Effects of sex steroids on the positive estrogen feedback mechanism in intact women and castrate men

We studied the effects of prolonged testosterone treatment on ovulatory function and positive estrogen feedback in women and of prolonged estrogen priming on gonadotropin feedback in castrate men. An estrogen provocation test was carried out in 4 groups of transsexual subjects: 12 female transsexuals in their early follicular phase (days 3-5; group 1A), 8 females who had been treated with Depo-testosterone (T) for 3-6 months (group 1B), 11 men who had been castrated 3 months previously (group 2A), and 4 male castrates treated with oral estrogen for 3 months starting 3 months after castration (group 2B). The estrogen provocation test consisted of 3 GnRH tests (100 micrograms) carried out immediately before (0 h) and 44 and 92 h after an im injection of estradiol valerate (10 mg). Responses to the estrogen provocation test in women with normal menstrual cycles (group 1A) were typically female. After initial suppression at 44 h, a LH surge (positive feedback) occurred at 92 h. Pituitary responsiveness, however, was amplified both at 44 and 92 h. Prolonged T priming of women in group 1B did not inhibit the estrogen-induced LH surge, nor was the amplitude of the surge blunted. Removal of androgens and other testicular factors (group 2A) did not result in the appearance of an estrogen-induced LH surge. On the other hand, prolonged estrogen priming in male castrates (group 2B) resulted in activation of the positive feedback mechanism; a LH surge in response to the estrogen provocation occurred. The results of the present study imply that 1) contrary to an earlier suggestion, testosterone does not block or blunt the LH surge, indicating that it is probably not responsible for suppressing the LH surge in normal men; 2) testosterone can cause ovulatory failure without suppressing the LH surge in women; and 3) prolonged estrogen priming may be involved in activation of the positive feedback mechanism in humans.     

Variation in levels of serum inhibin B,testosterone, estradiol,luteinizing hormone,follicle-stimulating hormone,and sex hormone-binding globulin in monthly samples from healthy men during a 17-month period: possible effects of seasons

To obtain information on the scale of the intraindividual variation in testicular hormone, blood samples for inhibin B determination were collected monthly in 27 healthy male volunteers during a 17-month period. In addition, the traditional reproductive hormones FSH, LH, testosterone, estradiol, and SHBG were measured. The intraindividual variation in inhibin B over the study period was, on the average, 10%, corresponding to the assay variation of the inhibin B assay, indicating that most of the observed day to day variation in inhibin B levels in men could be explained by assay variation. A seasonal variation was observed in LH and testosterone levels, but not in the levels of the other hormones. The seasonal variation in testosterone levels could be explained by the variation in LH levels. The seasonal variation in LH levels seemed to be related to the mean air temperature during the month before blood sampling, but not to the length of daylight or the hours of sunshine. In conclusion, our data showed that day to day levels of inhibin B are relatively constant in men and do not seem to be influenced by seasonal factors. In contrast, we found a seasonal variation in LH and testosterone levels in men. The peak levels of both LH and testosterone were observed during June-July, with minimum levels present during winter-early spring. Air temperature, rather than light exposure, seems to be a possible climatic variable explaining the seasonal variation in LH levels.     

TESTOSTERONE REDUCES THE BIOACTIVITY OF LUTEINIZING HORMONE (LH) IN MAN

The effect of testosterone on LH bioactivity was investigated in six adult men (aged 40-56 years) with primary hypogonadism. Two men received a 400 mg testosterone implant, another two 800 mg, and the final two patients received both doses in consecutive courses separated by at least 4 weeks. Plasma samples, obtained before and at 1, 2, 4, 8, 16 and 24 weeks after treatment, were analysed for bioactive LH by the mouse Leydig cell bioassay, and immunoreactive LH and testosterone (T) by standard radioimmunoassays. The bioactive to immunoreactive (B:I) LH ratio, an index of LH biopotency, was calculated and the results compared with those from a group (n = 17) of healthy adult men. Before treatment, both bioactive and immunoreactive LH levels in the patients were higher and T levels lower than in the normal men (P less than 0.001). The mean +/- SD B:I LH ratio (3.5 +/- 0.6) in the patients was greater (P less than 0.05) than in the controls (2.7 +/- 0.7), indicating that in primary testicular failure, increased amounts of LH with enhanced bioactivity are secreted. Following T administration, a dose-related increase in circulating T and a reciprocal decrease in LH levels was observed between 1 and 16 weeks of treatment. However, there was a more pronounced decline in bioactive rather than immunoreactive LH levels, so that the B:I LH ratios decreased (P less than 0.001) from basal values after treatment. There was a negative correlation (r = -0.82, P less than 0.001) between circulating T levels and B:I LH ratios; the stronger the feedback signal, the lower the B:I LH ratio. It is concluded that testosterone negative feedback modifies not only the quantity but also the biological quality of secreted LH.     

Modulation of gonadotropin secretion by corticosterone: interaction with gonadal steroids and mechanism of action

The effects of corticosterone (B) on pituitary responsiveness to LHRH and on gonadal steroid modulation of gonadotropin secretion were investigated using primary cultures of rat pituitary cells. Cultures were treated for 2 days with steroids and then challenged with LHRH for 4 h. B inhibited LH secretion, increasing the EC50 for LHRH from 1.40 to 4.96 nM. The reduction in LH release was accompanied by an increase in cell LH, so that the total amount of LH present in the cultures was unchanged. The EC50 for the effect of B on LH secretion was 0.57 microM. B increased the total amount of FSH present in the cultures. At high concentrations of B (10-100 microM), this effect was associated with an increase in FSH secretion. Testosterone inhibited LH secretion in both the absence and the presence of B. B had no effect in the presence of maximal concentrations of testosterone but augmented the inhibitory effect of lower concentrations. Estradiol (E) stimulated LH secretion in both the absence and the presence of B. However, the stimulatory effect of E was reduced by B, so that cultures treated with both B and E secreted no more LH than untreated cultures. B inhibited the LH secretory responses to Ca2+ influx and protein kinase C activation but did not affect the response to arachidonic acid, suggesting that the mechanism of B action may involve an inhibition of arachidonic acid release. Together these results indicate that the inhibitory effects of stress on reproduction are mediated at least partially by the inhibitory effects of B on LH secretion.     

Absence of steroid-dependent, endogenous opioid peptide suppression of pulsatile luteinizing hormone release between diestrus 1 and diestrus 2 in the rat estrous cycle

The objective of this study was to determine whether the negative feedback action of ovarian steroids on pulsatile luteinizing hormone (LH) release in the diestrous 1 (D1)-diestrous 2 (D2) interval of the rat estrous cycle is mediated by endogenous opioid peptides (EOPs), by examining the pulsatile LH release response to naloxone infusions in the presence or absence of D1-D2 levels of estradiol (E2) and progesterone (P). As plasma E2 and P levels increased between D1 and D2, mean blood LH levels decreased due solely to a decrease in LH pulse amplitude as frequency remained stable. However, ovariectomy increased both parameters of pulsatile LH release, indicating the effect of loss of ovarian steroid-negative feedback in this interval. Replacement of D1-D2 plasma levels of E2 and P restored D2 values for both parameters of pulsatile LH release, and E2 + P did not alter in vivo pituitary responsiveness to LH-releasing hormone (LHRH). In ovariectomized rats lacking the negative feedback provided by E2 + P in this cycle interval, continuous infusion of naloxone caused a further dose-dependent augmentation in both LH pulse amplitude and frequency. This stimulatory action of naloxone was prevented by simultaneous infusion with morphine, and was not associated with any change in in vivo pituitary responsiveness to LHRH, indicating that this was an action exerted through centrally located EOP receptors. Naloxone also increased both parameters of pulsatile LH release in E2 + P-treated rats. However, the magnitudes of the naloxone-induced increments in LH pulse amplitude and frequency in ovariectomized, steroid-treated rats were not greater than those seen in ovariectomized, nonsteroid-treated rats given naloxone versus saline. In addition, mean values for both parameters of pulsatile LH secretion during EOP receptor blockade in steroid-treated rats were reduced when compared to values in ovariectomized, nonsteroid-treated rats infused with naloxone. Thus the stimulatory effect of naloxone on pulsatile LH release was similar in the presence or absence of the negative feedback action of D1-D2 plasma levels of E2 + P. This indicates that the negative feedback effect of E2 + P on pulsatile LH release in this interval is not mediated by EOPs whose actions are blocked by naloxone.     

On the structure of the luteinizing hormone/chorionic gonadotropin receptor

In this review we have tried to argue that the evidence indicating that the LH/CG receptor is composed of a single polypeptide is stronger than the evidence indicating that the LH/CG receptor is a more complex structure composed of several subunits. Clearly, however, this issue has not been resolved and probably will not be resolved by performing additional experiments similar to those summarized here. It is our opinion that this issue will be resolved only by 1) reconstitution experiments in which the ability of the purified LH/CG receptor to bind hCG and activate adenylyl cyclase activity is tested; and/or 2) isolation and expression of a full length complementary DNA (cDNA) for the LH/CG receptor and the demonstration of hCG binding and adenylyl cyclase activation by the expressed receptor. Similar experiments will also clarify the proposed structures for the FSH and TSH receptors. As the second decade of work on the LH/CG receptor draws to an end it appears that these experiments are now possible, and hopefully a resolution of the existing controversy will be forthcoming in the near future.     

Serotonin inhibits luteinizing hormone release via 5-HT1A receptors in the zona incerta of ovariectomised, anaesthetised rats primed with steroids

The zona incerta (ZI), an area in the dorsal hypothalamus, contains neuronal systems that appear to control gonadotropin release. Previous findings show that there is an inverse relationship between serotonin (5-HT) activity in the ZI and plasma luteinizing hormone (LH) levels, indicating that the 5-HT system in this area has an inhibitory effect on LH release. Employing anaesthetised, ovariectomised rats primed with 5 microg oestradiol benzoate followed at 48 h by 0.5 mg progesterone, we have shown that 2 microg/side 5-HT in the ZI inhibits the LH surge that normally occurs 4 h after the progesterone treatment. This effect was mimicked by 2 microg/side 8-OH-DPAT, a 5-HT1A agonist, but not by DOI, a 5-HT2 agonist, BMY7378, a presynaptic 5-HT1A agonist or MCPP, a 2B & 2C agonist. The inhibitory effect of 5-HT and 8-OH-DPAT was prevented by pretreatment, 1 h before, with either 2 mg/kg i.p. WAY100135, a 5-HT1A antagonist or 0.25 mg/kg i.p. ritanserin, a 5-HT2 antagonist. These results indicate that 5-HT in the ZI exerts its inhibitory effect on LH release via 5-HT1A receptors but that another 5-HT subtype may also be involved.     

The changing ratio of bioactive to immunoreactive luteinizing hormone (LH) through puberty principally reflects changing LH radioimmunoassay dose-response characteristics

The ratio (B/I) of bioactive to immunoreactive LH in plasma varies during pubertal maturation. To elucidate the basis for these changes, we compared the dose-response characteristics of LH standards to those of plasma LH before and after GnRH infusion in normal males at various pubertal stages and girls with Turner's syndrome. We used a human LH (hLH) RIA modified to optimize specificity for LH bioactivity by employing a hLH tracer maximally bioactive in the rat interstitial cell testosterone production (RICT) bioassay. The pituitary hLH standards LER-907, NHPP I-1, and NHPP I-2 were not parallel to one another in the RIA, but were parallel in the RICT. Their relative slopes in the RIA were 1:1.35:1.49, respectively. The B/I for immunoreactive LH in these standards were 1 (LER-907):3 (I-1):5 (I-2). Dose-response characteristics varied greatly by patient category in the RIA. In contrast to the RICT, in which plasma from all subject groups, except the prepubertal basal group, gave parallel dose-response slopes, the groups differed in the steepness of their plasma LH RIA dose-response curves in the following order: adult post-GnRH congruent to adult basal congruent to pubertal post-GnRH greater than pubertal basal congruent to prepubertal post-GnRH congruent to prepubertal basal greater than Turner's syndrome. Prepubertal basal samples most closely resembled LER-907 in dose-response characteristics, while adult samples were most similar to NHPP I-1 and I-2. These characteristics were not related to the absolute LH concentration. Although the RIA dose-response characteristics of plasma LH changed during puberty, the B/I of basal LH did not increase through puberty using the modified hLH RIA, although B/I still rose in GnRH-stimulated samples during puberty. Our data demonstrate that the principal cause of variability in B/I is the changing dose-response characteristics of plasma LH in the RIA. We suggest that the source of this variability is a change in the molecular characteristics of LH in different states of pubertal maturation and gonadal function. The results imply that better ways of assaying LH are needed.     

Temporally and spectrally resolved subpicosecond energy transfer within the peripheral antenna complex (LH2) and from LH2 to the core antenna complex in photosynthetic purple bacteria

We report studies of energy transfer from the 800-nm absorbing pigment (B800) to the 850-nm absorbing pigment (B850) of the LH2 peripheral antenna complex and from LH2 to the core antenna complex (LH1) in Rhodobacter (Rb.) sphaeroides. The B800 to B850 process was studied in membranes from a LH2-reaction center (no LH1) mutant of Rb. sphaeroides and the LH2 to LH1 transfer was studied in both the wild-type species and in LH2 mutants with blue-shifted B850. The measurements were performed by using approximately 100-fs pulses to probe the formation of acceptor excitations in a two-color pump-probe measurement. Our experiments reveal a B800 to B850 transfer time of approximately 0.7 ps at 296 K and energy transfer from LH2 to LH1 is characterized by a time constant of approximately 3 ps at 296 K and approximately 5 ps at 77 K. In the blue-shifted B850 mutants, the transfer time from B850 to LH1 becomes gradually longer with increasing blue-shift of the B850 band as a result of the decreasing spectral overlap between the antennae. The results have been used to produce a model for the association between the ring-like structures that are characteristic of both the LH2 and LH1 antennae.     

Mutually independent effects of adrenocorticotropin on luteinizing hormone and testosterone secretion

In this study, we examined the effect of ACTH on the sensitivity of the testes to gonadotropin and determined the role of the testosterone (T) negative feedback system in mediating the inhibitory effect of ACTH on LH secretion in adult male rats. ACTH infusion for 3 days reduced basal levels of serum T and the T response to GnRH, but did not alter basal levels of serum LH (immunoreactive) or the LH response to GnRH. These effects required the presence of the adrenal glands. Infusion of corticosterone (B) at a dose that increased serum B concentrations 9-fold had an effect similar to that of ACTH on basal serum T levels and the serum T response to GnRH. Basal levels of serum LH and the serum LH response to GnRH were not affected by B administration. These data suggest that ACTH administration reduces the sensitivity of the testes to LH, resulting in a lower basal level of T and a reduced T response to GnRH. This effect was independent of basal serum LH levels or the LH response to GnRH. It appears that B mediates the effect of ACTH on testicular sensitivity to gonadotropin. In another experiment, ACTH administration for 4 days did not alter serum LH values, but reduced serum T levels in sham-castrated male rats. In contrast, ACTH treatment blunted the increase in serum LH after castration by day 2 of treatment, despite the absence of detectable levels of serum T within 6 h after castration. These data suggest that T is not essential for the inhibitory effect of ACTH on LH secretion to occur. They do not support the hypothesis that ACTH enhances the sensitivity of the hypothalamus and/or pituitary to the negative feedback effects of T.     

Absence of an estradiol-induced surge of luteinizing hormone in pigs receiving unvarying pulsatile gonadotropin-releasing hormone stimulation

The goal of this study was to pharmacologically block central nervous system (CNS) input to gonadotropes in mature ovariectomized gilts to determine the direct actions of estradiol (E2) on pituitary LH release when given at a dose sufficient to elicit a gonadotropin surge. Feeding AIMAX [N-methyl-N'-(1-methyl-2-propenyl)1,2-hydrazinedicarbothioamide; 125 mg/day] for 7 days reduced serum LH concentrations from 1.25 +/- 0.13 (mean +/- SE) to less than 0.18 ng/ml, abolished LH pulses, but did not compromise LH release in response to exogenous GnRH. Serum FSH concentrations were reduced by 27%, whereas serum concentrations of PRL, GH, thyroid hormones and cortisol were not affected after 7 days of AIMAX treatment. Behavior was not altered, aside from a slightly reduced appetite. The LH surge that peaked 48-80 h after injecting E2 benzoate (E2B) into control gilts was blocked in five of eight gilts given AIMAX. Giving GnRH pulses (1 microgram every 45 min) to AIMAX-treated gilts restored mean serum LH concentrations as well as the frequency and amplitude of LH pulses to those of untreated ovariectomized gilts. E2B suppressed the LH response to these GnRH pulses by 88% at 12 h, whereas from 24-96 h after E2B treatment, the LH response to GnRH and mean serum concentrations of LH were again similar to those of controls not given estradiol. These data indicate that induction of the gonadotropin surge by E2 in the gilt requires CNS input. The action of E2 on the pituitary in the presence of unvarying GnRH pulsation may, however, be limited to an early transient inhibition of responsiveness to GnRH, with no subsequent direct stimulation during the period of the surge.     

Roles of estradiol and progesterone in eiliciting the midcycle luteinizing hormone and follicle-stimulating hormone surges.

The positive feedback effects of estradiol (E2) and progesterone (P) on LH and FSH release were studied under novel experimental conditions in three women of reproductive age who had undergone oophorectomy and received uninterupted E2 replacement by subdermal implants. Basal serum E2 levels were in the midfollicular phase range, while LH and FSH levels were normal or slightly elevated. Each volunteer underwent seven experiments at 2- to 4-week intervals, receiving im injections of increasing amounts of estradiol benzoate (E2B) alone and in combination with P. The time and dose of P (administered via P-impregnated polysiloxane intravaginal rings) were varied. In two of the seven experiments, P was given without E2B injections. In all three subjects, increasing serum E2 levels mimicking the preovulatory E2 peak were followed by a surge of LH but not of FSH. However, when serum P levels rose after an increase in serum E2 concentrations had occurred, the LH surge occurred earlier and was accompanied by an FSH peak. When serum P levels rose before serum E2 concentrations had risen or when P levels increased without a rise in serum E2, neither a serum LH nor FSH peak was observed. When administered concomitantly, E2B and P suppressed FSH but not LH levels, while P alone did not affect serum LH or FSH concentrations. These data indicate that an acute rise in serum E2 is a necessary condition for the midcycle LH and FSH surges, that P facilitates or blocks the positive feedback response of gonadotropin release in a time-dependent manner, and that P is required for the preovulatory FSH peak.     

Effects of corticosterone and testosterone on pituitary gonadotropin content, secretion, bioactivity and messenger RNA levels in the presence or absence of GnRH in male rats

The effects of corticosterone (B) and testosterone (T) on pituitary and serum bioactive and immunoreactive gonadotropins and on gonadotropin hormone subunit messenger RNA levels were compared in the absence of GnRH. Male rats were implanted with pellets of either cholesterol, B or T. At implantation, 2 and 4 days later half of each group received GnRH antagonist and animals were killed 5 days after implantation. As expected, GnRH antagonist lowered bioactive and immunoreactive serum FSH and LH, pituitary FSH, LHβ and FSHβ mRNA. B treatment alone lowered bioactive and immunoreactive serum FSH and immunoreactive serum LH. B reversed the antagonist effect on bioactive and immunoreactive pituitary FSH and FSHβ mRNA. T alone lowered bioactive and immunoreactive serum FSH and LH levels. T reversed the antagonist effect on bioactive and immunoreactive pituitary FSH. T lowered bioactive and immunoreactive pituitary LH and LHβ mRNA and partially reversed the antagonist effect on FSHβ mRNA. The data suggest that either B or T enhance FSH synthesis by acting directly at the gonadotrope, but that B does not affect LH variables to the same extent as T. The results suggest that in stressed animals, when T levels are reduced, B can substitute for T in sustaining FSH synthesis.     

Reaction center and light-harvesting I genes from Rhodopseudomonas capsulata

Five structural genes coding for the reaction center (RC) L, M, and H subunits and the two light-harvesting (LH) I polypeptides, B870α and B870β, have been mapped on two restriction fragments from the R-prime plasmid pRPS404. It has been recently shown that enhanced near-infrared fluorescence mutants of Rhodopseudomonas capsulata typically lack RC or LH I polypeptides and that these lesions are marker-rescued by two restriction fragments from the R-prime plasmid: the 7.5-kilobase-pair EcoRI F fragment and the 4.75-kilobase-pair BamHI C-EcoRI fragment. We have now determined the nucleotide sequence of two restriction fragments and have found that the BamHI C-EcoRI B fragment carries the structural genes for the RC L and M subunits and both LH I polypeptides. Forty kilobase pairs away from this locus, the BamHI F fragment (within the EcoRI F fragment) carries the RC H subunit. The structural genes on the BamHI C-EcoRI B fragment are probably transcribed as part of a polycistronic mRNA. All of the structural genes begin with a consensus Shine-Dalgarno sequence and separate AUG start codons, indicating that the structural polypeptides are not cleaved from larger precursor polypeptides.     

Molecular basis of recognition by the glycoprotein hormone-specific N-acetylgalactosamine-transferase.

Lutropin (LH) bears asparagine-linked oligosaccharides terminating with the unique sequence SO4-4GalNAc beta 1-4GlcNAc beta 1-2Man alpha, whereas follitropin (FSH) bears oligosaccharides terminating predominantly with the sequence Sia alpha-Gal beta 1-4GlcNAc beta 1-2Man alpha, where Sia is sialic acid. We previously identified a glycoprotein-hormone-specific N-acetylgalactosamine-transferase (GalNAc-transferase) that recognizes a peptide-recognition marker(s) present on the common glycoprotein hormone alpha subunit and beta subunits of human chorionic gonadotropin and LH but not on the beta subunit of FSH. We have now identified an amino acid sequence motif, Pro-Leu-Arg, that is essential for recognition by the GalNAc-transferase. This tripeptide sequence is found 6-9 residues on the amino-terminal side of a glycosylated asparagine on the alpha subunit and beta subunits of LH and human chorionic gonadotropin but is not present on the beta subunit of FSH. The presence of this motif accounts for the differences in LH and FSH oligosaccharide structures. Additional proteins containing this recognition motif have been identified and were determined to bear sulfated oligosaccharides with the same structures as those on the glycoprotein hormones, indicating that these structures are not restricted to the glycoprotein hormones.     

Inhibition of P450 aromatase enhances gonadotropin secretion in early and midpubertal boys: evidence for a pituitary site of action of endogenous E

In early pubertal boys, E concentrations are very low. We studied the role and site of action of endogenous E in the regulation of gonadotropin secretion in early and midpubertal boys by inhibiting the action of E with a potent and specific P450 aromatase inhibitor, letrozole. A total of 35 boys who were referred to us because of suspicion of delayed puberty were included in the study. The boys were in either early or midpuberty, and they composed 3 groups: 10 boys did not receive any treatment, 12 boys received T alone, and 13 boys received T and letrozole. In the untreated group during the 5-month follow-up, no changes were observed in 17beta-E2, T, basal gonadotropin, or inhibin B concentrations or in the GnRH-induced gonadotropin responses. In the T-treated group during the 5-month treatment, the T concentration increased by 55% (P < 0.05), and the 17beta-E2 concentration increased by 130% (P < 0.02). Concurrently, basal gonadotropin concentrations were suppressed, but the GnRH-induced gonadotropin responses and the inhibin B concentration remained unchanged. In the T- plus letrozole-treated group during the 5-month treatment, an increase in T concentration of 606% was observed (P < 0.001), but the 17beta-E2 concentration remained unchanged. The changes in the 17beta-E2 concentration within 5 months in the untreated and the T- plus letrozole-treated groups were different (P < 0.02), indicating significant inhibition of endogenous E synthesis during letrozole treatment. During the T plus letrozole treatment, basal gonadotropin concentration, the GnRH-induced LH response, and inhibin B concentration increased, and the GnRH-induced FSH response did not change significantly. Serum nocturnal gonadotropin pulses were determined in 5 boys treated with T and in 5 boys treated with T plus letrozole. In the T- plus letrozole-treated group, the nocturnal LH pulse amplitude increased, and the LH pulse frequency and interpulse interval remained unchanged. In conclusion, in early and midpubertal boys, suppression of the action of E by the P450 aromatase inhibitor increased LH concentration, LH pulse amplitude, and the GnRH-induced LH response, which indicates that in boys during early and midpuberty, endogenous E regulates LH secretion at the site of the pituitary.     

Barium-induced LH release from chicken pituitary cells: synergism with phorbol ester

Barium is known to elicit secretion in a number of cell systems. The mechanism of Ba2+ stimulation of LH release in cultured chicken pituitary cells was investigated in the present study. Barium-stimulated LH release was inhibited by extracellular Ca2+, indicating that Ba2+ does not act by stimulating Ca2+ entry. Simultaneous stimulation of the cells with Ba2+ and phorbol ester produced a synergistic response, similar to the synergism obtained with phorbol ester and treatments which increase cytosolic Ca2+. Both Ba2+-stimulated LH release and the synergism of Ba2+ with phorbol ester were inhibited by calcium channel blockers (Co2+, methoxyverapamil and nifedipine) and by calmodulin antagonists (trifluoperazine and chlorpromazine). These results indicate that the actions of Ba2+ are dependent on its entry through Ca2+ channels, and suggest that calmodulin activation is necessary for the synergism between Ba2+ and phorbol ester. Thus, synergism does not result from a direct effect of divalent cations on C-kinase.     

Suppression of the preovulatory luteinizing hormone surge in the rat by 2-hydroxyestrone: relationship to endogenous estradiol levels

Injection of 100 micrograms 2-hydroxyestrone (2OHE1) at various times on the morning of proestrus into normal 4-day-cycling rats results in abolition of the preovulatory LH surge in a number of animals tested. The greatest response was observed when the administration of 2OHE1 coincided with endogenous estradiol (E2) levels that were close to but not at their maximal proestrous levels. The catechol estrogen failed to abolish the LH surge if given much earlier or after the E2 maximum had already been reached. The effectiveness of 2OHE1 inhibition of the LH surge was greatly increased by the administration of 1 microgram E2 1 h before the catechol estrogen. 2OHE1 did not interfere with LH secretion in response to LHRH administration, indicating that the inhibitory action of the catechol estrogen is exercised at the hypothalamic level. In contrast to its inhibition of the positive feedback, 2OHE1 administered either before or after the injection of E2 to ovariectomized rats had no effect on the negative feedback of the hormone on pituitary LH secretion. The narrow and specific "time window" on proestrus when an injection of 2OHE1 results in the abolition of the preovulatory LH surge and its relation to the endogenous E2 preovulatory secretion suggest that the catechol estrogen interferes with a brief neuronal triggering event obligatory for LHRH release. The evidence also indicates that this action does not involve conventional competition for the E2 receptor.     

Effects of antenatal corticosteroid treatment on pulmonary ventilation and circulation in neonatal lambs with hypoplastic lungs

Our aim was to determine whether antenatal corticosteroids improve perinatal adaptation of the pulmonary circulation in lambs with lung hypoplasia (LH). LH was induced in 12 ovine fetuses between 105 and 140 days gestation (term approximately 147 days); in 6 of these the ewe was given a single dose of betamethasone (11.4 mg im) 24 hr before delivery (LH + B). All lambs, including a control group (n = 6), were delivered at approximately 140 days and ventilated for 2 hr during which arterial pressures, pulmonary blood flow (PBF), and ventilating pressure and flow were recorded. During ventilation, respiratory system compliance was lower in both LH + B and LH groups than in controls. Pulmonary vascular resistance (PVR) was lower in LH + B lambs than in LH lambs and similar to controls; PBF was reduced in LH lambs but was restored to control levels by betamethasone. The mean density of small arteries of LH + B lambs was similar to that of LH lambs (P = 0.06) and lower than in controls; the thickness of the media of small pulmonary arteries from LH + B lambs was similar to that in LH lambs and thicker than in controls. VEGF mRNA levels were not different between groups. PDGF mRNA levels in LH + B lambs were higher than in LH lambs; a similar trend (P = 0.06) was seen for PECAM-1. SP-C mRNA levels were greater in both LH and LH + B lambs than in controls. Effects of betamethasone were greater on indices of pulmonary circulation than ventilation. We conclude that a single dose of maternal betamethasone 24 hr prior to birth has significant favorable effects on the postnatal adaptation of the pulmonary circulation in lambs with LH.