Pharmacokinetic Assessment of Peramivir in a Hospitalized Adult Undergoing Continuous Venovenous Hemofiltration

Objective:

To report a sieving coefficient for peramivir in a patient receiving continuous venovenous hemofiltration (CWH).

Case Summary:

An 16-year-old male presented with chills, myalgias, and dyspnea and was hospitalized. Nasal secretions were positive for influenza by rapid antigen test at an outside facility and oseltamrvir was commenced. Oral absorption was predicted to be unreliable, and Intravenous peramivir was accessed as an emergency investigational new drug application (eIND). CWH was initiated after the development of acute renal failure, with blood samples collected to determine peramivir concentrations.

Discussion:

Peramivir, an intravenous investigational neuraminidase inhibitor with activity against Influenza viruses, has limited data for dosing in the setting of CWH. A single patient received 600 mg of peramivir intravenously and had blood and ultrafittrate concentrations measured serially. A sieving coefficient of approximately 0.9 was identified.

Conclusions:

Peramivir is well cleared by CWH, and drug exposure is potentially predictable based on flow rates. Further study is necessary.

Clinical experience in adults and children treated with intravenous peramivir for 2009 influenza A (H1N1) under an Emergency IND program in the United States

BACKGROUND

Peramivir, an investigational intravenous neuraminidase inhibitor in Phase 3 trials for hospitalized patients, was made available during the 2009 H1N1 influenza pandemic under the Emergency Investigational New Drug (eIND) regulations. We describe the clinical characteristics and outcomes of all patients for whom peramivir was requested under the eIND.

METHODS

After obtaining eIND approval from the Food and Drug Administration and local institutional review board approval, clinicians caring for hospitalized patients with influenza administered intravenous peramivir and collected information on demographic characteristics, clinical characteristics, and outcomes.

RESULTS

From April through October 2009, peramivir was requested for 42 patients and administered to 20 adults and 11 children. At hospitalization, all patients had rapidly progressing, radiographically confirmed viral pneumonia with respiratory failure, and all but 1 patient required mechanical ventilation. In most patients, including 1 person with documented oseltamivir-resistant infection, the illness had progressed despite oseltamivir treatment. Peramivir was administered for 1-14 days (median duration, 10 days). The 14-day, 28-day, and 56-day survival rates were 76.7%, 66.7%, and 59.0%, respectively. Peramivir was generally well tolerated.

CONCLUSIONS

Intravenous peramivir was well tolerated and was associated with recovery in most patients hospitalized with severe 2009 H1N1 influenza viral pneumonia and treated under an eIND.

Peramivir pharmacokinetics in two critically ill adults with 2009 H1N1 influenza A concurrently receiving continuous renal replacement therapy

STUDY OBJECTIVE

To determine the pharmacokinetics of intravenous peramivir-an investigational neuraminidase inhibitor for the treatment of 2009 H1N1 infection or nonsubtypable influenza A thought to be the 2009 H1N1 virus-in patients concurrently receiving continuous renal replacement therapy (CRRT).

DESIGN

Pharmacokinetic analysis.

SETTING

Critical care unit at a university-affiliated hospital.

PATIENTS

Two critically ill women with 2009 H1N1 influenza A treated with compassionate-use intravenous peramivir administered as a daily infusion of 600 mg over 30 minutes while receiving continuous venovenous hemodiafiltration (CVVHDF), a form of CRRT.

MEASUREMENTS AND MAIN RESULTS

Plasma samples were collected from the two patients before and 30 minutes after the fourth (first patient) and ninth (second patient) peramivir infusion to estimate minimum (C(min)) and maximum (C(max)) plasma concentrations, respectively. Two additional postinfusion concentrations were measured from each patient to estimate noncompartmental pharmacokinetic parameters of peramivir while receiving CVVHDF. In the two patients, respectively, C(min) was 2170 and 251 ng/ml, C(max) was 18,400 and 20,300 ng/ml, area under the plasma concentration-time curve from 0-24 hours (AUC(0-24)) was 178,000 and 94,400 ng·hour/ml, drug clearance was 56 and 106 ml/minutes, and plasma half-life was 7.6 and 3.7 hours. The volume of distribution adjusted for ideal body weight at steady state was 0.51 and 0.54 L/kg, respectively.

CONCLUSION

The first patient had a slower peramivir plasma clearance compared with the second patient, but both patients had higher peramivir clearances as calculated from AUC(0-24) than those predicted by CRRT. Thus, the dosage of intravenous peramivir was appropriate in these patients. Additional pharmacokinetic data are needed to confirm these results and help guide dosing in patients receiving various forms of CRRT.

The emergency use authorization of peramivir IV: a view from the manufacturer

The 2009 H1N1 influenza pandemic prompted the US Food and Drug Administration (FDA) to issue an emergency use authorization (EUA) for the intravenous antiviral peramivir, an unapproved neuraminidase inhibitor (NAI) currently under development. Peramivir use was limited to patients for whom other NAI therapy had failed or in whom oral or inhalational drug absorption was believed to be unreliable. This introduced a patient selection bias that precluded safety and efficacy assessment. Despite the challenges and risks, there was a compelling public health need for an intravenous agent during the 2009 H1N1 pandemic.

Atrial wall tension changes and the release of atrial natriuretic factor on relief of cardiac tamponade

Cardiac tamponade causes elevation and equalization of cardiac filling pressures, sodium and water retention, and a paradoxically low plasma atrial natriuretic factor (ANF) concentration despite increased intraatrial pressures. Recent reports suggested that plasma ANF concentrations rise after relief of tamponade. The purposes of the present study were (1) to determine the time course and extent of ANF release on relief of cardiac tamponade; (2) to measure the atrial transmural wall pressures, atrial sizes, and atrial wall tension changes associated with relief of tamponade; and (3) to determine the biologic activity of elevated plasma ANF during and after relief of tamponade. We sampled blood for ANF and cyclic guanosine monophosphate (cGMP) immediately before and up to 24 hours after relief of cardiac tamponade in 10 patients. Atrial and pericardial pressures were measured immediately before and shortly after pericardiocentesis, and atrial dimensions were determined by two-dimensional echocardiography before and within 1 hour after the tap. Urine volumes were measured in 8-hour increments before and after the procedure. Relief of cardiac tamponade was associated with a prompt and massive increase in plasma ANF concentrations, reaching pharmacologically active levels. The rise in ANF was negatively correlated with atrial pressures but positively correlated with atrial transmural pressures, atrial size, and calculated wall tension. Plasma ANF levels peaked at 515 +/- 95 pg/ml 40 minutes after relief of tamponade and leveled off at 140% to 180% of the pretap concentrations. Plasma cGMP exhibited a slightly delayed but similar time course to the rise in ANF levels, and urine flow rate increased fourfold in the 8 hours after relief of tamponade.(ABSTRACT TRUNCATED AT 250 WORDS)

The diagnosis and treatment of baroreflex failure

BACKGROUND

Baroreflexes originate in the great vessels of the neck and thorax and prevent arterial pressure from rising or falling excessively.

METHODS

This study was undertaken to clarify the cause, clinical spectrum, and therapy of this disorder. We studied 11 patients with baroreflex failure presenting as severe, labile hypertension and hypotension, often with headache, diaphoresis, and emotional instability, and characterized by the failure of exogenous vasoactive substances to alter heart rate. Each underwent hemodynamic monitoring and biochemical, physiologic, and pharmacologic testing.

RESULTS

The patients' maximal systolic blood pressures ranged from 164 to 280 mm Hg, and their minimal systolic pressures ranged from 58 to 96 mm Hg. Plasma norepinephrine and epinephrine concentrations were sometimes many times normal during blood-pressure surges. All the patients had excessive pressor and tachycardia responses to the mental-arithmetic and cold pressor tests and marked hypersensitivity to clonidine. The underlying causes of baroreflex failure included the familial paraganglioma syndrome, neck surgery or radiation therapy for pharyngeal carcinoma, bilateral lesions of the nucleus tractus solitarii, and surgical section of the glossopharyngeal nerves; in two patients the cause was unknown. Therapy with clonidine reduced the frequency of attacks by 81 percent and attenuated the elevated blood pressure and heart rate in the attacks that occurred.

CONCLUSIONS

The syndrome of baroreflex failure should be considered in patients with otherwise unexplained labile hypertension. Clonidine attenuates the pressor and tachycardic surges in baroreflex failure.

Orthostatic hypotension. Causes, evaluation, and management

Chronic orthostatic hypotension is caused by a variety of disorders. Frequently patients withdraw from social interactions, are prone to adverse drug reactions and inappropriate diagnoses, and are bed-bound by the time of diagnosis. Applying basic principles of cardiovascular physiology and pharmacology usually permits these patients to lead active lives and to live longer. Much of the management is based on common sense and knowledge of the basic pathophysiology of the disorder and depends on thorough patient education and close monitoring of blood pressure in many of the activities of daily living.

Humoral factor, intraplatelet calcium, and hypertension

Plasma humoral factors which modulate the transmembrane distribution of sodium and calcium have been identified in hypertensive patients and have been hypothesized to be involved in the etiology of essential hypertension. In cross-incubation experiments, we have found that plasma of hypertensive subjects elevated basal and stimulated intraplatelet calcium levels, while plasma of normal subjects has an opposite effect on platelets from hypertensives. Basal intraplatelet calcium in normal platelets was 108 +/- 5 nmol/L and rose to 142 +/- 3 nmol/L (P less than .001) after incubation in plasma from hypertensive patients. Platelets from hypertensive patients had basal calcium levels of 182 +/- 11 nmol/L which fell to 127 +/- 11 nmol/L (P less than .01) after incubation in normal plasma. Hypertensive plasma potentiated the rise in intraplatelet calcium in response to ADP and PAF. Hypertensive patients treated experimentally with plasmapheresis exhibited a disappearance of the plasma factor responsible for elevating intraplatelet calcium. These results indicate the presence of a plasma humoral factor in hypertensives which elevates intraplatelet calcium and sensitizes platelets to agonist stimuli.

Role of cyclic AMP in adenosine inhibition of intracellular calcium rise in human platelets. Comparison of adenosine effects on thrombin- and epinephrine-induced platelet stimulation

Thrombin-induced platelet aggregation is associated with an increase in intracellular calcium. Epinephrine provokes aggregation in the absence of a rise in intracellular calcium. Adenosine has been postulated as an endogenous inhibitor of platelet aggregation. In this study, the authors examine the effect of adenosine on the rise in intracellular calcium and on platelet aggregation, and the role of cyclic AMP (cAMP) in these actions. Human platelets were obtained from citrated plasma containing 5 micrograms/mL of indomethacin. Intracellular calcium was determined by fura-2 fluorescent dye. Adenosine inhibited thrombin-induced platelet aggregation and the rise in intracellular calcium in a dose-dependent manner. At a concentration of 100 mumol/L, adenosine completely inhibited thrombin-induced aggregation, but only partly inhibited the rise in intracellular calcium (55%). Adenosine also partially inhibited the rise in calcium produced by thrombin in both calcium-containing and calcium-free media, suggesting that adenosine inhibits both calcium influx and calcium mobilization. The effects of adenosine on intracellular calcium, as in the case of platelet aggregation, appear to be linked to adenylate cyclase, since they were prevented by the adenylate cyclase inhibitor 2',5'-dideoxyadenosine (1-mmol/L) and were potentiated by phospho-diesterase inhibition with papaverine (1 mumol/L). Adenosine and dibutyryl-cAMP also inhibited epinephrine-stimulated platelet aggregation in a dose-dependent manner. Thus, it appears that adenosine may inhibit platelet aggregation independently of its ability to decrease cytosolic free calcium.

Atrial natriuretic factor and hypertension. A review and metaanalysis

Atrial natriuretic factor (ANF) is a recently discovered, volume responsive hormone with multiple potent antihypertensive actions. This article reviews data supporting hypothetical associations between ANF and essential hypertension, examines reports of plasma ANF concentrations in hypertension, discusses the efficacy of ANF and its analogs in the treatment of hypertension, and reviews future issues in ANF research. ANF has been shown to elicit vasodilatation, suppress plasma renin activity, inhibit the synthesis and release of aldosterone, antagonize sympathetically-mediated release of norepinephrine, and promote diuresis and natriuresis. A metaanalysis of plasma ANF concentrations reported in normal and hypertensive subjects reveals a 5 +/- 19 pg/mL (pooled, weighted mean and standard deviation) higher ANF level in age-matched, untreated hypertensives without evidence of end-organ damage. This difference may be inappropriately low given the increase in atrial filling pressures found in hypertension. Low doses of ANF elicit greater reductions in blood pressure in hypertensive subjects than in normals. Recently, inhibitors of the ANF-degrading enzyme, neutral endopeptidase, and of the ANF "clearance" receptor have enhanced the antihypertensive actions of endogenous or exogenously administered ANF. Human studies are currently in progress testing the antihypertensive efficacy of orally administered neutral endopeptidase inhibitors. The discovery of ANF has led to the elucidation of a family of natriuretic peptides from brain, heart, and kidney, and promises to enlarge our understanding of volume regulation in normal and pathophysiological states. The possibility that essential hypertension is associated with inappropriately low plasma ANF levels or altered responsiveness to ANF may offer new insights into the pathogenesis and treatment of hypertension.

Direct vasodilator effect of hyperventilation-induced hypocarbia in autonomic failure patients

Hyperventilation produces small decreases in blood pressure in normal subjects and larger decreases in patients with autonomic failure. The authors studied the mechanism for this observation by measuring mean arterial pressure (MAP) and arterial blood gas (ABG) changes in eight patients with severe primary autonomic failure after various maneuvers designed to alter PaCO2, PaO2, and pH. Maneuvers included voluntary hyperventilation, breathing a 5% CO2/95% O2 mixture, breathing 12% O2, breathing through a 1 meter tube to increase dead space, breathing 100% O2, and infusion of 120 mEq NaHCO3 over 30 minutes. All maneuvers led to expected changes in ABGs. Voluntary hyperventilation lowered MAP by 23 +/- 4 (p less than 0.01) mmHg but MAP was raised 11 +/- 3 and 7 +/- 1 mmHg by hyperventilation resulting from increasing breathing dead space or from breathing 5% CO2, respectively. Breathing 100% O2 or 12% O2 had no significant effect on MAP, and NaHCO3 infusion raised MAP by 8 +/- 4 (p less than 0.05) mmHg. With all maneuvers, change in MAP correlated with change in PaCO2 (r = 0.72, p less than 0.001) and change in pH (r = -0.57, p less than 0.01) but not with PaO2. Multiple regression analysis showed that only changes in PaCO2 predicted the change in MAP for all maneuvers. The authors conclude that a decrease in PaCO2 causes the observed decreases in MAP with hyperventilation. This most likely represents a direct peripheral vasodilator effect of hypocarbia rather than a reflex or centrally-mediated mechanism since our patient population is characterized by inadequate or absent autonomic cardiovascular reflex responses.

Adenosine inhibits the rise in intracellular calcium and platelet aggregation produced by thrombin: evidence that both effects are coupled to adenylate cyclase

Platelet aggregation and secretion are associated with a rise in intracellular calcium concentration ([Ca2+]i). Adenosine has been postulated as an endogenous inhibitor of platelet aggregation. The antiaggregatory effects of adenosine are related to activation of adenylate cyclase. We studied the effect of adenosine on the rise in [Ca2+]i and platelet aggregation produced by thrombin. Human platelets were obtained from dextrose/citrate-treated plasma. [Ca2+]i was determined by fluorescence-dye techniques (fura-2). Adenosine inhibited the slope of the first phase of aggregation and the rise in [Ca2+]i produced by thrombin, in a dose-dependent manner. The dose that produced 50% inhibition of both aggregation and the rise in [Ca2+]i was approximately 500 nM. The effects of adenosine on [Ca2+]i were shared by its stable analogs, 5'-N-ethylcarboxamidoadenosine being approximately 10-fold more potent than (-)N6-phenylisopropyladenosine, suggesting that these effects were mediated through adenosine A2 receptors. Furthermore, caffeine antagonized the inhibitory effects of adenosine on platelet aggregation and [Ca2+]i. The effects of adenosine on [Ca2+]i appear to be mediated through a rise in intracellular cAMP, because they were prevented by the adenylate cyclase inhibitor 2',5'-dideoxyadenosine (1 mM) and were potentiated by phosphodiesterase inhibition with papaverine (1 microM). Adenosine also inhibits the rise in [Ca2+]i produced by thrombin in a calcium-free medium, suggesting that adenosine inhibits both calcium influx and the release of calcium from intracellular stores.

Clearance of atrial natriuretic factor by lung, liver, and kidney in human subjects and the dog

We determined human and canine plasma clearance of atrial natriuretic factor (ANF) by lung, liver, and kidney from arteriovenous differences in plasma ANF and measured organ plasma flow. Human subjects had lower plasma ANF concentrations in the pulmonary vein or the pulmonary capillary wedge position when compared with the pulmonary artery, and both sites yielded pulmonary ANF extraction ratios of 24%. Canine lung ANF extraction was 19 +/- 3% and pulmonary ANF clearance was 328 +/- 78 ml/min per m2 vs. 357 +/- 53 ml/min per m2 in man. Hepatic plasma ANF clearance was 216 +/- 26 ml/min with an extraction ratio of 30 +/- 3% in humans and 199 +/- 89 ml/min and 36 +/- 6% in the dog. Renal plasma ANF clearance in human subjects was 78 +/- 12 ml/min per kidney and correlated well with each kidney's creatinine clearance (r = 0.58, P less than 0.05). The mean renal ANF extraction ratio was 35 +/- 4% in human subjects and 42 +/- 6% in the dog. These data quantitate the specific organ ANF clearances by lung, liver, and kidney in human subjects and in dogs and provide a rationale for elevated plasma ANF levels in cirrhosis, renal failure, and diseases accompanied by reduced perfusion of these organs. These findings support the conclusion that plasma ANF concentrations are dependent upon both the stimuli for ANF secretion as well as the specific organ clearances of ANF.

Plasma human atrial natriuretic factor in cirrhosis and ascites with and without functional renal failure

Functional renal failure of cirrhosis (FRFC) is a usually fatal syndrome of acute renal failure occurring in patients with advanced liver disease. Although not conclusively proven, most evidence suggests that renal arterial and arteriolar vasoconstriction is the cause of the renal failure in these patients. However, the mediators of the vasoconstriction remain unknown. Human atrial natriuretic factor (hANF) is a hormone with potent natriuretic, diuretic, and vasorelaxant properties. A deficiency of hANF could lead to renal arterial vasoconstriction and avid renal sodium retention as seen in FRFC. This study was undertaken to determine if patients with FRFC are deficient in circulating hANF. Seven patients with advanced alcoholic liver disease and renal failure of unknown cause (FRFC) were compared with 7 patients with advanced alcoholic liver disease, ascites, and normal serum creatinine as well as with 14 healthy volunteers. Plasma hANF was measured by radioimmunoassay. Plasma hANF was 742 +/- 227 pg/ml (mean +/- SEM) in patients with FRFC compared with 360 +/- 70 pg/ml in patients with liver disease and normal serum creatinine (p greater than 0.05) and 28 +/- 5.7 pg/ml in healthy volunteers (p less than 0.005 vs. FRFC and chronic liver disease, ascites, and normal serum creatinine). Thus, FRFC is not caused by a deficiency of circulating hANF. The elevated plasma hANF levels in patients with chronic liver disease and continued sodium retention may suggest a renal insensitivity to the natriuretic effects of hANF.

Norepinephrine and epinephrine in human erythrocyte plasma membranes

Acetone extract prepared from the plasma membranes of human erythrocytes contained substances which induced the contraction of the thoracic aortic strip of rabbit in vitro and caused blood pressure elevation in rat upon intravenous injection. The contractile response was inhibited by the alpha 1-adrenergic antagonist prazosin. By HPLC/electrochemical detection as well as radioenzymatic assay, large amounts of norepinephrine (NE) (14 +/- 4 [SE] ng/ml packed cells) and epinephrine (E) (16 +/- 2 ng/ml packed cells) were found in the extract. Using the same amounts as in the extract, we were able to demonstrate additive effect between NE and E. The possibility that erythrocyte membranes may play a role in the regulation of NE and E in circulation is suggested.

Both alpha- and beta-adrenoreceptors contribute to the central depressor effect of catecholamines

We compared the effects of alpha 2- and beta-adrenoreceptor blockade on the central actions of catecholamines and metabolites of alpha-methyldihydroxyphenylalanine, epinephrine, alpha-methylnorepinephrine, and alpha-methylepinephrine were studied. I.c.v. and nucleus tractus solitarii (NTS) injections were carried out under anesthesia. Following i.c.v. injection, both epinephrine and methylepinephrine rapidly reduced blood pressure and heart rate, but the effects of methylnorepinephrine occurred somewhat later. Following microinjection into the nucleus of the solitary tract, epinephrine, methylepinephrine, and methylnorepinephrine all caused hypotension and bradycardia. The hypotensive effects of all 3 amines in the NTS were attenuated in additive fashion by yohimbine, an alpha 2 adrenoreceptor antagonist, and timolol, a beta-adrenoreceptor antagonist, whereas only yohimbine attenuated the bradycardia. The combination of yohimbine and timolol abolished the effects of the amines. These data suggest that in the NTS both alpha 2 and beta adrenoreceptor stimulation contribute to the hypotensive effects of these amines, but that only alpha 2 adrenoreceptors are principally involved in the heart rate response.

Depletion of brainstem epinephrine stores by alpha-methyldopa: possible relation to attenuated sympathetic outflow

The antihypertensive effect of alpha-methyldopa (MD) is believed to be critically dependent on its ability to deplete endogenous catecholamines or cause the synthesis of false neurotransmitters. We used liquid chromatography with electrochemical detection (LCEC) and negative chemical ionization gas chromatography-mass spectrometry (GC-MS) for quantitation of catecholamines and MD metabolites in rat. MD intraperitoneally (100 mg/kg q12 hr X 12 days), significantly increased alpha-methylnorepinephrine (MNE) in brain (1.02 +/- 0.33 micrograms/g), heart (1.67 +/- 0.57 micrograms/g) and adrenal glands (114.93 +/- 50.47 micrograms/g) Endogenous norepinephrine (NE), epinephrine (E) and dopamine (DA) were reduced. ME levels were 2.19 +/- 0.44 micrograms/g (n = 6) in the adrenal gland but only 99 +/- 26 pg/g (n = 3) in the brainstem. MD-induced endogenous brainstem NE depletion was more than compensated by MNE production, but brainstem E depletion was not compensated for by a stoichiometric production of brainstem ME. We conclude (1) although ME is a metabolite of MD, it is present in extremely low concentrations in brainstem and (2) central epinephrine-containing neurons are depleted of neurotransmitter by MD therapy. If this selective epinephrine depletion occurs in the bulbospinal tract neurons responsible for maintaining sympathetic tone, then this effect could contribute to the antihypertensive effect of MD.

Cardiovascular and respiratory effects of adenosine in conscious man. Evidence for chemoreceptor activation

The cardiovascular and respiratory effects of intravenous adenosine were studied in conscious normal volunteers. Bolus injections of adenosine increased systolic and diastolic pressures initially (+15 and +13 mm Hg after 100 micrograms/kg) followed by a subsequent reduction in systolic and diastolic pressures (-12 and -16 mm Hg). Heart rate increased during trough blood pressure (R-R interval shortening of 298 msec after 100 micrograms/kg). Adenosine steady-state infusions increased heart rate (+30 beats/min during 140 micrograms/kg/min), systolic pressure (+16 mm Hg), and pulse pressure (+21 mm Hg) but decreased diastolic pressure slightly (-5 mm Hg), resulting in no significant change in mean arterial pressure. Adenosine stimulated respiration, resulting in decreased PaCO2 (41 to 31 mm Hg), increased PaCO2 (101 to 113 mm Hg), and increased pH (7.42 to 7.50). The increased ventilation was not explained by bronchoconstriction, hypotension, or hypoxia. The observed pressor and tachycardic effects are mediated through reflex autonomic mechanisms since they are completely abolished in patients with severe autonomic failure. These autonomic mechanisms probably involve chemoreceptor activation since adenosine is pressor when infused in the aortic arch proximal to the origin of the carotid arteries but depressor when infused in the descending aorta. It is concluded that the hemodynamic and respiratory effects of adenosine observed in normal volunteers are in part due to chemoreceptor stimulation. These findings raise the possibility that adenosine is an endogenous modulator of respiration in man.

Neck tumour with syncope due to paroxysmal sympathetic withdrawal

A patient with recurrent squamous carcinoma metastatic to the neck after radical neck dissection and high dose radiation therapy developed paroxysmal hypotensive episodes that were severe, spontaneous and characterised by suppressed sympathetic but not enhanced parasympathetic activity. Intravenous pressors were successful in treating acute episodes but neither drug therapy nor surgical neck exploration reliably prevented syncopal attacks. Glossopharyngeal and/or vagal nerve infiltration by tumour with episodic activation of the afferent limb of the baroreflex arc producing vasodilatation primarily due to sympathetic withdrawal is the likely mechanism of life threatening syncope in this patient.

Oral yohimbine in human autonomic failure

Yohimbine is an alpha 2-adrenoreceptor antagonist that acts to enhance sympathetic nervous system discharge and potentiate sympathetically mediated cardiovascular reflex responses. We therefore assessed the ability of yohimbine to increase sympathoadrenal discharge and raise blood pressure (BP) in patients with autonomic failure characterized by profound orthostatic hypotension. Yohimbine 5 mg orally in eight seated patients significantly elevated mean systolic BP by 33 mm Hg from 136 +/- 15 (mean +/- SD) to a maximum of 169 +/- 23 mm Hg (p less than 0.01), mean diastolic BP by 16 mm Hg from 77 +/- 9 to a maximum of 93 +/- 15 mm Hg (p less than 0.01), and mean heart rate (HR) by 10 beats per minute (BPM) from 68 +/- 12 to a maximum of 78 +/- 17 BPM (p less than 0.01). Plasma norepinephrine (NE) increased from 104 +/- 71 to a maximum of 196 +/- 182 pg/ml (p less than 0.05), but plasma epinephrine (E) did not increase significantly (31 +/- 18 versus a maximum of 39 +/- 21 pg/ml). In five patients given yohimbine 2.5 mg orally, BP, HR, NE, and E tended to increase, but the changes were not significant. Plasma yohimbine levels correlated significantly with the changes in mean arterial pressure (r = 0.61, p less than 0.01). Yohimbine raises BP and HR in patients with autonomic failure. These effects are dose- and concentration-dependent and mediated through increased sympathetic discharge. Yohimbine may be useful in the treatment of orthostatic hypotension associated with autonomic failure. It is unique among current modes of therapy for this disorder in that it enhances discharge of the patient's own sympathetic system.

Postjunctional vascular smooth muscle alpha-2 adrenoreceptors in human autonomic failure

Animal studies have suggested that vasoconstrictor alpha-2 adrenoreceptors exist on vascular smooth muscle cells. We tested this hypothesis in a patient with severe autonomic failure who demonstrated a pressor response to oral clonidine (a selective alpha-2 adrenoreceptor partial agonist). After clonidine 0.8 mg orally, mean arterial pressure rose by 54 mm Hg. After pretreatment with prazosin (a selective alpha-1 adrenoreceptor antagonist) and confirmation of alpha-1 blockade, clonidine 0.8 mg still raised mean arterial pressure by 43 mm Hg. After pretreatment with yohimbine (a selective alpha-2 adrenoreceptor antagonist), clonidine 0.8 mg elevated mean arterial pressure by 13 mm Hg. Since alpha-1 antagonism does not block, and alpha-2 antagonism does not block the pressor effect of clonidine, we conclude that clonidine raised blood pressure in this severely affected autonomic failure patient by vascular alpha-2 adrenoreceptor stimulation. Thus, this provides pharmacological evidence that postjunctional vascular smooth muscle alpha-2 adrenoreceptors exist in man and can modulate blood pressure.

Cardiovascular effects of adenosine infusion in man and their modulation by dipyridamole

In man, intravenous infusion of adenosine has been useful in inducing sustained hypotension during anesthesia. Bolus injections terminate supraventricular tachyarrhythmias by delaying AV node conduction. It has been proposed that some of its cardiovascular effects are related to inhibition of noradrenergic neurotransmission. We assessed the cardiovascular and sympathoadrenal effects of intravenous infusion of adenosine (10 to 140 micrograms/kg/min) in 7 conscious normal subjects. At the highest infusion rate achieved, adenosine increased heart rate by 33 bpm (p less than 0.005), increased systolic blood pressure by 13 mm Hg (p less than 0.02) and decreased diastolic blood pressure by 8 mm Hg (p less than 0.02). Plasma norepinephrine and epinephrine increased 44% and 213% respectively. Basal plasma renin activity was 0.7 +/- 0.09 ng AI/ml/hr and remained unchanged. Higher doses were not given due to the appearance of subjective side effects (headache, nervousness, flushing and an urge to breathe deeply). During dipyridamole administration, 4-fold lower doses were required to produce equivalent cardiovascular effects. We conclude that in conscious man, intravenous infusion of adenosine is associated with activation rather than inhibition of the sympathoadrenal system. The possible mechanisms of this sympathetic activation are discussed.

Reduction of liver plasma flow by caffeine and theophylline

Caffeine and theophylline block the vasodilating effects of adenosine and may act to enhance sympathoadrenal discharge and activate the renin-angiotensin system. To determine if these methylxanthines might thereby have effects on regional blood flow, we studied the influence of caffeine and theophylline on apparent liver plasma flow (LPF) in normal subjects as assessed by indocyanine green clearance. Oral caffeine, 250 mg, reduced LPF by 19% from 630 +/- 150 to 510 +/- 120 ml/min (P less than 0.001). Intravenous theophylline (4.3 mg/kg) reduced LPF by 15% from 550 +/- 50 to 470 +/- 90 ml/min (P less than 0.05). These methylxanthine-induced falls in LPF may alter the disposition of concomitantly administered drugs. Because of their widespread use in Western society, caffeine and theophylline may be major determinants of liver blood flow in the general population. They may therefore prolong the t1/2 and increase steady-state levels of hepatically eliminated drugs.

Use of alpha 2 adrenoreceptor agonists and antagonists in the functional assessment of the sympathetic nervous system

We studied the effects of clonidine, an alpha 2-adrenoreceptor agonist, and yohimbine, an alpha 2-adrenoreceptor antagonist, on blood pressure, heart rate, and plasma catecholamines in 12 patients with autonomic dysfunction. Clonidine (0.1 mg, orally) lowered blood pressure 18 +/- 3 torr in six subjects and raised it 5 +/- 1 torr in six. The change in blood pressure in response to this dose of clonidine was inversely proportional to the supine level of norepinephrine (P less than 0.05). Yohimbine (4-64 micrograms/kg) raised plasma norepinephrine and blood pressure in six patients with degenerative autonomic dysfunction. Yohimbine also attenuated by 50% (P less than 0.05) the hypotensive response to head-up tilt of patients with degenerative autonomic dysfunction. Clonidine depends upon postjunctional hypersensitivity and the degree of autonomic insufficiency to elicit its pressor response. In contrast, the pressor response to yohimbine is related to the capacity of the sympathetic nervous system to be activated and release norepinephrine. If plasma norepinephrine levels following yohimbine administration are monitored, the biochemical and hemodynamic response to the drug may provide a sensitive indication of the capacity of the sympathetic nervous system to be activated in patients with autonomic dysfunction.

Atrial pressure and secretion of atrial natriuretic factor into the human central circulation

Atrial natriuretic factor, a peptide found in mammalian cardiac atria, has natriuretic and vasodilatory properties that may be important in the regulation of intravascular volume. To study factors related to its release in human subjects, intracardiac pressures and plasma atrial natriuretic factor concentrations in the central circulation were measured in 34 patients with a variety of cardiovascular disorders. Plasma atrial natriuretic factor concentration increased from the inferior vena cava to the right atrium (76 +/- 24 to 162 +/- 37 pg/ml, p less than 0.001) and from the vena cava to the aorta (76 +/- 24 to 177 +/- 46 pg/ml, p less than 0.001). Mean right atrial pressure was positively correlated with atrial natriuretic factor concentration in the pulmonary artery (r = 0.58, p less than 0.001), and mean pulmonary capillary wedge pressure was positively correlated with concentration in the aorta (r = 0.64, p less than 0.001). In six patients whose atrial natriuretic factor concentrations were measured at two different levels of atrial pressure, increased atrial pressure was accompanied by increased atrial natriuretic factor concentration in the pulmonary artery (p less than 0.01) and aorta (p less than 0.01). Atrial natriuretic factor levels measured in fresh myocardium from a patient undergoing cardiac transplantation showed tissue concentrations in the atria 500-fold higher than tissue concentrations in the ventricles. These data document that atrial natriuretic factor is found in human atrial myocardium and suggest that it may be released in response to increased atrial pressure. Such a secretory release mechanism is consistent with the hypothesis that atrial natriuretic factor plays a role in the regulation of circulatory volume.

Sodium loading and posture modulate human atrial natriuretic factor plasma levels

Atrial natriuretic factor is postulated to act through atrial stretch receptors as a volume regulatory hormone that stimulates diuresis and natriuresis in response to increased atrial pressure. To characterize the stimuli associated with the release of atrial natriuretic factor in humans, we studied 14 normal subjects, both in the supine position and after 10 minutes in an upright posture, while they were on a regular diet (Day 0) and during 3 days of supplemental sodium chloride intake (8 g/day). Radioimmunoassay of plasma atrial natriuretic factor was performed with rabbit antibody to the human hormone amino acids (102-126). Urinary sodium excretion increased from 111 +/- 13 mEq/day (mean +/- SEM) on Day 0 to 275 +/- 15 mEq/day by the third day (Day 3) of high sodium intake. The level of atrial natriuretic factor in the supine position rose from 17 +/- 4 pg/ml (Day 0) to 76 +/- 13 pg/ml on Day 3 (p less than 0.001) and after 10 minutes in an upright posture on Day 3, the level fell to 32 +/- 10 (p less than 0.005). Plasma concentrations of atrial natriuretic factor correlated positively with spot and 24-hour urinary sodium excretion and weight gain, and correlated negatively with plasma aldosterone and renin activity. We conclude that the response of atrial natriuretic factor to sodium loading and posture change in humans is appropriate for a volume regulatory hormone.

Plasma catecholamine modulation of alpha 2 adrenoreceptor agonist affinity and sensitivity in normotensive and hypertensive human platelets

We measured alpha 2-adrenoreceptor density as well as affinity for and sensitivity to agonist on intact platelets of normotensive and hypertensive subjects before and after physiological increases in plasma catecholamines. In normotensives, posture-induced rises in plasma catecholamines correlated with reduced alpha 2-adrenoreceptor agonist affinity and fewer high affinity state receptors. Platelet aggregation and inhibition of adenylate cyclase by L-epinephrine also was reduced. Hypertensive subjects had similar rises in plasma catecholamines with upright posture, but showed no change in receptor affinity or sensitivity. No change in platelet alpha 2-adrenoreceptor number occurred in these studies. In vitro incubation with L-epinephrine revealed that platelets from hypertensives had slower desensitization than those from normotensives. Binding studies at different temperatures and with varying sodium concentrations found no thermodynamic or sodium-dependent differences between normotensive and hypertensive groups. These studies demonstrate that platelets from hypertensive subjects exhibit a defect in the ability of physiological concentrations of agonist to desensitize the alpha 2-adrenoreceptor.

Management of chronic orthostatic hypotension

Chronic orthostatic hypotension is characterized by recurrent symptoms of cerebral hypoperfusion due to low upright blood pressure levels. The initial approach should be to identify and correct reversible causes. Persistence of orthostatic hypotension suggests autonomic failure. The goal of management is to minimize symptoms and maximize functional capacity; therefore the magnitude of blood pressure fall is not as important as the advent of symptoms. Therapy is based upon the underlying pathophysiology and the risk/benefit ratio of interventions. Patient education and nondrug measures form the cornerstone of management. Drug therapy is often limited by unacceptable supine hypertension. Rational drug use can be governed by individualized trials of therapy. Patients with moderate or severe orthostatic hypotension are difficult to treat, but can be helped toward resumption of a normal life.

Hemodynamic and humoral effects of caffeine in autonomic failure. Therapeutic implications for postprandial hypotension

We examined the effects of caffeine and meals on blood pressure and heart rate in 12 patients with autonomic failure. The influence of caffeine on plasma norepinephrine, epinephrine, and renin activity was also studied. Caffeine 250 mg, raised blood pressure by 12/6 mm Hg, from 129 +/- 25/78 +/- 12 (mean +/- S.D.) to a maximum of 141 +/- 30/84 +/- 16 mm Hg at 45 minutes (P less than 0.01), but did not change heart rate, levels of norepinephrine, or epinephrine, or plasma renin activity. Blood pressure fell by 28/18 mm Hg after a standardized meal, from 133 +/- 32/80 +/- 15 to a minimum of 105 +/- 21/62 +/- 12 mm Hg at 60 minutes (P less than 0.01). After pretreatment with 250 mg of caffeine, the standardized meal induced a fall of only 11/10 mm Hg, from 140 +/- 33/79 +/- 7 to 129 +/- 31/69 +/- 13 mm Hg at 60 minutes (P less than 0.05 vs. values after the control per day for seven days) in five patients, postprandial blood pressures remained higher after caffeine than after placebo (P less than 0.05). We conclude that caffeine is a pressor agent and attenuates postprandial hypotension in autonomic failure, and that this effect is not primarily due to elevations in sympathoadrenal activity or activation of the renin-angiotensin system. Caffeine may be useful in the treatment of orthostatic hypotension due to autonomic failure, especially in the postprandial state.

Reflexes unique to myocardial ischemia and infarction

Although arrhythmias caused by myocardial ischemia are a well recognized cause of sudden death, the potential influence of cardiogenic reflexes originating in areas of ischemia has received less attention. In this study, 12 patients with well documented single vessel coronary artery spasm, with a total of 2,240 episodes of transient transmural ischemia, are described. Continuous electrocardiographic and hemodynamic recordings were analyzed to determine possible relations between the anatomic area of ischemia and patterns of change in blood pressure and heart rate. Of seven patients with ischemia of the posterior or inferior left ventricular wall, six had associated bradycardia and hypotension, an apparent Bezold-Jarisch response. Only one of five patients with anterior ischemia had a similar response. A hypertensive, tachycardiac response resembling the James reflex was seen in two of the patients with anterior ischemia, with an increase in blood pressure of 36/22 +/- 12/6 mm Hg and an increase in heart rate of 8 +/- 3 beats/min. This increase began before the onset of chest pain and was seen even in asymptomatic episodes. These reflexly mediated hemodynamic responses may modulate the direct effects of myocardial ischemia and could play a role in sudden cardiac death.

Antihypertensive metabolites of alpha-methyldopa

alpha-Methyldopa acts through a metabolite in the central nervous system. Of the three metabolites most prominently considered as potential mediators of alpha-methyldopa hypotension--alpha-methyldopamine, alpha-methylnorepinephrine (MNE), and alpha-methylepinephrine (ME)--ME is the most potent depressor agent following intravenous infusion into rats, following injection into the lateral ventricle, and following injection into the solitary tract nucleus (NTS). The depressor effect of ME in the NTS is attenuated as effectively by timolol as by yohimbine, while the combination of both timolol and yohimbine completely abolishes the pharmacological activity of ME in the NTS. ME is approximately eightfold more potent than MNE in the NTS and also has a greater susceptibility to timolol attenuation.

Caffeine and hypertension

The effect of prolonged caffeine administration on blood pressure in hypertensive subjects was assessed in a double-blind placebo-controlled study. Eighteen hypertensive subjects participated, nine of whom received placebo throughout the study and nine of whom received placebo during the first three days, caffeine during the subsequent seven days, and placebo during the final four days of the two-week study. Those who received caffeine were given 250 mg with meals three times daily. There were no untoward reactions in the course of the study, but one subject with unacceptably high blood pressures while receiving placebo had to be discharged from the study to resume antihypertensive therapy. Systolic blood pressure was immediately increased (9.2 +/- 3.4 mm Hg) within 15 minutes after the first dose of 250 mg of caffeine. On the first day of caffeine, systolic pressure was increased a mean of 7.3 +/- 4.0 mm Hg, but this was no longer significant after the initial day of caffeine administration. Diastolic pressure showed a trend toward increasing, but this never reached significance. The minor increases in plasma catecholamine levels and plasma renin activity were not significant on either a short- or long-term basis. After discontinuation of caffeine, no overshoot phenomena were observed. It is concluded that prolonged administration of caffeine is not associated with significant elevation in blood pressure, plasma catecholamine levels, or plasma renin activity in patients with borderline hypertension.

Increased vascular beta2-adrenoceptor responsiveness in autonomic dysfunction

Responsiveness to the vasopressor, vasodepressor and chronotropic effects of several sympathomimetic amines was assessed in 12 patients with severe autonomic dysfunction and in 8 age-matched control subjects. The patients with autonomic dysfunction showed a profound increase in responsiveness to both isoproterenol and phenylephrine as compared with control subjects. The mean bolus dose of isoproterenol required to increase heart rate by 25 beats/min was 0.9 + 0.2 microgram in the patients and 5.4 + 2.1 micrograms in the control subjects. The dose of isoproterenol required to reduce mean blood pressure by 25 mm Hg was 0.3 + 0.2 and 5.2 + 1.8 micrograms, respectively. Thus, although there is a 6-fold increase in responsiveness to the chronotropic effect of isoproterenol in autonomic dysfunction, the responsiveness to the drug's depressor effect is increased 17-fold. This enhanced depressor sensitivity is quite marked, even with oral beta-adrenoceptor agonists. Beta-adrenoceptor agonists must be used with caution in conditions associated with autonomic dysfunction if dangerous hypotension is to be avoided.

Central and peripheral cardiovascular effects of alpha-methylepinephrine

We compared peripheral and central cardiovascular effects of (+/-)-alpha-methylepinephrine (alpha-ME) and (+/-)-alpha-methylnorepinephrine (alpha-MNE), two putative active metabolites of alpha-methyldopa to those of (-)-epinephrine in urethane-anesthetized normotensive rats. Intravenous administration of 1 microgram doses of alpha-ME (4.3 nmol) lowered blood pressure whereas alpha-MNE (4.5 nmol) and (-)-epinephrine (3.0 nmol) raised blood pressure. Heart rate responses to each were opposite to the blood pressure response, consistent with reflex buffering. When administered into the cerebral ventricle (5-20 micrograms) (15-90 nmol) and nucleus of the solitary tract (0.3-10 nmol), each catecholamine caused marked reductions in both blood pressure and heart rate. alpha-ME was more potent than alpha-MNE and the endogenous catecholamine, (-)-epinephrine, in its central depressor effect. The greater potency of alpha-ME relative to alpha-MNE and (-)-epinephrine suggests that it could contribute to antihypertensive actions of alpha-methyldopa.

Acute reduction in human platelet alpha 2-adrenoreceptor affinity for agonist by endogenous and exogenous catecholamines

The binding characteristics of l-epinephrine to intact human platelets were assessed under conditions of physiological and pharmacological variations in plasma catecholamine concentration. In competition with the alpha 2-adrenoreceptor antagonist yohimbine, mean platelet receptor affinity for l-epinephrine was decreased 3.4-fold after 2 h of upright posture and exercise. This change in agonist affinity correlated significantly with the increases in plasma epinephrine and norepinephrine that were stimulated by upright posture and exercise. Supine subjects infused with l-norepinephrine or l-epinephrine for 2 h also averaged a 3.3- and 2.7-fold decrease in platelet alpha 2-adrenoreceptor affinity for agonist with no change in receptor number or antagonist affinity. The alpha 2-adrenoreceptor agonist affinity changes were specific for alpha-agonists since they were blocked by phentolamine, and incubation with 10(-5) M isoproterenol produced no change in alpha 2-adrenoreceptor affinity for l-epinephrine. In vitro exposure of intact human platelets to 10(-6) to 10(-10) M l-epinephrine for 2 h produced a concentration-related decrease in alpha 2-adrenoreceptor affinity for agonist. In all three paradigms, average slope factors approached 1.0 as affinity decreased, which is consistent with a heterogeneous receptor population that becomes more homogeneous after agonist exposure. Incubation of platelet-rich plasma with 10(-6) to 10(-8) M l-epinephrine resulted in a dose- and time-related loss of aggregatory response to l-epinephrine; this demonstrates that agonist affinity changes are correlated with changes in receptor sensitivity. These observations demonstrate that physiological variations in plasma catecholamines acutely modulate the intact human platelet alpha 2-adrenoreceptor's affinity for agonist, and can thereby alter the sensitivity of platelets to alpha 2-adrenergic agonist.

Influence of yohimbine on blood pressure, autonomic reflexes, and plasma catecholamines in humans

We studied the influence of the alpha 2-adrenoreceptor-blocking drug, yohimbine, on blood pressure, plasma norepinephrine, and other measures of autonomic function in normal male volunteers. These studies were designed to evaluate the role of alpha 2-receptors in the tonic regulation of sympathetic outflow in humans. In a dose-ranging study, we found that yohimbine HCl (0.016-0.125 mg/kg) elicited dose-related rises in mean, systolic, and diastolic pressures. At the maximal dose used (0.125 mg/kg), respective increments in mean, systolic, and diastolic pressures were 14 +/- 1 torr; 28 +/- 3 torr; and 8 +/- 1 torr (p less than 0.01) (mean +/- SE). No significant changes in heart rate occurred. Associated with the rise in blood pressure were enhanced pressor and heart rate responses to the cold pressor, isometric handgrip, and Valsalva maneuvers. In a double-blind study, yohimbine (0.125 mg/kg bolus, 0.001 mg/kg/min infusion) induced a two-to-threefold rise in plasma norepinephrine (p less than 0.01), without significantly altering plasma epinephrine or plasma renin activity. Ex vivo platelet aggregation in response to epinephrine was inhibited during yohimbine, showing that non-innervated alpha 2-adrenoreceptors were inhibited. Central effects of yohimbine were evaluated through use of linear analog mood rating scales which showed a shift from calm toward excited ends of these scales. If yohimbine is acting through blockade of alpha 2 receptors, then these receptors tonically suppress sympathetic outflow in humans.

Clonidine raises blood pressure in severe idiopathic orthostatic hypotension

The hemodynamic effects of clonidine were studied in four patients with sever idiopathic orthostatic hypotension and one patient with baroreceptor dysfunction. No depressor response to clonidine was observed in any patient with idiopathic orthostatic hypotension at any dosage. Rather, two patients responded to 0.4 mg of oral clonidine with a 40 mm Hg increment in systolic blood pressure lasting several hours. Each has been receiving clonidine, 0.4 mg twice daily, for one year with greatly increased functional capacity. The other two patients with idiopathic orthostatic hypotension had an even greater pressor response to 0.8 mg of oral clonidine, but adverse effects prevented continued therapeutic use. In marked contrast, the patient with baroreceptor dysfunction had a profound depressor response to 0.2 mg of clonidine. In the treatment of idiopathic orthostatic hypotension, the major advantage of clonidine over other pressor agents is its longer duration of action. The major adverse effects of the drug in these patients are sedation, dry mouth, altered mentation, and excessive hypertension. The drug should not be given to patients with mild idiopathic orthostatic hypotension or selective baroreceptor dysfunction, since severe hypotension may result.