Marianne K. Steele

Estrogens regulate angiotensin-converting enzyme and angiotensin receptors in female rat anterior pituitary

We studied the effects of the estrous cycle, ovariectomy and estrogen replacement on angiotensin-converting enzyme (ACE) (kininase II, EC 3.4.15.1) and angiotensin II (AT) receptors in the pituitary gland of the female rat. Quantitative autoradiography, with the use of consecutive pituitary sections, allowed for simultaneous determination of changes in binding and in the potential AT synthetic ability of individual pituitaries, and for a correlation between these two phenomena. In the anterior pituitary, ACE activity and binding of the ACE inhibitor [125I]-351A were not changed during the estrous cycle. Ovariectomy produced a significant increase in ACE activity and binding, and both of these parameters returned to normal after estrogen replacement. There were no changes in ACE activity or binding in the posterior pituitary during the estrous cycle or after ovariectomy or hormone replacement. AT receptors were characterized as of the AT1 type, since they were displaced by the selective AT1 antagonist DuP 753 and not by the AT2 competitor PD 123177. There were marked changes in the concentration of AT1 receptors during the estrous cycle, with highest numbers in metestrus, lower in estrus and diestrus, and lowest during proestrus. Estrogen replacement in ovariectomized rats decreased AT1 receptor number in the anterior pituitary. Our results indicate a dual effect of estrogen on anterior pituitary AT, physiologically on AT receptor expression and pharmacologically on ACE activity.

Immunocytochemical Localization of Angiotensin Immunoreactivity in Gonadotrops and Lactotrops of the Rat Anterior Pituitary Gland

Utilizing an immunocytochemical technique wherein two different antigens can be visualized on the same tissue section, angiotensin-like immunoreactivity was found in rat adenohypophyseal cells that showed staining with antisera to hLH, hFSH or human prolactin. Angiotensin-like immunoreactivity was not found in cells that showed staining with antisera to hTSH, C-terminal ACTH or hGH. The nature and possible physiologic significance of the angiotensin-like immunoreactivity is discussed.

Stimulatory or Inhibitory Effects of Angiotensin II upon LH Secretion in Ovariectomized Rats: a Function of Gonadal Steroids

The effects of intraventricular infusions of artificial cerebrospinal fluid (aCSF) or angiotensin II (AII) on LH secretion were investigated in rats that had been ovariectomized for 8 days. In untreated ovariectomized rats, the mean whole blood concentration of LH as well as the amplitude, frequency, and nadir of the LH pulses were not affected by infusion of aCSF or 15 ng AII/h, but were suppressed in a dose-dependent fashion by infusion of AII at doses of 150 or 600 ng/h. The AII receptor antagonist, saralasin, blocked the inhibitory effect of AII, demonstrating the specificity of the response to AII. In ovariectomized rats pretreated with estradiol, infusion of AII did not modify mean blood LH levels. However, in ovariectomized rats pretreated with both estradiol and progesterone, infusions of AII at 150 or 600 ng/h produced dose-dependent increases in mean LH concentrations. The results demonstrate both inhibitory and stimulatory effects of AII upon LH secretion, the direction of the effect being determined by gonadal steroids.

The role of brain angiotensin II in the regulation of Luteinizing Hormone and Prolactin secretion.

The renin-angiotensin system, both in the circulation and in the brain, is known for its role in the regulation of fluid balance and blood pressure. The brain angiotensin II (Ang II) system is also involved in the control of anterior pituitary hormone secretion, through affecting the secretion of releasing and inhibitory factors into the hypophyseal portal vessels. Ang II controls the release of LH and PRL in a manner that is modified by ovarian hormones, observed only under specific conditions, and localized to particular regions of the brain. The identification of Ang II systems in the pituitary gland and ovary, along with data showing effects of ovarian hormones on the activity of the brain Ang II system, suggests a feedback loop whereby the brain, pituitary, and gonads interact to affect reproductive function.

Further Studies on the Localization of Angiotensin-II-Like Immunoreactivity in the Anterior Pituitary Gland of the Male Rat, Comparing Various Antisera to Pituitary Hormones and Their Specificity

Using immunocytochemistry with the sensitive avidin-biotin complex method and an antibody to the beta subunit of rat luteinizing hormone (rat LH beta) provided by the National Hormone and Pituitary Program, NIADDK, to identify gonadotrops, the presence of angiotensin II-like immunoreactivity in the gonadotrops of the anterior pituitary gland of male rats was confirmed. However, using an antibody to rat prolactin, no angiotensin II-like immunoreactivity could be found in lactotrops. This finding is at variance with a previous report from this laboratory, in which the lactotrops were identified with an NIADDK antibody to human prolactin. The possibility that the antibodies to human prolactin cross-reacted with rat LH was therefore investigated by a noncompetitive radioimmunological binding assay. At the same dilution used in immunocytochemistry, two different NIADDK antihuman prolactin antisera displayed 49% and 24% binding of labeled rat LH beta. Similar cross reactions were demonstrated immunocytochemically. There was no cross reactivity between rat LH beta and the antibody to rat prolactin. Consequently, the staining reported to be in lactotrops in the previous study was probably in gonadotrops.

Effects of angiotensin II on LHRH release, as measured by in vivo microdialysis of the anterior pituitary gland of conscious female rats.

The present experiments assessed the effects of central administration of angiotensin II (Ang II) on mean levels of luteinizing hormone-releasing hormone (LHRH) in the extracellular fluid of the anterior pituitary gland, monitored by in vivo microdialysis. Ovariectomized rats were tested under two conditions: (1) nonhormone-treated where Ang II infusion inhibits luteinizing hormone (LH) release, and (2) ovarian hormone-treated where Ang II stimulates LH secretion. Animals were ovariectomized and chronic guide cannulae were implanted, one into the lateral cerebral ventricle for infusion of Ang II and one directed toward the anterior pituitary gland for the insertion of the microdialysis probe. Approximately 1 week later, the dialysis probe was inserted and cemented into place. The length of the dialysis probe transected the pituitary gland from its dorsal to ventral aspects. Dialysis samples were collected at 15-min intervals. Levels of LHRH were continuously monitored in nonhormone-treated animals, prior to and during intracerebroventricular (i.c.v.) infusion of Ang II. The dialysis probe was removed at the end of the experiment. One week later, the same animals were treated with estrogen and progesterone and dialysis of the anterior pituitary gland was performed 3 days later using a protocol identical to the first dialysis sampling session. A separate group of animals was tested to confirm the effects of lateral ventricle infusion of this dose of Ang II on LH release. There were no detectable values of LHRH in dialysis samples from non-hormone-treated animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of Angiotensins Injected into Various Brain Areas on Luteinizing Hormone Release in Female Rats

Angiotensin II (AII; 1.0 microliter of 10(-10) M) injected bilaterally into the anterior hypothalamus-preoptic area of the brain produced a 100% increase in mean whole blood LH concentrations in ovariectomized rats treated with estradiol and progesterone. Injections of equal volumes and concentrations of the peptide into the third cerebral ventricle or into the posterior hypothalamus did not affect blood LH values. The effects of AII administration into the rostral hypothalamus were dose-related and specific. Equimolar injections of angiotensin I or angiotensin III into this region did not significantly affect blood LH levels. Taken together, these data demonstrate that the anterior hypothalamus-preoptic area, compared to the third ventricle or more caudal brain sites, is particularly sensitive to the specific stimulatory actions of AII on LH release.

Brain Angiotensin II Receptor Subtypes and the Control of Luteinizing Hormone and Prolactin Secretion in Female Rats

The present experiments examined the role of the two recently identified angiotensin II (Ang II) receptor subtypes, AT, and AT2, in the central nervous system regulation of luteinizing hormone (LH) and prolactin secretion in estrogen‐ and progesterone‐treated ovariectomized rats. In this animal model, intracerebroventricular (icv) injection of Ang II stimulates LH and inhibits prolactin release. The specific Ang II receptor subtype antagonists losartan (AT1) or PD123177 (AT2) were administered (icv) in various doses (10 ng to 1,000 ng) 10 min prior to icv injection of Ang II (100 ng). Control animals were pretreated with artificial cerebrospinal fluid prior to Ang II administration. Blood samples for LH and prolactin determinations were taken from conscious, freely‐moving rats prior to and following injection of the antagonists and Ang II. Water intake was measured.

Additive effects of atrial natriuretic peptide and angiotensin II on luteinizing hormone and prolactin release in female rats.

The present experiments were designed to assess the effects of central administration of atrial natriuretic peptide (ANP), alone or in conjunction with angiotensin II (AII), on luteinizing hormone (LH) and prolactin release in ovarian steroid-primed female rats. Intracerebroventricular injection of ANP (200 ng/2 microliters over 1 min) induced an increase in plasma LH levels without significantly affecting prolactin release. Injection of AII (50 ng/2 microliters over 1 min) also resulted in an increase in LH levels and, in addition, reduced plasma prolactin titers. Coinjection of ANP and AII did not elevate LH levels beyond that achieved by either peptide alone, but produced a further suppression in prolactin levels than that seen with AII alone. Water intake induced by AII was attenuated by coadministration of ANP. In a second protocol utilizing lower doses of the peptides delivered by infusion, either ANP (800 ng/h; 13.3 ng/min) or AII (200 ng/h; 3.3 ng/min) alone produced 100% increases in LH levels. Coinfusion of the peptides induced an additive (250%) increase in plasma LH titers. In this protocol, the release of prolactin was unaffected by infusion of the peptides, either alone or as a comixture. Water intake induced by AII infusion was attenuated by coadministration of ANP. Taken together, these experiments demonstrate that, in contrast to the antagonistic interaction of ANP and AII on water intake, the peptides affect reproductive hormone secretion in the same direction and, under particular experimental conditions, display additive effects in stimulating LH and inhibiting prolactin release.

The Brain Renin‐Angiotensin System and the Regulation of Prolactin Secretion in Female Rats: Influence of Ovarian Hormones

This study was designed to investigate the effects of exogenous and endogenous angiotensin II (All) on prolactin release in ovariectomized rats, with and without estrogen and progesterone pretreatment. In the first series of experiments, All or vehicle was injected into the third cerebral ventricle of conscious, freely‐moving rats. In both treated and untreated rats, administration of 50 ng All suppressed prolactin levels within 15min of injection, compared to vehicle‐injected rats. In untreated rats, prolactin levels returned to baseline values by 30 min, while in treated rats, prolactin levels remained suppressed for an hour.