Independent receptors for pressor and drinking responses to central injections of angiotensin II and carbachol.

Angiotensin II and carbachol when injected in the brain ventricles of the rat produce similar responses of an increase in blood pressure and drinking behavior. The question of whether these effects are produced by independent receptors or via a cholinergic circuit is debatable for the drinking behavior and evidence is lacking for the blood pressure effect. We have used a chronic rat preparation for recording blood pressure and drinking at the same time during intraventricular injections (i.v.t.) of both angiotensin and carbachol and i.v.t. or intravenous infusions of appropriate antagonists. The results show that drinking and blood pressure response to angiotensin II can be blocked by P113 (500 ng 1.v.t.) an angiotensin antagonist; they are not blocked by atropine (10 mug i.v.t.) a cholinergic antagonist; carbachol effects, however, are not blocked by P113, but are totally blocked by atropine (10 mug i.v.t.), At high doses of atropine there is inhibition of both agents but this probably represents a general inhibition. The hormone and cholinomimetic administered together interact and both are inhibited by adrenergic stimulation. We conclude from these experiments that angiotensin and carbachol act upon independent receptors in the brain to produce blood pressure and drinking responses but at some point they share common, central effector pathways.     

The role of angiotensin, AT1 and AT2 receptors in the pressor, drinking and vasopressin responses to central angiotensin.

Angiotensin II (Ang II) given centrally produces an increase in blood pressure and motivation to drink. The physiological mechanisms that mediate the pressor response include release of vasopressin (AVP) and activation of the sympathetic nervous system. Using 2 new Ang II receptor antagonists, we were able to investigate the role of AT1 or AT2 receptors in mediating these effects. Adult male Sprague-Dawley rats were cannulated in the lateral ventricle and 5 days later catheterized in the carotid artery for blood pressure measurements. All experiments were carried out in conscious rats. Three treatments were given intraventricularly (i.v.t.), in 2 microliters artificial cerebrospinal fluid (ACSF) at 30 min intervals: (1) 50 ng Ang II, (2) 0.7 micrograms AT1 antagonist Losartan or 7.0 micrograms AT2 antagonist PD123177, followed by 50 ng Ang II, and (3) 50 ng Ang II, to test for recovery. Blood pressure and drinking measurements were recorded. Also, blood samples for assay of AVP were drawn at 1 or 3 min post-injection in 2 separate groups of rats. We found that both Losartan and PD123177 significantly reduced release of AVP to Ang II 1 min post-injection. Losartan significantly blocked the pressor response (P less than 0.001), while PD123177 had no significant effect. Drinking was also antagonized by Losartan (P less than 0.05) and reduced (n.s.) by PD123177. The results suggest that the pressor response to Ang II (i.v.t.) is predominantly AT1 mediated, while the drinking and AVP responses may be mediated by both receptor subtypes.     

Centrally regulated blood pressure response to vasoactive peptides is modulated by corticosterone

To investigate the role of brain glucocorticoid (GR) and mineralocorticoid receptors (MR) in centrally evoked blood pressure responses, the effects of intracerebroventricular (i.c.v.) administration of angiotensin II and vasopressin were studied in adrenalectomized rats with and without corticosterone or aldosterone replacement. Five groups were examined: (i) Adrenalectomy (ADX); (ii) ADX + a subcutaneously implanted 20-mg corticosterone pellet (low corticosterone); (iii) ADX + 100 mg corticosterone pellet (high corticosterone); (iv) ADX + 6 microg/24 h aldosterone via Alzet minipump (Aldo); and (v) Sham adrenalectomy (Sham). Pressor responses to 150 ng angiotensin II and 50 ng vasopressin i.c.v. were determined in freely moving rats using biotelemetry. The results show that, compared to sham rats, ADX rats showed significantly reduced pressor responses. This reduction of the pressor response to angiotensin II could be reversed and even further enhanced by replacement of the ADX rats with high corticosterone concentrations. In contrast, with aldosterone, a depressor type response was observed. Corticosterone replacement could not restore the pressor response to vasopressin. We conclude that the pressor response to centrally administered vasoactive substances is substantially attenuated by removal of the adrenals and that, in the case of angiotensin II, this is due to the lack of high concentrations of circulating corticosterone occupying both MR and GR. However, predominant MR occupancy appears to play an opposite role and attenuates the angiotensin II-induced pressor response.     

Injections of phentolamine into the anterior hypothalamus-preoptic area of rats blocks both pressor and drinking responses produced by central administration of angiotensin II

There are conflicting reports of a possible contribution of noradrenergic projections to the rostral hypothalamus to drinking and blood pressure regulation. The present study investigated the effects of injecting phentolamine into the anterior hypothalamus-preoptic region on drinking and blood pressure responses elicited by injecting angiotensin II into a lateral cerebral ventricle of the rat. Angiotensin II (250 ng or 25 ng) elicited water intakes averaging 9.25 +/- 0.52 ml and 4.35 +/- 0.44 ml respectively in 15 min with latencies of less than 3 min. Phentolamine, an alpha-adrenergic antagonist, injected into the rostral hypothalamus produced a dose-dependent reduction in water intake and number of laps taken accompanied by an increased latency to drink. In the urethane anaesthetized rat, angiotensin II produced significant increases in blood pressure. Injections of phentolamine into rostral hypothalamic sites in which drinking responses to angiotensin II were attenuated, also attenuated the pressor response to angiotensin II. These results indicate that alpha-adrenergic input to the rostral hypothalamus is involved in both the pressor and drinking responses elicited by central angiotensin II.     

Pressor response to CSF sodium in mice: mediation by a ouabain-like substance and renin-angiotensin system in the brain

Intracerebroventricular (i.c.v.) infusion of sodium in rats increases cerebrospinal fluid (CSF) [Na], mimicking the effects of a high salt diet in salt-sensitive strains and causing sympathetic hyperactivity and a pressor response that are mediated via both an endogenous brain ouabainlike substance (OLS) and the brain renin-angiotensin system (RAS). However, the concept that CSF sodium activates both the brain OLS and brain RAS to increase blood pressure has not been tested in any other species besides the rat. In the current study, it was established that continuous i.c.v. infusion of NaCl causes sustained increases in blood pressure and heart rate in both outbred (Swiss Webster, SW) and inbred (C57Bl/6) mouse strains. Subsequently, the mechanisms of the pressor effects were explored. In both SW and C57Bl/6, the i.c.v. administration of Fab fragments of an antibody with high affinity for ouabain and the OLS (Fab) abolished the pressor and tachycardic responses to i.c.v. sodium, as did the angiotensin II AT1 receptor antagonist losartan given i.c.v. In contrast, doses of NaCl, Fab and losartan that were effective i.c.v. were ineffective when given i.v. I.c.v. ouabain also caused the pressor and tachycardic responses, which were abolished by losartan (i.c.v.). In the reciprocal study, i.c.v. Fab had no effect on similar responses to i.c.v. angiotensin II. These studies demonstrate that the sustained blood pressure and heart rate responses caused by increases in CSF [Na] are mediated via both a brain OLS and the brain RAS. The RAS activation occurs downstream of the OLS effect.     

Sensitization to pressor effects by repeated central injections of vasopressin in conscious rats

Arginine vasopressin (AVP) injected intracerebroventricularly (i.c.v.) in the nanogram range elicits increases in mean arterial blood pressure (MAP), heart rate (HR) and efferent sympathetic nerve activity (SpNA) via central V1 AVP receptor stimulation. In this study in conscious rats we investigated, whether the cardiovascular and sympathetic responses can be augmented by repeated central applications of AVP, as has been previously shown for the convulsive responses to higher i.c.v. doses of the peptide. The AVP-induced pressor (0.1 and 1.0 ng) and the SpNA (0.1 ng) responses were significantly enhanced by a second AVP challenge 24 h after the first injection. With higher doses of the peptide (3 ng), the blood pressure responses were not different between two subsequent injections, but barrel rotation occurred in 21% of the animals upon the second challenge. The pressor responses to a threshold i.c.v. dose of 1 ng angiotensin II (ANG II) were not enhanced upon a second ANG II challenge. Our results demonstrate that AVP, unlike ANG II, can sensitize central mechanisms leading to increased MAP and SpNA responses.     

Functional role of brain AT1 and AT2 receptors in the central angiotensin II pressor response

Intracerebroventricular (i.c.v.) angiotensin II (ANG II) increase vascular resistance and elicits a pressor response characterized by sympathetic nervous system activation (SNS component) and increased vasopressin (VP) secretion (VP component). This study examines the role of brain AT1 and AT2 ANG II receptors in mediating the pressor and renal hemodynamic effects of i.c.v. ANG II in conscious Sprague-Dawley rats. Mean arterial pressure, heart rate and renal vascular resistance responses to i.c.v. ANG II (100 ng in 5 μl) were determined 10 min after i.c.v. injection of either the AT1 receptor antagonist, DuP 753 (1.0, 2.5, 5.0, 10.0 μg), the AT2 receptor ligand, PD 123319 (3.5 × [10⁻⁶, 10⁻⁴, 10⁻², 10⁰ μg), or both. In control rats, i.c.v. DuP 753 prevented the pressor response and the increase in renal vascular resistance that occurred following i.c.v. ANG II in a dose-dependent manner (P < 0.05), while i.c.v. PD 123319 was without affect. When the VP- and SNS components were studied individually, by preventing the SNS component with intravenous (i.v.) chlorisondamine or the VP component with a V1 receptor antagonist (i.v.) similar results were obtained; DuP 753 prevented the SNS component and significantly reduced the VP component. These results indicate that both central ANG II pressor components are mediated primarily by brain AT1 receptors. However, doses of DuP 753 were more effective when combined with 3.5 μg of PD 123319 than when given alone (P < 0.05), suggesting that the pressor effects of i.c.v. ANG II may involve activation of multiple ANG II receptor subtypes.     

Angiotensin II evokes noradrenaline release from the paraventricular nucleus in conscious rats.

In vitro and in vivo experiments have provided indirect evidence that some of the central actions of angiotensin II (ANG II) involve catecholaminergic pathways in the brain. In this study in conscious rats we investigated the effect of stimulation of periventricular ANG II receptors on blood pressure and on catecholamine release (microdialysis and HPLC) from the paraventricular nucleus (PVN), a hypothalamic area thought to be instrumental in the central pressor responses to ANG II through the release of vasopressin into the blood. Intracerebroventricular (i.c.v.) injections of pressor doses of ANG II (1 ng and 100 ng) led to significant dose-dependent increases of the noradrenaline (NA) release in the PVN (1 ng: 30.95 +/- 6.01 to 47.38 +/- 6.79 pg/sample, P less than or equal to 0.01; 100 ng: 32.93 +/- 5.38 to 73.18 +/- 11.4 pg/sample, P less than or equal to 0.01). These changes coincided in extent and duration with the respective pressor responses. A subpressor dose of ANG II (100 pg) did not release catecholamines from the PVN. Dopamine (DA) and the NA and DA, metabolites 3,4-dihydroxyphenylethylglycol and 3,4-dihydroxyphenylacetic acid, were not influenced by i.c.v. injections of ANG II at any dose. Pretreatment with the novel non-peptide ANG II-AT 1 receptor antagonist DuP 753 (5 micrograms, i.c.v.) abolished the effect of 100 ng ANG II on blood pressure and on NA release. Our results show for the first time in vivo that stimulation of periventricular ANG II-AT 1 receptors induces a selective NA release in the PVN. They further support the hypothesis that ANG II engages a noradrenergic pathway in the PVN to release vasopressin.     

Effects of intracerebroventricular losartan on angiotensin II-mediated pressor responses and c-fos expression in near-term ovine fetus

The renin-angiotensin system plays an important role in cardiovascular control. Intracerebroventricular (i.c.v.) angiotensin (ANG) II causes a reliable pressor response in the fetus at 90% gestation. To determine the roles of brain AT1 and AT2 receptors in this response, the effects of the central AT1 and AT2 receptor antagonists losartan and PD123319 were investigated in chronically prepared near-term ovine fetuses. Losartan at 0.5 mg/kg (i.c.v.) abolished central ANG II-induced pressor responses. High-dose losartan (5 mg/kg, i.c.v.) showed a potentiation of the pressor response to i.c.v. ANG II, accompanied by bradycardia. Associated with the pressor responses, c-fos expression in the cardiovascular controlling areas was significantly different between the low and high doses of losartan. These areas included the subfornical organ, median preoptic nucleus, organum vasculosum of the lamina terminalis, and paraventricular nuclei in the forebrain, and the tractus solitarius nuclei, lateral parabrachial nuclei in the hindbrain. Low-dose losartan markedly reduced c-fos in these areas after i.c.v. ANG II, while the high-dose losartan together with ANG II elicited a much stronger FOS-immunoreactivity in these areas than that induced by i.c.v. ANG II alone. This is a novel finding, that c-fos expression in the brain can be both activated and inhibited under the same condition. Central ANG II-induced fetal pressor responses were not altered by PD123319 (0.8 mg/kg). These results indicate that i.c.v. losartan at a high and a low dose has strikingly different effects on central ANG II-induced pressor responses in fetuses at late gestation, and that the AT1 mechanism plays an important role in fetal cardiovascular regulation.     

Effects of lidocaine injections into the lateral parabrachial nucleus on dipsogenic and pressor responses to central angiotensin II in rats

This study investigated the effects of bilateral injections of the local anesthetic, lidocaine, into the lateral parabrachial nucleus (LPBN) on the dipsogenic and pressor responses induced by intracerebroventricular (i.c.v.) injection of angiotensin II (ANG II). Centrally injected ANG II (50 ng/1 μl) induced water intake (10.2 ± 0.8 ml/h) and pressor responses (22 ± 1mmHg). Prior bilateral injection of 10% lidocaine (200 nl) into the LPBN increased the water intake (14.2 ± 1.4 ml/h), but did not change the pressor response (17 ± 1 mmHg) to i.c.v. ANG II. Lidocaine alone injected into the LPBN also induced a pressor response (23 ± 3 mmHg). These results showing that bilateral LPBN injection of lidocaine increase water intake induced by i.c.v. ANG II are consistent with electrolytic and neurotoxic lesion studies and suggest that the LPBN is associated with inhibitory mechanisms controlling water intake induced by ANG II. These results also provides evidence that it is feasible to reversibly anesthetize this brain area to facilitate fluid-related ingestive behavior.     

Functional roles of brain AT1 and AT2 receptors in the central angiotensin II pressor response in conscious young spontaneously hypertensive rats

Areas of adult rat brain that mediate the cardiovascular effects of central angiotensin II (ANG II) predominately express AT₁ ANG II receptors. In contrast, AT₂ receptor expression in young rats is transiently increased, reaching a maximum during the first few weeks of life. This study was designed to determine the roles of brain AT₁ and AT₂ receptors in mediating the central pressor effects of ANG II in young (4-week-old) conscious spontaneously hypertensive rats (SHR). Mean arterial pressure responses to intracerebroventricular (i.c.v.) ANG II (100 ng in 5 μl) were determined 10 minutes after i.c.v. injection of either the AT₁ receptor antagonist Losartan (1.0, 2.5, 5.0, and 10.0 μg), the AT₂ receptor ligand PD 123319 (3.5 × [10⁻⁶, 10⁻⁴, 10⁻², 10⁰] μg), or both. In control rats, i.c.v. Losartan prevented the pressor response to i.c.v. ANG II in a dose-dependent manner (P < 0.05), while i.c.v. PD 123319 alone was without effect. In other animals, pressor responses caused by i.c.v. ANG II-induced vasopressin secretion (VP-component) and sympathetic nervous system activation (SNS-component) were studied individually, with similar result; Losartan prevented the SNS-component, but reduced the VP-component by only 45%, indicating that both pressor components involve AT₁ receptor activation. However, doses of Losartan were more effective when combined with 3.5 μg of PD 123319 than when given alone (P < 0.05); nearly eliminating the VP-component. These results suggest that i.c.v. ANG II-induced pressor effects may involve activation of multiple receptor subtypes.     

Heightened blood pressure and drinking responsiveness to intracerebroventricularly applied angiotensins in the spontaneously hypertensive rat

The effects of bolus intracerebroventricular (i.c.v.) injections of angiotensin II (AII) and angiotensin III (AIII) on blood pressure and water consumption were investigated in Okamoto-Aoki spontaneously hypertensive rats (SHR), and Wistar-Kyoto (WKY) and Sprague-Dawley (SD) normotensive controls. Heightened sensitivity to i.c.v. administered AII and AIII was observed in the SHR as compared with WKY and SD strains for both pressor and drinking responses. The results are consistent with the notion that the SHR has a genetic defect that directly perturbs central angiotensinergic transmission. Two types of defects appear plausible, an alteration in the central angiotensin receptor and its associated transduction system and/or a decrease in the efficiency of signal termination. The present results are interpreted to primarily support the second possibility that a dysfunction in central aminopeptidase activity results in an extended life expectancy of angiotensin, and perhaps other peptides, that contribute to the hypersensitivity seen in the SHR.     

Role of prostaglandin,endothelin and sympathetic nervous system on the L-NAME-induced pressor responses in spontaneously hypertensive rats

We tested the hypothesis that in spontaneously hypertensive rat (SHR) NO produced centrally influences the resting arterial blood pressure by attenuating mechanisms involving prostaglandins, angiotensin II, endothelin and sympathetic nervous system. L-NAME (200 micro g/5 micro l), an inhibitor of NO synthase, administered intracerebroventricularly (i.c.v.) to awake and freely moving rats increased mean arterial blood pressure (MABP) in a biphasic pattern: an early transient increase within 1 min and a late prolonged response starting at 45 min and persisting for the duration of experiment (180 min). The two pressor responses involve different neurochemical mechanisms and, based on their latencies, they appear to reflect different anatomical sites of action of L-NAME. The late, but not the early pressor response, was prevented by pretreatment with chlorisondamine (2.5 mg/kg, i.v.), a ganglionic blocker, indicating its dependence on the sympathetic nervous system. Both pressor responses were abolished by i.c.v. pretreatment with indomethacin (200 micro g/5 micro l, i.c.v.), an inhibitor of cyclo-oxygenase, showing that they are mediated by prostaglandin(s). In contrast, losartan (25 micro g/5 micro l), an angiotensin II AT(1) receptor antagonist, had no effect. The initial pressor response was also attenuated by pretreatment with the endothelin ET(A)/ET(B) receptor antagonist, PD 145065 (48 micro g/2 micro l, i.c.v.). Intravenous pretreatment with another ET(A)/ET(B) receptor antagonist, L-754,142 (15 mg/kg as a bolus+15 mg/kg/h for 180 min), however, attenuated both responses to L-NAME. It is possible that L-754,142 crossed the blood-brain barrier and blocked, in addition, central ET(A)/ET(B) receptors. These studies show that NO synthesized in the brain attenuates pressor mechanisms involving prostaglandin, endothelin and sympathetic nervous system, but not angiotensin II, to modulate resting arterial blood pressure.     

Lesions of the diagonal band of broca enhance drinking in the rat

This study examined the role of the diagonal band of Broca (DBB) in drinking behaviour and vasopressin release. Adult male rats were anaesthetized (pentobarbital 50 mg/kg) and received DBB injections of either ibotenic acid (0.5 microl of 5 micro g/ microl) or vehicle (0.5 microl of phosphate-buffered saline). Although baseline drinking and urine output were not affected, drinking to 30% polyethylene glycol (MW 8000; 1 ml/100 g s.c.) and angiotensin II (0, 1.5 and 3.0 mg/kg s.c.) were significantly increased in ibotenic acid in phosphate-buffered saline (DBBX) rats. Drinking to hypertonic saline (0.9, 4 and 6%; 1 ml/100 g), and water deprivation were not significantly affected. DBBX rats had significantly lower basal heart rates than controls but the cardiovascular responses to infusions of angiotensin II (100 ng/kg/min i.v. for 45 min) were not affected. DBBX rats had significantly higher basal vasopressin, but angiotensin-stimulated vasopressin release was not significantly different. Although the DBB is not involved in basal water intake, it is involved in dipsogenic responses to hypovolemic stimuli and possibly basal autonomic function and basal vasopressin release.     

Angiotensin binding and pressor activity in the rat ventricular system and midbrain

Specific angiotensin II (AII) binding capacity was investigated in regions of rat brain thought to be involved in the central pressor effects of AII. In the midbrain specific angiotensin II binding capacity was principally localised in the superior colliculi. When the superior colliculi was divided into rostral and caudal portions, the caudal portion had a higher binding capacity than the rostral portion. In the hypothalamus AII binding capacity was only a third of that in the superior colliculi and was localised principally in the lateral hypothalamus. The medial hypothalamus and the preoptic recess each had about a quarter of the binding capacity of the lateral hypothalamus.To investigate whether AII binding capacity could be correlated with pressor activity AII was applied locally (local) to the surface of the superior colliculi (5 μg/5 μl saline) of rats with femoral artery cannulas and anaesthetised with pentobarbitone. These responses were compared with responses to the same dose given intraventricularly (IVT). When the surface of the midbrain was intact there was a pressor response to the local application not significantly different in magnitude and latency from the response to IVT AII. The midbrain was aspirated at the site of AII application and pressor responses to AII evoked during aspiration. As the midbrain lesion was extended the latency of the local pressor response was prolonged (P < 0.05) but its magnitude did not change. Both local and IVT AII became ineffective after extensive aspiration in 5 out of 9 rats. Histological examination of the brains of these 9 rats showed that the central pressor responses to AII depended on the integrity of a discrete region of the midbrain and periaqueductal gray which included the caudal region of the superior colliculi.     

Differential effects of aminopeptidase inhibitors on angiotensin-induced pressor responses

Recent iontophoretic data suggest that conversion of angiotensin II (AII) to angiotensin III (AIII) may be necessary before the peptide can activate central angiotensin-sensitive neurons. Furthermore, this conversion may be inhibited by the aminopeptidase A inhibitor, amastatin. In the present study we investigated the importance of aminopeptidase activity on central angiotensin-induced pressor responses. Intracerebroventricular (i.c.v.) pretreatment with amastatin, suppressed i.c.v. AII-induced pressor responses. Pretreatment with the aminopeptidase B inhibitor, bestatin, increased pressor responses to AIII. Pressor responses induced by the aminopeptidase-resistant analogue, [Sar1]angiotensin II, were not affected by pretreatment with angiotensin inhibitors. These results support the hypothesis that AII must be converted to AIII to be active in the brain.     

Amastatin potentiation of drinking induced by blood-borne angiotensin: evidence for mediation by endogenous brain angiotensin

Angiotensinergic synapses in the central nervous system (CNS) have been proposed to be involved in drinking induced by both intracerebroventricular (i.c.v.) and peripheral administration of angiotensins. In the present studies, we tested this hypothesis with i.c.v. application of amastatin, an aminopeptidase A inhibitor, to block peptide degradation. Potentiation of i.c.v. angiotensin II (Ang II)-induced drinking responses was observed when amastatin and Ang II were administered. Amastatin did not potentiate drinking to carbachol which demonstrates that the enhancement is specific to peptides. Centrally administered amastatin also potentiated drinking following systematic administration of Asn¹ angiotensin II. (Asn¹ Ang II). The results are consistent with the hypothesis that CNS angiotensin synapses are involved in the dipsogenic response that results from elevated levels of circulating angiotensin.     

Effects of drinking and central angiotensin II stimulation on organ blood flow in conscious rats

While the hemodynamic response pattern accompanying feeding behavior has been well characterized, there is less information about the hemodynamic changes associated with drinking. In the present study, we have measured organ blood flows in conscious, unrestrained rats during schedule-induced drinking behavior, using the tracer microsphere technique (diameter of spheres 15 +/- 3 microns; labels: 141Ce, 113Sn). In addition, we determined the hemodynamic response pattern following intracerebroventricular (i.c.v.) injection of 100 ng angiotensin II (ANG II) (a dose known to be dipsogenic) in rats that were not allowed to drink during the experiment. The hemodynamic responses during drinking behavior included (a) significant increases in blood flow through the kidney, stomach and small intestine, (b) a decrease in blood flow through skeletal muscle, and (c) no significant changes in the rest of the organs. ANG II i.c.v. elicited (a) significant decreases in blood flow through the kidney, stomach, small intestine and skin, (b) a significant increase in blood flow through the liver (hepatic artery), and (c) no significant changes in blood flow through the brain, heart, lung (bronchial arteries), colon, skeletal muscle (biceps) and testis. We conclude that spontaneous drinking behavior in rats is associated with a characteristic hemodynamic drinking response, which resembles a classical feeding reaction. In non-drinking rats the hemodynamic response pattern following ANG II i.c.v. was different from the drinking response, providing further evidence, that the behavioral and cardiovascular effects of the neuropeptide can be dissociated.     

NO and angiotensin II effects on blood pressure and fluid homeostasis

Angiotensin II (50 ng/5 microl) and L-NAME (250 microg/5 microl), an inhibitor of NO synthase (NOS), were administered intracerebroventricularly alone or in combination to conscious rats. Mean arterial blood pressure (MABP) increased reaching a peak within 5 min in all groups compared to controls treated with the vehicle, artificial CSF (5 microl). MABP returned to basal levels at 30 min after angiotensin II and remained stable for the following 90 min. In animals treated with L-NAME alone, after the initial pressor response, MABP declined but began to increase progressively from 30 min until the end of the experiment at 120 min. When administered with angiotensin II, however, the initial pressor response was prolonged. Angiotensin II-induced drinking was significantly attenuated by L-NAME. In control rats, inhibiting NOS elevated plasma levels of oxytocin and vasopressin but in angiotensin II-stimulated animals, only oxytocin was further elevated after L-NAME. Thus, NO formed centrally inhibits basal secretion of oxytocin and vasopressin as well as the resting blood pressure. During stimulation with angiotensin II, NO facilitates drinking, limits the pressor response and selectively inhibits oxytocin release.     

Cardiovascular effects of centrally injected melittin in hemorrhaged hypotensive rats: the investigation of peripheral mechanisms

We have previously shown that centrally injected melittin, a phospholipase A(2) (PLA(2)) activator, increases blood pressure and decreases heart rate in the normotensive conscious rats. In the current study we aimed to determine the cardiovascular effects of melittin in hemorrhaged hypotensive rats and to investigate the mediation of peripheral adrenergic, vasopressinergic and renin angiotensin system in the pressor effect of centrally administrated melittin in both normotensive and hypotensive conditions. Acute hypotensive hemorrhage was performed by withdrawing a total volume of 2.2ml of blood/100g body weight over a period of 10min. Melittin was injected intracerebroventricularly (i.c.v.) at the doses of 1.5microg, 3.0microg or 6.0microg after the stabilization period of hemorrhage procedure. We also repeated previous experiments by injecting melittin (1.5microg, 3.0microg or 6.0microg; i.c.v.) to the normotensive animals. Melittin caused dose- and time-dependent increases in mean arterial pressure (MAP) in normal and hypotensive conditions and decreases in heart rate (HR) in normotensive conscious animals. In hypotensive rats, melittin injected at the dose of 6.0microg completely restored the decrease in blood pressure. Plasma adrenaline, noradrenaline, vasopressin levels and renin activity increased after melittin (3.0microg; i.c.v) administration in normal conditions. Hemorrhage, itself, produced an increase in these plasma hormone levels and melittin (3.0microg; i.c.v.) caused additional increases in plasma adrenaline, noradrenaline, vasopressin levels and renin activity in hypotensive conditions. Intravenous pretreatments of rats with prazosin (0.5mg/kg), an alpha(1) adrenoceptor antagonist, [beta-mercapto-beta,beta-cyclopentamethylenepropionyl(1), O-Me-Tyr(2)-Arg(8)]-vasopressin (10microg/kg), a vasopressin V(1) receptor antagonist, or saralasin (250microg/kg), an angiotensin II receptor antagonist, partially blocked the pressor response to melittin (3.0microg; i.c.v.) in both normotensive and hypotensive conditions. Besides, the combined administration of these three antagonists before melittin completely abolished the pressor responses to drug in both conditions. Results show that centrally administered melittin, a PLA(2) activator, increases blood pressure and reverses hypotension in hemorrhagic shock. The increases in plasma adrenaline, noradrenaline, vasopressin levels and renin activity mediate the pressor responses to melittin in normal and hypotensive conditions.