Effects of calcium and sodium on ATP-induced vasopressin release from rat isolated neurohypophysial terminals.

ATP-receptors (P2X2, P2X3, P2X4 & P2X7) are found in neurohypophysial terminals (NHT). These purinergic receptor subtypes are known to be cation selective. Here we confirm that both sodium (Na+ ) and calcium (Ca2+ ) are permeable through these NHT purinergic receptors, but to varying degrees (91% vs. 9%, respectively). Furthermore, extracellular calcium inhibits the ATP-current magnitude. Thus, the objective of this study was to determine the effects of extracellular Na+ vs. Ca2+ on ATP-induced vasopressin (AVP) release from populations of rat isolated NHT. ATP (200 µM) perfused exogenously for 2 minutes in Normal Locke's buffer caused an initial transient increase in AVP release followed by a sustained increase in AVP release which lasted for the duration of the ATP exposure. Replacing extracellular NaCl with NMDG-Cl had no apparent effect on the ATP-induced transient increase in AVP release but abolished the sustained AVP release induced by ATP. Furthermore, removal of extracellular calcium resulted in no ATP-induced transient increase in AVP release, but had no effect on the delayed, sustained increase in AVP release. The ATP-induced calcium-dependent transient increase in AVP release was >95% inhibited by 10 µM of the P2X purinergic receptor antagonist PPADS, a dose sufficient to block P2X2 and P2X3 receptors but not P2X4 or P2X7 receptors. Interestingly, the ATP-induced calcium-independent, sodium-dependent sustained increase in AVP release was not affected by 10 µM PPADS. The ATP-induced calcium-dependent transient increase in AVP release was not affected by the P2X7 receptor antagonist BBG (100 nM). However, the ATP-induced sodium-dependent sustained AVP release was inhibited by 50%. Therefore, these results show that rat isolated NHT exhibit a biphasic response to exogenous ATP that is differentially dependent on extracellular calcium and sodium. Furthermore, the initial transient release appears to be through P2X2 and/or P2X3 receptors and the sustained release is through a P2X7 receptor. This article is protected by copyright. All rights reserved.

Adenosine trisphosphate appears to act via different receptors in terminals versus somata of the hypothalamic neurohypophysial system.

ATP-induced ionic currents were investigated in isolated terminals and somata of the hypothalamic neurohypophysial system (HNS). Both terminals and somata showed inward rectification of the ATP-induced currents and reversal near 0 mV. In terminals, ATP dose-dependently evoked an inactivating, inward current. However, in hypothalamic somata, ATP evoked a very slowly inactivating, inward current with a higher density, and different dose dependence (EC(50) of 50 µm in somata versus 9.6 µm in terminals). The ATP-induced currents, in both the HNS terminals and somata, were highly and reversibly inhibited by suramin, suggesting the involvement of a purinergic receptor (P2XR). However, the suramin inhibition was significantly different in the two HNS compartments (IC(50) of 3.6 µm in somata versus 11.6 µm in terminals). Also, both HNS compartments show significantly different responses to the purinergic receptor agonists: ATP-γ-S and benzoyl-benzoyl-ATP. Finally, there was an initial desensitisation to ATP upon successive stimulations in the terminals, which was not observed in the somata. These differences in EC(50) , inactivation, desensitisation and agonist sensitivity in terminals versus somata indicate that different P2X receptors mediate the responses in these two compartments of HNS neurones. Previous work has revealed mRNA transcripts for multiple purinergic receptors in micropunches of the hypothalamus. In the HNS terminals, the P2X purinergic receptor types P2X2, 3, 4 and 7 (but not 6) have been shown to exist in AVP terminals. Immonohistochemistry now indicates that P2X4R is only present in AVP terminals and that the P2X7R is found in both AVP and oxytocin terminals and somata. We speculate that these differences in receptor types reflects the specific function of endogenous ATP in the terminals versus somata of these central nervous system neurones.

P2X purinergic receptor knockout mice reveal endogenous ATP modulation of both vasopressin and oxytocin release from the intact neurohypophysis.

Bursts of action potentials are crucial for neuropeptide release from the hypothalamic neurohypophysial system (HNS). The biophysical properties of the ion channels involved in the release of these neuropeptides, however, cannot explain the efficacy of such bursting patterns on secretion. We have previously shown that ATP, acting via P2X receptors, potentiates only vasopressin (AVP) release from HNS terminals, whereas its metabolite adenosine, via A1 receptors acting on transient Ca(2+) currents, inhibits both AVP and oxytocin (OT) secretion. Thus, purinergic feedback-mechanisms have been proposed to explain bursting efficacy at HNS terminals. Therefore, in the present study, we have used specific P2X receptor knockout (rKO) mice and purportedly selective P2X receptor antagonists to determine the P2X receptor subtype responsible for endogenous ATP induced potentiation of electrically-stimulated neuropeptide release. Intact neurohypophyses (NH) from wild-type (WT), P2X3 rKO, P2X2/3 rKO and P2X7 rKO mice were electrically stimulated with four 25-s bursts (3 V at 39 Hz) separated by 21-s interburst intervals with or without the P2X2 and P2X3 receptor antagonists, suramin or pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS). These frequencies, number of bursts, and voltages were determined to maximise both AVP and OT release by electrical stimulations. Treatment of WT mouse NH with suramin/PPADS significantly reduced electrically-stimulated AVP release. A similar inhibition by suramin was observed in electrically-stimulated NH from P2X3 and P2X7 rKO mice but not P2X2/3 rKO mice, indicating that endogenous ATP facilitation of electrically-stimulated AVP release is mediated primarily by the activation of the P2X2 receptor. Unexpectedly, electrically-stimulated OT release from WT, P2X3, P2X2/3 and P2X7 rKO mice was potentiated by suramin, indicating nonpurinergic effects by this 'selective' antagonist. Nevertheless, these results show that sufficient endogenous ATP is released by bursts of action potentials to act at P2X2 receptors in a positive-feedback mechanism to 'differentially' modulate neuropeptide release from central nervous system terminals.

A new in vivo model for luteolysis using systemic pulsatile infusions of PGF(2α).

A new in vivo model for studying luteolysis was developed in sheep to provide a convenient method for collecting corpora lutea for molecular, biochemical, and histological analysis during a procedure that mimics natural luteolysis. It was found that the infusion of prostaglandin F(2α) (PGF(2α)) at 20 µg/min/h into the systemic circulation during the mid luteal phase of the cycle allowed sufficient PGF(2α) to escape across the lungs and thus mimic the transient 40% decline in the concentration of progesterone in peripheral plasma seen at the onset of natural luteolysis in sheep. Additional 1h-long systemic infusions of PGF(2α), given at physiological intervals, indicated that two infusions were not sufficient to induce luteolysis. However, an early onset of luteolysis and estrus was induced in one out of three sheep with three infusions, two out of three sheep with four infusions, and three out of three sheep with five infusions. Reducing the duration of each systemic infusion of PGF(2α) from 1h to 30 min failed to induce luteolysis and estrus even after six systemic infusions indicating that, not only are the amplitude and frequency of PGF(2α) pulses essential for luteolysis, but the actual duration of each pulse is also critical. We conclude that a minimum of five systemic pulses of PGF(2α), given in an appropriate amount and at a physiological frequency and duration, are required to mimic luteolysis consistently in all sheep. The five pulse regimen thus provides a new accurate in vivo model for studying molecular mechanisms of luteolysis.

Evidence for a potential role of neuropeptide Y in ovine corpus luteum function.

Neuropeptide Y (NPY) is a neurohormone that is typically associated with food intake, but it has also been reported to affect the production of progesterone from luteal tissue in vitro. However, NPY has not been previously immunolocalized in the ovine ovary or in the corpus luteum (CL) of any species, and the effects of this neurohormone on luteal function in vivo are not known. Thus, we performed fluorescent immunohistochemistry (IHC) to localize NPY in the ovine ovary and used avidin-biotin immunocytochemistry (ICC) to further define the intracellular localization within follicles and the CL. We then infused NPY directly into the arterial supply of the autotransplanted ovaries of sheep to determine the in vivo effect of exogenous NPY on ovarian blood flow and on the luteal secretion rate of progesterone and oxytocin. Immunohistochemistry revealed that the NPY antigen was localized to cells within the follicles and CL, in the nerve fibers of the ovarian stroma, and in the vessels of the ovarian hilus. In the follicle, the NPY antigen was localized to nerves and vessels within the theca interna layer, and strong staining was observed in the granulosal cells of antral follicles. In the CL, NPY was localized in large luteal cells and in the vascular pericytes and/or endothelial cells of blood vessels, found dispersed throughout the gland and within the luteal capsule. In vivo incremental infusions of NPY at 1, 10, 100, and 1,000 ng/min, each for a 30-min period, into the arterial supply of the transplanted ovary of sheep bearing a CL 11 d of age increased (P< or =0.05) ovarian blood flow. The intra-arterial infusions of NPY also increased (P< or =0.05) in a dose-dependent manner the secretion rate of oxytocin, which was positively correlated (P< or =0.05) with the observed increase in ovarian blood flow. The infusions of NPY had a minimal effect on the secretion rate of progesterone, and similar intra-arterial infusions of NPY into sheep with ovarian transplants bearing a CL over 30 d of age had no significant effect on ovarian blood flow or on the secretion rate of progesterone. These results suggest that NPY acts on the luteal vascular system and the large luteal cells to rapidly stimulate blood flow and the secretion of oxytocin, respectively, which collectively implies a putative role for NPY during the process of luteolysis when increasing amounts of oxytocin are secreted from the ovine CL in response to uterine pulses of prostaglandin F2alpha.

mu-Opioid receptor modulates peptide release from rat neurohypophysial terminals by inhibiting Ca(2+) influx.

The activation of opioid receptors in neurones of the central nervous system leads to a variety of effects including the modulation of diuresis and parturition, processes that are directly controlled by the hypothalamic-neurohypophysial system (HNS). The effects of mu-opioid receptor activation on peptide release, voltage-gated Ca2+ currents and intracellular calcium levels ([Ca2+]i) were studied in isolated nerve terminals of the HNS. The mu-receptor agonist, DAMGO ([d-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin) inhibited high K+-induced peptide release in a dose-dependent manner, with oxytocin release being more sensitive to block than vasopressin release at all concentrations tested. The addition of the mu-receptor antagonist CTOP (d-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr amide) was able to overcome the inhibitory effects of DAMGO. By contrast to previous results, voltage-gated Ca2+ currents were sensitive to blockage by DAMGO and this inhibition was also prevented by CTOP. Furthermore, [Ca2+]i measurements with Fura-2 corroborated the inhibition by DAMGO of calcium entry and its reversal by the micro -receptor antagonist in these nerve terminals. Thus, the decrease in neuropeptide release, particularly for oxytocin, induced by the activation of mu-opioid receptors in neurohypophysial terminals is mediated, at least in part, by a corresponding decrease in Ca2+ entry due to the inhibition of voltage-gated Ca2+ channels.

Tolerance to acute ethanol inhibition of peptide hormone release in the isolated neurohypophysis.

BACKGROUND: Acute ethanol (EtOH) exposure reduces the evoked release of vasopressin (AVP) and oxytocin (OT) from excised neurohypophyses and from dissociated neurohypophysial terminals of the rat. METHODS AND RESULTS: Rats placed on a diet that maintained blood levels of 30 mM EtOH for 20 to 40 days developed tolerance to acute EtOH inhibition of release. In the presence of 10 mM EtOH, high (50 mM) K+-induced release of AVP from isolated neurohypophysial terminals of EtOH-naive rats was reduced by 77.7+/-1.4%, whereas in the chronic EtOH group, release was reduced by only 9.4+/-8.7%. Similar tolerance was evident during acute challenge with 75 mM EtOH, as well as for release of OT from isolated terminals. Animals treated with an intraperitoneal injection of EtOH and sacrificed 90 min postinjection did not exhibit the reduced EtOH inhibition of release from dissociated terminals during a 75 mM EtOH acute challenge. CONCLUSIONS: The altered component responsible for the tolerance to inhibition of release resides in the isolated terminal, because tolerance measured in vitro from intact neurohypophyses was similar to that seen in isolated terminals. The failure of EtOH-injected animals to exhibit reduced inhibition of release in response to an acute EtOH challenge indicates that short-term elevated blood alcohol level does not induce this tolerance. The finding of tolerance to EtOH-induced inhibition of release from the intact neurohypophysis and isolated terminals provides a physiological preparation in which to examine the molecular targets of acute drug action modified after chronic exposure to the drug.

Luteolysis: a neuroendocrine-mediated event.

In many nonprimate mammalian species, cyclical regression of the corpus luteum (luteolysis) is caused by the episodic pulsatile secretion of uterine PGF2alpha, which acts either locally on the corpus luteum by a countercurrent mechanism or, in some species, via the systemic circulation. Hysterectomy in these nonprimate species causes maintenance of the corpora lutea, whereas in primates, removal of the uterus does not influence the cyclical regression of the corpus luteum. In several nonprimate species, the episodic pattern of uterine PGF2alpha secretion appears to be controlled indirectly by the ovarian steroid hormones estradiol-17beta and progesterone. It is proposed that, toward the end of the luteal phase, loss of progesterone action occurs both centrally in the hypothalamus and in the uterus due to the catalytic reduction (downregulation) of progesterone receptors by progesterone. Loss of progesterone action may permit the return of estrogen action, both centrally in the hypothalamus and peripherally in the uterus. Return of central estrogen action appears to cause the hypothalamic oxytocin pulse generator to alter its frequency and produce a series of intermittent episodes of oxytocin secretion. In the uterus, returning estrogen action concomitantly upregulates endometrial oxytocin receptors. The interaction of neurohypophysial oxytocin with oxytocin receptors in the endometrium evokes the secretion of luteolytic pulses of uterine PGF2alpha. Thus the uterus can be regarded as a transducer that converts intermittent neural signals from the hypothalamus, in the form of episodic oxytocin secretion, into luteolytic pulses of uterine PGF2alpha. In ruminants, portions of a finite store of luteal oxytocin are released synchronously by uterine PGF2alpha pulses. Luteal oxytocin in ruminants may thus serve to amplify neural oxytocin signals that are transduced by the uterus into pulses of PGF2alpha. Whether such amplification of episodic PGF2alpha pulses by luteal oxytocin is a necessary requirement for luteolysis in ruminants remains to be determined. Recently, oxytocin has been reported to be produced by the endometrium and myometrium of the sow, mare, and rat. It is possible that uterine production of oxytocin may act as a supplemental source of oxytocin during luteolysis in these species. In primates, oxytocin and its receptor and PGF2alpha and its receptor have been identified in the corpus luteum and/or ovary. Therefore, it is possible that oxytocin signals of ovarian and/or neural origin may be transduced locally at the ovarian level, thus explaining why luteolysis and ovarian cyclicity can proceed in the absence of the uterus in primates. However, it remains to be established whether the intraovarian process of luteolysis is mediated by arachidonic acid and/or its metabolite PGF2alpha and whether the central oxytocin pulse generator identified in nonprimate species plays a mediatory role during luteolysis in primates. Regardless of the mechanism, intraovarian luteolysis in primates (progesterone withdrawal) appears to be the primary stimulus for the subsequent production of endometrial prostaglandins associated with menstruation. In contrast, luteolysis in nonprimate species appears to depend on the prior production of endometrial prostaglandins. In primates, uterine prostaglandin production may reflect a vestigial mechanism that has been retained during evolution from an earlier dependence on uterine prostaglandin production for luteolysis.

The central oxytocin pulse generator: a pacemaker for the ovarian cycle.

During luteolysis in sheep, episodic pulses of oxytocin (OT), contributed by the neurohypophysis and the corpus luteum (CL), stimulate uterine luteolytic pulses of prostaglandin (PG) F2 alpha via endometrial OT receptors. To distinguish relative contributions of neurohypophysial and luteal OT, ovariectomized sheep were given estradiol-17 beta (E) and progesterone (P) to stimulate levels during the cycle. In intact sheep, luteectomy was performed to exclude the CL as a source of OT and to initiate P withdrawal. In ovariectomized sheep, E (1 microgram/h) for 12 to 36 h) superimposed on basal E(0.05 microgram/h), caused a series of 4 to 6 episodes of high frequency pulses of OT, each episode lasting 1 to 2 h at intervals of 3 h, and commencing at 24 h. Withdrawal of P (500 micrograms/h), superimposed on basal E in ovariectomized sheep, or luteectomy in intact sheep, evoked similar episodes of high frequency pulses of OT beginning at 24 h. We conclude that (1) an increase in E levels, or the return of E action following P withdrawal, causes intermittent increases in the frequency of the central OT pulse generator. (2) high frequency pulses of OT initiate subluteolytic levels of uterine PGF2 alpha which trigger a supplemental release of luteal OT; (3) luteal OT amplifies the secretion of uterine PGF2 alpha which initiates luteolysis and causes more luteal OT to be secreted; and (4) in addition to the established hypothalamic-anterior pituitary-gonadal axis for initiating the ovarian cycle (via the gonadotrophins), there is now evidence for a hypothalamic-posterior pituitary-gonadal axis for terminating the ovarian cycle (via OT).

The central oxytocin pulse generator: a pacemaker for luteolysis.

Oxytocin (OT) is released from the neurohypophysis into the jugular vein of sheep in small 1-2 min pulses (ca. 10 pg/ml) in both cyclic and ovariectomized sheep. In intact cycling sheep, additional hour long bursts of OT (up to 200 pg/ml) occur in peripheral blood during luteolysis at intervals of 6 to 9 hrs which appear to regulate large luteolytic pulses of uterine prostaglandin F2a (PGF2a). Since the ovine corpus luteum (CL) also synthesizes OT, experiments were performed to distinguish between the relative contributions of the neurohypophysis and the CL to the large bursts of OT secreted during luteolysis. Two models were used. First, ovariectomized sheep were given exogenous E and/or P by constant infusion to simulate levels during the estrous cycle. Second, in tact cycling sheep, the CL was surgically excised during the luteal phase to exclude the CL as a source of OT and, at the same time, subject the animals to the withdrawal of P. Pulses of OT in jugular vein plasma were determined by RIA or biometry of the uterus. The findings are summarized as follows: In ovariectomized sheep, maintained on low E (0.05 g/hr) to preserve the OT pulse generator, infusion of E (1 microgram, 2 micrograms or 4 micrograms/hr) for 12 to 36 hr, caused a series (4 to 6) of rapid increases in OT pulse frequency each lasting 1 to 2 hrs at intervals of 3 hrs. The time of onset of high frequency pulses was dose-dependent. Withdrawal of 10 day infusions of P (500 micrograms/hr) superimposed on low E (0.05 microgram/hr) also evoked a series of high frequency episodes of OT pulses beginning 24 hrs after P withdrawal. In intact sheep, surgical removal of the CL resulted in a series of high frequency pulses similar in duration and frequency to those following the withdrawal of P in the ovariectomized animal. We conclude that: (1) an increase in E or returning E action causes the OT pulse generator to alter its frequency intermittently thus producing a series of 4 to 6 episodes of high frequency pulses of OT. (2) Similar changes can be evoked by withdrawal of P either by terminating an infusion of P in the presence of E in the ovariectomized sheep or by surgically removing the CL from the ovary in the intact sheep. (3) At the end of the reproductive cycle, the central OT pulse generator appears to act as a pacemaker which, acting on the endometrial OT receptors, triggers a series of pulses of PGF2a from the uterus and hence causes regression of the CL. In the sheep and other ruminants, an intermittent supplemental secretion of OT from the CL, triggered via the central OT pulse generator, may also be required to amplify the luteolytic pulses of PGF2a from the uterus. (4) In addition to the well established interaction of ovarian steroid hormones, and the hypothalamic pituitary system for the initiation of the reproductive cycle via the gonadotrophins, there is now good evidence for an interaction of ovarian steroids and the posterior pituitary for terminating the reproductive cycle.

Identification of functional high and low affinity states of the prostaglandin F2 alpha receptor in the ovine corpus luteumin vivo and their role in hormone pulsatility.

Equivocal evidence has accumulated for the presence of high and low affinity receptors for PGF(2α) in the corpus luteum based on binding affinities of(3)H-PGF(2α) to cell membranes or separated whole cells. Some studies report only high affinity sites, while others report the occurrence of both high and low affinity sites. We have previously demonstrated, using subluteolytic levels of PGF(2α), the existence of functional high affinity luteal PGF(2α) receptors which show desensitization and recovery after 6 to 9 h. The present study, using direct intra-arterial infusions of PGF(2α) into the autotransplanted ovary in conscious sheep, was designed to probe for the existence of functional high and low affinity states of the PGF(2α) receptor in the corpus luteumin vivo. Subluteolytic and luteolytic concentrations of PGF(2α) (100 pg/min and 2500 pg/min, respectively) were infused sequentially, each for 2 h, into the ovary during the luteal phase (n=7 sheep). The same low and high concentrations of the inactive metabolite of PGF(2α) (PGFM) were given over the same time periods as negative controls (n=4 sheep). During the 2 h intra-arterial infusion of 100 pg/min of PGF(2α) the secretion rate of oxytocin increased (P<0.01) while the secretion rate of progesterone was unaffected. In contrast, during the 2 h intra-arterial infusion of 2500 pg/min of PGF(2α), secretion rate of oxytocin increased (P<0.01) and secretion rate of progesterone now began to decline (P<0.05). During the 2 h infusions of identical concent-rations of PGFM, the secretion rate of oxytocin and progesterone remained unchanged. These results indicate the existence of functional high and low affinity states of the PGF(2α) receptor within the ovine corpus luteumin vivo.

Effect of melengestrol acetate (MGA) or progesterone-releasing intravaginal device (PRID) on follicular development, concentrations of estradiol-17 beta and progesterone, and luteinizing hormone release during an artificially lengthened bovine estrous cycle.

Two trials were conducted to determine whether 7-d progestogen treatment beginning on d 17 of the estrous cycle altered 1) ovarian follicular development, 2) serum concentration of estradiol-17 beta (E2) and progesterone (P4), and 3) patterns of release of luteinizing hormone (LH). In both trials, Angus, Angus x Holstein, or Holstein cows 2 to 6 yr of age were randomly assigned to receive either melengestrol acetate (MGA, .5 mg.animal-1.d-1; n = 23), a progesterone-releaseing intravaginal device (PRID, n = 26) or to serve as untreated Controls (n = 14). Ultrasonography and blood sampling were performed daily throughout the experiment beginning on d 3 (Trial 1) or d 9 (Trial 2) of the estrous cycle. In Trial 2, blood samples were collected every 15-min for 6 h on d 17 (all cows) and d 20 and 23 (MGA and PRID cows) for determination of LH. Estrous cycle length was 3 to 5 d greater (P < .05) for MGA- and PRID-treated cows characterized by two (MGA-2 and PRID-2) or three (MGA-3 and PRID-3) dominant follicles than for control cows exhibiting two (Control-2) or three (Control-3) dominant follicles. A greater proportion (P < .05) of MGA- than of PRID-treated cows ovulated the follicle that was dominant at the beginning of treatment on d 17 (10 of 23 vs 1 of 26). Serum P4 concentrations declined 3 d earlier in Control-2 and MGA-2 cows than in Control-3, MGA-3 or PRID-3 cows.(ABSTRACT TRUNCATED AT 250 WORDS)

Jugular plasma concentrations of 13,14-dihydro-15-keto-prostaglandin F2 alpha in prepubertal beef heifers treated with progestogen then challenged with oxytocin.

Prepubertal Angus crossbred heifers (n = 24) between 8 and 10 mo of age were used to determine if progestogen treatment would enhance jugular concentrations of 13,14-dihydro-15-keto-prostaglandin F2 alpha (PGFM) after oxytocin (OT) injections. Heifers were stratified by age and weight and allotted to randomized treatments in a 2 x 2 factorial arrangement. Heifers were treated with either a norgestomet (NOR) implant (6 mg) for 9 d or no implant (0 mg; BLK). On d 8 of NOR treatment, jugular veins were catheterized and, on d 9, blood samples were collected every 15 min for 165 min. The first four samples were used to determine basal PGFM concentrations (an indirect measure of uterine PGF2 alpha release). After collection of the fourth sample, either OT (100 IU) or saline (0 IU; SAL) was injected via the jugular catheter. After the 165-min sample was collected, NOR implants were removed. Beginning 48 h after implant removal, a second 165- min blood sampling period was initiated. Average progesterone concentrations were less than 1 ng/ml during both bleeding periods. Within treatment, PGFM concentrations were similar between the first and second sampling periods; therefore, data within treatment were combined. Basal PGFM concentrations were higher (P < .01) in NOR-treated than in BLK heifers. Oxytocin did not increase PGFM concentrations in BLK-OT heifers; however, a marked increase in PGFM was detected in the NOR-OT heifers in response to oxytocin. Average PGFM concentration was greatest (P < .0001) in NOR-OT heifers, and PGFM profiles differed (P < .0001) between NOR-OT and each of the other treatment groups. Results from this study indicate that NOR increases basal PGFM and may "condition" the uterus to respond to OT in prepubertal heifers.

Postpartum interval to estrus and patterns of LH and progesterone in first-calf suckled beef cows exposed to mature bulls.

Two trials were conducted in which Angus x Hereford first-calf cows were assigned randomly at calving to one of two treatments: exposure to mature penile-blocked bulls (BE) or isolation from bulls (NE). In Trial 1 (BE, n = 38; NE, n = 37), cow to bull ratio increased from 12:1 to 19:1 over a 14-d period; in Trial 2 (BE, n = 25; NE, n = 24), this ratio was maintained at 13:1. In both trials, blood samples were collected weekly for progesterone and ovaries and uteri of cows were examined rectally. Cows were observed for estrus twice daily (am:pm) beginning 10 d after calving. In Trial 2, intensive blood sampling for LH began 10 d after calving (eight cows per treatment) and continued at weekly intervals until estrus or the end of the trial. Postpartum weight change, condition score change and time to uterine involution did not differ (P greater than .10) between treatments in either trial. Interval to estrus was shorter (P less than .05) for BE cows than for NE cows in both trials. A greater proportion (P less than .05) of BE cows exhibited estrus by 60 and 90 d after calving and showed an increase in progesterone before first estrus. Mean and baseline LH concentrations and amplitude, frequency and duration of LH pulses were not altered (P greater than .10) by bull exposure. In conclusion, exposing first-calf suckled beef cows to bulls after calving hastened resumption of estrous cycles. Bull exposure did not alter patterns of LH concentrations but did increase proportions of cows that showed increased progesterone before first estrus.

Physiological mechanisms controlling anestrus and infertility in postpartum beef cattle.

Postpartum infertility is caused by four factors: general infertility, lack of uterine involution, short estrous cycles and anestrus. The general infertility component is common to any estrous cycle and reduces potential fertility by 20 to 30%. Incomplete uterine involution prevents fertilization during the first 20 d after calving but is not related to anestrus. Short estrous cycles prevent fertility during the first 40 d after calving by causing the cow to return to estrus before pregnancy recognition occurs. Anestrus is the major component of postpartum infertility and is affected by several minor factors: season, breed, parity, dystocia, presence of a bull, uterine palpation and carryover effects from the previous pregnancy as well as two major factors: suckling and nutrition. These major factors have direct effects on anestrus but also interact with one or more other factors to control postpartum anestrus. Physiological mechanisms associated with anestrus involve blockage of the GnRH "pulse generator" in the hypothalamus, but other pathways also must be involved because bypassing the pulse generator is not an effective treatment for all cows. The primary cause of anestrus probably is different for different stages of anestrus. The mediating mechanisms for anestrus are not involved with prolactin, oxytocin, the adrenal or direct neural input from the mammary gland but are at least partially involved with blood glucose and the endogenous opioid peptide system. Management options to decrease the impact of anestrus and infertility include: 1) restrict breeding season to less than or equal to 45 d; 2) manage nutrition so body condition score is 5 to 7 before calving; 3) minimize effects of dystocia and stimulate estrous activity with a sterile bull and estrous synchronization; and 4) judicious use of complete, partial or short-term weaning.