Search

Cellular interactions mediated by both contact-dependent and contact-independent mechanisms are probably important to maintain luteal function. The objective of the present study was to evaluate the role of cAMP in regulation of contact-dependent gap junctional intercellular communication (GJIC) of bovine luteal cells from several stages of luteal development. In experiment 1, corpora lutea (n = 5) from the mid-luteal phase of the estrous cycle were dissociated with collagenase, and cells were preincubated in a medium with serum. Then the medium was changed to serum-free media containing a cAMP agonist (dbcAMP; 1 mM) or antagonist (Rp-cAMPS; 0, 3, 10, 30, or 100 microM). In experiment 2, corpora lutea from the early (n = 7), mid- (n = 6), and late (n = 6) luteal phases of the estrous cycle were dissociated and preincubated as in experiment 1, and luteal cells were then incubated with no treatment, LH (100 ng/ml), dbcAMP (1 mM), forskolin (1 microM), Rp-cAMPS (100 microM), or LH+Rp-cAMPS. After incubation of luteal cells with treatments for 18-24 h, media were collected for determination of progesterone and cAMP concentrations. Then the rate of GJIC was evaluated for selected cells (small luteal cells in contact with small luteal cells, and large luteal cells in contact with small luteal cells) by using the fluorescence recovery after photo-bleaching technique and laser cytometry. In experiment 1, dbcAMP increased (p0.01) but Rp-cAMPS (p0.05) decreased GJIC between small luteal cells and between large and small luteal cells. In addition, dbcAMP stimulated (p0.01) but Rp-cAMPS did not affect progesterone secretion. In experiment 2, treatments affected (p0.05) GJIC and progesterone production of luteal cells from the mid- and late but not from the early luteal phase of the estrous cycle. GJIC between small luteal cells was increased (p0.01) by LH, dbcAMP, and forskolin. GJIC between large and small luteal cells was increased (p0.05) by dbcAMP and forskolin. Rp-cAMPS decreased (p0.01) GJIC between small luteal cells (mid-luteal phase) and between large and small luteal cells (mid- and late luteal phases). In addition, Rp-cAMPS inhibited (p0.05) the stimulatory effects of LH on GJIC between small luteal cells from the mid- and late luteal phases of the estrous cycle. For luteal cells from the mid- and late luteal phases, progesterone production was increased (p0.05) by LH, dbcAMP, forskolin, and LH+Rp-cAMPS, but was not affected by Rp-cAMPS. Across all stages of the estrous cycle, cyclic AMP accumulation in media was greater (p0.05) in LH- and forskolin-treated cultures than in control cultures and was greater (p0.01) in forskolin-treated than in LH-treated cultures. These data demonstrate that small and large luteal cells communicate with each other and that the rate of GJIC is modulated by LH and cAMP, as has been shown previously for other cell types. Thus, cAMP appears to be involved in the regulation of GJIC within the bovine corpus luteum, which probably is an important mechanism for coordinating luteal cell function.

Weight of placental tissues of cows increased exponentially from Day 100 to Day 250 of gestation, but at much slower relative and absolute rates than fetal weight. In addition, growth rate of fetal placental tissues was less than that of maternal placental tissues. Concentrations of DNA, RNA and protein, however, increased in fetal placental but not in maternal placental tissues. Fetal placental tissues therefore exhibited hyperplasia, which probably contributes to increased functional capacity of the placenta during late gestation. The rate of O2 uptake in vitro was greatest for maternal placental tissues, suggesting that the maternal portion of the placenta accounts for most of the large rate of placental O2 utilization in vivo. Compared with other placental tissues, rate of secretion of macromolecules by intercaruncular endometrium was high, but decreased from Day 100 to 250, suggesting that uterine glandular secretory activity may decrease as gestation advances. Rate of secretion of macromolecules also was high for intercotyledonary tissues and increased with day of gestation, suggesting a role for secretory products of chorioallantois in gravid uterine function.

Samples from corpus haemorrhagicum, mid-cycle corpus luteum (CL) and late-cycle CL were tested for their abilities to stimulate neovascularization of chorioallantoic membranes (CAM) of developing chicks. Responses were graded from 0 to 4 (4 being the greatest response). Luteal tissue implants from each stage of the oestrous cycle stimulated growth of CAM blood vessels, and vascular responses increased with age of CL. Implants from late-cycle CL were typically graded 3 or 4. Luteal tissues from several stages of development were also incubated for 6 h in serum-free medium containing no hormone, LH, PGF-2 alpha or both hormones. Media conditioned by luteal tissues were assayed for progesterone and tested for their ability to stimulate mitogenesis and migration of bovine aortic endothelial cells in vitro. All media conditioned by luteal tissues stimulated mitogenesis and migration of endothelial cells, but media from late-cycle CL exhibited the greatest activity. Luteinizing hormone significantly increased in-vitro secretion of a factor(s) that stimulated migration of endothelial cells. PGF-2 alpha alone had no effect on production of endothelial cell mitogen or migration-stimulating factor(s) from luteal incubations; however, the ability of LH to enhance secretion of the migration-stimulating factor(s) was blocked by PGF-2 alpha. This study demonstrates that angiogenic activity of bovine luteal tissues increases with age of the CL and in-vitro secretion of angiogenic factor is responsive to hormones known to regulate luteal function.

The objective of the following study was to examine the ability of frequent milking, the physical presence of the calf, and their combination to prevent a postweaning rise in tonic luteinizing hormone (LH) secretion, estrus, and ovulation. Thirty Hereford cows were allowed to suckle their calves ad libitum until 17-21 days post partum and confirmed as anestrus. They were then assigned alternately by order of calving to 1 of 5 treatment groups: (1) Suckled (S) ad libitum; (2) Nonsuckled (NS)--calf removed for 102 h; (3) Nonsuckled--calf present (NSC)--calf remained with cow, but muzzled to prevent suckling for 102 h; (4) Nonsuckled--milked 8 times a day (NSM)--calf removed for 102 h and cow hand-milked for 10 min every 2 h from 0700 to 2100 h; (5) Nonsuckled--calf present--milked 8 times a day (NSMC)--combination of 3 and 4. Luteinizing hormone secretion patterns, estrous activity, and ovulation were monitored throughout the experiment. Prior to treatment (Day 0), mean pulse frequency (pulses/6 h), mean concentrations (ng/ml), and median concentrations (ng/ml) of LH did not differ (p greater than 0.45) between groups, and were 0.7 +/- 0.15, 2.8 +/- 0.14, and 2.6 +/- 0.11, respectively. Marked rises (p less than 0.01--p less than 0.03) in LH pulse frequency were observed in all groups except S between 48 and 54 h after onset of treatment. Mean and median concentrations of LH were lower (p less than 0.02) in S cows than in all other groups at 48-54 h.(ABSTRACT TRUNCATED AT 250 WORDS)

In Exp. 1, maternal (caruncle) and fetal (cotyledon) portions of the placenta as well as uterine endometrium were obtained from cows at mid-gestation and evaluated for angiogenic activity by placing tissue samples on chick chorioallantoic membranes (CAM). Only caruncular tissues exhibited angiogenic activity in the CAM assay. In Exp. 2, lyophilized homogenates of caruncular tissues obtained from cows at mid-gestation were evaluated for angiogenic activity on CAM and for their ability to stimulate mitosis of bovine aortic endothelial cells in vitro. Homogenates of caruncular tissues again were angiogenic on the CAM and also were mitogenic for endothelial cells. In Exp. 3, maternal (caruncle and endometrium) and fetal (cotyledon and fetal membrane) portions of the placenta were obtained from cows at mid-gestation and fine minces (explants) of each were cultured for 24 h. Explant-conditioned media were then tested for angiogenic activity by their abilities to stimulate mitosis and migration of bovine aortic endothelial cells in vitro. Conditioned media from caruncular explants, but not from explants of other tissues, exhibited both mitogenic and migration-stimulating activities. When pools of caruncular explant-conditioned media were fractionated by ultrafiltration, mitogenic activity was not present in fractions of Mr less than 10,000, less than 30,000 and less than 100,000, but was retained in fractions of Mr greater than 10,000, greater than 30,000 and greater than 100,000. Mitogenic activity was not observed in any fractions subjected to heat treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

The current experiment was conducted with beef cows during the first 2 weeks postpartum (PP) to determine the effects of suckling and low-level increases of systemic progesterone on secretory characteristics of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) in peripheral plasma. Variables measured included mean gonadotropin concentrations, FSH/LH pulse frequencies, pulse amplitudes and synchrony of coincident release. Suckled (S) cows had lower (P less than 0.01-P less than 0.05) mean concentrations of FSH and LH in plasma, lower (P less than .05) FSH and LH pulse frequencies and a lower (P less than 0.05) pulse synchrony (21.6 vs 72.3% coincident pulses) than nonsuckled (NS) cows on Day 7 PP. Neither FSH nor LH pulse amplitude was affected by suckling. Similar differences existed for mean gonadotropin concentrations, pulse frequencies and pulse synchrony on Day 14 PP between S and NS cows. Implanting cows with progesterone implants on Day 7 PP, which chronically increased plasma progesterone to 0.5-0.6 ng/ml, increased (P less than 0.05) mean plasma FSH and LH concentrations, FSH and LH pulse frequencies and pulse synchrony (87.5 vs. 66.3%) in NS-implanted (NSI) versus NS-nonimplanted (NSNI) cows. Progesterone implants had no beneficial effect in S cows. Thus, three major findings seem pertinent: 1) associated with the suckling-induced depression of episodic gonadotropin release was a marked decline in FSH/LH pulse synchrony; 2) a high degree of FSH/LH pulse synchrony (72-88%) was restored in the absence of suckling when gonadotropin pulse frequency increased to 4-5/6 h; 3) the absence of suckling, followed by the provision of low-level progesterone stimulation for 7 days, appeared to have additive effects on FSH and LH secretion. These results provide evidence that the neuroendocrine block associated with suckling in early PP beef cows is, in addition to being associated with depressed LH release, comprised of characteristic anomalies in FSH secretion, FSH/LH pulse synchrony and failure to respond to a putative hypothalamo-hypophyseal potentiator, progesterone.