Attenuation of the central hypertonic NaCl pressor response by angiotensin II inhibition

Sensitivity of pressor responses to central hypertonic saline is greatly enhanced even in pre-hypertensive spontaneously hypertensive rats

It has been suggested that intracerebroventricular injection of hypertonic saline mimics the effects of a high salt diet in spontaneously hypertensive rats (SHR), a genetic model of hypertension. Intracerebroventricular injection of hypertonic saline produces an increase in blood pressure and the pressor response to hypertonic saline is enhanced in adult hypertensive SHR. In this study, we examined whether the intracerebroventricular hypertonic saline-induced pressor response is enhanced even in pre-hypertensive SHR. The basal mean blood pressure was almost the same in 4-week-old SHR and age-matched Wistar Kyoto rats (WKY), whereas it was greater in 15-16-week-old SHR than in age-matched WKY. Intracerebroventricular injection of hypertonic saline (10 microl of 230 mM NaCl) produced an increase in blood pressure in both 4-week-old and 15-16-week-old SHR, whereas it did not affect blood pressure in both age-matched WKY. Intracerebroventricular injection of hypertonic saline (10 microl of 260 mM NaCl) produced an increase in blood pressure in all rats but the pressor response was greater in both 4-week-old and 15-16-week-old SHR than in respective age-matched WKY. Intracerebroventricular injection of Phe-Met-Arg-Phe amide (FMRF), an FMRF-inducible sodium channel activator, produced an increase in blood pressure in all rats but the pressor response was greater in SHR than in WKY at both ages. These findings indicate that the sensitivities of pressor responses to intracerebroventricular hypertonic saline and FMRF are enhanced not only in hypertensive but also in pre-hypertensive SHR.

Central injection of hypertonic saline activates angiotensin II-sensitive neurons in the anterior hypothalamic area of rats

We have previously reported that microinjection of angiotensin II into the anterior hypothalamic area (AHA) produces pressor responses and that angiotensin II-sensitive neurons in the AHA are tonically activated by endogenous angiotensins in rats. Central injection of hypertonic saline causes pressor responses via release of angiotensins in brain. In this study, we examined whether angiotensin II-sensitive neurons in the AHA are responsive to intracerebroventricular injection of hypertonic saline and whether endogenous angiotensins in the AHA are involved in the central hypertonic saline-induced pressor response. Male Wistar rats were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Intraventricular injection of hypertonic saline increased the neural activity of angiotensin II-sensitive neurons, whereas pressure application of hypertonic saline onto angiotensin II-sensitive neurons themselves did not affect their neural activities. The intraventricular hypertonic saline-induced increase of unit activity of AHA neurons was inhibited by pressure application of the angiotensin AT1 receptor antagonist losartan onto the same neurons. The hypertonic saline-induced increase of unit firing was also blocked by intraventricular injection of the amiloride-sensitive sodium channel blocker benzamil. In conscious rats, intraventricular injection of hypertonic saline produced pressor responses, and the hypertonic saline-induced pressor response was inhibited by bilateral microinjection of losartan into the AHA. Repeated intraventricular injection of hypertonic saline caused an increase in the release of angiotensins in the AHA of anesthetized rats. These findings indicate that intracerebroventricular injection of hypertonic saline increases neural activity of angiotensin II-sensitive neurons trans-synaptically via endogenous angiotensins in the AHA. In addition, these findings also indicate that the intracerebroventricular injection of hypertonic saline produces a pressor response at least partly via release of angiotensins in the AHA.

Brain atrial natriuretic peptide family abolishes cardiovascular haemodynamic alterations caused by hypertonic saline in rats

1. Regional haemodynamic alterations caused by hypertonic NaCl solution (Hi-Salt; 10%, 10 microL) injected intracerebroventricularly (i.c.v.) were investigated by using radioactive microspheres in anaesthetized rats. 2. Intracerebroventricular injections of Hi-Salt increased regional vascular resistance of visceral organs, including the kidney, and elevated plasma levels of vasopressin. 3. Intracerebroventricular pretreatment with TCV-11974 (50 micrograms/10 microL/nat), an angiotensin AT1 receptor antagonist, attenuated the pressor response and vasopressin release to subsequently injected Hi-Salt, but did not affect regional haemodynamic effects of i.c.v. Hi-Salt on vascular resistance. 4. In contrast, i.c.v. pretreatment with atrial natriuretic polypeptide (ANP) or type-C natriuretic polypeptide (CNP) almost completely abolished the haemodynamic changes and vasopressin release caused by i.c.v. Hi-Salt. 5. The present findings indicate that a natriuretic family in the brain may be involved to a great degree in the central regulation of salt-induced hypertension in rats, while brain angiotensin II is likely to participate only in vasopressin release.

Role of central angiotensinergic mechanism in shaking stress-induced ACTH and catecholamine secretion

The role of central angiotensin II (AII) in the shaking stress-induced adrenocorticotropic hormone (ACTH), plasma catecholamine secretion and pressor response were investigated using conscious rats. We also studied whether or not vasopressin (VP) is involved in the shaking stress-induced pressor response. The shaking stress caused significant elevations in plasma ACTH, catecholamine, and systolic blood pressure. Intra-third ventricular administration of the AII antagonist, Sar1, Ile8-angiotensin II (saralasin) significantly attenuated pressor response and plasma noradrenaline elevation but not plasma ACTH elevation. Pretreatment with the vascular-type VP receptor (V1) antagonist, d(CH2)5Tyr(Me)AVP, did not attenuate pressor response nor plasma catecholamine elevation. These results indicate that the central angiotensinergic pathway at least partly mediates the shaking stress-induced activation of the sympathetic nervous system without VP involvement, and that central AII does not mediate the ACTH secretion evoked by shaking stress.

Sodium and vasopressin modulation of renal sympathetic nerve activity

A series of correlative studies show that sympathetic renal nerve activity (RNA) may be influenced by the concentration of sodium (Na+) in the plasma and the cerebrospinal fluid (CSF). Further study of the sites and characteristics of the interaction between Na+ and the renal nerves was undertaken in anesthetized dogs. Cerebroventricular (i.c.v.) injections of hypertonic sodium chloride (NaCl) produced a sympathetic vasoconstrictor response associated with tachycardia, increases in plasma norepinephrine and vasopressin (AVP), and a decrease in the electroneurographic activity recorded from post-ganglionic renal sympathetic nerves. Both bilateral vagotomy and intravenous administration of an AVP antagonist prevented the decrease in RNA caused by i.c.v. hypertonic NaCl, without markedly reducing the magnitude of the pressor response. The same phenomenon, however, could not be duplicated by delivery of the AVP antagonist i.c.v. In another group of dogs, the pressor activity of NaCl injected into the cisterna magna was compared before and after surgical ablation of the area postrema in the dorsal medulla. Removal of this circumventricular organ attenuated the pressor effects of NaCl given into the cisterna magna, but not those produced by i.c.v. delivery of the injectate. The data suggest that acute increases in CSF Na+ cause a differential activation of the sympathetic nervous system mediated in part by structures at or near the area postrema. At this site, circulating AVP apparently augments the inhibitory input from vagal afferents on the preganglionic renal nerve neurons.

Brain renin angiotensin system contributes to the salt-induced enhancement of hypertension in SHR

The study was performed to determine whether the brain renin angiotensin system may contribute to the acceleration in hypertension following long-term salt loading in spontaneously hypertensive rats (SHR). Five weeks old SHR and normotensive Wistar-Kyoto (WKY) were given 1% NaCl solution or plain tap water as drinking for 7 weeks. The salt treatment exaggerated the development of hypertension in SHR, but did not change the blood pressure (BP) in WKY. The hypotensive actions of intracerebroventricular (ICV) captopril was greater in SHR treated with salt than in those without treatment, whereas ICV AII increased BP to a similar degree between salt and control SHR. In WKY, the effects of ICV captopril and AII were not altered by the salt loading. The increases in BP induced by ICV hypertonic saline were not different between the rats with and without saline drinking in either SHR or WKY. The intravenous (IV) hexamethonium led to a greater fall in BP in SHR treated with saline than in those without salt, while it tended to produce a smaller decrease in BP in WKY with salt overload than in those without loading. Both duration and magnitude of the depressor effects of IV captopril were reduced by the chronic saline treatment in SHR. The plasma renin concentration (PRC) in both SHR and WKY was significantly suppressed by the salt load. The present results suggest that long-term salt overload may result in the enhanced activity of brain renin angiotensin system, which could be responsible for the exaggerated development of hypertension in SHR. Our observations also provide further evidence that the central renin angiotensin system is independent of the peripheral system.

GABA agonists inhibit central sodium-induced vasopressin-dependent increases in arterial pressure

Previous studies have demonstrated that intraventricular (i.v.t.) administration of low doses of GABA agonists reduced the central pressor effects of cerebrospinal fluid (CSF) made hypertonic with sodium (Na). The following studies were designed to determine if GABA agonists acted to decrease the pressor response of Na through inhibition of the vasopressin-dependent pressor component. Following pretreatment with vascular vasopressin antagonist, the pressor response of i.v.t. administered Na was reduced approximately 60%. Hypophysectomy produced a similar reduction in the pressor response elicited by hypertonic CSF. These results indicate that vasopressin contributed to approximately 60% of the pressor response of Na. In order to generate a vasopressin-dependent pressor response, the sympathetic nervous system was eliminated with ganglionic blockade by chlorisondamine. The increase in arterial pressure produced by i.v.t. injection of hypertonic CSF was augmented after ganglionic blockade compared to untreated rats. The augmented pressor effect of i.v.t. administered Na was markedly reduced by the vascular vasopressin antagonist or by hypophysectomy. Therefore, the pressor effect of Na after ganglionic blockade was caused almost entirely by the pressor actions of arginine-vasopressin (AVP). This AVP-dependent pressor effect of i.v.t. injected Na in rats subjected to ganglionic blockade was reduced by pretreatment with 100 micrograms of GABA or 50 ng of the GABA agonist muscimol. These doses of GABA and muscimol have previously been shown to reduce the pressor response of i.v.t. administered Na in untreated rats. Thus, pretreatment with low doses of GABA agonists reduced the pressor effect of Na in part through inhibition of the vasopressin component of the pressor response. GABA pretreatment also antagonized the increase in plasma AVP levels produced by i.v.t. administered hypertonic CSF.

GABA inhibition of central angiotensin II and hypertonic CSF pressor responses

Angiotensin II (AII), hypertonic cerebrospinal fluid (CSF) and serotonin produced an increase in arterial pressure when administered intraventricularly (IVT) in conscious rats. Injection of 25 and 100 micrograms (IVT) of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA), while producing slight hypotension, reduced the pressor effect of centrally administered AII and hypertonic CSF, but not serotonin. IVT-administered muscimol, a potent GABA agonist, also attenuated the pressor effect of IVT AII and hypertonic CSF. Thus, in addition to the profound depressor effect of large doses of centrally administered GABA, very low doses (25-100 micrograms, IVT) of this amino acid can alter the pressor responses caused by IVT injection of AII and hypertonic CSF.