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Central sympathetic stimulation produced by saline application into the nucleus tractus solitarii area of conscious rats
Neuroscience Letters, 92 (1988) 335 340
335
Elsevier Scientific Publishers Ireland Ltd.
NSL 05592
Central sympathetic stimulation produced by saline application into the nucleus tractus solitarii area of conscious rats Demetrios Vlahakos and Haralambos Gavras Department ~['Medicine, Boston City Hospital and Department ~f Medicine, Boston University Schmd ~1' Medicine, Boston, MA (U.S.A.) (Received 12 January 1988; Revised version received 5 June 1988; Accepted 10 June 1988)
Key words." Nucleus tractus solitarii; Hypertonic saline; Conscious rat; Hypertension: Sympathetic stimulation: Catecholamine Hypertonic saline 4% was microinjected into the area of the nucleus tractus solitarii (NTS) of conscious rats. The experiments were designed to explore the systemic hormonal responses and the contribution of various hormones to the hypertensive effect elicited by this procedure. A mean blood pressure rise to 155 _+3 m m H g (as compared to 119 + 2 in rats receiving the same volume ofequiosmolar dextrose solution, P < 0 . 0 5 ) was accompanied by significant rises in plasma norepinephrine (0.556_+0.128 ng/ml) and epinephrine (0.970 +_0.287 ng/ml) as compared to control rats receiving dextrose or control rats having only the cannula implanted, without microinjection. There was no significant change in plasma renin activity or plasma vasopressin levels, and the blood pressure rise could not be prevented or reversed by an angiotensin-converting enzyme inhibitor. The data are consistent with the hypothesis that the hypertensive response to this procedure is mainly due to an acute stimulation of central sympathetic neurons.
A htrge number of epidemiological, clinical and experimental studies have shown that salt excess leads to high blood pressure [3, 10, 12, 19]. Animal experiments have shown that either acute or chronic sodium loading produces hypertension which is mediated at least in part by sympathetic stimulation [9, 14, 25]. The nucleus tractus solitarii (NTS) is a relay center in the regulation of blood pressure via the baroreceptor reflexes and an important locus of ascending and descending pathways that are involved in the complex and highly integrated blood pressure control [13, 20~22]. Passo et al. [17] have shown in dogs that electrical stimulation of the dorsal portion of the medulla oblongata near the obex elicited a pressor response and elevated plasma renin activity (PRA) with a small increase in circulating epinephrine but no significant change in plasma norepinephrine. Nakai et al. [15]
Correspondence." H. Gavras, Boston University School of Medicine, 80 East Concord Street, L217, Boston, MA 02118, U.S.A.
336 have shown that electrical stimulation of the NTS area in vagotomized rats with spinal cord cuts at Cl elicited a pressor response which was entirely accounted for by increased vasopressin (AVP) release. Recently, we demonstrated that saline microinjection into the NTS area of conscious rats results in a sustained pressor response which is abolished by alpha-adrenoceptor blockade with phentolamine injected systemically, but not affected by an AVP antagonist injected systemically [6]. The present experiments were designed to explore further the systemic hormonal response to saline microinjection into the NTS area of conscious rats and the contribution of various hormones to the hypertensive effect elicited by this procedure. Responses were compared to those elicited by an isovolumic and equiosmotic non-ionic control solution (i.e. dextrose). Adult normotensive male Wistar rats (275-350 g) were anesthetized prior to surgery with ether inhalation or pentobarbital (42 mg/kg, i.p.) and the right iliac artery was catheterized with PE-50 tubing which was exteriorized to the rat's dorsum. The animal was then implanted with a 25 gauge single stainless steel guide cannula aimed at the NTS area. With the skull adjusted to 8°25 , above the horizontal level, the coordinates for implantation with respect to the bregma were 10.9 mm posterior, 1.4 mm lateral and 7.6 mm deep from the surface of the skull. The guide cannula was cemented into place with acrylic cement. A 30 gauge stylus kept the guide cannula patent during the recovery period (a minimum of 24 h). The solutions used in these microinjections were 4% NaCl and 25 % dextrose, both with an osmolality of approximately 1250 mOsm/l. Microinjection of a volume of 2 / d over 2 min was given through a 30 gauge cannula connected by PE-tubing to a Hamilton 10 pl syringe which was mounted on a syringe pump (Sage Instruments, Model 341). Teprotide, a converting enzyme inhibitor, was used to block the formation of angiotensin II, and was injected intra-arterially at a dose o f 4 mg/kg. On the day o f the experiment the rats were conscious and moving freely in a glass cylinder and were attached to a multichannel Hewlett-Packard recorder (Model 7702B) via a Statham P23DI strain gauge transducer. Heart rate and mean arterial blood pressure were automatically recorded. One group (n = 24) received a central microinjection o f 4 % saline. Immediately following the procedure, at the peak of the blood pressure rise, within less than 1 min after injection, 3 separate subgroups o f 6 animals each had 2 ml blood samples drawn for either plasma catecholamines, plasma vasopressin (AVP) or plasma renin activity (PRA). Animals in the 4th subgroup (n = 6) had their blood pressures recorded without intervention until they returned to baseline and remained stable for 60 min. At that point, the rats of this last subgroup received intra-arterial injections of teprotide. Ten min later, they received a second microinjection of hypertonic saline in the area of the NTS and had their blood pressure recorded until return to the baseline. The second group (n = 20) served as controls for the effect of isovolumic injection of equiosmotic non-ionic fluid and received a central microinjection of 25 % dextrose. It was then subdivided into 3 subgroups, each one having blood drawn for either catecholamines (n = 7), AVP (n = 7) or PRA (n = 6). The 3rd group (n = 17) served as controls for the effect of cannula implantation,
*P<0.05
Mean BP (mmHg) PRA (ng/ml/h) Plasma AVP (pg/ml) Plasma epinephrine (ng/ml) Plasma norepinephrine (ng/ml) Plasma dopamine (ng/ml) Mean BP post-teprotide (mmHg)
113.2 + 2.57 (n 17) 7.88+ 1.73 (n - 5) 3.36_+0.77 (n - 6) 0.202 _+0.061 (n = 6) 0.241 _+0.036 (n - 6) 0.048 _+0.004 ( n - 6)
Control animals, no mlcromjectmns
BP, blood pressure; PRA, plasma renin activity; AVP, vasopressin.
118.8 + 2.1 (n =20) 7.86___ 1.67 (n = 6) 4.48_+0.47 (n = 7) 0.503 ± 0.086 (n = 7) 0.288 _+0.020 (n = 7) 0.064 _+0.007 (n = 7)
After hypertonic dextrose microinjecton 155.1 + 3.28" (n = 24) 10.36+2.13 (n = 6) 2.71 _+0.71 (n - 6) 0.970_+ 0.287* (n - 6) 0.556 _+0.128" (n - 6) 0.136 + 0.063 (n = 6) 150.3 + 8.7 (n = 6)
After hypertonic saline mlcrolnjectlon
RESPONSES TO H Y P E R T O N I C SALINE A N D H Y P E R T O N I C DEXTROSE M I C R O I N J E C T I O N S INTO THE AREA OF THE NTS IN C O N S C I O U S RATS
TABLE l
,-M
338 receiving no microinjection. It also was subdivided into 3 smaller groups and had blood samples drawn for hormone levels (see Table I). Blood samples were collected in chilled tubes containing E D T A (for renin and AVP measurements) and promptly centrifuged in the cold. Plasma was stored at 80°C. PRA (expressed as ng/ml/h) was determined by a modification of the method of Sealey et al. [23]. Plasma AVP (expressed as pg/ml) was measured by a modification of the method of North et al. [16]. Plasma catecholamines (expressed as ng/ml) were determined by a radioenzymatic assay [18]. At the end of the experiment, the rats were anesthetized with pentobarbital, intracardially perfused with formalin, and decapitated. The brainstem was sectioned and stained. Placement of the cannula in the NTS or immediately adjacent areas was confirmed by examination under light microscopy by an independent observer. There was little evidence of anatomic destruction around the tip of the cannula. Results are reported as mean_+ S.E.M. Comparison of the difference between the means of two samples was made by Student's t-test. For multiple comparisons we used one way analysis of variance, followed by the Newman-Keuls multiple range test. Differences were considered significant if P < 0.05. Table I shows the changes in blood pressure and hormone levels elicited by each procedure in each of the three groups. Only the saline microinjection produced a significant blood pressure rise. This hypertensive response could not be prevented by prior angiotensin-converting enzyme (ACE) inhibition with teprotide. Plasma renin activity, vasopressin and dopamine levels were not affected by microinjection of either hypertonic solution. On the contrary, plasma epinephrine and norepinephrine levels increased significantly in response to hypertonic saline, but not hypertonic dextrose solutions. All hormone levels were unaffected by the placement of the steel cannula in the area of the NTS. We have recently shown that hypertonic saline microinjection into the area of the NTS produced a sustained elevation of blood pressure, which was adrenergically mediated, since it could be abolished by prior ~-adrenoceptor blockade with phentolamine; this blood pressure rise was not due to a systemic pressor effect of stimulated AVP, since it could not be reversed by systemic injection of an antivasopressor V~ AVP antagonist [6]. Other investigators have reported that electrical or chemical stimulation or lesions in the area of NTS produce a hypertensive response attributable in part to disinhibition of the sympathetic nervous system [4, 20] and in part to systemic pressor effect of stimulated AVP [2, 15] and the renin-angiotensin system [17]. The present study was designed to investigate the effect of hypertonic saline microinjection in the area of the NTS on the circulating levels of various pressor hormones. We found a significant elevation of epinephrine and norepinephrine consistent with the view that the hypertensive response elicited by local application of saline in the NTS has a major neurogenic sympathetically mediated component [4, 6, 20]. In another series of experiments, we have found that the local effect of norepinephrine in the NTS is to increase blood pressure, provided the animal is unanesthetized [24]. The fact that there was no rise in either AVP or PRA levels in the systemic circula-
339
tion is consistent with the failure o f either an A V P antagonist [6] or an ACE inhibitor (teprotide) to prevent the pressor response. Obviously, the local injection ofhypertonic saline activates a mechanism different from that o f the electric or chemical stimulation or lesions mentioned above. The lack of pressor response after microinjection of an isovolumic equiosmolar control solution o f dextrose in that area confirmed that the saline-induced blood pressure rise was not attributable to the space-occupying or osmotic properties of the injected fluid, but indeed to its ionic composition. The present data do not permit further conclusion as to the nature o f the mechanism involved. However, based on a large number of in vitro and in vivo studies in the literature regarding the effects of sodium on the c~2-adrenergic receptors [1, 7, 8, I I] we have proposed the following theory [5]: that salt may promote hypertension in part by attenuating the binding affinity o f central c~2-adrenoceptors for naturally occurring local agonists, thereby diminishing their action in certain areas of the brainstem, whose normal function is to tonically inhibit sympathetic impulses; the net result is an increase in central sympathetic outflow. The present findings are consistent with a transient disinhibition of central sympathetic neurons, leading to a hyperadrenergic state associated with a rise in blood pressure.
Supported in part by USPHS Grant HL-18318.
I Ashida, T., Tanaka, T., Yokouchi, M., et al., Effect of dietary sodium on platelet alpha2-adrenergic receptors in essential hypertension, Hypertension, 7 (1985) 972 978. 2 Blessing, W.W., Sved, A.F. and Reis, D.J., Destruction of noradrenergic neurons in rabbit brainstem elevates plasma vasopressin, causing hypertension, Science, 217 (1982) 661-662. 3 Dahl, L.K., Salt and hypertension, Am. J. Clin. Nutr., 25 (1972) 231 244. 4 Doba, N. and Reis, D.J., Role of central and peripheral adrenergic mechanism in neurogenic hypertension produced by brainstem lesions in rat, Circ. Res., 34 (1974) 293 301. 5 Gavras, H., How does salt raise blood pressure? A hypothesis, Hypertension, 8 (1986) 83 88. 6 Gavras, H., Bain, G.T., Bland, L., Vlahakos, D. and Gavras, I., Hypertensive response to saline microinjection in the area of the nucleus tractus solitarii of the rat, Brain Res., 343 (1985) 113 119. 7 Glossman, H., Lubbecke, F., Bellemann, P., Sattler, E.L. and Doell, G., Ionic modulation of alphaadrenoceptors, J. Cardiovasc. Pharmacol., 4 (1982) 551 557. 8 Greenberg, D.A., U'Prichard, D.C., Sheehan, P.O. and Snyder, S.H., Alpha-noradrenergic receptors 3 in the brain: differential effects of sodium ion on binding of [-H]-agonists and [-HI-antagonists, Brain Res., 140 (1978) 378 384. 9 Hatzinikolaou, P., Gavras, H., Brunner, H.R. and Gavras, I., Role of vasopressin, catecholamines, and plasma volume in hypertonic saline-induced hypertension, Am. J. Physiol., 240 (1981) H827 H831. 10 Kawasaki, T., Delea, C.S., Bartter, F.C. and Smith, H., The effect of high sodium and low sodium intakes on blood pressure and other related variables in human subjects with idiopathic hypertension, Am. J. Med., 64 (1978) 193 198 11 Kohlmann, O., Jr., Gavras, l., Biollaz, J., Biollaz, B. and Gavras, H., Sodium chloride-induced partial inhibition in vivo of alpha2-adrenoceptor agonist function, J. Hypertens., 3 (1985) 269 274. 12 Koletsky, S., Role of salt and renal mass in experimental hypertension, Arch. Pathol., 68 (1959) I 1 22. 13 Miura, M. and Reis, D.J., The role of the solitary and paramedian reticular nuclei in mediating cardiovascular reflex responses from carotid baro- and chemoreceptors, J. Physiol. (Lond.), 223 (1972) 525 548.
340 14 Miyajima, E. and Bunag, R.D., Chronic cerebroventricular infusion of hypertonic sodium chloride in rats reduces hypothalamic sympatho-inhibition and elevates blood pressure, Circ. Res., 54 (1984) 566575. 15 Nakai, M., Yamane, Y., Umeda, Y. and Ogino, K., Vasopressin-induced pressor response elicited by electrical stimulation of solitary nucleus and dorsal motor nucleus of vagus of rat, Brain Res., 251 (1982) 164-168. 16 North, W.G., LaRochelle, F.T., Jr., Haldar, J., Sawyer, W.H. and Valtin, H., Characterization of an antiserum used in a radioimmunoassay for arginine-vasopressin: implications for reference standards, Endocrinology, 103 (1978) 1976-1984. 17 Passo, S.S., Assaykeen, T.A., Otsuka, K., Wise, B.L., Goldfien, A. and Ganong W.F., Effect of stimulation of the medulla oblongata on renin secretion in dogs, Neuroendocrinology, 7 (1971) 1-10. 18 Peuler, J.D. and Johnson, G.A., Simultaneous single isotope radioenzymatic assay of plasma norepinephrine, epinephrine and dopamine, Life Sci., 21 (1977) 625-636. 19 Prior, I.A., Evans, J.G., Davidson, F. and Lindsay, M., Sodium intake and blood pressure in two Polynesian populations, New Engl. J. Med., 279 (1968) 515-520. 20 Reis, D.J., Granata, A.R., Joh, T.H., Ross, C.A., Ruggiero, D.A. and Park, D.H., Brain stem catecholamine mechanisms in tonic and reflex control of blood pressure, Hypertension, 6 Suppl. II (1984) II7 II15. 21 Ross, C.A., Ruggiero, D.A. and Reis, D.J., Afferent projections to cardiovascular portions of the NTS in the rat, Brain Res., 223 (1981) 402408. 22 Saper, C.B., Loewy, A.D., Swanson, L.W. and Cowan, W.M., Direct hypothalamo-autonomic connections, Brain Res., 117 (1976) 305 312. 23 Sealey, J.E., Gerten Banes, J. and Laragh, J.H., The renin system: variations in man measured by radioimmunoassay or bioassay, Kidney Int., 1 (1972) 240-253. 24 Vlahakos, D., Gavras, I. and Gavras, H., Alpha-adrenoceptor agonists applied in the area of the nucleus tractus solitarii in the rat: effect of anesthesia on cardiovascular responses, Brain Res., 347 (1985) 372-375. 25 Waeber, B., Gavras, H., Gavras, I., Chao, P., Kohlmann, O., Bresnahan, M.R., Brunner, H.R. and Vaughan, D., Evidence for a sodium-induced activation of central neurogenic mechanisms in one-kidney, one-clip renal hypertensive rats, J. Pharmacol. Exp. Ther., 233 (1982) 510-515.
335
Elsevier Scientific Publishers Ireland Ltd.
NSL 05592
Central sympathetic stimulation produced by saline application into the nucleus tractus solitarii area of conscious rats Demetrios Vlahakos and Haralambos Gavras Department ~['Medicine, Boston City Hospital and Department ~f Medicine, Boston University Schmd ~1' Medicine, Boston, MA (U.S.A.) (Received 12 January 1988; Revised version received 5 June 1988; Accepted 10 June 1988)
Key words." Nucleus tractus solitarii; Hypertonic saline; Conscious rat; Hypertension: Sympathetic stimulation: Catecholamine Hypertonic saline 4% was microinjected into the area of the nucleus tractus solitarii (NTS) of conscious rats. The experiments were designed to explore the systemic hormonal responses and the contribution of various hormones to the hypertensive effect elicited by this procedure. A mean blood pressure rise to 155 _+3 m m H g (as compared to 119 + 2 in rats receiving the same volume ofequiosmolar dextrose solution, P < 0 . 0 5 ) was accompanied by significant rises in plasma norepinephrine (0.556_+0.128 ng/ml) and epinephrine (0.970 +_0.287 ng/ml) as compared to control rats receiving dextrose or control rats having only the cannula implanted, without microinjection. There was no significant change in plasma renin activity or plasma vasopressin levels, and the blood pressure rise could not be prevented or reversed by an angiotensin-converting enzyme inhibitor. The data are consistent with the hypothesis that the hypertensive response to this procedure is mainly due to an acute stimulation of central sympathetic neurons.
A htrge number of epidemiological, clinical and experimental studies have shown that salt excess leads to high blood pressure [3, 10, 12, 19]. Animal experiments have shown that either acute or chronic sodium loading produces hypertension which is mediated at least in part by sympathetic stimulation [9, 14, 25]. The nucleus tractus solitarii (NTS) is a relay center in the regulation of blood pressure via the baroreceptor reflexes and an important locus of ascending and descending pathways that are involved in the complex and highly integrated blood pressure control [13, 20~22]. Passo et al. [17] have shown in dogs that electrical stimulation of the dorsal portion of the medulla oblongata near the obex elicited a pressor response and elevated plasma renin activity (PRA) with a small increase in circulating epinephrine but no significant change in plasma norepinephrine. Nakai et al. [15]
Correspondence." H. Gavras, Boston University School of Medicine, 80 East Concord Street, L217, Boston, MA 02118, U.S.A.
336 have shown that electrical stimulation of the NTS area in vagotomized rats with spinal cord cuts at Cl elicited a pressor response which was entirely accounted for by increased vasopressin (AVP) release. Recently, we demonstrated that saline microinjection into the NTS area of conscious rats results in a sustained pressor response which is abolished by alpha-adrenoceptor blockade with phentolamine injected systemically, but not affected by an AVP antagonist injected systemically [6]. The present experiments were designed to explore further the systemic hormonal response to saline microinjection into the NTS area of conscious rats and the contribution of various hormones to the hypertensive effect elicited by this procedure. Responses were compared to those elicited by an isovolumic and equiosmotic non-ionic control solution (i.e. dextrose). Adult normotensive male Wistar rats (275-350 g) were anesthetized prior to surgery with ether inhalation or pentobarbital (42 mg/kg, i.p.) and the right iliac artery was catheterized with PE-50 tubing which was exteriorized to the rat's dorsum. The animal was then implanted with a 25 gauge single stainless steel guide cannula aimed at the NTS area. With the skull adjusted to 8°25 , above the horizontal level, the coordinates for implantation with respect to the bregma were 10.9 mm posterior, 1.4 mm lateral and 7.6 mm deep from the surface of the skull. The guide cannula was cemented into place with acrylic cement. A 30 gauge stylus kept the guide cannula patent during the recovery period (a minimum of 24 h). The solutions used in these microinjections were 4% NaCl and 25 % dextrose, both with an osmolality of approximately 1250 mOsm/l. Microinjection of a volume of 2 / d over 2 min was given through a 30 gauge cannula connected by PE-tubing to a Hamilton 10 pl syringe which was mounted on a syringe pump (Sage Instruments, Model 341). Teprotide, a converting enzyme inhibitor, was used to block the formation of angiotensin II, and was injected intra-arterially at a dose o f 4 mg/kg. On the day o f the experiment the rats were conscious and moving freely in a glass cylinder and were attached to a multichannel Hewlett-Packard recorder (Model 7702B) via a Statham P23DI strain gauge transducer. Heart rate and mean arterial blood pressure were automatically recorded. One group (n = 24) received a central microinjection o f 4 % saline. Immediately following the procedure, at the peak of the blood pressure rise, within less than 1 min after injection, 3 separate subgroups o f 6 animals each had 2 ml blood samples drawn for either plasma catecholamines, plasma vasopressin (AVP) or plasma renin activity (PRA). Animals in the 4th subgroup (n = 6) had their blood pressures recorded without intervention until they returned to baseline and remained stable for 60 min. At that point, the rats of this last subgroup received intra-arterial injections of teprotide. Ten min later, they received a second microinjection of hypertonic saline in the area of the NTS and had their blood pressure recorded until return to the baseline. The second group (n = 20) served as controls for the effect of isovolumic injection of equiosmotic non-ionic fluid and received a central microinjection of 25 % dextrose. It was then subdivided into 3 subgroups, each one having blood drawn for either catecholamines (n = 7), AVP (n = 7) or PRA (n = 6). The 3rd group (n = 17) served as controls for the effect of cannula implantation,
*P<0.05
Mean BP (mmHg) PRA (ng/ml/h) Plasma AVP (pg/ml) Plasma epinephrine (ng/ml) Plasma norepinephrine (ng/ml) Plasma dopamine (ng/ml) Mean BP post-teprotide (mmHg)
113.2 + 2.57 (n 17) 7.88+ 1.73 (n - 5) 3.36_+0.77 (n - 6) 0.202 _+0.061 (n = 6) 0.241 _+0.036 (n - 6) 0.048 _+0.004 ( n - 6)
Control animals, no mlcromjectmns
BP, blood pressure; PRA, plasma renin activity; AVP, vasopressin.
118.8 + 2.1 (n =20) 7.86___ 1.67 (n = 6) 4.48_+0.47 (n = 7) 0.503 ± 0.086 (n = 7) 0.288 _+0.020 (n = 7) 0.064 _+0.007 (n = 7)
After hypertonic dextrose microinjecton 155.1 + 3.28" (n = 24) 10.36+2.13 (n = 6) 2.71 _+0.71 (n - 6) 0.970_+ 0.287* (n - 6) 0.556 _+0.128" (n - 6) 0.136 + 0.063 (n = 6) 150.3 + 8.7 (n = 6)
After hypertonic saline mlcrolnjectlon
RESPONSES TO H Y P E R T O N I C SALINE A N D H Y P E R T O N I C DEXTROSE M I C R O I N J E C T I O N S INTO THE AREA OF THE NTS IN C O N S C I O U S RATS
TABLE l
,-M
338 receiving no microinjection. It also was subdivided into 3 smaller groups and had blood samples drawn for hormone levels (see Table I). Blood samples were collected in chilled tubes containing E D T A (for renin and AVP measurements) and promptly centrifuged in the cold. Plasma was stored at 80°C. PRA (expressed as ng/ml/h) was determined by a modification of the method of Sealey et al. [23]. Plasma AVP (expressed as pg/ml) was measured by a modification of the method of North et al. [16]. Plasma catecholamines (expressed as ng/ml) were determined by a radioenzymatic assay [18]. At the end of the experiment, the rats were anesthetized with pentobarbital, intracardially perfused with formalin, and decapitated. The brainstem was sectioned and stained. Placement of the cannula in the NTS or immediately adjacent areas was confirmed by examination under light microscopy by an independent observer. There was little evidence of anatomic destruction around the tip of the cannula. Results are reported as mean_+ S.E.M. Comparison of the difference between the means of two samples was made by Student's t-test. For multiple comparisons we used one way analysis of variance, followed by the Newman-Keuls multiple range test. Differences were considered significant if P < 0.05. Table I shows the changes in blood pressure and hormone levels elicited by each procedure in each of the three groups. Only the saline microinjection produced a significant blood pressure rise. This hypertensive response could not be prevented by prior angiotensin-converting enzyme (ACE) inhibition with teprotide. Plasma renin activity, vasopressin and dopamine levels were not affected by microinjection of either hypertonic solution. On the contrary, plasma epinephrine and norepinephrine levels increased significantly in response to hypertonic saline, but not hypertonic dextrose solutions. All hormone levels were unaffected by the placement of the steel cannula in the area of the NTS. We have recently shown that hypertonic saline microinjection into the area of the NTS produced a sustained elevation of blood pressure, which was adrenergically mediated, since it could be abolished by prior ~-adrenoceptor blockade with phentolamine; this blood pressure rise was not due to a systemic pressor effect of stimulated AVP, since it could not be reversed by systemic injection of an antivasopressor V~ AVP antagonist [6]. Other investigators have reported that electrical or chemical stimulation or lesions in the area of NTS produce a hypertensive response attributable in part to disinhibition of the sympathetic nervous system [4, 20] and in part to systemic pressor effect of stimulated AVP [2, 15] and the renin-angiotensin system [17]. The present study was designed to investigate the effect of hypertonic saline microinjection in the area of the NTS on the circulating levels of various pressor hormones. We found a significant elevation of epinephrine and norepinephrine consistent with the view that the hypertensive response elicited by local application of saline in the NTS has a major neurogenic sympathetically mediated component [4, 6, 20]. In another series of experiments, we have found that the local effect of norepinephrine in the NTS is to increase blood pressure, provided the animal is unanesthetized [24]. The fact that there was no rise in either AVP or PRA levels in the systemic circula-
339
tion is consistent with the failure o f either an A V P antagonist [6] or an ACE inhibitor (teprotide) to prevent the pressor response. Obviously, the local injection ofhypertonic saline activates a mechanism different from that o f the electric or chemical stimulation or lesions mentioned above. The lack of pressor response after microinjection of an isovolumic equiosmolar control solution o f dextrose in that area confirmed that the saline-induced blood pressure rise was not attributable to the space-occupying or osmotic properties of the injected fluid, but indeed to its ionic composition. The present data do not permit further conclusion as to the nature o f the mechanism involved. However, based on a large number of in vitro and in vivo studies in the literature regarding the effects of sodium on the c~2-adrenergic receptors [1, 7, 8, I I] we have proposed the following theory [5]: that salt may promote hypertension in part by attenuating the binding affinity o f central c~2-adrenoceptors for naturally occurring local agonists, thereby diminishing their action in certain areas of the brainstem, whose normal function is to tonically inhibit sympathetic impulses; the net result is an increase in central sympathetic outflow. The present findings are consistent with a transient disinhibition of central sympathetic neurons, leading to a hyperadrenergic state associated with a rise in blood pressure.
Supported in part by USPHS Grant HL-18318.
I Ashida, T., Tanaka, T., Yokouchi, M., et al., Effect of dietary sodium on platelet alpha2-adrenergic receptors in essential hypertension, Hypertension, 7 (1985) 972 978. 2 Blessing, W.W., Sved, A.F. and Reis, D.J., Destruction of noradrenergic neurons in rabbit brainstem elevates plasma vasopressin, causing hypertension, Science, 217 (1982) 661-662. 3 Dahl, L.K., Salt and hypertension, Am. J. Clin. Nutr., 25 (1972) 231 244. 4 Doba, N. and Reis, D.J., Role of central and peripheral adrenergic mechanism in neurogenic hypertension produced by brainstem lesions in rat, Circ. Res., 34 (1974) 293 301. 5 Gavras, H., How does salt raise blood pressure? A hypothesis, Hypertension, 8 (1986) 83 88. 6 Gavras, H., Bain, G.T., Bland, L., Vlahakos, D. and Gavras, I., Hypertensive response to saline microinjection in the area of the nucleus tractus solitarii of the rat, Brain Res., 343 (1985) 113 119. 7 Glossman, H., Lubbecke, F., Bellemann, P., Sattler, E.L. and Doell, G., Ionic modulation of alphaadrenoceptors, J. Cardiovasc. Pharmacol., 4 (1982) 551 557. 8 Greenberg, D.A., U'Prichard, D.C., Sheehan, P.O. and Snyder, S.H., Alpha-noradrenergic receptors 3 in the brain: differential effects of sodium ion on binding of [-H]-agonists and [-HI-antagonists, Brain Res., 140 (1978) 378 384. 9 Hatzinikolaou, P., Gavras, H., Brunner, H.R. and Gavras, I., Role of vasopressin, catecholamines, and plasma volume in hypertonic saline-induced hypertension, Am. J. Physiol., 240 (1981) H827 H831. 10 Kawasaki, T., Delea, C.S., Bartter, F.C. and Smith, H., The effect of high sodium and low sodium intakes on blood pressure and other related variables in human subjects with idiopathic hypertension, Am. J. Med., 64 (1978) 193 198 11 Kohlmann, O., Jr., Gavras, l., Biollaz, J., Biollaz, B. and Gavras, H., Sodium chloride-induced partial inhibition in vivo of alpha2-adrenoceptor agonist function, J. Hypertens., 3 (1985) 269 274. 12 Koletsky, S., Role of salt and renal mass in experimental hypertension, Arch. Pathol., 68 (1959) I 1 22. 13 Miura, M. and Reis, D.J., The role of the solitary and paramedian reticular nuclei in mediating cardiovascular reflex responses from carotid baro- and chemoreceptors, J. Physiol. (Lond.), 223 (1972) 525 548.
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