Low-dose Estradiol Research Articles

Hormonal dependence of pinopode formation at the uterine luminal surface

The uterine luminal epithelium during the period of receptivity for nidation displays characteristic protrusions of the apical surface named pinopodes. The effects of oestradiol and progesterone, singly or in combination, on the formation and regression of pinopodes were investigated using scanning electron microscopy. The appearance of pinopodes was found to be strictly progesterone dependent. When given together with progesterone, before the development of pinopodes, high doses of oestradiol (plasma level approximately 300 pmol/l) inhibited pinopode formation; on the contrary, low doses of oestradiol (nidatory doses) did not interfere with the process until the 4th day of treatment. When oestradiol was given as a single injection, after pinopode formation, both doses were equivalent in inducing their regression 48-72 h later. It appears that the hormonal conditioning for pinopode formation and for the development of uterine receptivity for egg implantation is the same. These observations support the hypothesis that pinopodes could be an extremely useful tool to estimate uterine receptivity. The experiments we describe here, together with observations made a few years ago, in stimulated cycles in the human, suggest that implantation failure as a result of a hormonal imbalance during the time intervening between ovulation and nidation, may be a general phenomenon.

Effects of the nonsteroidal inhibitor R76713 on testosterone-induced sexual behavior in the Japanese quail ( Coturnix coturnix japonica)

A new triazole derivative, R76713 (6-[(4-chlorophenyl)(1 H-1,2,4-triazol-1-yl)methyl]-1-methyl-1 H-benzotriazole), was recently shown to inhibit aromatase selectively without affecting other steroid-metabolizing enzymes and without interacting with estrogen, progestin, or androgen receptors. This compound was tested for its capacity to intefere with the induction of copulatory behavior by testosterone (T) in castrated Japanese quail ( Coturnix coturnix japonica). In a first experiment, R76713 inhibited (range 0.01 to 1 mg/kg) the activation of sexual behavior by T silastic implants and hypothalamic aromatase activity in castrated male quail in a dose-dependent manner. The 5α- and 5β- reductases of T were not systematically affected. Stereotaxic implantation of R76713 in the medial preoptic area similarly blocked the behavior activated by systemic treatment with T, demonstrating that central aromatization of androgen is implicated in the activation of behavior. These inhibiting effects of R76713 on behavior were observed when implants were placed in the medial part of the nucleus preopticus medialis, confirming the implication of this brain area in the control of male copulatory behavior. Finally, the behavioral inhibition produced by R76713 could be reversed by simultaneous treatment with a dose of estradiol, which was not behaviorally effective by itself. This suggests that the behavioral deficit induced by the inhibitor was specifically due to the suppression of estrogen production. This also shows that the activation of copulatory behavior probably results from the interaction of androgens and estrogens at the brain level, as the two treatments separately providing these hormonal stimuli (T with the aromatase inhibitor on one hand and a low dose of estradiol on the other hand) had almost no behavioral effects but they synergized to activate copulation when given concurrently. These data confirm the critical role of preoptic aromatase in the activation of reproductive behavior and demonstrate that R76713 is a useful tool for the in vivo study of estrogen-dependent processes.

Effect of Naloxone on the Pulsatile Secretion of Luteinizing Hormone in Gonadectomized Male and Female Ferrets Before and After Oestradiol Replacement

Abstract Intravenous infusions of the opioid receptor antagonist, naloxone, caused a significant rise in luteinizing hormone (LH) pulse frequency in male and female ferrets which had been gonadectomized 10 days earlier while in breeding condition; mean LH concentrations and LH pulse amplitudes were not affected. By contrast, naloxone failed to stimulate LH pulse frequency, or other LH parameters in gonadectomized ferrets of either sex which received daily injections of a low dose of oestradiol. Our results for the ferret, in which the female is an induced ovulator, resemble those previously obtained in another induced ovulator, the rabbit. They contrast, however, with the results of numerous studies using spontaneously ovulating species in which sex steroids, if anything, facilitate the ability of naloxone to stimulate LH secretion.

Specific, Time-Dependent Actions of Low-Dose Ethinyl Estradiol Administration on the Episodic Release of Growth Hormone, Follicle-Stimulating Hormone, and Luteinizing Hormone in Prepubertal Girls with Turner's Syndrome *

To investigate the actions of acute and chronic low doses of ethinyl estradiol (EE) on the pulsatile characteristics of GH and gonadotropin release we studied seven girls with Turner's syndrome [mean age, 7.5 +/- 0.75 (+/- SE) yr] on 3 separate study days. At baseline (study I), blood was drawn every 20 min from 2000-0800 h for GH, LH, and FSH determinations. One month after study I the patients were started on 100 ng/kg EE, orally, daily, and an identical study was repeated 1 week (study II) and 5 weeks (study III) from the initiation of low dose EE therapy. A pulse detection algorithm, Cluster, was used to objectively analyze pulsatility profiles. There were consistent and significant increases in all seven patients after 5 weeks of low dose EE therapy in mean GH concentrations (study I, 7.0 +/- 1.1 micrograms/L; study III, 13.4 +/- 1.7; P = 0.008), mean area under the GH pulse (study I, 602 +/- 52 micrograms/L.min; study III, 1350 +/- 261; P = 0.026), and mean GH pulse amplitude (study I, 14.0 +/- 2.2 micrograms/L; study III, 32.8 +/- 6.0; P = 0.018); with no detectable changes in GH pulse frequency (study I, 5.3 +/- 0.6 pulses/12 h; study III, 5.3 +/- 0.4). These findings were not accompanied by any significant changes in plasma somatomedin-C or serum estradiol concentrations or urinary cytological maturation indexes. Conversely, the amount of radioimmunoassayable FSH activity was suppressed after low dose EE therapy, with a decrease in mean FSH concentrations (study I, 23.5 +/- 6.6 IU/L; study III, 5.9 +/- 1.2; P = 0.035) and mean pulse amplitude (study I, 28.6 +/- 8.6 IU/L; study III, 8.2 +/- 1.8; P = 0.038), with no detectable changes in FSH pulse frequency (study I, 7.6 +/- 0.6 pulses/12 h; study III, 7.3 +/- 0.6). Similar qualitative changes in LH pulsatility were observed after low dose estradiol administration. In conclusion, our results demonstrate that low dose EE therapy results in a significant augmentation of pulsatile GH activity, with reciprocal decreases in gonadotropin concentrations in girls with Turner's syndrome. Such observations indicate an exquisite sensitivity of gonadotrope and somatotrope function to low dose estrogen action in this prepubertal hypogonadal model.

Vaginal administration of low-dose oestradiol — effects on the endometrium and vaginal cytology

Twenty (20) post-menopausal women, mean age 62.4 yr, presenting with symptoms associated with urogenital atrophy were treated intravaginally with daily doses of 25 and 50 μg oestradiol (E2) in a double-blind, cross-over study. After randomization, the patients started daily treatment with pessaries containing either 25 or 50 μg E2 for 3 wk, followed by a maintenance period of 6 wk during which the pessaries were used only twice a week. A 4-wk wash-out period was followed by another treatment period of 9 wk. The effects on the karyopyknotic index (KPI) and on endometrial histopathology were assessed before and after 3, 9, 16 and 22 wk of treatment. In the case of the 25 μg dose the mean KPI values were 34.7 and 20.9% after 3 and 9 weeks of treatment, respectively, the corresponding figures after treatment with 50 μg E2 being 39.2 and 22.7%. No dose-effect relationship was apparent from the vaginal cytology findings. Endometrial biopsies could not be taken systematically in all patients. Weak proliferation of the endometrium was observed in 1 woman after 3 wk of daily treatment with the 50 μg dose. No endometrial stimulation was detected in any of the patients after treatment with 25 μg daily. Beneficial clinical and cytological effects were obtained with both dosage regimens. Daily intravaginal administration of 25 μg E2 can accordingly be advocated for the treatment of urogenital symptoms attributable to oestrogen deficiency in post-menopausal women.

Lack of correlation of serum estradiol with growth velocity during male pubertal growth.

The adolescent growth spurt in boys is under hormonal control. It is accepted that androgens and growth hormone contribute to male pubertal growth, but the role of estrogens is uncertain even though low-dose estradiol administration stimulates growth in prepubertal boys. In the present work, the correlation of serum testosterone and serum estradiol with growth velocity was studied in 16 pubertal normal boys. The study included correlations of growth velocity with serum nonsex hormone-binding globulin-bound testosterone and with serum nonsex hormone-binding globulin-bound estradiol, which are parameters of serum bioavailable sex hormones. A statistically significant positive correlation was found between serum testosterone and growth velocity but not between serum estradiol and growth velocity. These findings are against the hypothesis that estrogens play a growth promoting role during male puberty.

Progesterone effects on sexual behavior and neuronal ultrastructure in female rats

Treatment of female rats with a low dose of estradiol (5% estradiol in a 5 mm Silastic capsule) and progesterone (0.5 mg) increased the percentage of neurons in the ventromedial nucleus of the hypothalamus that contained stacks of rough endoplasmic reticulum, as compared with female rats receiving either estradiol alone or no treatment. Only females treated with both estradiol and progesterone displayed any sexual behavior, with these females responding at maximal levels. These results support the relationship between stacking of rough endoplastic reticulum in ventromedial hypothalamic neurons and levels of sexual behavior in female rats, and provide evidence that progesterone can produce rapid changes in the morphology of hypothalamic neurons in estrogen-treated animals.

Influence of progesterone and oestradiol-17 beta on blastocysts of the tammar wallaby (Macropus eugenii) during seasonal diapause.

Forty tammar wallabies, presumed to be carrying quiescent blastocysts, were injected with progesterone and oestradiol alone, or in combination, during seasonal quiescence when the corpus luteum is inactive. Plasma progesterone concentrations were increased to values equivalent to those of late pregnancy for the duration of the treatment in progesterone-treated groups but otherwise remained at values equivalent to seasonal quiescence. Tammars treated with low doses of oestradiol showed no measurable increase in plasma oestradiol concentrations but in those treated with high doses plasma concentrations were increased to oestrous levels. At autopsy on Day 18 after the start of treatment the embryos and reproductive tracts were assessed. While progesterone alone caused reactivation of about 50% of the embryos, blastocysts in tammars treated with oestradiol alone remained in diapause (low dose) or disappeared from the uterus (high dose): 2 blastocysts collapsed after some slight expansion. No synergistic effect on pregnancy was noted in tammars receiving both oestradiol and progesterone. We conclude that oestrogen alone is not capable of stimulating normal growth of blastocysts, and its role during early pregnancy in tammars remains unclear.

Oestrogen Therapy in Turner's Syndrome

Oestradiol stimulates growth and development in Turner's syndrome. Previous results with low-dose oestradiol on growth rate are reviewed. The effect of oestradiol in low concentrations on somatomedin generation, GH secretion and directly on osseous tissue, may explain the growth response. The observations presented here of 35 girls treated with 17-beta-oestradiol demonstrated a definite increase in growth rate in the first year of therapy. Bone maturation was accelerated, but a reduction in final height was not found.

Opioidergic modulation of the oestradiol-induced LH surge in the rat: roles of ovarian steroids.

Sex steroids convey information on the status of the reproductive system, which the brain is able to integrate to promote ovulation, in the form of the LH surge. The present studies examined the influence of alterations in central opioidergic tone to initiate the LH surge, and the roles of oestradiol and progesterone to effect changes in opioidergic tone, by antagonizing this activity using either naloxone or nalmefene (N-cyclopropylmethyl-6-desoxy-6-methylene-noroxy-morphone), a long-acting mu- and kappa-opiate antagonist. The timing and amplitude of the LH surge was examined in (1) cyclic rats in pro-oestrus and (2) ovariectomized rats with varying doses of oestradiol supplementation. Plasma was obtained hourly through an indwelling intra-atrial catheter between 13.00 and 19.00 h, and later assayed for LH and oestradiol concentrations by radioimmunoassay. Rats treated with either nalmefene or progesterone on pro-oestrus demonstrated similar advances in the time of initiation of the LH surge by 1-2 h compared with control rats. The effects of nalmefene and progesterone were evident within 2 and 3-5 h of their administration respectively. Conversely, rats treated with progesterone on dioestrus demonstrated low pro-oestrous oestradiol levels and abolition of the pro-oestrous LH surge, but continuous naloxone infusion restored the pro-oestrous LH surge, with raised oestradiol concentrations. In ovariectomized rats without oestradiol supplentation, nalmefene alone was able to increase basal LH levels, but unable to facilitate a spontaneous rise in LH amplitude indicative of an LH surge. Supplementation with low doses of oestradiol was itself ineffective in facilitating a spontaneous rise in LH concentration, but nalmefene co-administration significantly potentiated the ability of low doses of oestradiol to induce augmented LH secretion, in addition to advancing the timing of the spontaneous LH rise. Similarly, progesterone co-administration to ovariectomized, oestradiol-primed rats significantly advanced and augmented LH hypersecretion. The results of these experiments are consistent with the concept that central opioidergic systems normally restrain the initiation of the LH surge, and that blocking opiate receptors removes this inhibition. They advance the hypothesis that oestradiol, the essential signal for LH surge induction, has, as one consequence of its action, the time-specific inhibition of hypothalamic opiodergic tone in the afternoon, which would otherwise restrain the LH surge-generating mechanism.(ABSTRACT TRUNCATED AT 400 WORDS)

Photoperiodic modulation of steroid-induced lordosis in golden hamsters

The photoperiodic modulation of steroid-induced receptive behavior was investigated using ovariectomized female hamsters (Mesocricetus auratus) kept in short (6 hr of light/day) or long (16 hr of light/day) photoperiods. When a low dose of estradiol (E) was used, a significantly larger percentage of animals housed in long days displayed lordosis behavior than did animals housed in short days in tests with intact males as stimuli. This effect of photoperiod was reduced when a high dose of E was employed and was absent when E treatment was supplemented with progesterone. These observations suggest that a reduction in behavioral sensitivity to E may contribute to the reproductive quiescence induced by short photoperiods.

Gonadal steroids and the extinction of conditioned taste aversions in young domestic fowl

Three experiments using domestic chicks demonstrate that the extinction of a conditioned taste aversion to a green-coloured sucrose solution is slowed by the administration of testosterone or 5 alpha-dihydrotestostone but is unaffected by estradiol. Testosterone slows extinction in male and female chicks to an equal extent. Previously described effects of testosterone on attentional and memory mechanisms have also been produced by low doses of estradiol so a separate mechanism must be postulated to explain the present results. It is probably similar to the mechanism responsible for a steroid-dependent slowing of extinction in rats since those effects are also androgenic and present in both sexes.

Integration of the effects of estradiol and progesterone in the modulation of gonadotropin secretion

Estradiol secreted by the maturing follicle is the primary trigger for the surge of gonadotropins leading to ovulation. Progesterone has stimulatory or inhibitory actions on this estrogen-induced gonadotropin surge depending upon the time and dose of administration. The administration of progesterone to immature ovariectomized rats primed with a low dose of estradiol induced a well-defined LH surge and prolonged FSH release, a pattern similar to the proestrus surge of gonadotropins. A physiological role of progesterone is indicated in the normal ovulatory process because a single injection of the progesterone antagonist RU 486 on the day of proestrus in the adult cycling rat and on the day of the gonadotropin surge in the pregnant mare's serum gonadotropin stimulated immature rat resulted in an attenuated gonadotropin surge and reduced the number of ova per ovulating rat. Progesterone administration brought about a rapid LHRH release and an decrease in nuclear accumulation of estrogen receptors in the anterior pituitary but not the hypothalamus. The progesterone effect was demonstrated in vitro in the uterus and anterior pituitary and appears to be confined to occupied estradiol nuclear receptors. In in vivo experiments the progesterone effect on estradiol nuclear receptors appeared to be of approximately 2-h duration, which coincided with the time period of progesterone nuclear receptor accumulation after a single injection of progesterone. During the period of progesterone effects on nuclear estrogen receptors, the ability of estrogens to induce progesterone receptors was impaired. Based on the above results, a model is proposed for the stimulatory and inhibitory effects of progesterone on gonadotropin secretion.

Growth and development in girls with Turner's syndrome during early therapy with low doses of estradiol.

Early therapy with a low dose of estrogen (estradiol-17 beta) was given to 33 girls with Turner's syndrome (T.s.) for a period of 4 years. The dose (0.25-2 mg/day) was adjusted every 3 months to maintain plasma estradiol in the normal concentration range for bone age. Growth velocity was compared with that of untreated girls with T.s. All girls were above age 10 years. Bone age was below 10 years in 11 girls (group I) and above 10 years in 22 girls (group II). Growth velocity in the first year of treatment in group I 7.5 +/- 1.3 cm (SD) with mean SD score (SDS) of +4.3 and in group II 4.9 +/- 1.3 with mean SDS of +3.5. Growth velocity decreased in the following years to 1.6 +/- 1.0 cm, SDS -1.44 in group I and 0.9 +/- 0.6 cm, SDS -2.34 in group II during the fourth year. Withdrawal bleeding occurred in 16 girls of group II after the mean of 23 (range 15-33) months and in 3 girls of group I after 15 to 51 months of treatment. The treatment did not cause an inappropriate acceleration of pubertal development. Breast development appeared in most girls by 3 months of treatment. Pubic hair appeared by 12 months of treatment in group I; it was present in most girls in group II at start of treatment. Final height is known for 12 girls of group II; it was 144.2 +/- 4.5 cm. The final height as predicted at the start of therapy was 142.2 +/- 5.3 cm.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of sexual behaviour in castrated rats: the role of oestradiol.

Sexual behaviour was induced in castrated male rats with oestradiol-17 beta- or testosterone-filled constant-release implants. Testosterone-induced sexual behaviour was unaffected by treatment with the 5 alpha-reductase inhibitor 17 beta-N,N-diethylcarbamoyl-4-aza-5 alpha-androstan-3-one (4-MA; 16.7 mg/day) but treatment with the aromatization inhibitor 1,4,6-androstatriene-3,17-dione (ATD; 10 mg/day) prevented testosterone from inducing the behaviour. Sexual behaviour could be activated in castrated rats treated with testosterone plus ATD by treatment with 4-MA or with implants filled with a low dose of oestradiol. Lordosis behaviour induced in ovariectomized rats with testosterone-filled implants and progesterone was blocked by ATD treatment and could not be activated with 4-MA but oestradiol implants restored the display of lordosis in the testosterone plus ATD-treated females. 4-MA inhibited the in-vitro formation of [14C]5 alpha-dihydrotestosterone from [14C]testosterone by combined preoptic and hypothalamic tissue at all doses tested and a high dose of oestradiol exerted a similar effect. The results suggest that androgen aromatization is required for testosterone-activated female sexual behaviour but not for testosterone-activated male sexual behaviour. It is suggested that oestradiol normally acts to control the sexual behaviour of male rats by modifying neural androgen metabolism.

Effect of low doses of estradiol on 6-month growth rates and predicted height in patients with Turner syndrome

We randomly assigned 16 girls with Turner syndrome, age 5 to 15 years, to receive treatment with 100 ng/kg/day ethinyl estradiol or placebo for 6 months, with crossover after a 2-month interim period. We assessed growth by measurement of the 4-week lower leg growth rate and by height velocity. Bone age was determined at the beginning and end of treatment. Growth rate during ethinyl estradiol treatment was approximately 70% greater than during placebo (P less than 0.001), without any bone age advancement relative to that with placebo. The change in predicted height was significantly greater after 6 months treatment with ethinyl estradiol than after treatment with placebo (mean +/- SEM, +0.35 +/- 0.38 cm vs. -0.85 +/- 0.32 cm, P less than 0.03). Breast budding occurred in six patients. We conclude that it may be feasible to begin low-dose estrogen therapy to promote growth at an earlier age than would be conventionally used to induce pubertal development. These data are still relatively short term, however; until long-term data are available, it would be premature to make definitive recommendations regarding the dose and timing of estrogen treatment in Turner syndrome.

Effects of suckling and low doses of estradiol on luteinizing hormone secretion during the postpartum period in beef cows

An experiment was conducted to test if suckling acutely suppressed circulating levels of LH during the postpartum period in beef cows. In addition, the influence of exogenous administration of low concentrations of estradiol on LH secretion during the postpartum period was evaluated. Twelve mature cows were randomly assigned before parturition to one of three treatments. Four intact cows were used as controls (INT). Eight cows were ovariectomized within the first 7 days following parturition. Four of these cows received a silastic 17β-estradiol implant subcutaneously at the time of ovariectomy (OVX-E); the remaining four cows received no further treatment (OVX). All cows were allowed to nurse one calf for 30 min daily between 1200 and 1230 hours for the duration of the experiment. Blood samples were collected at 12 min intervals for 6 hr before and 6 hr after suckling on days 9, 30, 44 and 58 postpartum. Mean interval (mean ± SE) to the first increase in peripheral progesterone concentrations indicative of the onset of ovarian luteal activity was detected in INT cows 37 ± 4.9 days postpartum. An acute effect of suckling on LH secretion did not occur in INT and OVX cows but mean LH concentrations were reduced in OVX-E cows following suckling on days 44 and 58. Mean LH concentrations remained low in INT cows; whereas, in OVX and OVX-E cows LH concentrations increased linearly (P<0.05) as the interval from time of ovariectomy increased. Cows in the OVX-E group had a higher mean concentration of LH than cows in the OVX group at 30, 44 and 58 days postpartum (P<0.05). Frequency of LH pulses did not differ between cows in the OVX and OVX-E groups at any period. Data from this experiment support the concept that suckling is acting in a chronic fashion to inhibit LH secretion during the postpartum period. In the absence of ovaries, chronic administration of exogenous estradiol in low concentrations has a positive effect on secretion of LH in the postpartum cow.

Selective inhibitory effects of thyroid hormones on estrogen-induced protein synthesis in chick embryo liver

We have investigated the effect of thyroid hormones on estrogen-induced responses in embryonic chick liver. Administration of thyroid hormones inhibits estrogen induction of vitellogenin, as well as of apoprotein-II of very low density lipoprotein (VLDL apo-II). A proportionate decrease in the concentration of hepatic salt-soluble nuclear estrogen receptor is also observed. In contrast, estrogen stimulation of apoprotein-B (VLDL apo-B) synthesis is relatively resistant to inhibition. The inhibitory effects of the thyroid hormones could be due to increased metabolism and clearance of estradiol-17 beta in their presence. The relative resistance of estrogen-induced VLDL apo-B synthesis to thyroid hormone inhibition can be explained by its greater sensitivity to low doses of estradiol. In addition, experiments with the antithyroid agent thiourea suggest that, in vivo, estrogen-induced responses could be balanced by the selective inhibitory effects of thyroid hormones.

Induction of an estrogen-dependent early steroidogenic lesion in murine Leydig tumor cells.

The effects of low doses (37 pM to 3.7 nM) of 17 beta-estradiol on Leydig tumor cell steroidogenesis were studied in primary culture. This gonadotropin-responsive Leydig tumor line (M5480A) produces progesterone as the major steroid and lower levels of testosterone. It was found that these tumor cells possess a relatively high level of estradiol receptors, but only low levels of estradiol. We, therefore, maintained dispersed Leydig tumor cells in culture under basal or hCG-stimulated conditions for varying periods of time with or without graded doses of estradiol. The media from these cultures were analyzed for pregnenolone, progesterone, and testosterone by specific RIAs. Although testosterone levels were similar to control values, both pregnenolone and progesterone levels were significantly decreased by low doses of estradiol in a dose- and time-dependent manner. For example, basal progesterone levels were diminished 36% by 0.37 nM estradiol, and this effect could be reversed by the antiestrogen LY117018 [6-hydroxy-2-(p-hydroxyphenol)benzo-b-thien-3-yl-p-2-(1-pyrr olidinyl)- ethoxyphenyl ketone]. To evaluate whether the decreased medium progesterone level was due to increased metabolism, [3H] progesterone was added to estrogen-treated and control cells, and ether-extracted media were analyzed for steroid metabolites by HPLC. No significant difference in progesterone metabolism, including its conversion to testosterone, was detected between control and treated cells. Thus, the estradiol-mediated decrease in progesterone concentrations most likely reflects decreased synthesis rather than increased metabolism. These results provide the first indication of an estrogen-mediated effect at an early site in Leydig tumor cell steroidogenesis.

An evaluation of tamoxifen as a partial agonist by classical receptor theory — An explanation of the dual action of tamoxifen

Tamoxifen is known to have both agonist and antagonist properties. Classical receptor theory predicts that given the relative concentrations of a partial agonist and full agonist acting on the same receptor, the partial agonist may reduce the effect of the full agonist. The immature rat uterine model is an excellent system to evaluate the interactions of estradiol and tamoxifen by application of receptor theory. Using this model, tamoxifen demonstrates both additive and antagonistic effects to estradiol in the fashion predicted by theory. The effects of tamoxifen are additive at low doses of estradiol and antagonistic over higher estradiol doses. It is possible that the dualism of agonism and antagonism seen in other target organs and species is a function of these basic characteristics of a partial agonist.