Cationic amino acid transport in the renal medulla and blood pressure regulation

Previous studies have indicated that NO synthesis in isolated inner medullary collecting duct cells is reduced by cationic amino acids that compete with L-arginine for cellular uptake. In the present study, we investigated the effects of chronic renal medullary infusion of cationic amino acids on renal NO concentration and mean arterial pressure (MAP) in Sprague-Dawley rats. Renal medullary infusion of L-ornithine (50 microg/kg per min) or L-lysine (50 microg/kg per min) markedly decreased NO in the medulla (vehicle, 124 +/- 11 nmol/L; L-ornithine, 45 +/- 4 nmol/L; L-lysine, 42 +/- 6 nmol/L) and increased MAP (vehicle, 111 +/- 7 mm Hg; L-ornithine, 143 +/- 6 mm Hg; L-lysine, 148 +/- 3 mm Hg) after 5 days of infusion. In contrast, intravenous infusion of the same dose of L-ornithine or L-lysine for 5 days increased plasma concentration to levels similar to those observed with intramedullary infusion but did not change NO in the medulla or alter MAP. Furthermore, the NO-suppressing and hypertensive effects of medullary interstitial infusion of L-ornithine (50 microg/kg per min) were attenuated by simultaneous infusion of L-arginine (500 microg/kg per min; NO, 97 +/- 10 nmol/L; MAP, 124 +/- 3 mm Hg). A 5-day infusion of an antisense oligonucleotide against CAT-1 (18-mer, 8.3 nmol/h) significantly decreased CAT-1 protein in the medulla, decreased NO in the medulla (scrambled oligo, 124 +/- 10 nmol/L; antisense oligo, 67 +/- 11 nmol/L), and increased MAP (scrambled oligo, 113 +/- 2 mm Hg; antisense oligo, 130 +/- 2 mm Hg). These results suggest that uptake of L-arginine by cationic amino acid transport systems in the renal medulla plays an important role in the regulation of medullary NO and MAP in rats.     

Nitric oxide and central antihypertensive drugs: one more difference between catecholamines and imidazolines

NO is known to be involved in the peripheral and central regulation of the cardiovascular function. It plays a neuromodulatory role via a direct action on presynaptic nerve terminals, stimulating the release of gamma-aminobutyric acid, glutamate, and norepinephrine. Our aim was to study the possible role of NO in the cardiovascular effects of the central antihypertensive drugs clonidine, rilmenidine, and alpha-methyl-norepinephrine (alpha-MNA). Sites and mechanisms of the hypotensive action of these drugs were different; clonidine and rilmenidine acted on imidazoline receptors in the nucleus reticularis lateralis, whereas alpha-MNA acted upon alpha(2)-adrenoceptors in the nucleus tractus solitarius. The influence of N:(G)-nitro-L-arginine, an NO synthase inhibitor, on the central hypotensive effects of these drugs was investigated in pentobarbital-anesthetized rabbits. The intracisternal (IC) administration of alpha-MNA (30 microg/kg) induced hypotension (79+/-2 versus 103+/-4 mm Hg) and bradycardia (222+/-8 versus 278+/-4 bpm) (P:<0.05) (n=5). Clonidine (0.07 microg/kg IC) also induced hypotension (69+/-5 versus 99+/-4 mm Hg) and bradycardia (266+/-7 versus 306+/-10 bpm) (P:<0.05) (n=5). In addition to clonidine, rilmenidine (1 microg/kg IC) induced hypotension (64+/-4 versus 97+/-4 mm Hg) and bradycardia (264+/-11 versus 310+/-4 bpm) (P:<0.05) (n=5). Pretreatment with N:(G)-nitro-L-arginine (900 microg/kg IC) completely prevented the hypotensive effect of alpha-MNA but influenced the cardiovascular effects of neither clonidine nor rilmenidine. These results confirm that imidazoline drugs, such as clonidine, rilmenidine, and the catecholamine alpha(2)-adrenoceptor agonist alpha-MNA, have distinct mechanisms of action.     

Antihypertensive effect of the GABA receptor agonist muscimol in spontaneously hypertensive rats. Role of the sympathoadrenal axis

The antihypertensive action of central GABA-ergic stimulation was investigated in conscious stroke prone spontaneously hypertensive rats. Injection of the potent GABA agonist muscimol (0.01-1 microgram) into the lateral brain ventricle (icv) lowered mean arterial blood pressure (192.1 +/- 8.4 mm Hg) dose-dependently in stroke prone spontaneously hypertensive rats with a maximal fall of -52.7 +/- 5 mm Hg lasting for about 90 minutes. This was accompanied by bradycardia and sedation. Pretreatment with atropine (2 mg/kg, ip, or 15 micrograms/kg, icv) did not significantly influence the muscimol-induced fall in mean arterial pressure. In normotensive (109.3 +/- 1.9 mm Hg) Wistar-Kyoto controls, the maximal decrease in mean arterial pressure was -12.1 +/- 1.6 mm Hg from 109.3 +/- 1.9 mm Hg, and the duration of the effect was much less than in stroke prone spontaneously hypertensive rats, Following 1 microgram muscimol, icv, plasma noradrenaline did not fall significantly in stroke prone spontaneously hypertensive and Wistar-Kyoto rats, but in stroke prone spontaneously hypertensive rats, plasma adrenaline was fully suppressed (from 118.1 +/- 24.2 to 22.8 +/- 5.7 pg/ml) throughout the depressor response. The efferent sympathetic nervous activity as directly recorded from the n. splanchnicus was similar in conscious stroke prone spontaneously hypertensive and Wistar-Kyoto rats, and was moderately reduced in both strains by 1 microgram muscimol, icv.(ABSTRACT TRUNCATED AT 250 WORDS)     

Synergistic Antihypertensive Effects of Nifedipine on Endothelium : Concurrent Release of NO and Scavenging of Superoxide

Recent studies have suggested that part of the vasorelaxation caused by nifedipine, a 1,4-dihydropyridine Ca(2+) antagonist, depends on the endothelium. To study the effect of endothelium-dependent vasorelaxation, the release of NO and superoxide (O(2)(-)) in the presence of nifedipine in isolated cultured rabbit endothelial cells was measured. Highly sensitive electrochemical microsensors were placed onto the cell membrane, and the kinetics of NO and O(2)(-) were measured simultaneously with time resolutions of 0.1 and 0.05 ms, respectively. Nifedipine at its therapeutical concentrations stimulated NO release and scavenged O(2)(-) in endothelial cells. The linear relationship between NO concentration and nifedipine concentration was observed in the range between 0.01 and 1 nmol/L. NO concentration reached a maximum of 200+/-10 nmol/L at 1.2 nmol/L of nifedipine. The NO concentration was approximately 50% and 30% of the concentration measured in the presence of receptor-dependent (acetylcholine) and the receptor-independent (Ca(2+) ionophore A23187) NO synthase (eNOS) agonists, respectively. NO release stimulated by eNOS agonists was followed by the generation of the NO scavenger superoxide. The concentration of O(2)(-) was significantly lower after stimulation with nifedipine (peak 5+/-0.5 nmol/L) than after stimulation with acetylcholine (15+/-1 nmol/L) and Ca(2+) ionophore (25+/-1 nmol/L). The average rate of NO release by nifedipine is relatively slow (17 nmol/L per second). This is in sharp contrast to the fast rate of NO release by acetylcholine and Ca(2+) ionophore (40 and 300 nmol/L per second, respectively). These experiments show that nifedipine, apart from its well-known Ca(2+) antagonistic properties in vascular smooth muscle cells, stimulates the release of significant concentration of NO in endothelium and also preserves NO concentration. Both these effects may be beneficial in the treatment of hypertension.     

Central cardiovascular action of urotensin II in spontaneously hypertensive rats.

We have previously reported that urotensin II acts on the central nervous system to increase blood pressure in normotensive rats. In the present study, we have determined the central cardiovascular action of urotensin II in spontaneously hypertensive rats (SHR). Intracerebroventricular (ICV) injection of urotensin II elicited a dose-dependent increase in blood pressure in both SHR and normotensive Wistar-Kyoto rats (WKY). The changes in mean arterial pressure induced by ICV urotensin II at doses of 1 and 10 nmol in the WKY were 8 +/- 2 and 23 +/- 3 mmHg, respectively. ICV administration of urotensin II caused significantly greater increases in blood pressure in SHR (16 +/- 3 mmHg at 1 nmol and 35 +/- 3 mmHg at 10 nmol, respectively) compared with those in WKY. Urotensin II (10 nmol) elicited significant and comparable increases in heart rate in SHR (107 +/- 10 bpm) and WKY (101 +/- 21 bpm). Plasma epinephrine concentrations after ICV administration of 10 nmol urotensin II were 203 +/- 58 pmol/ml in SHR and 227 +/- 47 pmol/ml in WKY, which tended to be higher than those in artificial cerebrospinal fluid-injected rats (73+/- 7 and 87 +/- 28 pmol/ml, respectively, p < 0.1). The immunoreactivity of urotensin II receptor GPR 14 was expressed extensively in the glial cells within the brainstem, hypothalamus, and thalamus. These results suggest that central urotensin II may play a role in the pathogenesis of hypertension in SHR. Since GPR 14 was expressed in the glial cells of the brain, urotensin II may act as a neuromodulator to regulate blood pressure.     

Carvedilol action is dependent on endogenous production of nitric oxide

BACKGROUND: Carvedilol is known to be an adrenoreceptor blocker and free radical scavenger, used in hypertension and cardiac failure. However, its therapeutic actions cannot be fully explained by these mechanisms. In these studies, we tested the hypothesis that carvedilol action is associated with the synthesis/release of nitric oxide (NO). METHODS: Male Wistar rats (n = 22), 9 weeks old, were anesthetized with an intraperitoneal injection of sodium pentobarbital. Mean arterial pressure and arterial NO levels were monitored throughout the experiments. Carvedilol (1 mg/kg, intravenously [iv]) effects were evaluated before and after NO synthase (NOS) inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME, 5 mg/kg, iv). RESULTS: Carvedilol induced a significant decrease in basal arterial pressure (from 126.6 +/- 4.3 mm Hg to 75.9 +/- 3.0 mm Hg, P < .001) and significant increase in NO levels (from 17.9 +/- 1.7 micromol/L to 32.2 +/- 2.5 micromol/L, P < .001). After administration of L-NAME the arterial pressure increased (129.9 +/- 5.0 mm Hg, P < .001) with concomitant decrease in NO levels (13.4 +/- 1.6 micromol/L, P < .01). The second carvedilol administration (post-L-NAME) did not affect either arterial pressure (108.3 +/- 8.0 mm Hg) or NO levels (22.1 +/- 1.3 micromol/L). CONCLUSIONS: Our results suggest that the carvedilol-induced decrease of blood pressure is associated with an increase of plasma NO levels. Furthermore, NOS inhibition results in impairment of carvedilol hemodynamic effects and plasma NO levels. Therefore, these results are consistent with the hypothesis that the hemodynamic effect of carvedilol is in part dependent on endogenous NO production.     

Training-induced pressure fall in spontaneously hypertensive rats is associated with reduced angiotensinogen mRNA expression within the nucleus tractus solitarii

Knowing that exercise training reduces arterial pressure in hypertensive individuals and that pressure fall is accompanied by blockade of brain renin-angiotensin system, we sought to investigate whether training (T) affects central renin-angiotensin system. Spontaneously hypertensive rats (SHRs) and normotensive Wistar-Kyoto controls (WKY) were submitted to training or kept sedentary (S) for 3 months. After functional recordings, brain was removed and processed for autoradiography (brain stem sequential slices hybridized with (35)S-oligodeoxynucleotide probes for angiotensinogen [Aogen] and angiotensin II type 1 [AT(1A)] receptors). Resting arterial pressure and heart rate were higher in SHR(S) (177+/-2 mm Hg, 357+/-12 bpm versus 121+/-1 mm Hg, 320+/-9 bpm in WKY(S); P<0.05). Training was equally effective to enhance treadmill performance and to cause resting bradycardia (-10%) in both groups. Training-induced blood pressure fall (-6.3%) was observed only in SHR(T). In SHR(S) (versus WKY(S)) AT(1A) and Aogen mRNA expression were significantly increased within the NTS and area postrema (average of +67% and +41% for AT(1A) and Aogen, respectively; P<0.05) but unchanged in the gracilis nucleus. Training did not change AT(1A) expression but reduced NTS and area postrema Aogen mRNA densities specifically in SHR(T) (P<0.05 versus SHR(S), with values within the range of WKY groups). In SHRs, NTS Aogen mRNA expression was correlated with resting pressure (y=5.95x +41; r=0.55; P<0.05), with no significant correlation in the WKY group. Concurrent training-induced reductions of both Aogen mRNA expression in brain stem cardiovascular-controlling areas and mean arterial pressure only in SHRs suggest that training is as efficient as the renin-angiotensin blockers to reduce brain renin-angiotensin system overactivity and to decrease arterial pressure.     

Pulmonary blood flow alters nitric oxide production in patients undergoing device closure of atrial septal defects

OBJECTIVE: To determine the effect of pulmonary blood flow (Qp) on nitric oxide (NO) production in patients with increased Qp due to an atrial septal defect (ASD). BACKGROUND: Alterations in pulmonary vascular NO production have been implicated in the development of pulmonary hypertension secondary to increased Qp. In vitro, acute changes in flow or shear stress alter NO production. However, the effect of Qp on lung NO production in vivo is unclear. METHODS: Nineteen patients (2.4-61 years of age, median 17) with secundum ASD undergoing device closure were studied. Before, and 30 min after ASD closure, exhaled NO and plasma nitrate concentration were measured by chemiluminescence (NOA 280, Sievers, Boulder, Colorado). RESULTS: Before ASD closure, all patients had increased Qp (Qp: systemic blood flow [Qs] of 2.0 +/- 0.7) and normal mean pulmonary arterial pressure (13.4 +/- 3.1 mm Hg). Atrial septal defect device closure decreased Qp from 6.0 +/- 2.5 to 3.6 +/- 1.3 L/min/m2 (p < 0.05). Mean pulmonary arterial pressure was unchanged. Associated with the decrease in Qp, both exhaled NO (-22.1%, p < 0.05) and plasma nitrate concentrations (-17.9%, p < 0.05) decreased. CONCLUSIONS: These data represent the first demonstration that acute changes in Qp alter pulmonary NO production in vivo in humans. Exhaled NO determinations may provide a noninvasive assessment of pulmonary vascular NO production in patients with congenital heart disease. Potential correlations between exhaled NO, pulmonary vascular reactivity and pulmonary hypertension warrant further study.     

Effect of methyldopa on brain cholinergic neurons involved in cardiovascular regulation. A study in conscious spontaneously hypertensive rats

Chemical stimulation of brain cholinergic neurons in many species can produce hypertension. Recent studies in this laboratory have demonstrated that clonidine inhibits this central cholinergic pressor response by inhibiting the biosynthesis of brain acetylcholine. This study was designed to determine whether methyldopa, like clonidine, could inhibit brain cholinergic neurons involved in cardiovascular regulation in freely-moving spontaneously hypertensive rats (SHR). Intravenous (i.v.) injection of methyldopa (50-200 mg/kg) produced a dose-related fall in blood pressure (29/15-54/33 mm Hg) in SHR. Intracerebroventricular (i.c.v.) injection of hemicholinium-3 (HC-3) in SHR evoked a fall in arterial pressure through inhibition of acetylcholine synthesis. Doses of HC-3 (10 micrograms, or 15 micrograms, i.c.v.) and methyldopa (50 mg/kg, i.v.) were administered to produce small reductions in arterial pressure in SHR (7-14 mm Hg diastolic, respectively). When the two agents were injected simultaneously, however, a greater than additive response was obtained (p less than 0.05). Central injection of echothiophate (a long-acting cholinesterase inhibitor) to potentiate brain cholinergic activity resulted in a sustained hypertensive response (greater than 40 mm Hg) in SHR for at least 150 minutes. Simultaneous injection of or pretreatment with methyldopa (100 mg/kg, i.v.) inhibited the pressor response to echothiophate over a time course similar to its antihypertensive response in untreated SHR. Methyldopa, however, was completely ineffective in altering the hypertensive response to central injection of carbachol (1 microgram, i.c.v.). This difference in methyldopa susceptibility between the indirect-acting (echothiophate) and direct-acting (carbachol) cholinergic agonists may be related to an inhibiting effect of methyldopa on brain acetylcholine release.(ABSTRACT TRUNCATED AT 250 WORDS)     

Nitric oxide, peroxynitrite and cGMP in atherosclerosis-induced hypertension in rabbits: beneficial effects of cicletanine

We studied the effect of the furopyridine derivative antihypertensive drug, cicletanine, on blood pressure, vascular nitric oxide (NO) and cyclic guanosine 3':5'-monophosphate (cGMP) content in the aorta and the renal and carotid arteries, aortic superoxide production, and serum nitrotyrosine level in hypertensive/atherosclerotic rabbits. The effect of cicletanine was compared to that of furosemide. Rabbits were fed a normal or a cholesterol-enriched (1.5%) diet over 8 weeks. On the 8th week, the rabbits were treated per os with 2 x 50 mg/kg daily doses of cicletanine, furosemide, or vehicle for 5 days (n = 5-6 in each groups). The cholesterol diet increased mean arterial blood pressure (MABP) from 86 +/- 1 to 94 +/- 2 mm Hg (p < 0.05). Cicletanine decreased MABP in atherosclerotic rabbits to 85 +/- 1 mm Hg (p < 0.05), but it did not affect MABP in normal animals. Furosemide was without effect in both groups. In normal animals, NO content (assessed by electron spin resonance after in vivo spin trapping) in the aorta and the renal and carotid arteries was increased by cicletanine, and the drug increased cGMP in the renal artery as measured by radioimmunoassay. The cholesterol-enriched diet decreased both vascular NO and cGMP and increased aortic superoxide production assessed by lucigenin-enhanced chemiluminescence and serum nitrotyrosine determined by ELISA. In atherosclerotic animals, cicletanine increased NO and cGMP content in the aorta and the renal and carotid arteries and decreased aortic superoxide production and serum nitrotyrosine. Furosemide did not influence these parameters. We conclude that cicletanine lowers blood pressure in hypertensive/atherosclerotic rabbits. The antihypertensive effect of the drug in atherosclerosis may be based on its beneficial effects on the vascular NO-cGMP system and on the formation of reactive oxygen species.     

Tubulovascular nitric oxide crosstalk: buffering of angiotensin II-induced medullary vasoconstriction

Studies were designed to determine the source of NO responsible for buffering of the angiotensin II (Ang II)-mediated decrease of blood flow in the renal medulla. Intracellular Ca2+ concentration ([Ca2+]i) and NO production ([NO]i) of pericytes and endothelium of the vasa recta were independently measured with the use of fura 2-AM and 4,5-diaminofluorescein diacetate (DAF-2DA), respectively, in microtissue strips of the vascular bundles of the outer medullary vasa recta. Disruption of the endothelium of the vasa recta by perfusion with latex microspheres enabled imaging of the pericytes. Ang II (1 micromol/L) produced an increase of [NO]i of 19+/-6 U in pericytes of the vasa recta when the vessels were adjacent to medullary thick ascending limbs (mTALs). Pericytes of isolated vasa recta without surrounding mTALs showed a rapid peak increase in [Ca2+]i of 248+/-107 nmol/L, with a sustained elevation of 107+/-75 nmol/L, but did not show an increase in [NO]i to either Ang II (1 micromol/L) or the Ca2+ ionophore 4-bromo-A23187 (5 micromol/L). These observations indicated the lack of Ang II and Ca2+-sensitive NO production in pericytes of the vasa recta. In isolated vasa recta with intact endothelium, Ang II reduced [Ca2+]i from 128+/-28 to 62+/-13 nmol/L and failed to increase [NO]i. However, the Ca2+ ionophore did increase [NO]i in the endothelium (47+/-8 U), indicating the presence of Ca2+-sensitive NO production. Significant increases of [NO]i were observed in single isolated mTALs in response to both Ang II (33+/-6 U) and the Ca2+ ionophore (51+/-18 U). We conclude that Ang II increases [Ca2+]i in pericytes of the descending vasa recta as part of its constrictor action and that this vasoconstriction is buffered by the NO from the surrounding tubular elements, such as mTALs.     

Central cardiovascular action of urotensin II in conscious rats

OBJECTIVE: To examine the central cardiovascular action of urotensin II in conscious rats. METHODS: Intracerebroventricular (ICV) injections of urotensin II (1 and 10 nmol) were carried out in conscious Wistar rats. The effects of intravenous (i.v.) urotensin II (10 nmol) were also determined. RESULTS: The ICV injection of urotensin II at a dose of 1 nmol did not alter the arterial pressure or heart rate significantly, while 10 nmol urotensin II increased the arterial pressure and heart rate. The mean arterial pressure at 5 min of ICV urotensin II was 121 +/- 4 mmHg, which was significantly higher than that obtained by ICV injection of artificial cerebrospinal fluid (107 +/- 3 mmHg, P <0.05). In addition, significant increases in heart rate were observed 5-15 min after ICV urotensin II. Pre-treatment with pentolinium (5 mg/kg, i.v.) significantly attenuated the increases in mean arterial pressure (20 +/- 3 versus 8 +/- 2 mmHg, P <0.01) and heart rate (78 +/- 18 versus 7 +/- 5 beats/min, P <0.05) induced by ICV urotensin II. On the other hand, i.v. injection of urotensin II (10 nmol) elicited a depressor response associated with tachycardia; mean arterial pressure 5 min after injection was significantly lower in the urotensin II-injected rats (89 +/- 5 mmHg) than in the control rats (102 +/- 2 mmHg, P <0.05), and the heart rate was significantly higher in the former (402 +/- 11 versus 360 +/- 9 beats/min, respectively, P <0.05). CONCLUSIONS: Central urotensin II produces pressor and tachycardic responses through sympathetic activation, while peripheral urotensin II exerts a vasodilation-mediated depressor response in conscious rats.     

Reduced production of nitric oxide during haemodialysis.

OBJECTIVE: To examine the influence of systemic nitric oxide (NO) synthesis on blood pressure in patients with chronic renal failure undergoing haemodialysis, since nitric oxides are susceptible to renal excretion or are dialysed, a different indicator that is unaffected by renal function, such as the level of exhaled NO was evaluated. We examined the levels of the endogenous NO before and after a haemodialysis session. DESIGN AND METHODS: We evaluated the serum concentrations of nitrite/nitrate and the rate of nitric oxide release into exhaled air in 10 patients with hypertension who were receiving maintenance haemodialysis. RESULTS: The serum concentrations of nitrite/nitrate before haemodialysis were significantly higher than those in 10 normal controls (183 +/- 151 microM vs 42 +/- 17 microM, P < 0.05). These levels decreased significantly by the end of haemodialysis (42 +/- 26 microM). Because the amount of nitric oxide in the deepest expirate correlated well with the duration of exhalation, we were able to derive the rate of release of NO. The rate of NO release was 0.034 +/- 0.012 nmol/sec before haemodialysis, similar to that in normal controls (0.031 +/- 0.013nmol/sec). The rate was significantly reduced after dialysis (0.023 +/- 0.010 nmol/sec) (P < 0.05). The mean pre-dialysis mean blood pressure (109 +/- 11 mm Hg) and the post-dialysis blood pressure (106 +/- 9 mm Hg) were the same. CONCLUSIONS: These data indicate that NO production does not appear to have a critical role in control of arterial blood pressure across haemodialysis in patients with chronic renal failure.     

Contribution of nitric oxide to arterial pressure and heart rate variability in rats submitted to high-sodium intake

The aim of this study was to determine the contribution of NO to arterial pressure and heart rate variability in normotensive rats subjected to high sodium intake. Arterial pressure, heart rate, and arterial pressure and heart rate variability, baroreflex sensitivity, and pressure responsiveness were measured in male Wistar rats treated for 6 weeks (control and high sodium [1%] intake groups), before and after acute NO synthesis blockade. After treatment, no changes were observed in arterial pressure or heart rate. Arterial pressure variability was increased after sodium intake; however, heart rate variability and baroreflex sensitivity were not modified in high-sodium rats. NO synthase blockade increased arterial pressure in both groups but was higher in the high-sodium group (from 110+/-5 to 162+/-1.5 mm Hg) compared with the control group (from 109+/-6.7 to 144+/-10 mm Hg). The increase in arterial pressure was accompanied by a decrease in heart rate (from 354+/-28 to 303+/-25 bpm in control rats and from 380+/-34 to 298+/-30 bpm in high-sodium rats). NO synthase blockade increased the tachycardic response to sodium nitroprusside in high-sodium rats. Arterial pressure variability, evaluated by a nonlinear method (3D return maps), showed a larger reduction in response to NO synthase inhibition in the high-sodium group (from 162+/-26 to 34.8+/-8.6 for general index of beat-to-beat blood pressure variability) than in the control group (from 58+/-9.6 to 36+/-4.7 for general index of beat-to-beat blood pressure variability). Heart rate variability, evaluated by the SD of the R-R intervals, was not changed in control rats but was increased by NO synthase inhibition in the high-sodium rats (from 9.5+/-0.2 to 21.9+/-1.7 milliseconds). These findings suggest an important role for increased NO production in adaptation to high-sodium intake. The increase in NO system sensitivity in high-sodium intake may contribute to changes in the autonomic nervous system regulating heart rate and, especially, arterial pressure variability.     

Cerebral blood flow autoregulation and edema formation during pregnancy in anesthetized rats

Eclampsia is considered a form of hypertensive encephalopathy in which an acute elevation in blood pressure causes autoregulatory breakthrough, blood-brain barrier disruption, and edema formation. We hypothesized that pregnancy predisposes the brain to eclampsia by lowering the pressure of autoregulatory breakthrough and enhancing cerebral edema formation. Because NO production is increased in pregnancy, we also investigated the role of NO in modulating autoregulation. Cerebral blood flow autoregulation was determined by phenylephrine infusion and laser Doppler flowmetry. Four groups were studied: untreated nonpregnant (n=7) and late-pregnant (days 19 to 21; n=8) Sprague-Dawley rats and nonpregnant (n=8) and late-pregnant (n=8) animals treated with an NO synthase inhibitor (N(G)-nitro-l-arginine methyl ester; 0.5 to 0.7 g/L). Brain water content and blood-brain barrier permeability to sodium fluorescein were determined after breakthrough. Pregnancy caused no change in autoregulation or the pressure of breakthrough. However, treatment with the NO synthase inhibitor significantly increased the pressure of autoregulatory breakthrough (nonpregnant: 183.6+/-3.0 mm Hg versus 212.0+/-2.8 mm Hg, P<0.05; late-pregnant: 180.8+/-3.2 mm Hg versus 209.3+/-4.7 mm Hg, P<0.05). After autoregulatory breakthrough, only late-pregnant animals showed a significant increase in cerebral edema formation, which was attenuated by NO synthase inhibition. There was no difference in blood-brain barrier permeability between nonpregnant and late-pregnant animals in response to acute hypertension, suggesting that pregnancy may predispose the brain to eclampsia by increasing cerebral edema through increased hydraulic conductivity.     

Relation between renal interstitial ATP concentrations and autoregulation-mediated changes in renal vascular resistance

The present study was performed to examine the hypothesis that autoregulation-related changes in renal vascular resistance (RVR) are mediated by extracellular ATP. By use of a microdialysis method, renal interstitial concentrations of ATP and adenosine were measured at different renal arterial pressures (RAPs) within the autoregulatory range in anesthetized dogs (n=12). RAP was reduced in steps from the ambient pressure (131+/-4 mm Hg) to 105+/-3 mm Hg (step 1) and 80+/-2 mm Hg (step 2). Renal blood flow and glomerular filtration rate exhibited efficient autoregulation in response to these changes in RAP. RVR decreased by 22+/-2% in step 1 (P<0.01) and 38+/-3% in step 2 (P<0.01). The control renal interstitial concentration of ATP was 6.51+/-0.71 nmol/L and decreased to 4. 51+/-0.55 nmol/L in step 1 (P<0.01) and 2.77+/-0.47 nmol/L in step 2 (P<0.01). In contrast, the adenosine concentrations (117+/-6 nmol/L) were not altered significantly. Changes in ATP levels were highly correlated with changes in RVR (r=0.88, P<0.0001). Further studies demonstrated that stimulation of the tubuloglomerular feedback (TGF) mechanism by increasing distal volume delivery elicited with acetazolamide also led to increases in renal interstitial ATP concentrations, whereas furosemide, which is known to block TGF responses, reduced renal interstitial fluid ATP concentrations. The data demonstrate a positive relation between renal interstitial fluid ATP concentrations and both autoregulation- and TGF-dependent changes in RVR and thus support the hypothesis that changes in extracellular ATP contribute to the RVR adjustments responsible for the mechanism of renal autoregulation.     

Clonidine and carotid baroreflex in essential hypertension

Clonidine is believed to reduce blood pressure by a neural action and animal experiments suggest that this consists in potentiation of baroreflexes. In 16 patients with essential hypertension we studied the effects of alterations in carotid sinus baroreceptor activity (neck chamber technique) on arterial blood pressure (catheter measurements) and heart rate, before and after intravenous administration of 150 microgram and 300 microgram of clonidine. The magnitude of the reflex responses was assessed by the slope of the linear regressions relating applied increase and decrease in tissue pressure at the carotid sinus (and therefore applied decrease and increase in carotid sinus transmural pressure) and resulting changes in mean arterial pressure and R-R interval. Clonidine caused a marked reduction in mean arterial pressure (-26 +/- 3 mm Hg) and a slight but significant reduction in heart rate (-5 +/- 1 b/min). There was no evidence for a potentiation of the baroreceptor influence on blood pressure, although a slight potentiation of the baroreceptor influence on heart rate was observed in few instances. We conclude that in man clonidine can exert a pronounced hypotensive effect without potentiating baroreceptor influence on blood pressure. Therefore this mechanism does not play a prominent role in the clinical antihypertensive action of the drug.     

Combined central and peripheral sympathetic blockade: absence of additive antihypertensive effects

Two antihypertensive agents that inhibits sympathetic activity by differing pharmacologic mechanisms, the centrally-acting alpha-adrenergic agonist, clonidine, and the peripheral alpha-adrenergic antagonist, prazosin, were compared and also given in combination in 24 patients with essential hypertension. The patients were randomized into two groups; 11 received chlorthalidone (50 mg) daily throughout the protocol, whereas 13 received no diuretic. In the 13 patients not receiving the diuretic, supine mean blood pressure fell by 11.0 +/- 3.5 mmHg (p less than 0.01) with clonidine (0.3 mg daily), 5.5 +/- 2.9 mm Hg (p less than 0.05) with prazosin (15 mg daily), and 12.7 +/- 5.2 mmHg (p less than 0.02) with the combination. In the 11 patients concurrently receiving chlorthalidone, supine mean blood pressure fell by 9.5 +/- 2.0 mmHg (p less than 0.01) when clonidine was added, 2.3 +/- 1.9 mmHg (ns) when prazosin was added, and 7.9 +/- 2.1 mmHg (p less than 0.01) with the combination. In both the presence and absence of concurrent diuretic treatment, the blood pressure-lowering effects of clonidine were greater than those of prazosin, but were not different from the antihypertensive effects of the combination of clonidine and prazosin. Therefore, the simultaneous administration of clonidine and prazosin in the doses used in this study does not appear to be more effective than clonidine alone in the treatment of patients with hypertension.     

Prolactin-releasing peptide releases corticotropin-releasing hormone and increases plasma adrenocorticotropin via the paraventricular nucleus of the hypothalamus

Intracerebroventricular (ICV) injection of prolactin-releasing peptide (PrRP) is known to increase plasma adrenocorticotropin (ACTH) and cause c-fos expression in the hypothalamic paraventricular nucleus (PVN). We hypothesize that this is the site at which PrRP acts to increase plasma ACTH. We have used ICV injection and direct intranuclear injection of PrRP into the PVN to investigate the sites important in the stimulation of ACTH release in vivo. To investigate the mechanism of action by which PrRP increases ACTH, we have used primary culture of pituitary cells and measured neuropeptide release from in vitro hypothalamic incubations. ICV administration of PrRP increased plasma ACTH 10 min post-injection (PrRP 5 nmol 81.0 +/- 23.5 pg/ml vs. saline 16.8 +/- 14.1 pg/ml, p < 0.05). Intra-PVN injection of PrRP increased ACTH 5 min post-injection (PrRP 1 nmol 22.9 +/- 5.0 pg/ml vs. saline 10.3 +/- 1.4 pg/ml, p < 0.05). This effect continued until 40 min post-injection (PrRP 1 nmol 9.9 +/- 1.5 pg/ml vs. saline 6.2 +/- 0.5 pg/ml, p < 0.05). In vitro PrRP (1-100 nmol/l) did not effect basal or corticotropin-releasing hormone (CRH)-stimulated ACTH release from dispersed anterior pituitary cells. PrRP increased hypothalamic release of CRH (PrRP 100 nmol/l 1.4 +/- 0.2 nmol/explant vs. the basal 1.1 +/- 0.2 nmol/explant, p < 0.05) but not arginine vasopressin. PrRP also stimulated neuropeptide Y release (PrRP 100 nmol/l 56.5 +/- 11.8 pmol/explant vs. basal 24.0 +/- 1.9 pmol/explant, p < 0.01), a neuropeptide known to stimulate the hypothalamo-pituitary-adrenal axis. Our data suggest that in vitro PrRP does not have a direct action on the corticotrope but increases plasma ACTH via the PVN and this effect involves the release of hypothalamic neuropeptides including CRH and neuropeptide Y.     

Endothelial NO/cGMP system contributes to natriuretic peptide-mediated coronary and peripheral vasodilation

We tested the hypothesis that the endothelial nitric oxide (NO)-soluble guanylyl cyclase system is involved in atrial natriuretic peptide (ANP) and C-type natriuretic peptide (CNP) mediated regulation of coronary and peripheral vascular resistance. Rat hearts were perfused via the aorta at constant flow and the effect of ANP and CNP on coronary perfusion pressure and release of cGMP was determined in the absence and presence of the nitric oxide synthase inhibitor NG-nitro-L-arginine (L-NNA; 0.2 mmol/L) and the specific inhibitor of soluble guanylyl cyclase ODQ (20 micromol/L), respectively (n = 6). ANP (10-300 nmol/L) reduced perfusion pressure from 133 +/- 2 to 53 +/- 2 mm Hg (-60%; control) in the presence of L-NNA from 132 +/- 1 to 71 +/- 1 mm Hg (-46%) and in the presence of ODQ from 133 +/- 1 to 85 +/- 2 (-36%) (n = 6; P < 0.05). Disruption of the coronary endothelium by perfusion of hearts with collagenase reduced the relaxant effect of ANP to a similar extent as L-NNA. Basal release of cGMP was increased up to sixfold by ANP and this increase was reduced by L-NNA and ODQ (n = 6; P < 0.05). The coronary relaxant effect of CNP (0.1-3 micromol/L) was similarly attenuated by L-NNA and ODQ (n = 6). In conscious mice, a low dose of L-NNA (30 nmol) consistently reduced the blood pressure lowering effect of ANP (30 nmol) by approximately 40% (n = 7), whereas the hypotensive effect of nitroprusside (0.15 micromol) was not affected (n = 5). We conclude that the coronary dilatory and hypotensive action of natriuretic peptides involves the endothelium and is partly mediated by soluble guanylyl cyclase. The data may explain previous observations in humans with congestive heart failure showing impaired vascular ANP responses.