The Events of Normal and Abnormal Postpartum ReproductiveEndocrinology and Uterine Involution in Dairy Cows: A Review

K.E. LESLIE

Department of Clinical Studies, Ontario Veterinary College,University of Guelph, Guelph, Ontario NI G 2 WI

SUMMARY

The results of numerous hormonalprofile studies in the dairy cow arereviewed with respect to specific hor-mone changes in the early postpartumperiod, and factors contributing to theonset of cyclic reproductive activity.Variations from the normal pattern ofpostpartum hormonal activity or ute-rine involution are reviewed also. Thenormal involutionary changes of thebovine uterus are discussed.

R SUM

Une revue de rendocrinologie repro-ductive puerperale normale et anor-male, ainsi que de l'involution uterine,chez la vache laitiereCet article presente une revue desresultats de plusieurs etudes hormo-nales effectuees chez la vache laitiere etrelatives aux changements hormo-naux qui se produisent des le debut dela periode puerperale, ainsi qu'aux fac-teurs qui contribuent au retour de l'ac-tivite reproductrice cyclique. II passeegalement en revue les variations duprofil normal de l'activite hormonalepuerperale et de l'involution uterine. I1presente aussi des commentaires rela-tifs 'a l'involution uterine bovinenormale.

NORMAL POSTPARTUMR E P R O D U C T I V EENDOCRINOLOGY ANDUTERINE INVOLUTION

Normal Postpartum ReproductiveEndocrinologyA balanced, coordinated endocrine

system is important for normal repro-ductive function. This involveshomeostasis among gonadotrophinreleasing hormones (GnRH) from thehypothalamus; follicle stimulatinghormone (FSH), luteinizing hormone(LH) and prolactin (PRL) from theadenohypophysis; prostaglandin F2-

alpha (PGF2) from the uterus and thegonadal steroids (36).

During pregnancy one of the ovariescontains a well developed corpusluteum (CL) and both ovaries havesmall follicles that are barely detect-able on the ovarian surfaces. Occa-sionally larger follicular structures(> 10 mm) are found. Approximatelysix weeks prior to parturition, the bio-synthetic functions of the fetoplacen-tal unit result in gradually increasingplasma levels of estrogens (11). Theperipheral blood level of a metaboliteof PGF2a undergoes a dramaticincease shortly before parturition andearly in the postpartum period (57).This was viewed as an indication thatPGF2a is involved in the prepartumluteolysis of the CL of pregnancywhich is in agreement with otherreports (15,27).

This increase in PGF concentra-tion appears to be extremely impor-tant for normal uterine involution.The PGF2< levels parallel the rate ofuterine involution with a peak at day 4postpartum and thereafter remain ele-vated for up to 20 days (27,31).

Progesterone levels decline rapidlyduring the last 48 hours before parturi-tion and remain at very low levels(< 0.5 ng/ mL) throughout the earlypostpartum period (25,26). Thisdecline is related to regression of theCL of pregnancy.

After parturition, the plasma estro-gen levels decrease sharply to valuesbelow those found during the normalestrous cycle (18). Increases in estro-gen levels have not been reported inthe early postpartum period, eventhough follicular activity is initiatedearly. It has been suggested that thesame feedback mechanism which con-trols the pituitary during the normalestrous cycle is responsible for initiat-ing the first postpartum estrus (23).

This feedback mechanism involves fol-licular growth and estrogen secretion.

Follicle stimulating hormone ismaintained at relatively constant levelsthroughout the life of the postpubertaldairy cow (50). Plasma FSH concen-trations fluctuate between 30 and70 ng/mL and are somewhat inde-pendent of other hormonal changes.Follicle stimulating hormone release,in response to exogenous GnRH injec-tions, is reduced during the periodfrom nine days before to six days afterparturition (49). It has been demon-strated also that FSH levels are slightlyincreased and are maintained at con-stant levels through parturition andinto the postpartum period (13). Thus,it would appear that delay of onset ofcyclic function is unlikely to be due tolow FSH levels.

Several investigators have studiedLH concentrations during variousreproductive stages. Luteinizing hor-mone follows a release pattern that ischaracterized by a rapid increase fol-lowed by a more gradual return tonormal levels (3,44). These spasmodicepisodes of rapid increase in LH levelare described as pulsatile LH release(20). A tonic release maintains basalLH levels. Superimposed on thisrelease pattern is a rapid and markedincrease in basal LH levels just prior toovulation. This has been called thepreovulatory LH surge. The samebasic LH concentrations and releasepatterns found during the normalestrous cycle have been observed incows during the early postpartumperiod (6,20,49,61).

Follicular activity has been detectedas early as five to ten days postpartum(48). The follicles continue to growuntil at least one becomes ready torespond to a preovulatory LH surge.Extensive investigation has beennecessary to accurately determine the

Can Vet J 1983; 24: 67-71. 67

factors which initiate the first postpar-tum ovulation. It has been suggestedthat the ovary regulates LH secretionin cattle (44). However, from studies inprimates, it has been proposed thatLH secretion, and ovulation are underneural control (30).

In sexually mature female cattle,both estradiol 17-beta (51) and GnRH(49) can induce the release of LH fromthe anterior pituitary gland. The inter-action of these two hormones to causea LH surge depends primarily on theestradiol 17-beta concentration. Ova-riectomized heifers display only a pul-satile LH release pattern (3).

Gradually increasing estrogen con-centrations to markedly elevated levelsmay have an inhibitory effect on theproduction and/or release of a LHsurge by the pituitary. This effect hasbeen demonstrated in late pregnancyand in animals with cystic ovarian dis-ease and is different from the effect ofasynchronous rapid elevation in estro-gen during late proestrus ofthe normalcycle. Firstly, in late pregnancy from40 to 60 days prepartum until six daysafter calving, estrogen levels are highand there is a decrease in LH peakvalues. Several workers have exam-ined the pituitary response to exogen-ous GnRH during the prepartum andpostpartum periods and have de-scribed LH release in response toGnRH injection to be substantiallyreduced from five weeks before untileight to ten days after parturition(19,26,49). Full responsiveness re-turned by day 10 postpartum. A fullLH response has been artificially pro-duced at day 2 postpartum by using acombination of estrogens, progeste-rone, and then, GnRH (2). Secondly,elevated estrogen levels have beendfni-n cowswith-ovarian cysts--(24)3Such animals have elevated basallevels of LH, but no LH surge (38).This negative feedback activity is incontrast to the one day estrogen peakthat occurs during a normal estrouscycle which is followed by the pre-ovulatory LH surge (23).On the other hand, high levels of

progesterone appear to suppress thehypothalamus and not the pituitary.Progesterone levels are constantly ele-vated during gestation (42), except fora small decrease from day 60 to day 90after conception (47). Yet, the pitui-tary can fully respond to exogenous

GnRH throughout pregnancy (49).In summary, the endocrine changes

responsible for the resumption ofpostpartum cyclic activity are com-plex. A particular set of hormonal cir-cumstances are necessary for the firstpostpartum cycle to be initiated. Aftera short-term elevation, plasma proges-terone levels must be low and estradiol17-beta must be at an appropriatethreshold level (5 1). The rise anddecline of progesterone, followed bythe rise in estradiol 17-beta that occurprior to the first postpartum cycle, aresimilar to the changes during proestrusof a normal estrous cycle. It is specu-lated that GnRH initiates the preovu-latory LH surge in late estrus. Thepresence of a large mature follicle isnecessary for all of these changes tooccur and the ovulation which follows.The need for specific hormonal

conditions to precede and induce ovu-lation is supported by the research onthe use of GnRH in the early postpar-tum period (21,26). In one of thesestudies the ovaries were examined bylaparoscopy, and then the cows weretreated with GnRH. Induced ovula-tion occurred only if a large follicle(> 10 mm) was present on the ovary(21).The first ovulation usually occurs

between ten and 25 days postpartum(33,40). Approximately 50% of cowsexhibit short cycles during the earlypostpartum period (39,50). It has beensuggested that immature corpora luteamay be the cause of these short cycles(39). The first postpartum endogenouspreovulatory LH surges have beenshown to be too low to produce com-plete luteinization (14). It is also possi-ble that reduced blood flow to theovary may interfere with normal CLformatfion-aslhas-besnAemonstated insheep (41).The CL of pregnancy is still palpa-

ble on the ovary until ten to 14 dayspostpartum, even though functionalactivity ceases before parturition (40).The regressed CL of pregnancy doesnot appear to influence the timing ofthe first postpartum ovulation. Short-term increases in plasma progesteronehave been reported prior to the initia-tion of the first cycle, which are specu-lated to be from luteinized follicles(61). These workers suggest that adelay in the time of first ovulation maybe due to suppression of GnRH activ-

ity by these small increases in proges-terone (61). This might, in turn, inhibitpulsatile LH release.Hormonal profiles of the first

estrous cycle have been described ingreat detail (14). Few endocrinologicaldifferences exist compared to subse-quent estrous cycles (62). The estradiollevels preceding first estrus were lowerthan in subsequent cycles. Likewise,LH levels during the first estrus andprogesterone concentrations follow-ing first ovulation were comparativelylower (50,53).The ovary ipsilateral to the non-

gravid horn has been shown to resumeactivity more rapidly (33). When ovu-lations occurred between day 10 andday 15 postpartum, 90% of these ovu-lations were on the ovary contralateralto the gravid horn. Whether this is dueto a local inhibitory effect of the CL ofpregnancy or a uterine inhibitoryeffect of reduced bloodflow remains amatter for speculation.

Several studies indicate that fewanimals (< 30%) show behavioralsigns of estrus accompanying first ovu-lation (5,28,29). The incidence of silentestrus decreases to less than 30% at thesecond and third ovulatory cycles (5).

In beef cows, the return to cyclicovarian function is markedly delayed,the cause of which is poorly under-stood. The presence of a sucking calfpostpones the commencement of cy-clic activity (43). Suckling inhibits thepulsatile release of LH and assurescontinued high PRL levels. In dairycows, PRL increases at parturition(1 1), but declines to lower levels withintwo or three days of calving (58). Ele-vated PRL levels have been recordedin a selected group of higher producingdairy cows (57). The suppression of

--PRL-does-notalter the-rate ofre-rn4-o--cyclic function (49). Thus, its role incontrolling the onset of cyclic activityis suggested to be minimal.

Normal Postpartum UterineInvolution

Several authors have describednormal and abnormal involution ofthe uterus (4,22,39,40,45,46,55).

Shortly after parturition, the uterusis a large organ measuring roughly onemeter in length and weighing 8 to10 kg (46). The most profound involu-tion occurs between the time of calvingand day 3 postpartum (38). The rapid

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decrease in size is due to vasoconstric-tion and peristaltic contractions,which occur at three to four minuteintervals and gradually diminish byday 4 postpartum.

Events during involution of the ute-rus include necrosis of the caruncularstalk, breakdown of the superficiallayer of the caruncle and formation ofthe lochial discharge (22,46). This pro-cess is generally complete by day 12postpartum. Normal involution can bean aseptic process, but this is theexception rather than the rule (54).Spontaneous puerperal infection, withmassive bacterial growth in the lochia,is common (45). It has also beenreported that 93% of bovine uteri areinfected up to day 15, 78% up to day30, 50% up to day 45 and 9% up to day60 postpartum (16). Corynebacteriumpyogenes and Escherichia coli are themost commonly cultured bacteria.An extensive study of postpartum

involution of the uterus and cervix hasbeen carried out (40). The size of thebovine uterus decreases slowlybetween days 4 and 9 postpartum. Atthis time, the diameter of the pre-viously gravid horn ranges from 12 to14 cm in normal cows and by day 10the uterus could be completely definedby rectal palpation. Normal cowsexhibit a marked increase in uterinetone and decrease in ute-rine size fromdays 10 to 14 postpartum which coin-cide with the onset of first estrus. Theuterine horn averages 7 to 8 cm indiameter by day 14. Subsequent invo-lution was rapid and by day 25 adiameter of 2 to 4 cm was usuallyattained with the uterine horns beingalmost equal in size (40).

Regression in uterine weight, in asimilar pattern and proportion to thereduction in size, has been reported(34). Histological evidence of repair ofthe endometrium is reported to pro-ceed at a slightly slower rate than grossuterine involution (32).

Involution of the cervix is slowerthan that of the uterus (40). Normalcows, on average attain complete cer-vical involution by day 30 postpartum.

ABNORMALITIES OFR E P R O D U C T I V EENDOCRINOLOGY ANDUTERINE INVOLUTION IN THEEARLY POSTPARTUM PERIODAbnormal hormonal (7,10,18) and

uterine involutionary changes (40,55)

during the early postpartum periodhave been reported.

Primary predisposition of cows toretained placenta (RP) has been asso-ciated with numerous infectious,nutritional, genetic and physiologicaletiologies. However, it has been sug-gested that these predisposing factorscould lead to a common prepartumtemporal change of certain hormones(7). Synchronized hormonal changeshave been described in relation to lac-togenesis (11), parturition (17), andplacental release (10). Interferencewith these changes by ovariectomyduring pregnancy has resulted in ahigh frequency of RP (8,37). Similarlyinduction of premature parturition byglucocorticoid therapy has been asso-ciated with a high incidence of RP(9,37,60).Hormonal profile studies have

demonstrated that prepartum plasmaprogesterone values were elevated inRP cows compared to non-RP cowsthat calved either spontaneously (1,10)or following glucocorticoid induction(9). Prepartum plasma estrogens werelower in RP cows than in non-RPcows either after both spontaneousand induced parturition (7,8).The balance between plasma levels

of progesterone and estradiol 17-betaon day 6 prepartum was significantlyassociated with the subsequent occur-rence of RP (7). When plasma proges-terone was at an intermediate level (4to 8 ng/ mL) and estradiol 17-beta washigh (exceeding 99 pg/ mL) there was atenfold lower incidence of RP thanwhen progesterone was high or whenprogesterone and estradiol 17-betawere both low. Plasma profiles ofPRLand estrone were almost identical inRP and non-RP groups. This experi-ment was conducted as a case controlstudy with peer matching on severalparameters (7).

It has been demonstrated that cowswith prolonged uterine involutiontimes also had relatively extendedperiods of elevated plasma PGF2a lev-els in comparison to cows with normaluterine involution times (15,3 1).The fetoplacental unit produces

large amounts of estrogen in late preg-nancy, which sensitizes the myome-trium to both oxytocin and prosta-glandin. A decrease in the estradiol17-beta/ 17-alpha ratio has recentlybeen reported to result in a reduced

rate of release of prostaglandins fromthe uterus and a slower rate of uterineinvolution (18). In other words, alower concentration of biologicallyactive estrogens may result in a slowerrate of involution possibly due toreduced PGF2a release. In this regard,an increase in PGF release into theutero-ovarian vein occurred inresponse to the administration ofestradiol 17-beta in the goat (12).

Uterine involution is more rapid insuckled beef cows than in milkingdairy cows (48). Involution was com-plete in 15 to 25 days in suckled cowsand in 25 to 30 days in milking cows.The sucking stimulus was believed tocause frequent release of oxytocinfrom the posterior pituitary, whichenhanced the reduction in size of theuterus through its myometrial con-tractile effect. Milking also causesoxytocin release, but the frequencywould be less than that with suckling.It has been shown that milking Hol-stein cows four times a day until day 25postpartum improved uterine involu-tion, without affecting the pulsatileLH release and the onset of first estrus(6).Normal uterine involution and

resumption of cyclic ovarian activityprogressed more slowly in cows withperiparturient diseases, such as dys-tocia, milk fever, ketosis, RP and met-ritis, than in cows which had expe-rienced normal parturition (40). Theuterus did not reach nongravid sizeuntil approximately day 30 in cowswith such diseases. The uterus was sig-nificantly larger at ten and 20 dayspostpartum after an abnormal parturi-tion. Similar results have beenreported by other investigators (4).

Shortly after 30 day postpartum, thedifferences in involutionary statusbetween cows which had calved nor-mally and those with periparturientabnormalities, were no longer detec-table (40). Age and parity are impor-tant considerations when evaluatingthe size of uterine horns on a postpar-tum examination (4,39). Otherauthors have also shown that RP sig-nificantly delayed uterine involution(34).

Prolonged luteal function has beendemonstrated in cows with pyometra(52). It was hypothesized that thismight result from failure of PGF2,syn-thesis due to endometrial damage. It is

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therefore tempting to assume that ute-rine infection in the immediate post-partum period may reduce PGF2.release and consequently delayinvolution.

In studies, in which sequentialplasma progesterone concentrationswere plotted, ovarian function andestrous activity during early lactationhave been monitored (28). It wasfound that 57 of 69 cows establishedearly regular cycles and their ovarianactivity was considered normal. How-ever, the duration of the estrous cyclesin some of the normal cows was consi-dered aberrant, being less than 17 daysor more than 25 days in length. Theincidence of aberrant cycles was 25%after first ovulation, 12% after secondovulation and 8% after third ovulation(27).The 12 abnormal cows in this study

presented a wide variety of postpar-tum abnormalities (27). Follicularcysts, with plasma progesterone levelsof less than 1 ng/ mL, were observedfollowing none, one or two normalovulations. Smooth nonfunctioningovaries resulted after failure of CLformation subsequent to first ovula-tion in one case studied. Even in theabsence of clinical uterine pathology, apersistent CL was occasionally foundafter first or second ovulation. Allcows commenced the postpartumperiod with basal plasma progesteronelevels, indicating the CL of pregnancydid not remain functional after partu-rition (27).

Others have reported similar find-ings using the milk progesterone assay(5,35,59). Incidence rates of 5 to 10%were found for both ovarian cysts andpostpartum anestrus.

It has been found that a normal CLformed during only 62.5% of postpar-tum cycles (40). A CL was consideredindicative of a normal estrous cycle.Follicular ovarian cysts developedduring 12.3% of the observed cyclesand occurred more frequently in thefirst 30 days postpartum than in the31-60 day period. Spontaneous recov-ery, however, was common (40).

Differences between cows withperiparturient diseases and cows thatcalved normally were also reported(40). Ovarian cysts were more com-mon in the former group. They alsorequired more services per conceptionand had longer calving to conception

intervals than the normal cows. Theauthors emphasized that normal cowshad two ovulations during the first 30to 35 days after parturition, whereascows with periparturient diseases hadonly one ovulation during this period(40). These findings were in agreementwith the results of other workers (33)and are important in view of the find-ing that the number of cycles prior today 30 postpartum was an importantindicator of reproductive performance(56).

It is evident from this review ofreproductive endocrinology and ute-rine involution of the postpartumdairy cow that normal ranges havebeen extensively described. Reports ofhormonal values and uterine eventsare less frequently reported withrespect to abnormalities of the post-partum period, such as might exist incases of retained placenta or metritis.The underlying endocrinologicalmechanisms necessary for initiation ofcyclic ovarian activity after parturitionare still not well understood and thereasons for the delay or advancementof resumption of reproductive activityare poorly described. Many questionsremain to be answered concerning thepostpartum period of the dairy cow.For example, resumption of cyclicovarian activity in RP cases has notbeen well documented. Concurrenthormonal profile studies and repro-ductive tract examinations could pro-vide useful information on this impor-tant period in the reproductive life ofboth dairy and beef cattle.

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