serum PRL fractions were identified and termed big-big, big (> 67 kDa), native (23 kDa) and fragmented (< 20) kDa) PRL. The percentages of these fractions were roughly similar in CTRL and PSPT animals, irrespective of their serum PRL levels (higher in PSPT than in CTRL bitches). A large proportion of glycosylated PRL (between 69 and 100%) was also detected in these sera. We conclude that in dogs, circulating PRL occurs in multiple molecular forms, whose relative abundance is comparable in covertly and overtly pseudopregnant bitches.     

Effect of calf suckling on oxytocin,prolactin, growth hormone and milk yield in crossbred Gir x Holstein cows during milking

This study evaluated the effect of different milking managements on oxytocin, prolactin and growth hormone release in Gir x Holstein cows. Six cows were suckled by their calves, before and after milking (SM group); another six cows were submitted to exclusive milking (M group). High OT levels were observed during suckling of the SM group, however both groups had similar levels of OT during milking. The SM group presented PRL levels significantly higher than the M group, however only during suckling. For GH levels, the SM group showed higher levels than the M group, however this difference was significant only during the first suckling, cleaning of the teats and at the beginning of machine milking. Despite these results, SM cows produced more milk (milking plus suckling) than M cows, however milk obtained by milking was similar for both groups.     

Effects of TRH on circulating growth hormone, prolactin and triiodothyronine levels in the bovine.

The effects of administration of synthetic thyrotropin-releasing hormone (TRH) on circulating growth hormone (GH), PROLACTIN (PRL) and triiodothyronine (T3) levels of lactating dairy cows, non-lactating dairy heifers, and beef cows were studied. Intravenous administration of 0.1, 1, and 5 microgram of TRH per kg of body weight (bw) elevated plasma GH and PRL levels of lactating cows within 5 min. The plasma GH and PRL levels increased in proportion to the dose of TRH and reached a peak 10 to 30 min after TRH injection. Intravenous administration of 1 microgram of TRH per kg of bw to 7 non-lactating heifers, 14 lactating dairy cows, and 5 non-lactating beef cows elevated plasma GH level to peak values after 15 min, the increase rates being 6.9, 5.6, and 3.8 times as high as those in the pretreatment levels. The mean maximum vale was also in that order. Plasma T3 levels of non lactating dairy heifers at pre- and post-injection of TRH were significantly higher than those of lactating cows. The peak values of plasma PRL were obtained between 5 to 30 min after TRH administration. The increase rates of lactating dairy cows, heifers, and beef cows were 19.2, 13.9, and 20.9 times as high as those in the pretreatment. In contrast to GH and T3, plasma PRL levels of both pre- and post-injection with TRH in lactating cows and heifers were significantly higher in May than in October, though the increase rates were similar. Plasma PRL levels of lactating dairy cows at pre- and post-injection with TRH were significantly higher than those of non-lactating heifers. Subcutaneous administration of TRH was also effective to increase plasma TH, rl, and T3 levels in lactating cows. No significant change of GH or PRL response to TRH was observed after a short-term pretreatment of thyroid hormones.     

I. A bibliographic database for quantitative analysis of phosphorus flow in ruminants

The present study describes a quantitative analysis of phosphorus metabolism in ruminants. The database compiles 100 sources either published (95) or unpublished (5) involving studies on true or apparent phosphorus balances carried out before 1999. The database reports 652 distinct experimental conditions (lines) described with experimental variables (column) involving a total of 2982 animals: 414 lambs (33 references), 1078 sheep (37 references), 64 pregnant ewes (3 references), 212 lactating ewes (5 references), 528 calves (20 references), 4 maintenance cows (1 reference), 519 lactating cows (6 references), 42 kids (1 reference) and 121 lactating goats (3 references). The reliability of the database was tested using some general relationships.     

Effects of restricted nursing on milk production and collection, kid growth and plasma prolactin and growth hormone concentrations in dairy goats

The milk production of dairy goats under various regimes of mother-young contact from day 4 post partum were studied during the first 2 months of lactation, together with the prolactin (PRL) and growth hormone (GH) responses to udder stimulation. In the control group, 13 goats and their kids were left in permanent contact and did not undergo milking. In two additional groups, goats were machine milked once a day in the morning (at 0800 h) and kids were allowed 10 hours (from 1000 to 2000 h; 10H group, n = 11) or 5 h (from 1000 to 2000 h; 5H group, n = 11) of mother-young interaction per day. In the last group (MO, n = 10), mothers were permanently separated from their kids on day 4 post partum and milked once a day. Milk production during a 24-h period at 37 days post partum performed by controlled nursing and weighing of the kids (groups with kids) or by two machine milking 12 h apart (milking only group) revealed a higher production in the three groups with some mother-young contact than in the MO group. Total milk collected by milking over the 2 months of the study did not differ between the three groups that underwent milking. Kid weights at 2 months were 3.4 to 4.8 kg. lighter in the groups that underwent milking than in the control group. Hormonal profiles were significantly affected by restricted mother-young contact, with highest pre-stimulation concentrations of PRL and GH in the 5H group. Restricting mother-young contact from the first week postpartum can permit an early collection of milk without major effects on kid growth, when compared with one daily milking in goats totally separated from their young.     

Diet and cooling interactions on physiological responses of grazing dairy cows, milk production and composition

The objective of this trial was to evaluate the effects of diet and cooling in the holding pen before milking on rectal temperature, respiration rate and milk production and composition. Fifty-eight lactating Holstein cows were used in a factorial split-plot design, at Rafaela Experimental Station from 12 January to 3 March 2003. The treatments were combinations of two diets: control (CD) and balanced (BD) with two levels of cooling before milking: none (NSF) and a sprinkler and fans (SF). Forage:concentrate ratios for CD and BD were 81:19 and 68:32, respectively. Cows were milked twice daily. Milk production was recorded daily, and milk composition (fat, protein, lactose and urea) was analysed twice a week. The physiological data were recorded once a week, before the cattle entered the holding pen and after milking, in the afternoon. Average maximum weekly temperature humidity index was 75.4 and ranged from 61.4 to 83. There were highly significant effects of cooling on physiological responses. Milk production was affected by diet and cooling, with no interaction; the highest and lowest production of milk was 22.42 and 20.07 l/cow per day, for BD+SF and CD+NSF, respectively. Protein was affected by diet, and was higher for BD (3.17 vs. 3.08%). There were interaction effects on milk fat at the 8% level, the highest concentration being 3.65% for BD+NFS. It was concluded that under grazing conditions, cooling by sprinkler and fans before milking improves the comfort of dairy cows, and that the effects on milk production and composition are enhanced when diets are specially formulated for heat-stress periods.     

Prolactin release and milk removal induced by suckling and milking in lactating ewes is prevented by L-dopa treatment

The effect of L-DOPA on milk removal and on prolactin release during suckling or milking was studied in lactating ewes. Various doses of L-DOPA (25, 50, 100 and 200 mg per animal) were injected iv 30 min before the suckling or milking period. Control ewes were injected with 0.9% NaCl solution only. Milking induced a significant long-lasting release of prolactin. An inhibition of milk removal was obtained with the dose of 200 mg of L-DOPA. An inhibition of prolactin secretion was observed related to the dose of drug administered. The inhibitory effect of 200 mg of L-DOPA on the secretion of prolactin after milking lasted for about 120 min, and thereafter a significant increase in serum prolactin level occurred. This increase in serum prolactin was not due to a "rebound" effect of L-DOPA, since the milking stimulus had to be present to induce the delayed increase in prolactin. Doses of 25 or 50 mg of L-DOPA prevented the surge of prolactin observed immediately after milking, but a long-lasting release of prolactin was obtained thereafter. The inhibitory effect of L-DOPA on prolactin release could be overridden by the suckling or milking stimuli according to the dose administered. The suckling stimulus was more effective than milking in overriding the inhibitory effect of the low dose of L-DOPA. The results indicate that milk removal and prolactin release induced by milking or suckling in lactating ewes is inhibited by an increase in monoamines at the hypothalamic-hypophyseal level.     

Physiological and productive responses of multiparous lactating Holstein cows exposed to short-term cooling during severe summer conditions in an arid region of Mexico

Heat stress generates a significant economic impact for the dairy industry in arid and semi-arid regions of the world, so that heat abatement is an important issue for dairy producers. The objective of this study was to evaluate effects of two short-term cooling periods on physiological and productive status of lactating Holstein cows during hot ambient temperatures. Thirty-nine multiparous cows were blocked by milk yield and assigned to one of three treatments including: control group (C), cows cooled before milking time (0500 and 1700 h daily, 1 h cooling); AM group, cows cooled at 1000 h and before milking (2 h cooling); and AM?+?PM group, cows cooled at 1100, 1500 and 2200 h, as well as before milking (4 h cooling). The cooling system was placed in the holding pen which the cows were moved through for cooling. Respiratory rate, and temperatures of thurl and right flank, were lower (P?P?P?P?相似文献   

Sub-clinical mastitis and associated risk factors on lactating cows in the Savannah Region of Nigeria

ABSTRACT: BACKGROUND: Sub-clinical mastitis limits milk production and represents an important barrier to profitable livestock economics worldwide. Milk production from cows in Nigeria is not at optimum levels in view of many factors including sub-clinical mastitis. RESULTS: The overall herd-level prevalence rate for SCM was 85.33% (256/300 heads of cows) while the quarter-level prevalence rate of SCM was 43.25% (519/1,200 quarters). The prevalence of SCM was 50.67%, 43.67%, 39.67% and 39.13% for the left fore-quarter, right hind-quarter, left hind-quarter and right fore-quarter, respectively. The Rahaji breed had the highest prevalence of SCM with 65.91% (29/44), Sokoto while the White Fulani breed had the least with 32.39% (57/176). A total of 32.33% (97/300) had only one mammary quarter affected, 30.33% (91/300) had two quarters affected, 16.00% (48/300) had three quarters affected while 6.67% (20/300) had all the four quarters affected. A total of 53.00% had SCM in multiple quarters (159/300). The risk of SCM decreased significantly among young lactating cows compared to older animals (OR = 0.283; P < 0.001; 95%CI = 0.155; 0.516). The Rahaji breed had significantly higher risk compared with the White Fulani breed (OR = 8.205; P = 0.013; 95%CI = 1.557; 43.226). Improved sanitation (washing hands before milking) will decrease the risk of SCM (OR = 0.173; P = 0.003; 95%CI = 0.054; 0.554). CONCLUSION: SCM is prevalent among lactating cows in the Nigerian Savannah; and this is associated with both animal characteristics (age, breed and individual milk quarters) and milking practices (hand washing).Good knowledge of the environment and careful management of the identified risk factors with improved sanitation should assist farm managers and veterinarians in implementing preventative programmes to reduce the incidence of SCM.     

Once-a-day milking of Holstein and Montbéliarde cows for 7 weeks in mid-lactation

This study aimed at comparing the effects of once-a-day (OAD) milking during the descending phase of lactation between cows from the two most common breeds in France (Holstein and Montbéliarde). This study was carried on during two successive summers on a total of 50 Holstein and 38 Montbéliarde cows. During 7 weeks, half of the cows from each breed was milked OAD while the other half was milked twice a day (TAD). The animals were also followed for the next 3 or 5 weeks when they were all milked TAD, to check for any residual effect of OAD milking. The behaviour of OAD cows was observed around milking time. The incidence of diseases, the main performance variables (milk production, milk flow rate, live weight and body condition score), the detailed composition of milk (fat, protein, lactose, somatic cells, minerals, pH, free fatty acid (FFA), nitrogen fractions and enzymes) and some technological variables (clotting time and curd firmness) were measured on all cows.Some signs of disturbance were observed in the OAD cows at the time when milking was omitted: some cows mooed, some went close to the exit of the paddock, some leaked milk prior to milking. However, these signs disappeared after 2 days. After the experimental period, the live weight and the body condition score of TAD and OAD cows did not differ significantly. OAD cows produced 4.5 kg/day less than TAD (P < 0.001), this being more marked in Holstein (5.7 kg/day, P < 0.001) than Montbéliarde (3.3 kg/day, P < 0.001) cows. The milk contents of fat, whey protein, casein, total protein and phosphorus, and its plasminogen activity, were higher with OAD cows while lactose and FFA contents, and lipoprotein lipase activity were lower, with no interaction observed with breed. During the subsequent 3 weeks, when all cows were again milked TAD, OAD cows still produced 1.7 kg/day less milk (P < 0.01) with slightly higher fat and protein content.OAD milking for 7 weeks during the descending phase of lactation decreased milk production but increased milk content of most components, with a low residual effect. Montbéliarde cows were less affected by OAD milking than Holstein cows.     

Blood melatonin and prolactin concentrations in dairy cows exposed to 60 Hz electric and magnetic fields during 8 h photoperiods

Two experiments were conducted to test the hypothesis that electric and magnetic field (EMF) exposure may result in endocrine responses similar to those observed in animals exposed to long days. In the first experiment, 16 lactating, pregnant Holstein cows were assigned to two replicates according to a crossover design with treatment switchback. All animals were confined to wooden metabolic cages and maintained under short day photoperiods (8 h light/16 h dark). Treated animals were exposed to a vertical electric field of 10 kV/m and a horizontal magnetic field of 30 microT (EMF) for 16 h/day for 4 weeks. In a second, similar experiment, 16 nonlactating, nonpregnant Holstein cows subjected to short days were exposed to EMF, using a similar protocol, for periods corresponding to the duration of one estrous cycle. In the first experiment, circulating MLT concentrations during the light period showed a small numerical decrease during EMF exposure (P < .05). Least-square means for the 8 h light period were 9.9 versus 12.4 pg/ml, SE = 1.3. Melatonin concentrations during the dark period were not affected by the treatment. A similar trend was observed in the second experiment, where MLT concentrations during the light period tended to be lower (8.8 pg/ml vs. 16.3 pg/ml, P < .06) in the EMF exposed group, and no effects were observed during the dark period. Plasma prolactin (PRL) was increased in the EMF exposed group (16.6 vs. 12.7 ng/ml, P < .02) in the first experiment. In the second experiment, the overall PRL concentrations found were lower, and the mean plasma PRL concentration was not affected by treatment. These experiments provide evidence that EMF exposure may modify the response of dairy cows to photoperiod.     

Milking- and suckling-induced secretion of oxytocin and prolactin in parturient dairy cows

Serum concentrations of prolactin were unaffected by either suckling or milking on Day 2 or 3 postpartum in cows housed with their calves following parturition. In contrast, among cows housed without their calves milking elicited a four- to sixfold increase in serum prolactin concentrations. Serum oxytocin levels increased in response to both suckling and milking among cows housed with their calves with suckling being a more potent stimulus (257 ± 32 vs 189 ± 23 pg/ml at peak). However, the greatest increase in oxytocin levels accompanied milking in cows housed without their calves (375 ± 36 pg/ml at peak). These results suggest that stimuli associated with the presence or the absence of the calf can alter maternal secretion of oxytocin and prolactin. Greater understanding of factors which regulate secretion of these hormones may result in techniques to modify milk synthesis and milk ejection in dairy cows.     

Offering a forage crop at pasture did not adversely affect voluntary cow traffic or milking visits in a pasture-based automatic milking system

Feed is a strong incentive for encouraging cows in automatic milking systems (AMS) to voluntarily move around the farm and achieve milkings distributed across the 24 h day. It has been reported that cows show preferences for some forages over others, and it is possible that offering preferred forages may increase cow traffic. A preliminary investigation was conducted to determine the effect of offering a forage crop for grazing on premilking voluntary waiting times in a pasture-based robotic rotary system. Cows were offered one of two treatments (SOYBEAN or GRASS) in a cross-over design. A restricted maximum likelihood procedure was used to model voluntary waiting times. Mean voluntary waiting time was 45.5±6.0 min, with no difference detected between treatments. High and mid-production cows spent <44 min/milking in the premilking yard compared with >55 min/milking for low-production cows, whereas waiting time increased as queue length increased. Voluntary waiting time was 23% and 80% longer when cows were fetched from the paddock or had a period of forced waiting before volunteering for milking, respectively. The time it took cows to return to the dairy since last exiting was not affected by treatment, with a mean return time of 13.7±0.6 h. Although offering SOYBEAN did not encourage cows to traffic more readily through the premilking yard, the concept of incorporating forage crops in AMS still remains encouraging if the aim is to increase the volume or quantity of home-grown feed rather than improving cow traffic.     

The effects of conditioning to suckling, milking and of calf presence on the release of oxytocin in dairy cows

The aim of the study was to record the oxytocin (OT) release during milking (M) without or with calf presence, suckling (S) and finally calf removal just before the next milking in cows during postpartum or early lactation periods. Furthermore, the release of OT was examined during S and M in unknown surroundings (parlour). A total of 20 Brown Swiss cows kept under loose housing environment were used in our experiment. In both periods, the cows were milked twice daily at 07.00 and 18.00h and suckled three times daily at 09.00, 14.00 and 20.00h in the stall (tie housing). In the postpartum period, 13 cows were suckled and milked in the presence of their calves in the stall for the first 5 days of postpartum. Five from seven primiparous cows were additionally suckled by their own calves in the parlour on day 5 at 20.00h. On day 6, calves were separated and moved from mothers to the calf barn 10min before morning M. After evening milking cows were relocated to the herd within the same stable and milked in the parlour for a period of 4 weeks without contact to their calf. For control, additional seven primiparous cows without calf presence (not suckled) were also milked in the tie housing. In the early lactation period, suckled cows were moved back to the tie housing 2 days before the start of two consecutive days of S by their own calves and milking. Afterwards, 10min before M calves were separated again. Before S, two consecutive M were considered as controls. Results: The S stimulus during postpartum resulted in a higher OT release as compared with M in the calf presence and M after calf separation but not during M of not suckled cows. S in parlour reduced OT release. However, when not suckled primiparous cows were first milked in parlour, OT release was more reduced and in some cows total inhibition was observed. In early lactation during the first S, release of OT was lower than during control M, but increased gradually with repeated S and reached a maximum already on the second day. After two S, during evening M, the M related OT release was reduced as compared with controls. Calf removal 10min before M reduced OT release as compared with control M or M in calf presence. In conclusion, the calf presence and its removal can negatively influence OT secretion during M. Conditioning to machine milking caused a short-term reduction of OT release during first suckling, which normalised within 1 day.     

The effect of omitted milking on the behaviour of cows in the context of cluster attachment failure during automatic milking

In robotic milking there is always a slight chance of failure to attach the milking cluster. Attachment failure is most likely for cows whose udder conformation is less convenient for robot attachment. In general, after milking failure cows try to revisit the milking robot if they are not sent to a separate area. Since it is difficult to estimate the effect of milking failure on such a cow and her welfare in conditions of robotic milking, a specific 16-day trial was conducted on 12 cows. These cows were milked in a milking parlour with six milking stalls. Each afternoon milking, three cows were not milked. All the cows were closely observed in the cubicle house for 1 h after the afternoon milking. Thereafter, all cows were brought to the milking parlour the third time and the three unmilked cows were milked. In total, each cow was observed 12 times after milking and four times after omitted milking. The following behavioural traits were registered: time budget for the 1 h, occurrence and time until eating, drinking, lying, urination and defecation, and aggressive interactions. Milking order was defined on the basis of how often a cow came to the milking parlour in the first batch of six cows. Moreover, the data related to the milk yield and the use of the automatic feeding installation with the complete diet were analysed. After omitted milking, only the cows from the first batch stood longer in cubicles (14.2 min of 1 h) and lay less (5.4 min of 1 h) than milked cows of the same batch (respectively 7.0 min and 16.3 min for standing and lying in cubicles) (P<0.01). After omitted milking, cows urinated earlier and more frequently (64.5%) than milked cows (36.3%) (P<0.002) (both batches). There were no statistically significant differences in eating time and feed intake after milking and omitted milking. Milk yield per cow averaged 24.9 kg during days with omitted (delayed by 1 h) milking and 25.3 kg during the days without omitted milking (P<0.05). It was concluded that cows show some signs of discomfort after omitted milking (urination); this discomfort seemed to be greater in cows coming earlier to the milking parlour (afterwards they preferred to stand rather than to lie). The 60% of cases of milk leakage found after omitted milking indicates that failed cluster attachment can be accompanied by an extra risk factor for the occurrence of mastitis. However, omitted milking as a treatment did not influence feeding and aggressive behaviour or milking order when unmilked cows were brought to the milking parlour the third time together with the milked cows. Our methods and results can be useful for estimating the effects of robot milking failures on a cow. Future studies should pay particular attention to high-yielding cows and to longer periods of delayed milking.     

The effect of the intramammary infusion of Escherichia coli endotoxin on ovulation in lactating dairy cows

The purpose of this experiment was to determine if intramammary inflammation during the periovulatory period affects the occurrence of ovulation in lactating dairy cows. Ten lactating, cyclic, Holstein dairy cows received 2 injections of prostaglandin F2alpha at eleven-day intervals, to synchronize luteolysis. The day of the second injection was designated as day 0. Ovulation was anticipated to occur 3-5 days later (on days 3-5). Beginning at the morning milking on day 1, cows received intramammary infusions of either Escherichia coli endotoxin (10 microg; n=5) or infusion vehicle (pyrogen free Hank's balanced salt solution; n=5) into 2 quarters immediately after milking. The same quarters were infused after each milking through day 4. Venous blood samples were collected daily from day -1 through 13 for determination of progesterone to monitor luteolysis and formation of a new corpus luteum. Blood samples were also collected at 4-h intervals (days -1 to 2), then at 2-h intervals (days 2 to 5) to measure concentrations of luteinizing hormone. Ovaries were examined ultrasonographically on days -1 through 5 and on day 12 to monitor follicular growth and formation of the corpus luteum. Collectively, these observations were used to determine if and when ovulation occurred. Intramammary infusion of E. coli endotoxin induced an immediate increase in the concentration of somatic cells in milk from treated quarters. However, this treatment had no effect on the occurrence or timing of ovulation. Based on ultrasonography and concentrations of progesterone, four of five cows in each treatment group appeared to have ovulated. Preovulatory surges of LH were detected within the intensive bleeding periods for three cows in each treatment group. The magnitude of the LH surge was reduced in cows receiving endotoxin.     

Lactational [correction of Lacational] anovulation in rats and its dependency on progesterone

The relationship between prolactin (PRL) secretion and anovulation in lactating rats was studied. Normal lactating rats and lactating rats treated with antiserum against luteinizing hormone-releasing hormone at the time of postpartal ovulation were used. Normal lactating rats were treated with either a dopamine agonist (CB-154, 150 micrograms/rat) on Day 10 or 13, or pups removal on Day 7 or 10, and thereafter luteolysis and inhibition on PRL secretion were assessed. With the CB-154 treatment, the incidence of luteolysis increased as the lactational period advanced (42% vs 72%), whereas it decreased (73% vs 14%) with the pups removal. Thus, dopamine effectively inhibited PRL secretion during the later lactational stage, but could not do so during the earlier stage when there were mechanisms other than dopamine stimulating PRL secretion. Following luteal regression induced by CB-154, ovulation did not occur if the rats were treated with CB-154 on Day 10, whereas 50% of the rats ovulated within 4 days if treated on Day 13. Furthermore, in the lactating rats treated with anti-luteinizing hormone-releasing hormone serum during late pregnancy, ovulation was not observed until Day 10 of lactation. Since the serum progesterone levels were low in these rats due to the absence of ovulation and lactational corpora lutea, the blockade of ovulation was not due to elevated circulating progesterone during the early lactational period. The mechanism of ovulation blockade during lactation thus seems to shift from being progesterone independent to progesterone dependent at a similar period when the neuroendocrine control of PRL secretion shifts from dopamine independent to dependent.