Deoxycorticosterone-salt Hypertensive Rats Research Articles

Abstract 201: Direct but not Indirect Activation of Sympathetic Ganglia Leads to Higher Neuronal Activation in Normotensive Than Deoxycorticosterone-salt Hypertensive Rats

Sympathetic activity is chronically elevated in hypertensive human patients and in many animal models of hypertension, including the deoxycorticosterone (DOCA)-salt rat model. Sympathetic ganglia receive signals from the central nervous system and relay them to blood vessels and peripheral organs. How sympathetic ganglia respond to activation signals differently in hypertension is not fully understood. We examined how direct and indirect activators of sympathetic ganglia would influence neuronal activation. 2-deoxy-D-glucose (2-DG) is a glucose analog that causes hypoglycemia leading to centrally-mediated, indirect activation of celiac ganglia, a prevertebral sympathetic ganglion. Nicotine directly activates sympathetic ganglia by acting on cholinergic receptors found on most sympathetic neurons. Acute neuronal activation was examined by measuring c-fos immunoreactivity (ir). We hypothesized that since there is higher chronic sympathetic activity in DOCA-salt hypertensive than normotensive rats, both direct and indirect acute activators will induce higher c-fos expression in sympathetic ganglia of normotensive than hypertensive rats. Maximal c-fos expression for 2-DG (800mg/kg i.p.) and nicotine (2mg/kg s.c.) treatments was observed 2hrs post-treatment and for all following experiments, celiac ganglia were dissected and fixed 2hrs post-treatment. Following nicotine treatment, c-fos expression was significantly higher in celiac ganglia of normotensive (25.5 ± 9.7 c-fos ir neurons/m^2) than hypertensive (12.0 ± 4.8 c-fos ir neurons/m^2) rats. However, following 2-DG treatment, c-fos expression was similar in celiac ganglia of normotensive (49.1 ± 17.6 c-fos ir neurons/m^2) and hypertensive (49.4 ± 16.0 c-fos ir neurons/m^2). In contrast with our hypothesis, only the direct but not the indirect activation of sympathetic ganglia induced higher sympathetic neuronal activation in normotensive than hypertensive rats. Thus, a centrally-mediated metabolic stimulus does not activate pathways to sympathetic ganglia differentially in normotension versus hypertension. However, direct ganglionic stimulation leads to greater neuronal activation in normotensive than hypertensive ganglia.

Altered Cardiac Endothelin Receptors and Protein Kinase C in Deoxycorticosterone-Salt Hypertensive Rats

J. Fareh, R. M. Touyz, E. L. Schiffrin and G. Thibault. Altered Cardiac Endothelin Receptors and Protein Kinase C in Deoxycorticosterone-Salt Hypertensive Rats. Journal of Molecular and Cellular Cardiology (2000) 32, 665–676. The aim of the present study was to assess the status of ET-1 receptor subtypes (ETAand ETB) in ventricular myocytes and fibroblasts and to determine the role of PKC-dependent pathways in ET-1-stimulated cardiac cells in deoxycorticosterone acetate (DOCA)-salt hypertensive rats. Systolic blood pressure and relative heart to body weight were significantly increased in DOCA-salt rats. In unilaterally nephrectomized (Uni-Nx) control rats, more than 90% of cardiomyocyte ET receptors were of the ETAsubtype, whereas in fibroblasts ETAand ETBreceptors were present in a 1:3 ratio. In DOCA-salt rats, the density of the ETAreceptor subtype was reduced by 31% in cardiomyocytes and in cardiac fibroblasts only ETBreceptor density was decreased by 29%. Affinity was unchanged. The relative expression of immunoreactive PKC α, γ and ϵ was significantly increased, whereas PKC δ was not altered in cardiac extracts of DOCA-salt rats. In cardiac fibroblasts from DOCA-salt rats PKCδ was significantly increased and PKC ϵ was not translocated after ET-1 stimulation. The hearts of DOCA-salt hypertensive rats are thus characterized by: (1) decreased density of cardiomyocyte ETAreceptors and fibroblast ETBreceptors; (2) cell-specific enhanced expression of some PKC isoenzymes (α, γ,δ and ϵ); and (3) unresponsiveness of PKC ϵ to translocate in the presence of ET-1. Together with alterations of ET-1-induced Ca2+handling in cardiac myocytes and fibroblasts, which we previously reported, results from the present study indicate a marked modification of the cardiac ET-1 system of DOCA-salt hypertensive rats.

O-141 - Increased Ca2+ sensitivity in α-toxin permeabilized mesenteric artery from deoxycorticosterone-salt hypertensive rats

O-141 - Increased Ca2+ sensitivity in α-toxin permeabilized mesenteric artery from deoxycorticosterone-salt hypertensive rats

Gender Differences in Vascular Reactivity to Endothelin-1 in Deoxycorticosterone-Salt Hypertensive Rats

Gender Differences in Vascular Reactivity to Endothelin-1 in Deoxycorticosterone-Salt Hypertensive Rats

Effects of vasodilatory antihypertensive agents on endothelial dysfunction in rats with ischemic acute renal failure.

Ischemic acute renal failure is associated with vascular endothelial dysfunction. We examined whether vasodilatory antihypertensive agents would improve endothelial function in rats with ischemia/reperfusion renal injury. Rat kidneys were isolated and perfused after clipping of the bilateral renal arteries for 45 min and reperfusion for 24 h, and renal perfusion pressure and nitric oxide concentration in the venous effluent (chemiluminescence assay) were monitored. Preischemic administration of celiprolol (a beta-blocker; 100 mg/kg p.o.), benidipine (a calcium channel blocker; 1 mg/kg p.o.), or imidapril (an angiotensin converting-enzyme inhibitor; 3 mg/kg p.o.) restored endothelial function in rats subjected to acute renal ischemia (deltarenal perfusion pressure [10(-8) M acetylcholine]: sham -42+/-3%, ischemia -31+/-1%, ischemia +celiprolol -39+/-1%*, ischemia+benidipine -38+/-2%*, ischemia+imidapril -42+/-2%*; *p<0.05 vs. ischemia). Serum urea nitrogen and creatinine levels were also lower in the treated groups. Furthermore, ischemia-induced decreases in the response to acetylcholine and renal excretory function were smaller in SHR than in deoxycorticosterone-salt hypertensive rats, in which endothelial damage was marked. These results suggest that preischemic endothelial function may influence the degree of ischemic renal injury. Calcium channel blockers, converting-enzyme inhibitors, and endothelial NO synthase-activating beta-blockers had beneficial effects on renovascular endothelial dysfunction due to ischemia.

Gender differences in vascular reactivity to endothelin-1 in deoxycorticosterone-salt hypertensive rats.

In experimental models of hypertension, blood pressure reaches a higher level in male than in female rats. Because endothelin-1 (ET-1) seems to play a role in blood pressure elevation in deoxycorticosterone acetate (DOCA)-salt hypertension, we hypothesized that male DOCA-salt rats would display a greater vascular responsiveness to ET-1 than female DOCA-salt rats. Male and female Wistar rats were uninephrectomized, received DOCA injections (50 mg/kg/week) and water plus 1.0% NaCl/0.2% KCl. Control rats received vehicle and tap water. Responses to ET-1, norepinephrine (NE), serotonin (5-HT), IRL-1620, a selective endothelin-B- (ET(B)) receptor agonist, and acetylcholine (ACh) were evaluated in isolated aortic rings and also in vivo in the mesenteric microcirculation. Endothelium-intact aortas from male and female DOCA rats displayed increased sensitivity (p < 0.05) to NE and 5-HT, but decreased relaxation to ACh in comparison to aortas from respective control male and female rats. Endothelium-denuded, but not endothelium-intact, arteries from male DOCA rats displayed increased sensitivity (-log EC20) to ET-1, but no changes in ET-1 sensitivity were observed in female DOCA aortas. IRL-1620 induced contraction in male DOCA aortas, but not in female DOCA or control endothelium-denuded aortas. In the microcirculation, IRL-1620 induced vasodilation in male and female control rats, but marked vasoconstriction in male DOCA and minimal changes in vessels diameter in female DOCA rats. Bosentan, an ET(A)/ET(B)-receptor antagonist, induced a greater decrease in mean arterial blood pressure in male than in female DOCA-salt hypertensive rats. These data support the hypothesis that DOCA-salt rats exhibit gender differences in ET-1 vascular reactivity, which probably result from functional changes in ET(B)-receptors. The increased ET(B) responses in male DOCA-salt hypertensive rats may play a role in their higher blood pressure levels.

Cardiovascular effects of the novel potassium channel opener UR-8225.

We compared the hemodynamic and electrophysiological effects of UR-8225, a new potassium channel opener with those of levcromakalim. UR-8225 and levcromakalim (0.03-1 mg/kg) dose-dependently decreased mean arterial pressure (MAP) in conscious spontaneously hypertensive rats (SHR) and deoxycorticosterone-salt (DOCA) hypertensive rats and in conscious and anesthetized normotensive rats. The decrease in MAP was accompanied by a dose-dependent increase in heart rate (HR). Levcromakalim was about twice as potent as an antihypertensive agent, and its hypotensive and tachycardic effects were of longer duration than those of UR-8225. In conscious normotensive rats, the hypotensive response to UR-8225 (1 mg/kg) did not diminish with repeated dosing for 16 days. In conscious SHR, propranolol antagonized the tachycardic response of UR-8225 without affecting its hypotensive response. In anesthetized rats, glibenclamide (3, 10, and 20 mg/kg) dose-dependently reduced the hypotensive effect of the drug. In anesthetized dogs, UR-8225 and levcromakalim (0.01-30 micrograms/kg) dose-dependently decreased MAP and total peripheral resistance (TPR): UR-8225 was approximately 10 times less potent than potent than levcromakalim. UR-8225 had no effect on HR, left ventricular end-diastolic pressure (LVEDP), stroke volume (SV), or cardiac output (CO) but decreased rate-pressure product (RPP) and cardiac work (CW). In contrast, levcromakalim decreased HR, RPP, and CW and increased SV and LVEDP, whereas CO remained unaltered. UR-8225 and levcromakalim (0.1-10 microM) had no effect on the ventricular action potentials (APs), whereas at higher concentrations they shortened the AP duration (APD), an effect that was antagonized by glibenclamide. UR-8225 and levcromakalim decreased the Vmax and shortened the APD of the slow ventricular APs elicited by isoprenaline in K(+)-depolarized ventricular muscle fibers. These results indicated that UR-8225 decreases blood pressure (BP) by reducing TPR. Its electrophysiological and hemodynamic effects were blocked by glibenclamide, which suggests that they may be mediated largely through the activation of ATP-sensitive K+ channels.

Effect of an intracerebroventricularly administered vasopressin V1 antagonist on blood pressure and heart rate in deoxycorticosterone-salt hypertensive rats

It is well known that peripheral vasopressin (VP) is essential for the development and maintenance of DOC-salt hypertension. It is, however, still unclear whether central VP is involved in this type of hypertension. Therefore, the aim of this study was to clarify the role of central VP in the regulation of blood pressure in DOC-salt hypertension. In order to examine this issue, three series of investigations were performed. First, a novel vasopressin V1 antagonist (OPC21268) was administered intravenously to DOC-salt hypertensive rats, and mean arterial blood pressure (MABP) and heart rate (HR) were recorded. Second, the concentration of VP in the perfusate of microdialysis of cerebrospinal fluid (CSF) was determined in DOC-salt hypertensive and control rats. Finally, intracerebroventricular (i.c.v.) administration of a V1 antagonist was performed in DOC-salt hypertensive rats to determine the central mechanism of hypertension. Intravenous administration of a V1 antagonist had no effect on MABP and HR. There was no difference in VP in the perfusate of CSF between DOC-salt hypertensive and control rats. I.c.v. administration of a V1 antagonist significantly decreased MABP and HR in a dose-dependent manner in DOC-salt hypertensive rats ( P < 0.05-0.01). These results suggest that central VP is involved in the maintenance of DOC-salt hypertension, and the mechanism is, in part, mediated by upregulation of the V1 receptor.

High Calcium Diet Effectively Opposes the Development of Deoxycorticosterone-Salt Hypertension in Rats

The effects of increased dietary calcium intake on blood pressure and arterial function were investigated in one-kidney deoxycorticosterone-salt hypertensive Wistar rats. The calcium content of the control diet was 1.1%, and that of the high calcium diet, 2.5%. During the 10-week study calcium supplementation markedly attenuated the steroid-salt-induced rise in blood pressure and the associated cardiac hypertrophy. Responses of mesenteric arterial rings in vitro were examined at the end of the study. In deoxycorticosterone-salt-treated rats, the contractile sensitivity of endothelium-denuded preparations to norepinephrine, 5-hydroxytryptamine, and KCl, and the inhibitory effect of nifedipine on KCl-evoked responses were enhanced. It is interesting that the high calcium diet alleviated the steroid-salt-induced increase in sensitivity to KCl but did not significantly affect it to the receptor-mediated agonists norepinephrine and 5-hydroxytryptamine. Thus, sensitivity to membrane depolarization was reduced by calcium supplementation. Smooth muscle responses were also studied by challenging the preparations with KCl in a calcium-free solution, after which calcium was added to the organ bath in increasing concentrations. In steroid-salt-treated rats, these calcium contractions were attenuated, but concomitant calcium supplementation normalized the responses, suggesting improved cell membrane handling of calcium. In addition, the mineralocorticoid-salt treatment impaired relaxation responses of endothelium-intact arterial rings to acetylcholine, sodium nitroprusside, and isoproterenol.(ABSTRACT TRUNCATED AT 250 WORDS)

119 Calcium supplements inhibit the expression of parathyroid hypertensive factor in deoxycorticosterone-salt hypertensive rats

Lin, C. M.; Saito, Komei; Hsieh, S. T.; Miyajima, T.; Tsujino, T.; Yokoyama, MatsuhiroAuthor Information

152 Impairment of endothelium-dependent relaxation in the mesenteric arteries of deoxycorticosterone-salt hypertensive rats

152 Impairment of endothelium-dependent relaxation in the mesenteric arteries of deoxycorticosterone-salt hypertensive rats

Angiotensin II receptors in the solitary-vagal area of hypertensive rats.

Angiotensin II (Ang II) has been proposed to be an endogenous neuromodulator of the baroreceptor reflex at the level of the brain stem solitary-vagal area. Elevated activity of the brain Ang II system has been implicated in the development and maintenance of hypertension in spontaneously hypertensive rats and deoxycorticosterone acetate-salt hypertensive rats. In the present study, we sought to determine if Ang II receptors in the solitary-vagal area exhibited altered binding kinetics in spontaneously hypertensive rats or deoxycorticosterone-salt hypertensive rats. Ang II receptors were examined by quantitative autoradiographic analysis of iodine-125-labeled [Sar1,Ile8]Ang II binding in the solitary-vagal area in six groups of animals: 1) spontaneously hypertensive rats, 2) normotensive Wistar-Kyoto rats, 3) uninephrectomized rats, 4) uninephrectomized rats with a 1% solution of saline for drinking water, 5) uninephrectomized and deoxycorticosterone-treated rats, and 6) uninephrectomized and deoxycorticosterone-treated rats given a 1% solution of saline for drinking water. Blood pressure was significantly elevated in the spontaneously hypertensive rats and deoxycorticosterone-salt rats relative to control animals. There was a significant decrease in the binding affinity (increased KD) for 125I-[Sar1,Ile8]Ang II and a significant increase in the maximum binding density for 125I-[Sar1,Ile8]Ang II in the solitary-vagal area of spontaneously hypertensive rats relative to Wistar-Kyoto rats. Deoxycorticosterone-salt rats also exhibited significantly higher KD and maximum binding density values compared with controls. These results indicate that Ang II receptor binding is altered in the solitary-vagal area of two different models of experimental hypertension and suggest that these changes could contribute to the expression of the hypertensive state.

No rarefaction of cerebral arterioles in hypertensive rats

A reduction in the density of small arterioles (rarefaction) has been reported in several vascular beds of the spontaneously hypertensive rat (SHR). There have been conflicting reports on the existence of rarefaction in the pial vasculature of SHR. In this study, we determined whether there was rarefaction of pial arterioles in several models of hypertension. We studied SHR; two-kidney, one-clip Goldblatt hypertensive rats; deoxycorticosterone-salt hypertensive rats; and Dahl salt-sensitive rats fed high salt diet. The two groups of normotensive controls were Wistar--Kyoto rats and Dahl salt-sensitive rats fed low salt diet. The duration of hypertension was about 2 months. Density of first-, second-, third-, and fourth-order arterioles was determined by counting the number of vessels from enlarge photographs. We also measured the lengths of segments of the arterioles. We did not observe any evidence of rarefaction of arterioles in the pial vasculature in any of the hypertensive groups of rats. We conclude that (i) rarefaction of arterioles does not occur in the pial microvasculature after approximately 2 months of hypertension and (ii) rarefaction of pial arterioles does not account for abnormalities in the cerebral circulation of hypertensive rats such as protection of the blood-brain barrier or changes in autoregulation of cerebral blood flow.

&amp;beta;-Adrenergic Reactivity in Conscious DOCA-Salt Hypertensive Rats

The endocrine (plasma renin activity, insulin and ADH) and hemodynamic responses (heart rate and mean arterial pressure) to isoprenaline infusion were examined in conscious deoxycorticosterone-salt hypertensive rats (DS) and compared with uninephrectomized-salt control rats (US). A dose-related rise in plasma renin activity and plasma insulin values was found in US rats, while no change in either parameter was observed in DS rats after 30 min of isoprenaline infusion. ADH was not increased in US rats at any dose of isoprenaline infusion. However, in DS rats the largest dose (450 ng/kg/min) produced a significant rise. Isoprenaline infusion increased the heart rate in both groups, but the increases in the DS group were significantly lower than in the US group for the 200-ng/kg/min dose (p less than 0.01). The drop in mean arterial pressure was found to be more pronounced in DS rats than in US rats at 50, 100 and 200-ng/kg/min isoprenaline doses. Recovery of the mean arterial pressure to basal levels was also found in US rats with the various doses of isoprenaline administered. However, in DS rats the different doses of isoprenaline produced a progressive drop in mean arterial pressure with no recovery at the end of 30 min of isoprenaline infusion. The present results provide no evidence of subsensitivity to isoprenaline in the resistance vessels of conscious DS rats and suggest that the greater hypotensive response observed in these rats may be due to the inability of the renin-angiotensin system to adequately compensate the vasodilation produced by isoprenaline.

P-429 - Enhancement of norepinephrine-induced vasoconstriction and phosphatidylinositol turnover in the mesenteric artery from deoxycorticosterone-salt hypertensive rats

P-429 - Enhancement of norepinephrine-induced vasoconstriction and phosphatidylinositol turnover in the mesenteric artery from deoxycorticosterone-salt hypertensive rats

Cytoplasmic free [Ca2+] is not increased in the platelets of deoxycorticosterone-salt and spontaneously hypertensive rats.

The cytoplasmic free calcium concentration ([Ca2+]i) in the platelets of spontaneously hypertensive rats (SHR), Wistar-Kyoto rats (WKY), deoxycorticosterone-salt hypertensive rats (DOC) and normotensive Sprague-Dawley rats (SD) was measured with the fluorescent dye, quin-2-tetra-acetoxymethyl ester. No significant difference in platelet [Ca2+]i was found between SHR and WKY or between DOC and SD rats. No correlations were found between systolic blood pressure and [Ca2+]i. These results suggest that the elevation of platelet [Ca2+]i does not necessarily accompany hypertension in rats.

Inhibitors of Phenylethanolamine-N-Methyltransferase: Effects on Brain Catecholamine Content and Blood Pressure in DOCA-Salt Hypertensive Rats

Inhibitors [2-cyclooctyl-2-hydroxyethylamine (CONH), 1 aminomethylcycloundecanol (CUNH), 7,8-dichloro-1,2,3,4-tetrahydroisoquinoline (SKF64139), 2,3-dichloro-alpha-methylbenzylamine (DCMB), 8,9-dichloro-2,3,4,5-tetrahydro-1H-2-benzazepine(LY134046)] of phenylethanolamine N-methyltransferase (PNMT) were found to reduce blood pressure in deoxycorticosterone-salt (DOCA-salt) hypertensive rats. CONH, CUNH and DCMB, but not SKF64139 and LY134046, also lowered blood pressure in normotensive control rats. All of the PNMT inhibitors tested lowered hypothalamic epinephrine (Epi) content in both DOCA-salt hypertensive and normotensive rats. DCMB, SKF64139 and LY134046 also lowered brainstem Epi in both animal groups. From these data a good correlation could not be made between the blood pressure lowering effects of PNMT inhibitors and their effects on hypothalamic Epi content as had been observed in other animal models of hypertension (e.g. spontaneously hypertensive rats).

Chronic effects of a beta-adrenoceptor blocking drug, 4-[3-(tert-butylamino)-2-hydroxypropoxy]-N-methylisocarbostyril hydrochloride (N-696), in hypertensive rats.

4-[3-(tert-Butylamino)-2-hydroxypropoxy]-N-methylisocarbostyril hydrochloride (N-696) is a new beta-adrenoceptor blocking drug with direct vasodilatory activity, and may be classified into the fourth generation. Antihypertensive effects of N-696 were studied for 12 weeks in spontaneously (SHR), two kidney, one clip (CLIP), and deoxycorticosterone-salt (DOC) hypertensive rats. Propranolol (PPL) was used as the reference drug. In indirect blood pressure (BP) determination at the tail, milder prewarming condition was employed to observe antihypertensive effects clearly. Rats were prewarmed in rat holders on a hot plate for 30-60 min. Surface temperature of the hot plate was 35-45 degrees C. N-696 (20 mg/kg per day p.o.) and PPL (100 mg/kg per day, p.o.) treatments significantly decreased heart rate (HR) and maximum BP determined indirectly in SHR rats, even at the early stage of the experiments. These antihypertensive effects were shown also by mean BP determined directly at the 12th week. N-696 and PPL treatments showed no significant antihypertensive effects in CLIP rats, and slight antihypertensive effects in DOC rats. N-696 treatments showed a tendency to decrease plasma renin concentration (PRC) in SHR and DOC rats, whereas PPL treatments significantly decreased PRC in these hypertensive rats. N-696 and PPL treatments prevented nephrosclerosis and vascular lesions in SHR and DOC rats only slightly.

Distributions of microvascular pressure in skeletal muscle of one-kidney, one clip, two-kidney, one clip, and deoxycorticosterone-salt hypertensive rats.

Studies were performed on the cremaster skeletal muscle in rats to investigate the microvascular changes that are associated with established one-kidney, one clip (1K1C) and two-kidney, one clip (2K1C) Goldblatt hypertension and with deoxycorticosterone (DOC)-salt hypertension. Rats were anesthetized with urethane and chloralose; and cremaster muscles with intact circulation and innervation were suspended in a controlled Krebs bath. Microvascular pressures and vessel diameters were measured at three consecutive arteriolar (A) and venular (V) branch levels. Arteriolar diameters (means +/- SEM) in normotensive (NT) rats were 119 +/- 7, 86 +/- 5, and 31 +/- 3 micron respectively for 1A, 2A, and 3A arterioles; and venule diameters were 218 +/- 12, 141 +/- 15, and 53 +/- 7 micron respectively for 1V, 2V, and 3V venules. As compared to NT rats, there was a selective decrease in lumen size (percent reduction from control) for 1A and 2A (23% to 38%) in 1K1C and 2K1C rats and for 1A, 2A, and 3A (42% to 44%) in DOC rats. Venule diameters were not significantly different between normotensive and hypertensive animals at any branch level. Femoral artery pressures were significantly elevated (greater than or equal to 43%) in all three forms of hypertension; however, this increase in pressure was not proportionally transmitted throughout the microcirculation. This was evidenced by normal pressure in 3A arterioles and in all venules for 1K1C and 2K1C rats and by normal pressures in 3V and larger venules for DOC rats. Our findings indicate that elevated arterial pressure in chronic renal hypertension is not transmitted uniformly across all microvascular segments.(ABSTRACT TRUNCATED AT 250 WORDS)

Blood pressure response to central and/or peripheral inhibition of phenylethanolamine N-methyltransferase in normotensive and hypertensive rats.

We studied the effects on blood pressure and heart rate of two different phenylethanolamine N-methyltransferase (PNMT) inhibitors in normotensive, in two-kidney renal hypertensive, and in deoxycorticosterone-salt (DOC-salt) hypertensive rats. One compound (SK&F 64139) blocks the conversion of norepinephrine to epinephrine in both the central and the peripheral nervous system, whereas the other (SK&F 29661) does not cross the blood-brain barrier and therefore is active mostly in the adrenal glands. In the rats given SK&F 29661, practically no acute blood pressure changes were in the adrenal glands. In the rats given SK&F 64139 induced only a minor blood pressure and heart rate response in normotensive and two-kidney renal hypertensive rats. However, in DOC-salt hypertensive rats, it reduced arterial pressure to approximately normal levels and concomitantly slowed pulse rate. There was a close correlation between the magnitude of the blood pressure response observed in all SK&F 64139-treated animals and the control plasma norepinephrine (4 = -0.795, P less than 0.001) and epinephrine (r = -0.789, P less than 0.001) levels. These results suggest an important role for central epinephrine in regulating the peripheral sympathoadrenomedullary and the baroreceptor reflex activity, particularly when the maintenance of the high blood pressure is not renin-dependent.