Epinephrine and the genesis of hypertension

Basal and stimulated sympathetic responses after epinephrine: no evidence of augmented responses

Delayed facilitation of norepinephrine release through the action of epinephrine (NE) at presynaptic beta-adrenoceptors has been postulated to account for the delayed hemodynamic effects of epinephrine and to be a mechanism causally related to the development of hypertension. To determine whether a short-term increase in epinephrine concentrations resulted in subsequent facilitation of sympathetic responses, 9 healthy subjects (age, 21+/-0.9 years) were studied at rest and during physiological stress on 2 occasions when they received an infusion of either saline or epinephrine (20 ng/kg per minute) in random order. Heart rate, blood pressure, forearm blood flow, epinephrine concentrations, and NE spillover were measured at rest, during mental stress (Stroop test), and during a cold pressor test. Measurements were performed before, during the 1-hour infusion of epinephrine or placebo, and 1 hour after the infusion. A radioisotope dilution method was used to measure NE spillover. Hemodynamic measurements and NE spillover were increased during the infusion of epinephrine, but 1 hour after discontinuation of epinephrine there was no significant augmentation of hemodynamic or sympathetic responses. NE spillover 1 hour after saline or epinephrine infusion was similar (0.85+/-0.2 versus 0. 87+/-0.2 microg/min; P=0.92). In addition, there was no delayed facilitation of stress-induced hemodynamic or NE responses after epinephrine. These findings do not support the hypothesis that epinephrine results in delayed facilitation of NE release.

Adrenaline responsiveness in mild hypertension: no evidence for altered beta-adrenoceptor sensitivity

The effects of circulating adrenaline on cardiovascular function were studied in 14 subjects (mean age, 36.5 years; range, 19-46 years) with mild hypertension and in 14 normotensive controls, matched for age and sex. Adrenaline was infused i.v. in step-wise increasing doses (0.1, 0.2, 0.4, and 0.8 nmol/kg/min). Cardiovascular responses were evaluated by echocardiography and noninvasive blood pressure measurements. Noradrenaline, adrenaline, potassium, and cyclic adenosine monophosphate (cAMP) were determined in venous plasma. Systolic and diastolic blood pressure responses to adrenaline were similar in both groups. Adrenaline increased myocardial contractility and stroke volume, but less so in the hypertensive patients. Cardiac output was increased in the hypertensive patients at rest, but the signs of increased myocardial contractility disappeared during adrenaline infusion, most likely because of a reduced myocardial compliance. Increased heart rate and systemic vascular resistances were displayed by the hypertensive patients at all adrenaline concentrations studied, but the responses were similar in both groups. The adrenaline-induced decreases in potassium and increases in cAMP were also similar in both groups. The increases in myocardial contractility and in heart rate are compatible with an increased arousal in mild hypertension at rest. Mild hypertension does not appear to be associated with alterations of beta2-adrenoceptor sensitivity, and the findings do not support that adrenaline is involved in the pathogenesis of primary hypertension.

Angiotensin II mediates systemic rebound hypertension after cessation of prostacyclin infusion in sheep

Prostacyclin (or epoprostenol), an arachidonic acid metabolite, is an effective treatment for patients with primary pulmonary hypertension. Interruption of chronic prostacyclin infusion can result in recurrent symptoms of dyspnea and fatigue. The etiology of this phenomenon is unknown. We hypothesized that sympathoadrenal activation could lead to increased vascular tone after abrupt termination of the infusion. To evaluate this effect, we monitored six chronically instrumented, awake sheep during and after infusion of prostacyclin. Prostacyclin decreased mean arterial pressure (MAP) by 14% and increased cardiac output by 33%. After the infusion ceased, MAP rebounded 23% above baseline, and cardiac output decreased by 28% from peak values within 10 min. We were unable to demonstrate an increase in norepinephrine levels after cessation of prostacyclin, nor did alpha-adrenergic blockade affect postinfusion hemodynamics. However, plasma renin activity increased >10-fold at peak infusion and remained elevated for up to 2 h after discontinuation of prostacyclin. Coinfusion of the angiotensin II-receptor antagonist L-158,809 resulted in complete abrogation of the postcessation rise in MAP. We conclude that renin-angiotensin system activation is primarily responsible for systemic hypertension occurring after abrupt cessation of prostacyclin infusion in sheep and that angiotensin II receptor blockade prevents this response. Our data do not support a role for sympathetic nervous system activation in the systemic pressor response after prostacyclin infusion.

Oxytocin pathways mediate the cardiovascular and behavioral responses to substance P in the rat brain

Stimulation of brain periventricular and hypothalamic substance P receptors induces a pressor response and tachycardia associated with mesenteric and renal vasoconstriction and hindlimb vasodilation resembling thus the classical defense reaction. This cardiovascular response is brought about by the activation of the sympathoadrenal system and is accompanied by grooming behavior. To address the role of oxytocinergic pathways in the brain in the mediation of these responses, we investigated the effects of central pretreatment of rats with oxytocin antisense, mixed base, and sense oligodeoxynucleotides on mean arterial pressure, heart rate, and grooming behavior induced by intracerebroventricular injections of substance P (50 pmol). Central pretreatment of conscious rats with the oxytocin antisense oligodeoxynucleotide (intracerebroventricular injections, 8 and 4 hours before administration of substance P) attenuated the mean arterial pressure (by 55%) and heart rate responses (by 58%) as well as grooming behavior induced by the peptide. A complete recovery of all substance P-induced responses was observed 28 hours after antisense oligodeoxynucleotide pretreatment. Intracerebroventricular pretreatment of rats with mixed base and sense oligodeoxynucleotides did not affect the cardiovascular and behavioral responses to substance P. The signal for oxytocin mRNA in the paraventricular nucleus was reduced only in rats pretreated with the antisense oligodeoxynucleotide. These results demonstrate that oxytocin neurons in the paraventricular nucleus, which innervate the cardiovascular centers in the hindbrain and the spinal cord, mediate the increases in blood pressure and heart rate induced by stimulation of substance P receptors in the forebrain. These neurons may also transmit signals, which are generated by substance P in the hypothalamus and are responsible for the sympathoadrenal activation in response to stress.

Stress reduction and preventing hypertension: preliminary support for a psychoneuroendocrine mechanism

Our objective was to identify endocrine-related mechanisms capable of mediating preventive effects of stress reduction in hypertensive heart disease. Since beneficial effects of stress reduction accrue over time, this cross-sectional, descriptive study sought differences between healthy students not practicing a systematic technique for reducing stress (the average stress, or AS, group, n = 33) and a similar group who for 8.5 years had practiced the Transcendental Meditation (TM) technique, used widely to reduce stress (the low stress, or LS, group, n = 22). The two groups of students, matched for age and area of study, performed timed collections of urine that included (separately) the entire waking and sleeping portions of 1 day. They also completed the Profile of Mood States and the State-Trait Anxiety Inventory, self-report instruments sensitive to subjective level of stress. Urine samples were analyzed for adrenocortical steroids by radioimmunoassay, for Na+, K+, Mg2+, Ca2+, and Zn2+ by atomic absorption spectrometry, and for neurotransmitter metabolites by reverse-phase, high-performance, liquid chromatography, and spectrophotometry. The two groups differed significantly on most measures. Specifically, the LS group was lower in cortisol and aldosterone and higher in dehydroepiandrosterone sulfate (DS) and the serotonin metabolite, 5-hydroxyindoleacetic acid (5-HIAA). Excretion of sodium, calcium, zinc, and the norepinephrine metabolite, vanillylmandelic acid (VMA), was also lower in this group, as were Na+/K+ ratio, mood disturbance, and anxiety. In women practicing TM, cortisol correlated inversely and DS directly with number of months of TM practice. The results identify improvements in mood state, adrenocortical activity, and kidney function as probable factors in the preventive and treatment effects of stress reduction. Because suboptimal levels of these parameters result from chronic, subjective stress, the findings add mechanistic support to the contention that hypertensive heart disease is avoidable, even in modern industrialized societies.

Norepinephrine release in the human forearm: effects of epinephrine

It has been postulated that delayed facilitation of norepinephrine release by epinephrine is causally related to the development of hypertension. It has been proposed that a brief increase in epinephrine concentrations results in the uptake of epinephrine into the sympathetic nerve terminal. Subsequent rerelease of epinephrine stimulates presynaptic beta-adrenergic receptors, resulting in a prolonged increase in plasma norepinephrine (NE) concentrations, with amplified sympathetic responses and vasoconstriction. To determine whether such epinephrine-induced, delayed facilitation of NE release occurs in a vascular bed draining resistance vessels and, if it occurs, whether that facilitation differs in hypertension, we used a radioisotope dilution method to measure unstimulated and isoproterenol-stimulated forearm NE spillover before, during, and after a 50 ng/min infusion of epinephrine for 30 minutes directly into the brachial artery. No delayed facilitatory effects of epinephrine on forearm NE spillover were observed in either 6 normotensive (NT) or 8 borderline hypertensive (BHT) subjects (NT unstimulated forearm NE spillover preepinephrine 1.79+/-0.41 ng/min versus postepinephrine 2.36+/-0.65 ng/min, P=.38; BHT preepinephrine 2.24+/-0.70 ng/min versus postepinephrine 1.93+/-0.46 ng/min, P=.51; NT isoproterenol-stimulated forearm NE spillover preepinephrine 4.61+/-1.01 ng/min versus postepinephrine 4.4+/-0.98 ng/min, P=.9; BHT preepinephrine 4.04+/-1.36 ng/min versus postepinephrine 4.69+/-1.49 ng/min P=.5). We conclude that the short-term local infusion of epinephrine does not have a delayed facilitatory effect on forearm NE spillover in NT or BHT subjects. Therefore, the prolonged increase in NE concentrations after epinephrine infusion previously shown systemically, and not seen locally in the forearm, suggests that the delayed facilitatory response to epinephrine may occur in other organs.

Cardiorenal epinephrine kinetics: evidence for neuronal release in the human heart

There are experimental data suggesting that epinephrine (Epi) may act as a cotransmitter in sympathetic nerves, stimulating presynaptic beta 2-receptors and enhancing norepinephrine (NE) release. To examine neuronal Epi release, patients with congestive heart failure and hypertension and healthy subjects were examined with the isotope-dilution method. At baseline, small cardiac and renal Epi spillovers were found in patients. During intense supine exercise, cardiac NE and Epi spillovers increased concomitantly with similar magnitude, whereas no renal Epi spillover could be detected. Blockade of neuronal uptake 1 caused a consistent decrease in both cardiac and renal fractional extractions of NE and Epi. The present study demonstrates baseline cardiorenal Epi release in patients with congestive heart failure and renal Epi release in hypertensive patients. Furthermore, Epi is removed by neuronal uptake in both the heart and kidney, and cardiac Epi spillover increases during exercise. This study, in contrast to other results, provides evidence for cardiac neuronal Epi release.

The cardiac beta-adrenoceptor-mediated signaling pathway and its alterations in hypertensive heart disease

Hypertension-induced cardiac hypertrophy is a predictor of the development of cardiac failure. It is unknown which cellular markers contribute to the progression from compensated hypertrophy to failure. In heart failure, several signal transduction defects leading to adenylate cyclase desensitization have been demonstrated, such as beta-adrenoceptor downregulation, increase of inhibitory G protein expression, and uncoupling of beta-adrenergic receptors, presumably by an increase of receptor kinase activity. In hypertensive heart disease, most studies have been performed in rat models of hypertension. As in heart failure, heterologous adenylyl cyclase desensitization occurs. The mechanisms are often different between the heterogeneous models for acquired and genetic hypertension, but Gi protein alterations and beta-adrenoceptor downregulation have been observed frequently. The underlying mechanism for desensitization is most likely a sympathetic activation in established hypertension rather than genetic alterations of signal transduction proteins. The data available suggest that beta-adrenergic desensitization could represent a mechanism that contributes to the progression from hypertrophy to failure. The key question remains whether those hypertensive patients who develop heart failure are more prone to beta-adrenergic desensitization or whether early intervention to reduce sympathetic activity is more effective in preventing or delaying the transition from compensated hypertrophy to overt failure.

Smooth muscle apoptosis during vascular regression in spontaneously hypertensive rats

We previously reported that apoptosis is increased in smooth muscle cells cultured from the aorta of spontaneously hypertensive rats versus normotensive controls. As an initial in vivo exploration, we now examined smooth muscle cell apoptosis regulation during the regression of vascular hypertrophy in the thoracic aorta media of spontaneously hypertensive rats receiving the antihypertensive drug enalapril (30 mg.kg-1.d-1), losartan (30 mg.kg-1.d-1), nifedipine (35 mg.kg-1.d-1), hydralazine (40 mg.kg-1.d-1), propranolol (50 mg.kg-1.d-1), or hydrochlorothiazide (75 mg.kg-1.d-1) for 1 to 4 weeks starting at 10 to 11 weeks of age. Three criteria were used to evaluate smooth muscle cell apoptosis: (1) oligonucleosomal fragmentation of the extracted aortic DNA, (2) reduction in aortic DNA content, and (3) depletion of smooth muscle cells in the arterial media. Arterial DNA synthesis was evaluated by [3H]thymidine incorporation in vivo. After 4 weeks of treatment, systolic blood pressure was reduced significantly by > 42% with losartan, enalapril, and hydralazine, and by 23% with nifedipine, versus control values of 220 +/- 5 mm Hg. However these agents affected vascular growth and apoptosis differently. Losartan, enalapril, and nifedipine stimulated smooth muscle cell apoptosis threefold to fivefold before there was a significant reduction in DNA synthesis (> 25%), vascular mass (> 19%), or vascular DNA content (> 38%), and these treatments markedly reduced (by 38% to 50%) medial cell number as measured at 4 weeks by the three-dimensional disector method. Losartan and nifedipine stimulated smooth muscle cell apoptosis before reducing blood pressure. In contrast, hydralazine did not affect vascular mass, apoptosis, or DNA synthesis, although blood pressure was lowered. Propranolol or hydrochlorothiazide failed to affect hypertension or vascular growth. Thus, smooth muscle cell apoptosis represents a novel therapeutic target for the control of hypertensive vessel remodeling in response to therapeutic agents.

Talking effect and white coat phenomenon in hypertensive patients

Both white coat effect (the tendency of blood pressure to rise during a medical visit) and talking effect were analyzed in 42 patients with essential hypertension. Blood pressure was measured during the clinic visit and over the subsequent 24-hour ambulatory period, with the physician performing 49 +/- 4 measurements for each patient. Three silent periods and two talking periods (stress and relaxation) were randomly allocated in a crossover design and studied, using analysis of variance. During the initial 11-minute silent period, systolic/diastolic blood pressures increased by 6 mm Hg/5 mm Hg. During the subsequent talking periods, these variations were significantly greater: +22 mm Hg/+17 mm Hg. Measures of systolic/diastolic blood pressure were higher during stressful talking than during relaxed talking. The talking and its emotional contents seemed to explain 70% of the white coat phenomenon. To minimize the white coat phenomenon in the clinic, physicians, nurses, and clinicians are advised to measure blood pressure during an initial period of silence.

Acute effects of beta 1-selective and nonselective beta-adrenergic receptor blockade on cardiac sympathetic activity in congestive heart failure

BACKGROUND

beta-Blockers may reduce cardiac sympathetic activity in patients with heart failure by antagonizing beta-adrenergic receptors that facilitate sympathetic outflow to the heart. To explore this possible effect of beta-blockade, we measured cardiac norepinephrine spillover responses in patients with heart failure after the acute administration of either propranolol, a nonselective beta-blocker, or metoprolol, a beta 1-selective agent.

METHODS AND RESULTS

Eighteen patients were studied. Repeated intravenous doses of propranolol (0.5 mg; nine patients; left ventricular ejection fraction, 14 +/- 2%) or metoprolol (1.0 mg; nine patients; left ventricular ejection fraction, 18 +/- 2%) were administered until one of the following end points was reached: a 15% decrease in heart rate, left ventricular +dP/dt, or mean arterial blood pressure or a 5 mm Hg increase in left ventricular end-diastolic pressure. Propranolol (mean dose, 2.0 mg) and metoprolol (mean dose, 3.6 mg) caused similar reductions in heart rate, +dP/dt, and coronary sinus plasma flow. Cardiac norepinephrine spillover was reduced after propranolol (277 +/- 55 to 262 +/- 53 pmol/min, P < .05) but was increased after metoprolol (233 +/- 57 to 296 +/- 82 pmol/min, P < .05). In a comparison of the two groups, the decrease in spillover after propranolol was significantly different than the increase seen after metoprolol (P < .01).

CONCLUSIONS

The administration of a beta 1-selective antagonist was associated with increased cardiac norepinephrine spillover. In contrast, the administration of a nonselective beta-blocker until similar hemodynamic end points were reached caused a reduction in norepinephrine spillover. This suggests that in patients with heart failure, nonselective beta-blockade may have favorable inhibitory effects on cardiac sympathetic activity.

Forearm vasodilator mechanisms during mental stress: possible roles for epinephrine and ANP

The contribution of epinephrine (Epi) to forearm vasodilator responses to mental stress was evaluated in 12 healthy men by comparing hemodynamic and plasma catecholamine responses to mental stress and to intravenous and intra-arterial infusions of epinephrine. Mental stress decreased forearm vascular resistance (FVR) by 45%, increased arterial Epi from 0.23 to 0.44 nmol/l in arterial plasma, and increased forearm norepinephrine overflow. Intra-arterial Epi infusion decreased FVR concentration dependently by up to 43%. Intravenous Epi infusion decreased diastolic arterial pressure and increased heart rate and systolic blood pressure dose dependently. FVR decreased by up to 39% at 4.60 nmol/l Epi in arterial plasma. The average Epi contribution to forearm vasodilation during mental stress was calculated to be between 9 and 30%, depending on if responses to stress were compared with intravenous or intra-arterial Epi infusion. Arterial atrial natriuretic peptide immunoreactivity increased by 23% during stress, supporting a vasodilator influence, whereas vasopressin immunoreactivity was unaffected. Thus secretion of Epi explains only part of the stress-induced forearm vasodilation. Intravenous infusion of Epi appears to activate sympathetic counterregulation.

Modifications induced by -10 degrees Trendelenburg's posture in sodium tubular handling in patients with essential hypertension

In essential hypertensive patients "exaggerated natriuresis" is a response to acute volume expansion. However, the underlying mechanisms for this remain to be determined. We studied 19 patients with essential hypertension (HP) and 9 normotensive subjects (NS). In all examined subjects the response to acute central volume expansion, without the plasma compositional change that Trendelenburg's position involves, was evaluated during 90 min (period T) after a similar period of deambulation (period D). Mean blood pressure (MBP), tubular sodium handling by the lithium clearance technique, plasma renin activity (PRA), plasma aldosterone (PA), plasma catecholamines and urine prostaglandine E2 and kallikrein were assessed after D and T. MBP was significantly higher in HP than in NS (p = 0.00001). HP showed "exaggerated natriuresis" after T (fractional excretion of sodium increased from 0.55 +/- 0.1% after D to 1.20 +/- 0.2% after T, p < 0.01). This was because of a decrease in their proximal fractional reabsorption of sodium (from 74.96 +/- 1.8% after D to 62.50 +/- 2.8% after T, p < 0.01). Plasma epinephrine and plasma dopamine after T were significantly lower than in standing position in HP (p < 0.01) but no in NS. The decrease in plasma renin activity after T in HP was 53%, and 32% in NS. There were not any significant differences between groups in the other neurohormonal systems studied. We conclude that the major determinant of "exaggerated natriuresis" in hypertensive patients is a higher stimulation of the cardiopulmonary receptors following Trendelenburg's position and consequently stronger reflex inhibition of sympathetic system activity and renin-angiotensin II activity. The "exaggerated natriuresis" after Trendelenburg's position in HP was an expression of abnormal pressure natriuresis.

Beta-adrenergic, angiotensin II, and bradykinin receptors enhance neurotransmission in human kidney

The aim of this study was to investigate angiotensin II (Ang II) receptor-, bradykinin receptor-, and beta-adrenergic receptor-mediated modulation of norepinephrine release from human renal sympathetic nerves and to characterize the respective receptor subtypes involved. Human cortical kidney slices were incubated with [3H]norepinephrine, placed in superfusion chambers between two platinum electrodes, and superfused with Krebs-Henseleit solution. The sympathetic nerves were stimulated electrically at 2.5 Hz for 1 minute, and the stimulation-induced outflow of radioactivity was taken as an index of endogenous norepinephrine release. Ang II and its precursor Ang I (both 0.01 to 1 mumol/L) enhanced stimulation-induced outflow of radioactivity in a concentration-dependent manner, with EC50 values of 0.03 and 0.05 mumol/L, respectively. The enhancement by Ang I but not that by Ang II was inhibited by the angiotensin-converting enzyme inhibitor captopril (3 mumol/L). The concentration-response curves of Ang I and Ang II were shifted to the right by EXP 3174 (0.01 mumol), the in vitro active form of the Ang II type 1 receptor antagonist losartan, with affinity estimates of 8.72 and 9.30, respectively. A higher concentration of EXP 3174 (0.1 mumol/L) abolished the facilitatory effects of Ang I and Ang II. The Ang II type 2 receptor antagonist PD 123319 (10 mumol/L) did not alter the facilitation by Ang II. In the absence of other drugs, bradykinin (0.01 to 1 mumol/L) failed to modulate stimulation-induced outflow of radioactivity but in the presence of captopril (3 mumol/L) enhanced it in a concentration-dependent manner, with an EC50 of 0.1 mumol/L.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypertension and enhanced beta-adrenoceptor-mediated facilitation of noradrenaline release produced by chronic blockade of adenosine receptors

1. The study was undertaken to compare the beta-adrenoceptor-mediated facilitation of noradrenaline release in the tail artery of vehicle-treated rats and of rats rendered hypertensive by chronic administration of 1,3-dipropyl-8-sulphophenylxanthine (DPSPX). Artery rings were loaded with [3H]-noradrenaline, and five periods of electrical stimulation (1 Hz for 2 min) were applied. To eliminate the influence of prejunctional alpha 2-adrenoceptors, the tissues were pre-exposed to 1 microM phenoxybenzamine. 2. Isoprenaline caused a concentration-dependent increase of tritium overflow elicited by electrical stimulation. It was more effective in arteries from DPSPX-treated than in those from vehicle-treated rats; isoprenaline (27.8 nM) increased by 30% tritium overflow in vessels from vehicle-treated rats whereas isoprenaline (7.0 nM) produced a 30% increase in vessels from DPSPX-treated animals. Furthermore, the maximal effect of isoprenaline was a 32.6% increase in control rats but a 48.6% increase in DPSPX-treated rats. 3. These results show that the sympathetic nerve endings of the rat tail artery are endowed with prejunctional beta-adrenoceptors which mediate facilitation of noradrenaline release elicited by electrical stimulation. They also suggest that adenosine receptors and beta-adrenoceptors interact at the prejunctional level and that impairment of this 'talk' may lead to the development of a hypertensive state.

Renal fluid and electrolyte handling in streptozotocin-diabetic rats

Male Sprague-Dawley rats (300-400 g) were made diabetic by an i.p. injection of streptozotocin (STZ, 60 mg/kg in citrate buffer, n = 8) to examine renal function in well-established diabetes mellitus. Vehicle-injected animals (n = 8) acted as controls. The mean weekly total amounts of food taken by control and STZ-diabetic rats did not differ, but diabetic rats exhibited diarrhea and drank more water. STZ-diabetic rats progressively lost weight from the first to the third week but gained weight in the fourth week. At 5 weeks the weight stabilized, plasma glucose concentration was elevated, and this was associated with increased kidney weight. The weekly urine volume from STZ-diabetic rats was elevated throughout the 5-week period of study and this was associated with significantly lower urinary outputs of Na+ and higher outputs of K+ than those of control animals. After 5 weeks of STZ diabetes, plasma corticosterone and aldosterone concentrations in unanesthetized rats did not differ significantly from values seen in controls. To examine renal function in more detail, groups of Inactin-anesthetized 5-week STZ-diabetic rats (n = 7) and control rats (n = 7) were placed on continuous jugular infusion of 0.077 M NaCl at 150 microL/min for 8.5 h. Following a 3.5-h equilibration period, urine flow and Na+, K+, and Cl- outputs were determined for the subsequent 5-h period, with mean arterial pressure and glomerular filtration rate (GFR). STZ-diabetic rats voided significantly less of the infused fluid and the urinary excretions of Na+ and K+ were lower than those of controls. Mean arterial blood pressure and GFR values in STZ-diabetic rats did not differ statistically from those seen in controls. Following hypotonic saline infusion for 8.5 h the levels of aldosterone were elevated significantly (p < 0.01) in STZ-diabetic rats by comparison with control animals (5.36 +/- 1.58 nmol/L, n = 7 vs. 2.36 +/- 0.12 nmol/L, n = 7). It is concluded that a challenge of hypotonic saline load to rats with 5 weeks of STZ diabetes mellitus elevates plasma aldosterone to reduce the ability to excrete Na+.

Facilitation by procaterol, a beta-adrenoceptor agonist, of noradrenaline release in the pithed rat independently of angiotensin II formation

1. The effects of the beta 2-adrenoceptor agonist, procaterol, on sympathetic neuroeffector transmission were studied in the pithed adrenal demedullated rat to determine if generation of angiotensin II was involved in its effect. Pressor responses were elicited by either electrical stimulation (20 V, 2 Hz) of the entire spinal sympathetic outflow or methoxamine (0.1 mg kg-1, i.v.). 2. Sodium nitroprusside (3 and 5 micrograms kg-1 min-1, i.v.) produced hypotension and the pressor responses to both sympathetic nerve stimulation and methoxamine were reduced. This indicates that decreasing blood pressure in pithed rats reduces pressor responses. Procaterol (10 and 30 ng kg-1 min-1, i.v.) also produced hypotension but did not alter pressor responses to sympathetic nerve stimulation. Nevertheless, procaterol (10 and 30 ng kg-1 min-1, i.v.) did reduce pressor responses to to methoxamine. Together these results suggest that procaterol may have enhanced sympathetic neurotransmitter release. This was confirmed in another series of experiments where procaterol (30 ng kg-1 min-1, i.v.) increased plasma noradrenaline levels during sympathetic nerve stimulation. 3. Captopril (5 mg kg-1, i.v.) produced hypotension and as expected reduced pressor responses to sympathetic nerve stimulation. When the hypotensive effect of captopril was abolished by concomitant vasopressin infusion (1.5-4.5 i mu kg-1 min-1, i.v.), pressor responses to sympathetic nerve stimulation were restored to pre-captopril levels. In this situation procaterol (10 and 30 ng kg-' min', i.v.) reduced basal blood pressure and did not alter pressor responses to sympathetic nerve stimulation whereas the pressor responses were reduced by an equihypotensive infusion of sodium nitroprusside (3 and 5 jig kg-' min' , i.v.). The lack of reduction of pressor responses after procaterol in the presence of captopril is indirect evidence that procaterol may have enhanced noradrenaline release independently of angiotensin II.4. In another series of experiments, plasma noradrenaline levels elicited by sympathetic nerve stimulation were not altered by captopril (5 mg kg', i.v.). In the presence of captopril (5 mg kg-', i.v.),procaterol (30 ng kg- min-1, i.v.) no longer enhanced plasma noradrenaline levels during sympathetic nerve stimulation. However, since the dose of captopril is well above that required to block angiotens in converting enzyme (ACE) the effect may be non-specific. Therefore, the selective AT, receptor antagonist, losartan (10mgkg'1, i.v.), was also used. Losartan (10mgkg'1, i.v.) did not alter plasma noradrenaline levels during sympathetic nerve stimulation, and in the presence of losartan procaterol(30 ng kg-I min-', i.v.) enhanced plasma noradrenaline levels during sympathetic nerve stimulation. This result further suggests that 1-adrenoceptor facilitation of noradrenaline release from sympathetic nerves in the pithed rat occurs by a mechanism independent of angiotensin II generation.

Sustained prejunctional facilitation of noradrenergic neurotransmission by adrenaline as a co-transmitter in the portal vein of freely moving rats

1. The duration of the facilitatory effect of adrenaline on the electrically evoked overflow of noradrenaline was studied in the portal vein of permanently adreno-demedullated freely moving rats. 2. Rats were infused with adrenaline (20 or 100 ng min-1) for 2 h. After an interval of 1 h, when plasma adrenaline had returned to undetectable levels, electrical stimulation resulted in an enhanced catecholamine overflow amounting to 219% (noradrenaline) and 241% (noradrenaline plus adrenaline) of control (saline infusion), respectively. 3. When stimulation was applied again, in the same animal, at 24, 48 and 72 h after the first stimulation episode, the evoked noradrenaline overflow was 150, 111 and 102% (after 20 ng ml-1 adrenaline) and 158, 134 and 105% (after 100 ng min-1 adrenaline) of control. 4. The beta 2-adrenoceptor antagonist, ICI 118,551 (0.3 mg kg-1), blocked the facilitatory effect obtained after the 100 ng min-1 adrenaline infusion on all days. 5. The results show that adrenaline, after being taken up by and released from sympathetic nerve terminals, is able to facilitate the evoked noradrenaline overflow through activation of prejunctional beta 2-adrenoceptors for at least 48 h after administration.

Postnatal development of vascular beta-adrenoceptor-mediated responses and the increase in the adrenaline content of the adrenal gland have a parallel time course

The present study was undertaken to analyse the relationship between postnatal development of vascular beta 2-adrenoceptor-mediated responses and the content of adrenaline in the adrenal gland and its concentration in plasma. Dog saphenous vein tissue from newborn, two-weeks old and adult animals were either preloaded with 3H-noradrenaline (or 3H-adrenaline) to study prejunctional beta-adrenoceptor-mediated effects or mounted in organ baths to determine isoprenaline-induced relaxation of preparations contracted by phenylephrine to about 65% of the maximum. The adrenal glands and samples of blood from the same animals were taken for estimation of adrenaline and noradrenaline. At birth, there were no beta-adrenoceptor-mediated effects pre- or postjunctionally. At two weeks, while the results at the prejunctional level were not significantly different from those obtained in newborns, at the postjunctional level there was a relaxant response to isoprenaline, which antagonised about 35% of the previous contraction to 1.75 mumol.l-1 phenylephrine. In adults, isoprenaline (50 nmol.l-1) increased by 24% tritium overflow evoked by electrical stimulation of tissues preloaded with 3H-noradrenaline but not that of tissues preloaded with 3H-adrenaline. On the other hand, propranolol (1 mumol.l-1) reduced by 21% the overflow of tritium evoked by electrical stimulation of tissues preloaded with 3H-adrenaline but not that of tissues preloaded with 3H-noradrenaline; postjunctionally, the maximal response to isoprenaline antagonised 70% of the previous contraction to 1.75 mumol.l-1 phenylephrine.(ABSTRACT TRUNCATED AT 250 WORDS)

Proenkephalin gene expression is altered in the brain of spontaneously hypertensive rats during the development of hypertension

Enkephalins have been discovered in various regions of the brain involved in cardiovascular regulation. Sympathoadrenal hyperactivity and altered baroreflex activity have been implicated in the development of hypertension. The purpose of this investigation was to determine whether proenkephalin gene expression is altered in the arterial baroreceptor reflex region of the brain and in neurons involved in regulating sympathetic outflow, during the development of hypertension. Proenkephalin mRNA levels were compared, using in situ hybridization, in 4- and 14 week-old spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). Systolic blood pressure was measured by tail-cuff impedance plethysmography. There were no differences in blood pressure at 4 weeks, however by 14 weeks resting systolic blood pressure was approximately 40% higher in SHR (162.5 +/- 1.6 vs. 117.3 +/- 1.5 mmHg). Proenkephalin gene expression in the nucleus tractus solitarius (NTS), caudal (CVLM) and rostral ventrolateral medulla (RVLM) was lower (approximately 67, 50, and 55%, respectively) in the SHR at 14 weeks. However, in the locus coeruleus (LC), anterior (AH) and lateral hypothalamus (LH), proenkephalin mRNA was significantly increased (approximately 50, 100 and 100%, respectively) in the SHR. The decrease in proenkephalin mRNA in the NTS, CVLM, and RVLM may attenuate arterial baroreceptor reflex activity, while the increase in proenkephalin mRNA in the LC, AH and LH may increase sympathetic tone by inhibiting the activity of sympathodepressor preganglionic neurons in the intermediolateral cell column of the spinal cord.

Clinical application of noradrenaline spillover methodology: delineation of regional human sympathetic nervous responses

The proportionality which in general exists between rates of sympathetic nerve firing and the overflow of noradrenaline into the venous drainage of an organ provides the experimental justification for the use of measurements of noradrenaline in plasma as a biochemical measure of sympathetic nervous function. Static measurements of noradrenaline plasma concentration have several limitations. One is the confounding influence of noradrenaline plasma clearance on plasma concentration. Other drawbacks include the distortion arising from antecubital venous sampling (this represents but one venous drainage, that of the forearm), and the inability to detect regional differentiation of sympathetic responses. Clinical regional noradrenaline spillover measurements, performed with infusions of radiolabelled noradrenaline and sampling from centrally placed catheters, and derived from regional isotope dilution, overcome these deficiencies. The strength of the methodology is that sympathetic nervous function may be studied in the internal organs not accessible to nerve recording with microneurography. Examples of the regionalization of human sympathetic responses disclosed include the preferential activation of the cardiac sympathetic outflow with mental stress, cigarette smoking, aerobic exercise, cardiac failure, coronary insufficiency, essential hypertension and in ventricular arrhythmias, and the preferential stimulation or inhibition of the renal sympathetic nerves with low salt diets and mental stress, and with exercise training, respectively. By application of the same principles, regional release of the sympathetic cotransmitters neuropeptide Y and adrenaline can be studied in humans. Cotransmitter release, however, is detected only with some difficulty. In restricted circumstances we find evidence of regional cotransmitter release to plasma, such as the release of neuropeptide Y from the heart at the very high rates of sympathetic nerve firing occurring with aerobic exercise, and cardiac adrenaline release also with exercise and after loading of the neuronal adrenaline pool by intravenous infusion of adrenaline.

Strong activation of vascular prejunctional beta 2-adrenoceptors in freely moving rats by adrenaline released as a co-transmitter

The effect of adrenaline on the electrically evoked noradrenaline overflow in the portal vein of adrenal demedullated freely moving rats was studied. Adrenaline (100 ng/min) was infused for 2 h into the portal vein. After a 1-h interval when plasma adrenaline had returned to pre-infusion undetectable levels, the portal vein nervus plexus was electrically stimulated. During stimulation (2 Hz, 3 ms, 5 mA) adrenaline and noradrenaline were released. The stimulus-evoked noradrenaline overflow was facilitated to 194% of the control-evoked overflow (infusion of saline). Total catecholamine overflow (noradrenaline plus adrenaline) was enhanced to 258%. The facilitation of the evoked overflow of both noradrenaline and adrenaline was blocked by the selective beta 2-adrenoceptor antagonist, ICI 118,551 (0.3 mg/kg). Cocaine (2.5 mg/kg plus 0.05 mg/kg per min) infused together with adrenaline prevented the evoked release of adrenaline and no facilitation of the stimulus-induced noradrenaline overflow occurred. Inhibition of prejunctional inhibitory alpha 2-adrenoceptors with yohimbine (0.5 mg/kg) further enhanced, to 577%, the electrically evoked catecholamine overflow after adrenaline infusion. The results demonstrate that adrenaline can be taken up by sympathetic nerve endings through cocaine-sensitive uptake carriers, and is released from these nerves during electrical stimulation of the portal vein nervus plexus. Neuronally released adrenaline can strongly facilitate electrically evoked neurotransmitter overflow through activation of prejunctional beta 2-adrenoceptors.

Clinical aspects of sympathetic activation and parasympathetic withdrawal in heart failure

Proposed reflex mechanisms for generalized neurohumoral activation in heart failure include decreased input from inhibitory baroreceptor afferent vessels and increased input from excitatory afferent vessels arising from arterial chemoreceptors, skeletal muscle metaboreceptors or the lungs. Not all subjects with left ventricular dysfunction have increased sympathetic nerve activity, but the magnitude of sympathoneural activation appears to independently predict survival. This association suggests both a causative mechanism linking sympathetic activation with adverse outcome and a therapeutic opportunity to improve the prognosis of such patients by inhibiting central sympathetic outflow. Generalized sympathetic activation is not unique to heart failure, and its functional consequences appear to be both organ- and condition-specific. Sympathetic activation is present in other disorders such as mild hypertension, cirrhosis and aging that do not share the dim prognosis of congestive heart failure. The adverse effects of adrenergic activation on the diseased myocardium may be a function of the magnitude and time course of increases in cardiac sympathetic nerve activity, the mechanical and electrophysiologic consequences of nonuniform abnormalities of sympathetic innervation in the failing heart and the absence of specific countervailing mechanisms present in other conditions also characterized by increased sympathetic traffic. The hypotheses that activation of adrenergic drive to the diseased myocardium is the causative mechanism linking sympathoexcitation to adverse outcome and that interventions that inhibit sympathetic outflow to the heart will improve the prognosis of patients with congestive heart failure have not been specifically tested. Greater understanding of the mechanisms responsible for the heterogeneity of sympathetic activation in response to ventricular dysfunction, for cardiac-specific and generalized activation of the sympathetic nervous system and for the stimulation or suppression of countervailing mechanisms capable of resisting its adverse effects is fundamental to the development of better therapies for congestive heart failure.

Comparison between thermoregulatory effects mediated by alpha 1- and alpha 2-adrenoceptors in normothermic and febrile rabbits

1. In order to better delineate the profile of thermoregulatory action of alpha 1-adrenoceptor antagonists; corynanthine (CRN) and BMY 20064 (BMY), and alpha 2-adrenoceptor agonist; medetomidine (MDT) and B-HT 920 (BHT), the effect of intravenous administration of two doses of these drugs on rectal (Tre) and ear skin (Te) temperatures, metabolic rate (M), respiratory evaporative heat loss (Eres) and respiratory rate (Rr) were examined in febrile and non-febrile rabbits. 2. Results indicate that alpha 1-adrenoceptor antagonists as well as alpha 2-adrenoceptor agonists markedly lowered body temperature exhibiting antipyretic and hypothermic actions. The hypothermic and antipyretic effect after the CRN or BMY, and BHT or MDT, treatment was associated with inhibition of metabolic rate and/or with body heat redistributed to peripheral tissues and an increase of the potential for heat loss to the environment. 3. BMY also abolished the thermogenic response to cold. However, BMY did not affect metabolic heat production on exposure to a cold ambient temperature. This unexpected phenomenon is difficult to explain at the present moment. Possible mechanisms responsible for the thermoregulatory activity of BMY are discussed. 4. These results indicate that alpha 1- and alpha 2-adrenoceptor drugs' thermoregulatory actions are similar in event and suggest that both subtypes of alpha-adrenoceptor might be implicated functionally in a variety of thermoregulatory processes.

Plasma catecholamines--analytical challenges and physiological limitations

Catecholamines in plasma may be measured to assess sympathoadrenal activity. Numerous assay methodologies have been published, illustrating the fact that there are many analytical problems. Different methodologies are discussed briefly. A plea for better validation, especially with regard to specificity (which should not be confused with sensitivity or reproducibility), is made. Plasma NA is a frequently used marker for sympathetic nerve activity in humans, but the data obtained are often misinterpreted due to lack of appreciation of the physiological determinants of the NA concentration measured. NA overflow from an organ gives a good reflection of nerve activity in that organ. However, sympathetic nerve activity is highly differentiated, particularly during stress, and conventional plasma NA levels (usually forearm venous samples) cannot be taken as an indication of 'sympathetic tone' in the whole individual. NA is rapidly removed from plasma, resulting in meaningless net veno-arterial concentration differences over organs unless its removal from arterial plasma is taken into account. In the forearm, for example, 40-50% of catecholamines are removed during one passage; about half of the NA in a venous sample is derived from the arm and half from the rest of the body. Therefore, conventional venous sampling overemphasizes local (mainly skeletal muscle) nerve activity. Whole-body sympathetic nerve activity may be monitored in arterial or mixed venous (i.e. pulmonary arterial) samples, which reflect NA overflow from all organs in the body. NA levels are determined both by overflow to plasma and clearance from plasma. NA turnover studies with 3H-NA infusions may be needed to assess clearance, but the simpler concentration measurements usually yield adequate information if the sampling site is relevant. NA overflow from an organ can be assessed (using 3H-NA or ADR as a marker for NA extraction in the organ) and provides valuable information on local sympathetic activity. Mental stress elicits marked circulatory responses, with mainly cardiorenal sympathetic activation and minor elevations of conventional venous plasma NA levels, thus illustrating the differentiated firing pattern of the sympathetic nerves. Circulating ADR is less important than neurogenic mechanisms in the responses to stress. Concentration-effect studies for infused catecholamines may be used for receptor sensitivity studies in vivo, but reflexogenic contributions to responses need to be determined. However, prejunctional mechanisms cannot be assessed without knowledge of the nerve activity present; for example, ADR infusion leads to increased nerve activity. When correctly sampled, measured and interpreted, plasma catecholamines can yield very valuable information on sympathoadrenal activity.

Catecholamines and essential hypertension

Given the ubiquitous distribution of catecholamines in mammals, and their importance in a range of physiological processes pivotal to blood pressure regulation, the subject of catecholamines and essential hypertension has a broader context than simply consideration of sympathetic nervous system and adrenal medullary dysfunction. These further matters are the likely involvement in hypertension pathogenesis of the CNS catecholaminergic neurones influencing peripheral sympathetic outflow, the possible pathogenetic significance of adrenaline released as a cotransmitter in sympathetic nerves, and the natriuretic renal tubular dopamine mechanisms for regulating body sodium balance which appear to be impaired in patients with essential hypertension. The central consideration, however, remains the important issue of the causes and consequences of the now well-documented sympathetic nervous overactivity which characterizes the early developmental phases of essential hypertension.

α- and β-Adrenoceptors in Hypertension: Molecular Biology and Pharmacological Studies

Recent years have witnessed astonishing progress in our understanding of the molecular basis of adrenoceptor structure, function and regulation and revealed an unexpected heterogeneity of adrenoceptors demonstrating the existence of at least 11 subtypes. This paper discusses the implications of these advances on studies regarding a specific role of adrenoceptors in the development of genetic hypertension. The available data indicate that among the alpha-adrenoceptor subtypes the alpha 2A-adrenoceptor is the most likely candidate for an alteration specifically linked to genetic hypertension in the animal model of the spontaneously hypertensive rat and possibly in some patients. Alterations of other alpha-adrenoceptor subtypes may be specific for some forms of genetic hypertension but are unlikely to play an important role for blood pressure regulation. Most beta-adrenoceptor alterations appear to occur secondary to blood pressure elevation independently of whether hypertension has occurred on a genetic basis or not. Moreover, the mechanisms regulating alpha- and beta-adrenoceptor responsiveness upon prolonged agonist exposure may be altered in hypertension and thereby contribute to the pathophysiology of this disease.