Intravenous angiotensin induces brain c-fos expression and vasopressin release in the near-term ovine fetus

The effect of intravenous angiotensin II (ANG II) on fetal brain c-fos expression and arginine vasopressin (AVP) release was studied in the near-term ovine fetus. Fetuses with chronically implanted catheters received an intravenous infusion of ANG II or saline. Fetal plasma AVP concentrations were significantly increased after the peripheral administration of ANG II, with peak levels (3-fold) at 30 min after the intravenous infusion. There was no change in fetal plasma osmolality, sodium, and hematocrit levels between the control and experimental groups or between the periods before and after the infusion of ANG II. Intravenous ANG II administration induced Fos immunoreactivity (Fos-IR) in the circumventricular organs and the median preoptic nucleus of the fetal brain. Fos-IR was also demonstrated in the fetal supraoptic nuclei (SON). Double labeling demonstrated that the AVP-containing neurons in the SON were expressing c-fos in response to intravenous ANG II. These results indicate that the peripheral ANG II in the fetus may play a significant role in stimulating the central hypothalamic-neurohypophysial system during late gestation. It supports the hypothesis that circulating ANG II may act at the fetal AVP neurons in the hypothalamus in body fluid balance via the circumventricular organs, which are situated outside the blood-brain barrier, and the central neural pathway between these two brain structures has been relatively established in utero, at least at near-term.     

Angiotensin-induced vasopressin release and activation of hypothalamic neuron in pre-term fetuses

Our previous studies have shown that central administration of angiotensin II (ANG II) causes vasopressin release in the near-term fetus in utero as evidence that the hypothalamic-neurohypophysial system has relatively matured before birth. However, it is still unknown whether the vasopressin controlling centers have been functionally developed in younger fetuses. This study determined fetal plasma vasopressin levels and hypothalamic vasopressin neuron activity in the chronically instrumented pre-term ovine fetuses. Introcerebroventricular (i.c.v.) administration of ANG II did not affect fetal plasma osmolality and sodium concentrations. However, fetal plasma vasopressin levels were significantly increased ( approximately 3-fold) in response to central injection of ANG II. Central ANG II also induced vasopressin-neuron activity marked with c-fos expression in the fetal hypothalamus at pre-term. In addition, the fetal organum vasculosum of the lamina terminalis and the subfornical organ were activated. The results suggest that hypothalamic-neurohypophysial system has been relatively intact and functional at 70% gestational age, and that central angiotensin is important in inducing fetal vasopressin release in utero.     

Osmotic threshold and sensitivity for vasopressin release and fos expression by hypertonic NaCl in ovine fetus

In adults, hyperosmolality stimulates central osmoreceptors, resulting in arginine vasopressin (AVP) secretion. Near-term fetal sheep have also developed mechanisms to respond to intravascular hypertonicity with stimulation of in utero AVP release. However, prior studies demonstrating fetal AVP secretion have utilized plasma tonicity changes greater than those required for adult osmotically induced AVP stimulation. We sought to examine near-term fetal plasma osmolality threshold and sensitivity for stimulation of AVP secretion and to correlate plasma hormone levels with central neuronal responsiveness. Chronically instrumented ovine fetuses (130 +/- 2 days) and maternal ewes simultaneously received either isotonic or hypertonic intravascular NaCl infusions. Maternal and fetal plasma AVP and angiotensin II (ANG II) levels were examined at progressively increasing levels of plasma hypertonicity. Intravenous hypertonic NaCl gradually elevated plasma osmolality and sodium levels. Both maternal and fetal plasma AVP increased during hypertonicity, whereas ANG II levels were not changed. Maternal AVP levels significantly increased with a 3% increase in plasma osmolality, whereas fetal plasma AVP significantly increased only at higher plasma osmolality levels (over 6%). Thus the slope of the regression of AVP vs. osmolality was greater for ewes than for fetuses (0.232 vs. 0.064), despite similar maternal and fetal plasma osmolality thresholds for AVP secretion (302 vs. 304 mosmol/kg). Hyperosmolality induced Fos immunoreactivity (FOS-ir) in the circumventricular organs of the fetal brain. FOS-ir was also demonstrated in the fetal supraoptic and paraventricular nuclei (SON and PVN), and double labeling demonstrated that AVP-containing neurons in the SON and PVN expressed Fos in response to intravenous NaCl. These results demonstrate that, in the ovine fetus at 130 days of gestation, neuroendocrine responses to cellular dehydration are functional, although they evidence a relatively reduced sensitivity for AVP secretion compared with the adult.     

A central link between angiotensinergic and cholinergic systems; role of vasopressin

Participation of central cholinergic system in the effects of intracerebroventricular (i.c.v.) injection of angiotensin II (Ang II) on blood pressure and heart rate was studied in conscious, freely moving rats. Ang II dose-dependently increased blood pressure and decreased heart rate. Both atropine and mecamylamine (i.c.v.) pretreatments prevented the cardiovascular effects of Ang II. Pretreatment with a vasopressin V1 antagonist also prevented the cardiovascular responses to Ang II. Our data suggest that the central pressor effect of Ang II is mediated in part by central acetylcholine via both muscarinic and nicotinic receptors, and vasopressin participates in this effect through V1 receptors.     

Central angiotensin II-induced pressor responses and neural activity in utero and hypothalamic angiotensin receptors in preterm ovine fetus

The central renin-angiotensin system is important in the control of blood pressure in the adult. However, few data exist about the in utero development of central angiotensin-mediated pressor responses. Our recent studies have shown that the application of ANG II into the fetal brain can increase blood pressure at near term. The present study determined fetal blood pressure and heart rate in response to a central application of ANG II in the chronically prepared preterm ovine fetus, determined the action sites marked by c-Fos expression in the fetal central pathways after intracerebroventricular injection of ANG II in utero, and determined angiotensin subtype 1 receptors in the fetal hypothalamus. Central injection of ANG II significantly increased fetal mean arterial pressure (MAP). Adjusted fetal MAP against amniotic pressure was also increased by ANG II. Fetal heart rate was subsequently decreased after the central administration of ANG II and/or the increase of blood pressure. ANG II induced c-Fos expression in the central putative cardiovascular area, the paraventricular nuclei in the brain sympathetic pathway. Application of ANG II also caused intense Fos immunoreactivity in the tractus solitarius nuclei in the hindbrain. In addition, intense angiotensin subtype 1 receptors were expressed in the hypothalamus at preterm. These data demonstrate that central ANG II-related pressor centers start to function as early as at preterm and suggest that the central angiotensin-related sympathetic pathway is likely intact in the control of blood pressure in utero.     

Mechanisms underlying the cardiovascular responses to intrathecal vasopressin administration in rats

Vasopressinergic pathways within the spinal cord have been implicated in the control of cardiovascular function. This study was undertaken to determine the mechanisms whereby intrathecally administered arginine vasopressin (AVP) increases blood pressure and heart rate in anesthetized rats. The cardiovascular responses to intrathecal AVP administration were significantly attenuated after intravenous administration of the ganglionic blocking agent, chlorisondamine chloride, as were the pressor responses following alpha-adrenergic receptor blockade with phentolamine and the heart rate responses following beta-receptor blockade with propranolol. Intrathecal administration of the V1 vasopressin receptor antagonist d(CH2)5Tyr(Me)AVP completely blocked the cardiovascular responses to intrathecal AVP injections, but did not significantly alter the responses to intrathecal substance P injections. There was no evidence for the involvement of the renin-angiotensin system in the pressor responses to intrathecal AVP, as (i) an angiotensin II receptor blocking agent, [Sar1, Val5, Ala8]angiotensin, failed to significantly alter the responses to intrathecal AVP, and (ii) plasma renin levels did not change following administration of the peptide. Intrathecal injections of [3H]AVP suggest that only small amounts of the peptide may cross into the plasma during the time in which the cardiovascular variables are changing. These data provide evidence that intrathecally administered AVP discretely activates the sympathetic outflow to the heart and vasculature, and confirm the neurally mediated nature of the response.     

Changes in central and peripheral renin-angiotensin system after furosemide injection

To examine the effects of acute stimulation on the peripheral and central renin-angiotensin system, simultaneous sampling of blood and cerebrospinal fluid (CSF) for measurements of plasma renin activity (PRA), plasma angiotensin I-immunoreactivity (PAng I-ir), plasma angiotensin II-immunoreactivity (PAng II-ir), plasma angiotensinogen and cerebrospinal fluid angiotensin II-ir (CSF Ang II-ir) and CSF angiotensinogen was carried out following intravenous injection of furosemide (5 mg/kg) in conscious dogs. Administration of furosemide induced marked increases in PRA, Ang I-ir, PAng II-ir and CSF Ang II-ir, however, neither plasma nor CSF angiotensinogen was changed. Furthermore, a relatively large dose (20 mg/kg/min) of intravenously infused synthetic Ang II for 20 min produced a five-fold increase in PAng II-ir compared with no significant increase in CSF Ang II-ir. In spite of significant suppression of PRA and PAng I-ir, there were no significant changes in either plasma or CSF angiotensinogen. These results primarily suggest that the peripheral and the brain renin-angiotensin systems may be linked and that acute changes in the peripheral renin-angiotensin system do not alter either plasma or CSF angiotensinogen.     

Central interaction of the brain atrial natriuretic polypeptide (ANP) system and the brain renin-angiotensin system in ANP secretion from heart--evidence for possible brain-heart axis

To elucidate the involvement of the brain renin-angiotensin system and the brain atrial natriuretic polypeptide (ANP) system in the regulation of ANP secretion from the heart, the effects of intracerebroventricular administration of angiotensin II and ANP on the plasma ANP level were examined in conscious unrestrained rats. The intracerebroventricular administration of angiotensin II at doses of 100 ng and 1 microgram significantly enhanced ANP secretion induced by volume-loading with 3-mL saline infusion (peak values of the plasma ANP level: control, 220 +/- 57 pg/mL; 100 ng angiotensin II, 1110 +/- 320 pg/mL, p less than 0.01; 1 microgram angiotensin II, 1055 +/- 60 pg/mL, p less than 0.01). The intracerebroventricular injection of angiotensin II at the same doses alone had no significant effect on the basal plasma ANP level. The enhancing effect of central angiotensin II on ANP secretion induced by volume-loading was significantly attenuated by pretreatment with the intravenous administration of the V1-receptor antagonist of vasopressin or with the intracerebroventricular administration of phentolamine. The intracerebroventricular administration of alpha-rANP(4-28) (5 micrograms) had no significant influence on the basal plasma ANP level; however, it significantly attenuated central angiotensin II potentiating effect of volume-loading induced ANP secretion. These results indicate that the brain renin-angiotensin system regulates ANP secretion via the stimulation of vasopressin secretion and (or) via the activation of the central alpha-adrenergic neural pathway, and that the brain ANP system interacts with the brain renin-angiotensin system in the central modulation of ANP secretion from the heart.(ABSTRACT TRUNCATED AT 250 WORDS)     

Hypotensive activity of TCV-116, a newly developed angiotensin II receptor antagonist, in spontaneously hypertensive rats.

TCV-116, a recently developed angiotensin II (Ang II) receptor antagonist, was administered orally (1 mg/kg per day) to 10-week-old spontaneously hypertensive rats (SHR) for 2 weeks. Blood pressure and plasma components of the renin-angiotensin-aldosterone system were determined in these rats. TCV-116 produced a marked reduction in blood pressure without altering heart rate. Whereas plasma renin concentration (PRC), angiotensin I (Ang I) and angiotensin II (Ang II) all were significantly increased, plasma aldosterone was decreased by approximately 70% compared with control animals. These results not only indicate therapeutic efficacy of this agent in the chronic treatment of human hypertension, but support also the concept that the renin-angiotensin system plays an important role in the control of blood pressure in this animal model of human essential hypertension.     

Central cholinergic signal-mediated neuroendocrine regulation of vasopressin and oxytocin in ovine fetuses

Background  

The hypothalamic-neurohypophysial system plays a fundamental role in the maintenance of body fluid homeostasis by secreting arginine vasopressin (AVP) and oxytocin (OT) in response to a variety of signals, including osmotic and nonosmotic stimuli. It is well established that central cholinergic mechanisms are critical in the regulation of cardiovascular responses and maintenance of body fluid homeostasis in adults. Our recent study demonstrated that intracerebroventricular (i.c.v.) injection of carbachol elicited an increase of blood pressure in the near-term ovine fetuses. However, in utero development of brain cholinergic mechanisms in the regulation of the hypothalamic neuropeptides is largely unknown. This study investigated AVP and OT neural activation in the fetal hypothalamus induced by central carbachol.     

The ratio of plasma bioactive to immunoreactive ACTH-like activity increases with gestational age in the fetal lamb.

The fetal ovine pituitary-adrenal axis plays an important role in the timing of parturition, in fetal lung maturation, and in fetal and neonatal responses to stress. While the ovine pituitary during the last third of gestation (term = 145 days) is capable of secreting immunoreactive ACTH (iACTH) in response to various stimuli, plasma cortisol levels frequently do not reflect the rise in plasma ACTH. Therefore, we examined the relationship between plasma iACTH and steroidogenic ACTH-like activity (bACTH) in a group of immature fetal lambs (Group I: gestational age = 97 +/- 2 days, mean +/- SEM, n = 16) and a group of near-term fetuses (Group II: gestational age = 136 +/- 1 days, n = 13) following acute exteriorization. Plasma iACTH was determined by RIA. Plasma bACTH was determined by the ability of glass-extracted material to stimulate corticosterone (B) production in an acutely dispersed rat adrenal bioassay. Plasma iACTH and bACTH levels varied among animals within age groups, with iACTH tending to be higher in immature fetal lambs (Group I) than near-term lambs (Group II) and bACTH being higher (P < 0.05) near term than earlier (Group I: iACTH = 807 +/- 273 pg/ml, bACTH = 173 +/- 44 pg/ml; Group II: iACTH = 405 +/- 85 pg/ml, bACTH = 371 +/- 96 pg/ml). The proportion of iACTH that had biologic activity (e.g. B/I ratio) was significantly greater in the older than in the younger fetuses (Group II: B/I = 0.862 +/- 0.109; Group I: B/I = 0.462 +/- 0.105 P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)     

Interrelations between vasopressin and the renin-angiotensin system.

The interrelationships between vasopressin and the renin-angiotensin system are reviewed. Vasopressin can inhibit the release of renin by the kidney. This effect can occur at physiological plasma concentrations of vasopressin. Centrally administered angiotensin II can stimulate the release of vasopressin, a response that may be partially mediated by brain prostaglandins. The significance of this action of angiotensin II depends on whether there is an effective brain renin-angiotensin system and on whether peripherally generated or administered angiotensin can reach sites in the brain where it can act on vasopressin release. Peripherally administered angiotensin II can under certain, but not all, conditions stimulate vasopressin release. Peripheral angiotensin II can also potentiate the vasopressin response to an osmotic stimulus and to dehydration, but has little effect the release of vasopressin and renin, there is a failure to demonstrate any correlation between the two. Blockade of the renin-angiotensin system fails to modify the vasopressin response to a reduction in blood volume. In conclusion, the physiological significance of the interactions between the vasopressin and the renin-angiotensin system is not as yet clearly established.     

Indomethacin reversal of ethanol-induced suppression of ovine fetal breathing movements and relationship to prostaglandin E2

The effects of indomethacin on the ethanol-induced suppression of fetal breathing movements and fetal arterial plasma and cerebrospinal fluid (CSF) PGE2 concentrations and maternal arterial plasma PGE2 concentration were determined in the near-term fetal lamb. Eight conscious instrumented pregnant ewes (between 130 and 133 days of gestation; term, 147 days) received 1-h maternal intravenous infusion of 1 g ethanol/kg total body weight, and the fetus received 6-h intravenous infusion of indomethacin (1 mg/h per kg fetal body weight) commencing 30 min later. Serial fetal and maternal arterial blood samples (n = 8) and fetal CSF samples (n = 5) were collected at selected times throughout the 12-h study for the determination of PGE2 concentration. Fetal breathing movements were monitored continuously throughout the experimental period. Maternal ethanol infusion resulted in initial suppression (P less than 0.05) of fetal breathing movements for 2 h below pretreatment value, followed by a rapid increase in the incidence of fetal breathing movements after the onset of fetal indomethacin treatment. Fetal and maternal plasma PGE2 concentrations and fetal CSF PGE2 concentration were increased (P less than 0.05) above the pre-infusion value during the administration of ethanol and 1 h thereafter. Fetal indomethacin treatment suppressed (P less than 0.05) to undetectable levels fetal plasma and CSF PGE2 concentrations, which then became similar (P greater than 0.05) to pretreatment by 12 h. There was a positive correlation between fetal plasma and CSF PGE2 concentrations. There was an inverse correlation between the incidence of fetal breathing movements and fetal CSF PGE2 concentration.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of i.c.v. losartan on the angiotensin II-mediated vasopressin release and hypothalamic fos expression in near-term ovine fetuses

Our previous studies have shown that central administration of angiotensin (ANG II) causes arginine vasopressin (AVP) release in the fetus at 70-90% gestation. This is evidence that the hypothalamic-neurohypophysial system is relatively mature before birth. However, few data exist regarding central ANG receptor mechanisms-mediated AVP response during fetal life. To determine roles of brain ANG receptor subtypes in this response, AT1 and AT2 receptor antagonists, losartan and PD123319, were investigated in the brain in chronically prepared ovine fetuses at the last third of gestation. Application of losartan intracerebroventricularly (i.c.v.) at 0.5 mg/kg suppressed central ANG II-stimulated plasma AVP release. Losartan at 5 mg/kg (i.c.v.) demonstrated a significant enhancement of AVP increase to i.c.v. ANG II. Associated with the increase of plasma vasopressin levels, c-fos expression in the hypothalamic neurons was significantly different between the low and high doses of losartan. The low dose losartan markedly reduced the dual immunoreactivity for FOS and AVP in the supraoptic nuclei and paraventricular nuclei after i.c.v. ANG II, whereas the high dose losartan together with ANG II, significantly increased the co-localization of positive FOS in the AVP-containing neurons than that induced by i.c.v. ANG II alone. Central ANG II induced fetal plasma vasopressin increase was not altered by PD123319. The data suggest that losartan in the fetal brain has remarkably different effects based on the doses administrated on central ANG II-related neuroendocrine effects at the late gestation, and that the AT1 mechanism is critical in the regulation of fetal body fluid homeostasis related to plasma AVP levels.     

Angiotensin-(1--7) in the ovine fetus

In the adult animal, ANG-(1-7) may counterbalance some effects of ANG II. Its effects in the fetus are unknown. Basal ANG-(1-7), ANG I, ANG II, and renin concentrations were measured in plasma from ovine fetuses and their mothers (n = 10) at 111 days of gestation. In the fetus, concentrations of ANG I, ANG-(1-7), and ANG II were 86 +/- 21, 13 +/- 2, and 14 +/- 2 fmol/ml, respectively. In the ewe, concentrations of ANG I were significantly lower (20 +/- 4 fmol/ml, P < 0.05) as were concentrations of ANG-(1-7) (2.9 +/- 0.6 fmol/ml), whereas ANG II concentrations were not different (10 +/- 1 fmol/ml). Plasma renin concentrations were higher in the fetus (4.8 +/- 1.1 pmol ANG I x ml(-1) x h(-1)) than in the ewe (0.9 +/- 0.2 pmol x ml(-1) x h(-1), P < 0.05). Infusion of ANG-(1-7) (approximately 9 microg/h) for a 3-day period caused a significant increase in plasma concentrations of ANG-(1-7) reaching a maximum of 448 +/- 146 fmol/ml on day 3 of infusion. Plasma levels of ANG I and II as well as renin were unchanged by the infusion. Urine flow rate, glomerular filtration rate, and fetal arterial blood pressure did not change and were not different than values in fetuses receiving a saline infusion for 3 days (n = 5). However, the osmolality of amniotic and allantoic fluid was significantly higher in fetuses that received ANG-(1-7). Also, compared with the saline-infused animals, mRNA expression levels of renin, the AT(1) receptor, and AT(2) receptor were elevated in kidneys of fetuses that received infusions of ANG-(1-7). Infusion of an ANG-(1-7) antagonist ([D-Ala(7)]-ANG-(1-7), 20 microg/h) for 3 days had no effect on fetal blood pressure or renal function. In conclusion, although infusion of ANG-(1-7) did not affect fetal urine flow rate, glomerular filtration rate, or blood pressure, changes in fetal fluids and gene expression indicate that ANG-(1-7) may play a role in the fetal kidney.     

Fetal functional capabilities in response to maternal hypertonicity associated with altered central and peripheral angiotensinogen mRNA in rats

Although a number of studies have shown neural, hormonal, and behavioral capabilities in the control of body fluid regulation under conditions of dehydration in adults, limited information is available on the development of fetal functional abilities in response to osmotic challenge in rats. This study was performed to investigate the influence of maternal hypertonicity on fetal osmoregulatory capabilities at late gestational time in rats. Maternal and fetal plasma osmolality and blood sodium levels were determined and compared at continuous time points from 0.5 to 9h following maternal injection of hypertonic NaCl. Subcutaneous administration of hypertonic saline evoked a rise in plasma osmolality and sodium concentrations in maternal rats and fetuses associated with an up-regulation in angiotensinogen gene mRNA in the fetal liver and down-regulation of the same gene in the fetal brain. The increased levels of fetal blood osmolality and sodium were less than that in their mothers, and the fetus took less time to balance the enhanced osmolality and sodium concentrations. The results suggest that there may exist additional mechanisms in utero at near-term in protecting fetuses from hypertonic challenge. In addition, molecular results in the present study provide new data on fetal angiotensinogen gene expressed differently in the liver and brain under the same condition of prenatal salt loading, indicating osmotic signals of intracellular dehydration related to an acute increase in angiotensinogen mRNA in the fetal liver, and subsequent decrease in angiotensinogen mRNA levels in the fetal brain.     

Osmotic regulation of prolactin secretion in the fetal sheep

The functions of prolactin in the fetus remain speculative. No obvious physiological role has been found for the prolactin present in the fetal or maternal plasma and amniotic fluid compartments. The aim of the present study was to investigate changes in fetal plasma prolactin following intracerebroventricular (i.c.r.) administration to the fetus of artificial cerebrospinal fluid of different tonicities. A lateral ventricle catheter was placed in 11 fetuses at 107-128 days of gestation. Either isotonic artificial cerebrospinal fluid (300 mOsm.1(-1);n = 9), 15% polyethylene glycol (340 mOsm.1(-1);n = 5), or 7% distilled water in isotonic artificial cerebrospinal fluid (270 mOsm.1(-1);n = 9) was infused i.c.v. at 35 mu1.min-1 for 240 min. At 180 min thyrotropin releasing hormone (TRH) was administered through a fetal a fetal jugular catheter. Fetal carotid arterial blood gases, plasma osmolality and concentrations of prolactin, vasopressin (AVP), and norepinephrine (NE) were measured. Administration of hypotonic artificial cerebrospinal fluid produced an increase in fetal plasma prolactin from 46.6 +/- 36 ng.ml-1 at 0 min to 83.3 +/- 49 ng.ml-1 at 180 min (mean +/- SEM; P less than 0.05). No changes in either AVP or NE were observed. Administration of hypertonic artificial cerebrospinal fluid produced a decrease in plasma prolactin from 85 +/- 57 at time 0 to 49 +/- 35 at 180 min (P less than 0.05). No changes in either AVP or NE were observed. Fetal plasma prolactin, AVP, and NE did not change during control infusion of isotonic artificial cerebrospinal fluid.(ABSTRACT TRUNCATED AT 250 WORDS)     

Angiotensinase and vasopressinase activities in hypothalamus, plasma, and kidney after inhibition of angiotensin-converting enzyme: basis for a new working hypothesis

Reducing angiotensin II (Ang II) production via angiotensin-converting enzyme (ACE) inhibitors is a key approach for the treatment of hypertension. However, these inhibitors may also affect other enzymes, such as angiotensinases and vasopressinase, responsible for the metabolism of other peptides also involved in blood pressure control, such as Ang 2-10, Ang III, Ang IV, and vasopressin. We analyzed the activity of these enzymes in the hypothalamus, plasma, and kidney of normotensive adult male rats after inhibition of ACE with captopril. Aspartyl- (AspAP), glutamyl- (GluAP), alanyl- (AlaAP) and cystinyl-aminopeptidase (CysAP) activities were measured fluorimetrically using arylamides as substrates. Systolic blood pressure (SBP), water intake, and urine flow were also measured. Captopril reduced SBP and increased urine flow. In the hypothalamus, GluAP and AspAP increased, without significant changes in either AlaAP or CysAP. In contrast with effects in plasma, GluAP was unaffected, AspAP decreased, while AlaAP and CysAP increased. In the kidney, enzymatic activities did not change in the cortex, but decreased in the medulla. These data suggest that after ACE inhibition, the metabolism of Ang I in hypothalamus may lead mainly to Ang 2-10 formation. In plasma, the results suggest an increased formation of Ang IV together with increased vasopressinase activity. In the kidney, there is a reduction of vasopressinase activity in the medulla, suggesting a functional reduction of vasopressin in this location. The present data suggest the existence of alternative pathways in addition to ACE inhibition that might be involved in reducing BP after captopril treatment.     

125I[Sar(1)Ile(8)] angiotensin II has a different affinity for AT(1) and AT(2) receptor subtypes in ovine tissues

Iodinated angiotensin II (Ang II) and its analogues are often assumed to have equal affinities for AT(1) and AT(2) receptor subtypes. However, using saturation and competition binding assays in several tissues from pregnant, nonpregnant, and fetal sheep, we found the affinity of 125I[Sar(1)Ile(8)] Ang II for Ang II receptors was different (P<0.05) between tissue types. The dissociation constants (Kd) and half maximal displacements of [Sar(1)Ile(8)] Ang II (Sar IC(50)) were directly related (P<0.05) to proportions of AT(1) receptors, and inversely related (P<0.05) to proportions of AT(2) receptors in tissues from all groups combined, in tissues from individual groups (pregnant, nonpregnant or fetal), and in some individual tissues (uterine arteries and aortae). This suggests that 125I[Sar(1)Ile(8)] Ang II has a different affinity for AT(1) and AT(2) receptors in ovine tissues. The Kds of 125I[Sar(1)Ile(8)] Ang II for "pure" populations of AT(1) and AT(2) receptors were 1.2 and 0.3 nM, respectively, i.e. affinity was four-fold higher for AT(2) receptors. We corrected the measured proportions of the receptor subtypes using their fractional occupancies. In tissues which contained at least 10% of each receptor subtype, the corrected proportions were significantly altered (P<0.05), even in some tissues, to the extent of being reversed.