Effects of estradiol on aldosterone secretion in ovariectomized rats

The effects and action mechanisms of estradiol on aldosterone secretion in female rats were studied. Replacement of estradiol benzoate (EB) increased the levels of plasma estradiol and aldosterone in ovariectomized (Ovx) rats. The aldosterone release from zona glomerulosa (ZG) cells was higher in EB-treated rats than in oil-treated animals. EB treatment potentiated the responses of aldosterone release to adrenocorticotropic hormone (ACTH), forskolin (FSK), and 8-bromoadenosine 3',5'-cyclic monophosphate (8-Br-cAMP). Administration of EB in vivo did not alter cAMP production in response to ACTH or FSK. Although angiotensin II (Ang II) increased aldosterone secretion by rat ZG cells, the stimulatory effect of Ang II on the release of aldosterone was not altered by EB treatment. The conversions of [3H]-deoxycorticosterone to [3H]-corticosterone and [3H]-corticosterone to [3H]-aldosterone in EB-treated groups were greater than those in the oil-treated group. These results suggest that estradiol increases aldosterone secretion in part through the mechanisms involving the activation of the post-cAMP pathway, 11beta-hydroxylase and aldosterone synthase activity.     

Effects of estradiol on corticosterone secretion in ovariectomized rats

The effects of estradiol benzoate (EB) on steroidogenesis in rat zona fasciculata-reticularis (ZFR) cells were studied. Female rats were ovariectomized (Ovx) for 2 weeks and then injected subcutaneously with oil or EB for 3 days before decapitation. ZFR cells were isolated and incubated with adrenocorticotropin (ACTH) or prolactin (PRL) for 1 h. Corticosterone concentrations in plasma and cell media, and adenosine 3',5'-cyclic monophosphate (cAMP) production in ZFR cells were determined by radioimmunoassay. The effects of EB replacement in vivo on the activities of steroidogenic enzymes in ZFR cells were measured by the amounts of intermediate steroidal products separated by thin-layer chromatography. Replacement of EB in vivo resulted in a dose-dependent increase of plasma PRL and corticosterone in Ovx rats. The basal, ACTH-, and PRL-stimulated release of corticosterone by ZFR cells was greater in EB- than in oil-treated animals. Forskolin-induced production of cAMP was greater in the EB-replaced rats than in oil-treated animals, which correlated with the increase of corticosterone production. The 3-isobutyl-l-methylxanthine (IBMX) plus ACTH-, IBMX plus PRL-, and forskolin plus PRL-stimulated productions of cAMP were higher in EB- than in oil-treated rats. The enzyme activities of postpregnenolone were not affected by EB replacement in Ovx rats. These results suggest that the EB-related increase of corticosterone production in Ovx rats is associated with an increase of cAMP generation and the stimulatory effect of PRL on ZFR cells.     

Effects of neuromedin U on the pulsatile LH secretion in ovariectomized rats in association with feeding conditions

We examined the effects of intracerebroventricular injection of neuromedin U (NMU), at a dose that is reported to induce satiety in rats, on the pulsatile luteinizing hormone (LH) secretion in adult ovariectomized (OVX) rats under a normal feeding or a 48-h fasted condition. In OVX rats under the normal feeding condition, injection of NMU (1 nmol/3 microl) significantly decreased the mean LH concentration without affecting the frequency or amplitude of LH pulses, but under the 48-h fasted condition, it significantly decreased the mean LH concentration and the frequency of LH pulses without affecting the amplitude. The interpulse interval was significantly lengthened by NMU injection under the normal and the 48-h fasted condition, but the effect under the 48-h fasted condition was greater than under the normal feeding condition. We also confirmed that the 48-h fasted condition per se did not affect the pulsatile LH secretion in OVX rats. We suggest that NMU and fasting synergistically inhibit the pulsatile LH secretion, even though NMU has been said to act as a satiety factor.     

Age and the effect of adrenocorticotropic hormone on aldosterone secretion in rats

Age-related changes in functional activity of the glomerular zone were studied in the adrenal cortex of Wistar rats. It was shown that secretion of the basic mineralocorticoid hormone aldosterone was decreased with age. The glomerular zone of old rats showed the reduced sensitivity to the stimulant action of corticotropin, marked by the pronounced reaction to less doses of the hormone administered. At the same time the reactivity and the range of shifts in the course of ACTH dosage build-up decrease with aging.     

Modulatory effects of the amygdala on oestrogen-induced LH secretion in ovariectomized rats

An increase in LH secretion was induced in ovariectomized oestradiol benzoate-primed rats 5 h after a second injection of oestradiol benzoate. Lesions stereotaxically placed in the cortical and basomedial amygdala of steroid-primed rats abolished this rise. The results provide evidence for a facilitatory action of the amygdala upon LH release and an involvement of this region of the limbic system in oestrogen-feedback mechanisms.     

Effects of prolactin on aldosterone secretion in rat zona glomerulosa cells

Acute effects and action mechanisms of prolactin (PRL) on aldosterone secretion in zona glomerulosa (ZG) cells were investigated in ovariectomized rats. Administration of ovine PRL (oPRL) increased aldosterone secretion in a dose-dependent manner. Incubation of [3H]-pregnenolone combined with oPRL increased the production of [3H]-aldosterone and [3H]-deoxycorticosterone but decreased the accumulation of [3H]-corticosterone. Administration of oPRL produced a marked increase of adenosine 3',5'-cyclic monophosphate (cAMP) accumulation in ZG cells. The stimulatory effect of oPRL on aldosterone secretion was attenuated by the administration of angiotensin II (Ang II) and high potassium. The Ca2+ chelator, ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA, 10(-2) M), inhibited the basal release of aldosterone and completely suppressed the stimulatory effects of oPRL on aldosterone secretion. The stimulatory effects of oPRL on aldosterone secretion were attenuated by the administration of nifedipine (L-type Ca2+ channel blocker) and tetrandrine (T-type Ca2+ channel blocker). These data suggest that the increase of aldosterone secretion by oPRL is in part due to (1) the increase of cAMP production, (2) the activation of both L- and T-type Ca2+ channels, and (3) the activation of 21-hydroxylase and aldosterone synthase in rat ZG cells.     

Characteristics of pulsatile luteinizing hormone secretion in middle-aged ovariectomized rats

Experiments were performed to characterize the pulsatile patterns of circulating luteinizing hormone (LH) in the middle-aged ovariectomized (OVX) rat. Frequent blood samples were taken from OVX rats at 6, 7-8, and 9-10 mo of age, and LH was measured by radioimmunoassay. Rats had been OVX either 2 wk (STO) or 10-20 wk (LTO) previously. Mean LH levels were significantly lower with increasing age, reflecting effects on both pulse amplitude and pulse frequency. Mean LH levels were higher in LTO than STO groups, reflecting primarily an increase in pulse amplitude, but there was also a small, significant decrease in pulse frequency with increased time following OVX. In a second experiment, a random selection of the rats in the STO groups was tested again 10 wk after OVX. A significantly higher number of 9- to 10-mo-old rats had pulsatile patterns at 10 wk than at 2 wk following OVX. Furthermore, mean plasma LH concentrations were higher in all three groups. We conclude that decreases in several parameters of LH secretion are seen in middle-aged OVX rats, at the time when irregularities are first seen in the estrous cycle in the intact rat.     

染料木黄酮对去势大鼠骨骼矿化的影响

目的: 研究染料木黄酮对去势大鼠骨骼矿化的影响.方法: 雌性Wistar大鼠47只随机分为假手术组,去势对照组、去势 雌激素组(己烯雌酚20 μg.kg bw-1.d-1)、去势 染料木黄酮组(剂量分别为25、50、100 mg.kg bw-1.d-1).饲养三个月后处死,测定骨密度、骨矿化相关参数、骨钙、磷、锌、镁、锰、血清甲状旁腺激素、降钙素和雌激素含量.结果: 大鼠去势后,股骨骨密度降低,平均类骨质宽度增大,骨矿化延迟时间和类骨质成熟时间延长,骨中钙、磷、锌、镁和血清雌激素含量降低,与假手术组相比均有显著性差异(P＜0.05);补充染料木黄酮后,股骨骨密度有改善的趋势,平均类骨质宽度变窄,骨矿化延迟时间和类骨质成熟时间缩短,骨中钙、磷、镁含量升高.结论: 染料木黄酮通过促进类骨质矿化,减少骨中钙、磷、镁丢失,预防骨质疏松的发生.     

Effects of fasting on maximum thermogenesis in temperature-acclimated rats

To further investigate the limiting effect of substrates on maximum thermogenesis in acute cold exposure, the present study examined the prevalence of this effect at different thermogenic capabilities consequent to cold- or warm-acclimation. Male Sprague-Dawley rats (n=11) were acclimated to 6, 16 and 26C, in succession, their thermogenic capabilities after each acclimation temperature were measured under helium-oxygen (21% oxygen, balance helium) at –10C after overnight fasting or feeding. Regardless of feeding conditions, both maximum and total heat production were significantly greater in 6>16>26C-acclimated conditions. In the fed state, the total heat production was significantly greater than that in the fasted state at all acclimating temperatures but the maximum thermogenesis was significant greater only in the 6 and 16C-acclimated states. The results indicate that the limiting effect of substrates on maximum and total thermogenesis is independent of the magnitude of thermogenic capability, suggesting a substrate-dependent component in restricting the effective expression of existing aerobic metabolic capability even under severe stress.     

Effects of misoprostol on bone loss in ovariectomized rats.

This study was performed to investigate whether misoprostol (prostaglandin E1 analogue) (Cytotec, Searle, England) is effective for restoration of bone loss. Four-month-old parous female Sprague-Dawley rats (n = 30) were subjected either to bilateral ovariectomy (OVX, 24 rats) or to sham surgery (sham, 6 rats). The OVX rats were divided into four groups 60 days after the surgery. Six of them were killed, and dual-energy X-ray absorption (Norland xr-36, Norland Corporation, Fort Atkinson, WI, USA) measurements were performed, called pretreatment OVX group. The remaining groups (each had 6 rats) treated orally with 0 (control), 100, 200 micrograms/kg/day misoprostol for 60 days. All rats were killed 60 days after having treatment, and bone loss of the lumbar spine was measured by dual-energy X-ray absorption. The bone mineral density was decreased by 25.4% in control group and 23.6% in pretreatment group compared to sham group, but restored by 86% and 96% in groups treated with 100 and 200 micrograms/kg/day misoprostol, respectively. These results suggest that misoprostol restores bone loss in the lumbar spine of OVX rats in a dose-dependent manner.     

Pharmacological influences on aldosterone secretion

Besides the classical modulators of aldosterone secretion, new factors influencing positively or negatively aldosterone secretion have been described. These new factors and the effect of related drugs constitutes the aim of this review. The effect of dopamine agonists and H2-receptor antagonists on aldosterone secretion in normal volunteers as well as in different clinical situations characterized by an increased production of aldosterone opens a new field of investigation for the therapy of aldosterone secretion alterations.     

Effects of intracerebroventricular administration of opiate receptor antagonists on the suppressed pulsatile LH release during acute fasting in ovariectomized estradiol-treated rats.

The involvement of specific opiate receptors in the suppression of LH release during acute fasting in ovariectomized estradiol-treated rats was examined by intracerebroventricular (i.c.v.) administration of opiate receptor antagonists that exert a specificity directed mainly, although not absolutely, towards the delta-, kappa- or mu-opiate receptors. Fasting for 48 h significantly decreased mean plasma LH levels in estradiol-treated animals by increasing sensitivity to the negative feedback effect of estradiol. Injecting i.c.v. the mu-opiate receptor antagonist naloxone (10 or 100 nmol in 2 microliters of saline) blocked the inhibitory effect of fasting on pulsatile LH release and reinstated LH pulses. On the other hand, i.c.v. administration of the same dosages of a delta-opiate receptor antagonist ICI 174,864 or a kappa-opiate receptor antagonist WIN 44441-3 did not have any effect. These results suggest that the increased sensitivity of the LH-releasing mechanism to the negative feedback effect of estradiol during fasting involves the endogenous opioids mainly through the selective activation of the mu-opiate receptors.     

Relative and combined effects of estradiol and prolactin on corticosterone secretion in ovariectomized rats

In vivo and in vitro experiments were designed to assess the relationship of the estradiol (E2) and prolactin (PRL) on glucocorticoid secretion in ovariectomized (Ovx) rats. Female rats were Ovx for two weeks and then subcutaneously injected with oil or estradiol benzoate (EB) for 3 days before experimentation. Venous blood samples were collected from right jugular vein at 0, 30, 60, 90, and 120 min after challenge with adrenocorticotropin (ACTH). Adrenal zona fasciculata-reticularis (ZFR) cells from Ovx rats were isolated and incubated with E2 or PRL. In the morning and afternoon, EB enhanced the basal and ACTH-stimulated concentrations of plasma corticosterone (CORT) and PRL. Administration of E2 in vitro increased the basal and ACTH-stimulated release of CORT and production of adenosine 3', 5'-cyclic monophosphate (cAMP) in ZFR cells. E2 enhanced the forskolin-stimulated release of CORT by ZFR cells. However, the 3-isobutyl-l-methylxanthine (IBMX)- or 8-Br-cAMP-stimulated release of CORT was not affected by E2. E2 augmented the lower doses of PRL-stimulated release of CORT and cAMP accumulation as compared with the PRL-treated group alone. Incubation of higher doses of PRL increased the production of cAMP. Administration of nifedipine and R(+) BK8644 (classic L-type Ca2+ channel blocker) significantly attenuated the PRL-stimulated release of CORT. Taken together, these data indicate that E2- and PRL-related increase of CORT in Ovx rats is associated with the increase of cAMP accumulation and calcium influx in ZFR cells. In conclusion, E2 and PRL play a stimulatory role in the co-regulation of CORT secretion.     

Simulated microgravity impairs aldosterone secretion in rats: possible involvement of adrenomedullin

The prolonged exposure to microgravity (MG) or simulated MG (SMG) has been reported to cause hypotension, mainly due to reduced vascular contractility, and dysregulation of fluid and electrolyte balance. However, the mechanism(s) involved in these MG- or SMG-induced effects is not yet completely elucidated. Hence, we investigated in the rat the effect of prolonged (15 day) SMG, in the form of hindlimb unweighting, on the renin-angiotensin-aldosterone system (RAAS), as well as on atrial natriuretic peptide (ANP) and adrenomedullin (ADM), two hypotensive peptides that play a major role in the regulation of RAAS activity by inhibiting adrenal aldosterone secretion. SMG caused a mild hypotension in rats, associated with the blockade of body weight gain. Plasma aldosterone concentration and basal and agonist-stimulated in vitro aldosterone secretion from adrenal slices were decreased, and plasma renin activity was moderately increased. Neither Na(+) and K(+) serum concentrations nor ACTH and corticosterone blood levels were significantly affected. Plasma ANP concentration did not display significant alterations, while ADM blood concentration underwent a marked rise. The administration of the ADM-receptor antagonist ADM-(22-52) during the last 3 days of hindlimb unweighting reversed the SMG-induced hypotension and hypoaldosteronism. Collectively, these findings allow us to suggest that prolonged SMG impairs RAAS activity in rats, through a mechanism probably involving upregulation of the ADM system. Both hypoaldosteronism and increased ADM secretion may contribute to the development of hypotension during prolonged exposure to SMG.     

Effects of short-term fasting in male Sprague-Dawley rats

Fasting is a common procedure for animals in experiments. Although fasting may be necessary for scientific reasons, it should be minimized. In the current study, jugular-catheterized male Sprague-Dawley rats in metabolism cages were fasted for 0 to 24 h before measurement of various physiologic markers (serum chemistry, CBC analysis, serum corticosterone). When controlled for cohort, rats fasted for 6 and 16 h had significantly lower serum glucose than did nonfasted rats. Other values did not differ from controls. Only rats fasted for 24 h had elevated serum corticosterone levels. Therefore, fasting for as long as 16 h has fewer effects on rats that does fasting for 24 h. Fasting for 24 h or more therefore should receive appropriate consideration by both scientists and the IACUC in the experimental design and the animal-use protocol.     

Involvement of dopamine receptor subtypes in dopaminergic modulation of aldosterone secretion in rats

The postulation that dopamine (DA) may tonically inhibit aldosterone (ALDO) secretion has arisen from the finding that metoclopramide, a non-selective DA receptor antagonist with prominent non-dopaminergic actions, stimulates ALDO secretion. Experiments were performed to determine: (a.) the ability of several non-specific and subtype-specific DA receptor antagonists to stimulate ALDO secretion, (b.) the subtype DA receptor involved in regulating ALDO secretion, and (c.) if ALDO responses were associated with changes in plasma Na+(pNa), K+(pK), or osmolality (pOsm). Blood samples were withdrawn from carotid arterial catheters in conscious, fasted male Sprague-Dawley rats before and following intra-arterial administration of lactated Ringer's placebo, furosemide (10 mg/kg), or one of several DA receptor antagonists. Furosemide stimulated ALDO, decreased pK, and left pNa and pOsm unchanged. The non-selective DA receptor antagonists metoclopramide (0.2, 0.6 mg/kg), rs-sulpiride (0.2 mg/kg), and haloperidol (0.1 mg/kg), and the DA-2 receptor antagonists domperidone (0.1 mg/kg) and s-sulpiride (0.1 mg/kg) each stimulated ALDO, and left pNa, pK, and pOsm unchanged. Conversely, the DA-1 receptor antagonists SCH 23390 (0.03, 0.1 mg/kg) and r-sulpiride (0.1 mg/kg) failed to stimulate ALDO, and left pNa, pK, and pOsm unaltered. These studies suggest that ALDO secretion in rats is modulated by a mechanism involving DA-2, but not DA-1 subtype receptors, and that the ALDO responses to DA receptor antagonism are independent of changes in pNa, pK, and pOsm.     

Inhibitors of aldosterone secretion

Aldosterone secretion may be inhibited by potassium depletion, inhibitors of the renin-angiotensin system, dopamine and atrial natriuretic factor. The latter appears to be an important physiological regulator of aldosterone secretion. ANF inhibits basal, ACTH, Angiotensin II and potassium-stimulated aldosterone production in vitro by a direct action on the adrenal gland. In vivo data also support a direct inhibitions of aldosterone. The stimulation of aldosterone secretion by infusions of Angiotensin II and potassium is inhibited by simultaneous infusions of ANF. Infusions of ANF lower the basal aldosterone secretion in man. The mechanism by which ANF inhibits aldosterone is not known. No unifying first step has been identified to explain ANF's ability to inhibit all stimuli. In vivo, part of the lowering of aldosterone levels may be due to inhibition of renin secretion. This effect of ANF upon renin is inconsistent and appears to depend upon the experimental conditions.     

Effects of tamoxifen on myocardial ischemia-reperfusion injury model in ovariectomized rats

The purpose of this study is to examine the antiarrhythmic and antioxidant effects of tamoxifen, one of the selective estrogen modulators, in ovariectomized rats subjected to myocardial ischemia-reperfusion (I/R) injury. A month after ovariectomy, rats were divided into four groups: (I) ovariectomized controls without any treatment, (II) ovariectomized rats treated with vehicle dimethylsulfoxide (DMSO), (III)–(IV) ovariectomized rats treated with tamoxifen 1 or 10 mg/kg,sc daily for 14 days. To produce arrhythmia, the left main coronary artery was occluded for 7 min, followed by 7 min of reperfusion. The blood pressure (BP), heart rate (HR), electrocardiography (ECG) was recorded before and during the ischemia-reperfusion period. The blood levels of malondialdehyde (MDA), creatine kinase (CK), glutathione (GSH), glutathione peroxidase (GSH-Px), glutathione reductase (GR), and catalase (CAT) were measured after the rats were killed. Tamoxifen reduced the incidence of ventricular tachycardia (VT) on ischemia and reperfusion as well as the incidence and duration of reversible ventricular fibrillation (VF) on reperfusion. I/R injury caused a significant fall in GSH, GSH-Px as well as an increase in MDA and CK levels in the control group when compared to tamoxifen treated groups. The changes in levels of CAT and GR were however, not significant. In conclusion, our findings suggest that tamoxifen has cardioprotective effects against I/R injury in rats, likely its antioxidant properties.