The effects of intravenous endotoxin on various host-effector molecules

To understand better the pathogenesis of gram-negative infections, we administered a single intravenous injection of Escherichia coli RE-2 endotoxin (4 ng/kg) to five normal volunteers and periodically measured various host-effector molecules in blood. All subjects had a significant (p less than 0.05) increase in temperature (maximum of 1.3 +/- 0.1 degrees C at 4 hours) and white blood cell count (maximum of 7700 +/- 1800 cells per cubic millimeters at 8 hours). Thymocyte costimulatory activity in blood was markedly increased 2 hours after endotoxin administration but returned to baseline at 4 hours. Blood cortisol and beta-endorphin levels paralleled each other, peaking 4 hours after endotoxin (mean increases of 21 +/- 14 micrograms/dl and p less than 0.01; 18.6 +/- 5.9 pg/ml and p less than 0.05, respectively) and returning to baseline at 8 hours. Blood histamine levels did not significantly change at any time point after endotoxin administration. Since thymocyte costimulatory activity was the first of the measured parameters to achieve peak blood levels, we suggest that endotoxin-induced increases in various cytokines may contribute to the increases in the other measured parameters and thereby play a significant role in the pathogenesis of gram-negative infections.

Elevated BAL fluid histamine levels and parenchymal pulmonary disease in rheumatoid arthritis

To determine the amount of histamine in BAL fluid in subjects with RA and to ascertain if elevated histamine levels were associated with parameters of active pulmonary disease, we measured BAL fluid histamine levels in 31 subjects with RA and 36 normal subjects. The subjects with RA had a significantly greater mean BAL histamine level than the normal subjects, (313 +/- 154 pg/ml vs 18 +/- 8 pg/ml; p less than 0.05). When the subjects with RA were divided into three groups based on chest radiograms (1 = normal; 2 = pleural disease only; 3 = interstitial or nodular disease), we found that subjects in group 3 had significantly lower values for TLC and D. Subjects in group 3 also had higher percentages of BAL neutrophils and eosinophils and higher BAL histamine levels (group 1, 115 +/- 52 pg/ml; group 2, 30 +/- 30 pg/ml; and group 3, 1,182 +/- 709 pg of histamine per milliliter). Moreover, BAL histamine levels were negatively correlated with TLC (r = -0.46; p = 0.01) and FVC (r = -0.45; p = 0.01) and positively correlated with BAL neutrophils (r = 0.6; p = 0.0003) and BAL eosinophils (r = 0.89; p = 0.0001). These data suggest that the BAL histamine level may be a useful marker to determine the activity of pulmonary disease in RA.

Azelastine inhibits stimulated histamine release from human lung tissue in vitro but does not alter cyclic nucleotide content

To investigate the mechanism by which azelastine may be effective therapeutically in asthma, we studied its ability to inhibit anti-IgE- and calcium ionophore A23187-stimulated histamine release from human lung and to alter lung cyclic nucleotide levels. Significant inhibition of histamine release from both anti-IgE- and A23187-stimulated human tissue was apparent after 30 minutes preincubation of the lung tissue in azelastine. Significant inhibition of anti-IgE-stimulated histamine release was consistently seen in azelastine concentrations greater than or equal to 5 microM, and was dose dependent (r = 0.71, p less than 0.05) with maximal mean inhibition of 53 +/- 11%. For A23187-stimulated lung tissue, consistent inhibition of histamine release was not found until we used 30 microM azelastine, mean 35 +/- 11%. Inhibition in azelastine concentrations below 30 microM was variable and not significant. Lung cyclic AMP and cyclic GMP content was not significantly altered by incubation of lung tissue in 100 microM azelastine. We conclude that azelastine inhibits stimulated histamine release from human lung tissue in vitro but does not alter cyclic nucleotide content.

Use of segmental airway lavage to obtain relevant mediators from the lungs of asthmatic and control subjects

Studies have demonstrated that increased amounts of histamine in the airways of asthmatic patients are associated with increased airway reactivity. However, using routine bronchoalveolar lavage (BAL), histamine can be detected in only a portion of asthmatic subjects and a minority of control populations. To obtain relevant mediators from the airways in higher concentrations by avoiding the dilution inherent with a standard BAL, a technique was developed to lavage isolated airway segments of the human lung that employed a double-lumen bronchoscope and a balloon-tipped catheter. Lavage fluid obtained by this method yielded significantly higher concentrations of histamine than that obtained with routine BAL (asthmatic subjects, 2,403 +/- 633 pg/ml vs 188 +/- 42 pg/ml; rhinitis subjects, 533 +/- 187 pg/ml vs 113 +/- 53 pg/ml; normal subjects, 174 +/- 63 pg/ml vs 11 +/- 11 pg/ml). Similar findings were also noted for prostaglandin D2 (PGD2). Segmental airway lavage also resulted in higher lavage fluid concentrations of LTB, than routine BAL. Segmental airway lavage should help in studying the relationship of mast cell degranulation to airways reactivity in both asthmatic and other study populations.

Azelastine inhibits IgE-mediated human basophil histamine release

To examine the mechanism potentially contributing to therapeutic efficacy of azelastine in allergic rhinitis and asthma, we studied the effect of azelastine on stimulated histamine release from basophils prepared as a mixed leukocyte suspension from human blood. Azelastine was found to significantly inhibit anti-IgE-stimulated basophil histamine release. Time-course experiments indicated that the inhibitory effect of azelastine was immediate and that preincubation of basophils in azelastine was not necessary. In dose-response experiments with azelastine, 1 to 100 mumol/L, significant inhibition of histamine release was consistently observed in azelastine concentrations greater than or equal to 10 mumol/L. This inhibition was dose dependent (r = 0.96; p less than 0.001) with maximal mean inhibition of 91 +/- 8% at 100 mumol/L of azelastine.

The interaction of azelastine with human lung histamine H1, beta, and muscarinic receptor-binding sites

Azelastine has previously been demonstrated to inhibit histamine release, to antagonize histamine-mediated responses, and to be a bronchodilator. To determine the mechanism by which azelastine has antihistaminic and bronchodilatory actions, we studied its interaction with relevant human lung receptors. We performed competitive radioligand binding assays and determined the affinity of azelastine for [3H]pyrilamine (histamine H1), [125I]pindolol (beta), and [3H]quinuclidinyl benzilate (muscarinic) binding sites. Azelastine had a relatively high affinity for histamine H1 receptors with IC50 values consistently as low or lower than values measured for other antihistamines. In contrast, azelastine had a very low affinity for both beta-receptors and muscarinic receptors with IC50 values greater than 2 logs greater than those determined for beta-agonists and muscarinic antagonists, respectively. Thus, bronchodilatory activity of azelastine does not appear to result from either beta-agonist or muscarinic-antagonist properties. Azelastine does, however, have the ability to inhibit the release of histamine and to bind to histamine H1 receptors, thereby effectively antagonizing histamine H1 receptor-mediated responses in the lung. These characteristics make azelastine a potentially very useful drug to treat allergic responses in the respiratory tract.

Bronchoalveolar lavage fluid histamine levels in interstitial lung diseases

We measured bronchoalveolar lavage (BAL) fluid histamine in 36 normal subjects, 32 patients with sarcoidosis, and 28 patients with idiopathic pulmonary fibrosis (IPF). Patients with sarcoidosis and IPF had significantly greater BAL histamine (59 +/- 21 and 203 +/- 42 pg/ml, respectively) than did normal subjects (18 +/- 8 pg/ml). Sarcoidosis patients with Stage 3 chest radiographs had significantly greater BAL histamine than did sarcoidosis patients with radiographic Stages 0, 1, or 2. Sarcoidosis patients with higher BAL lymphocytes also had higher levels of BAL histamine. Furthermore, among the sarcoidosis patients, we found a marked interactive effect between higher BAL lymphocytes and Stage 3 radiographs on BAL histamine. In IPF, subjects with increased BAL lymphocytes had significantly less BAL histamine than subjects with normal levels of lymphocytes. These studies suggest that BAL histamine may be a useful marker (of more active disease and/or poorer prognosis) to be evaluated in prospective studies in patients with sarcoidosis and IPF.

Characterization of muscarinic receptor subtypes on human peripheral lung

To better define the role of muscarinic receptors in lung responses and airway diseases, we characterized the binding of the M1-specific antagonist, [3H]pirenzepine (PZ), and the nonspecific (M1- and M2-) antagonist, [3H]quinuclidinyl benzilate (QNB), to human peripheral lung tissue. Data obtained from 15 different lung specimens showed that the radioligands bound to single high-affinity sites with dissociation constant (Kd) values ranging from 1 to 9 nM for [3H]PZ and 0.03 to 0.46 nM for [3H]QNB. Comparison of total binding capacity values by equilibrium experiments with [3H]PZ, unlabeled PZ, and [3H]QNB indicates that approximately one-half of the total muscarinic binding sites in human peripheral lung binds PZ with high affinity (putative M1-subtypes). Kd values for muscarinic agents determined by competition experiments with [3H]PZ were consistent with the expected rank order of potency for interactions with muscarinic receptors. Characterization of the role of these muscarinic receptor subtypes in human lung responses may lead to the development of more selective therapeutic agents for the treatment of chronic obstructive airway diseases.

Acute systemic reactions to carbonless copy paper associated with histamine release

We report two cases of recurrent episodes of hoarseness, cough, flushing, pruritus, and rash occurring within 30 minutes of topical exposure to carbonless copy paper. Provocative challenges revealed that alkylphenol novolac resin was the ingredient responsible. Video endoscopy of the larynx was performed and plasma histamine levels were obtained prior to and 30 minutes after cutaneous challenge of a patient with alkylphenol novolac resin. We documented marked laryngeal edema and a sixfold increase in plasma histamine levels after challenge. We conclude that topical exposure to carbonless copy paper may cause mast cell/basophil-mediated acute systemic and potentially life-threatening reactions in susceptible patients.

Airway reactivity to methacholine in nonatopic asymptomatic adults

We studied 50 nonsmoking volunteers, ages 18-35 yr, with no past or present history or physical examination findings of asthma, rhinitis, allergic disease, or recent respiratory infections, to evaluate the usefulness of the methacholine bronchoprovocation challenge (MBPC) as a screening test for asthma. All were skin-test-negative to 29 aeroallergens and had base-line pulmonary function values greater than 80% predicted. Fourteen (28%) subjects had a drop in forced expiratory volume in 1 s (FEV1) of 20% or greater at a provocative dose (PD20FEV1) less than or equal to 225 breath units. Moreover, when these subjects were compared with 21 asymptomatic allergic asthmatics, there was significant overlap between the two groups in concentration of methacholine causing this decline in FEV1. A positive MBPC at methacholine concentrations less than or equal to 5 mg/ml was not diagnostic of asthma, and a negative MBPC at methacholine concentrations greater than or equal to 10 mg/ml did not rule out asthma. These data strongly suggest that MBPC should not be used as the sole factor for the diagnosis of clinically significant asthma. A positive MBPC is one indication of the presence of airway hyperresponsiveness and thus is only one of many factors that must be considered in the diagnosis of asthma.

Neuromechanisms of asthma

Autonomic nervous system abnormalities may indeed underlie bronchial hyperreactivity. Imbalances between excitatory (parasympathetic, alpha-adrenergic and noncholinergic excitatory) and inhibitory (beta-adrenergic and non-adrenergic inhibitory) nervous systems at one or more loci could lead to airway hyperreactivity. The current data, however, do not clearly identify any single abnormality in the autonomic nervous system that is ubiquitous in all asthmatic patients. Rather, it appears that bronchial hyperreactivity results from many factors and that distinct autonomic nervous system abnormalities may occur in individual subjects. Future studies of how autonomic control of airway function in asthma may be disordered should prove useful in gaining a better understanding of the pathophysiology of asthma and in designing new treatment modalities.

Direct evidence of a role for mast cells in the pathogenesis of antigen-induced bronchoconstriction

We measured bronchoalveolar lavage (BAL) fluid histamine levels in allergic asthmatics and nonallergic normal subjects after local airway antigen and cold 22 degrees C normal saline challenges. Immediately after instillation of antigen through a bronchoscope wedged into a subsegmental airway, all 17 allergic asthmatics but none of the nine normal subjects had visible airway constriction. The asthmatics had a concomitant mean increase in BAL histamine of 23% (P = 0.005), whereas the normals had no change in BAL histamine. Among the allergic asthmatics, the change in BAL histamine content in response to antigen directly correlated with the control (baseline) BAL histamine content (r = 0.66, P = 0.003). Moreover, asthmatics with large antigen-induced changes in BAL histamine had greater airway methacholine sensitivity than did asthmatics without measurable increases in BAL histamine (8 +/- 2 vs. 41 +/- 31 breath units). Neither asthmatics nor normal subjects had airway constriction or changes in BAL histamine levels in response to nonspecific challenge with cold saline. Our data suggest that when allergic asthmatics are exposed to relevant antigens they have in vivo lung mast cell degranulation which results in airway constriction and contributes to nonspecific airway hyperresponsiveness.

Comparison of two radiolabeled quinuclidinyl benzilate ligands for the characterization of the human peripheral lung muscarinic receptor

Quinuclidinyl benzilate, a muscarinic antagonist, has previously been used in its tritiated form ([3H]-QNB) to study the lung muscarinic receptor. We investigated whether a newer iodinated form of QNB ([125I]-QNB) of higher specific activity would be an appropriate ligand to study the human peripheral lung muscarinic receptor. Both the tritiated and iodinated ligands bound specifically to human lung at 23 degrees C. At 37 degrees C the specific binding of [3H]-QNB increased slightly, but no specific binding of [125I]-QNB was found. The data from multiple equilibrium binding experiments covering a wide range of radiolabeled QNB concentrations were combined and analyzed using the computer modeling program, LIGAND. The tritiated QNB identified a single affinity human lung binding site with a Kd of 46 +/- 9 pM and a receptor concentration of 34 +/- 3 fmol/mg protein. The iodinated QNB identified a single higher affinity human lung binding site (Kd = 0.27 +/- 0.32 pM) of much smaller quantity (0.62 +/- 0.06 fmol/mg protein). Competition studies comparing the binding of unlabeled QNB relative to labeled QNB indicated that unlabeled QNB had the same Kd as that measured for [3H]-QNB, but a 5 log greater Kd than that measured for [125I]-QNB. Other muscarinic receptor agonists and antagonists competed with [3H]-QNB, but not [125I]-QNB for binding to muscarinic receptors with the expected magnitude and rank order of potency. We conclude that of the 2 radiolabeled forms of QNB available, only the tritiated form should be used to study the human peripheral lung muscarinic receptor.

(-)[125I]pindolol binding to human peripheral lung beta-receptors

Beta-adrenergic receptors in human peripheral lung were characterized by biochemical and radioligand assays employing binding of the beta-antagonist (-)[125I]pindolol to plasma membrane preparations. The specific binding of (-)[125I]pindolol reached equilibrium by 45 min with an initial rate constant of 0.0282 min-1. Binding was reversible with a kinetic dissociation rate constant of 0.0146 min-1. The calculated kinetic Kd (dissociation constant) was 430 pM which agreed very well with the Kd of 394 pM obtained by Scatchard analyses of equilibrium binding data. Computer analyses of equilibrium binding experiments revealed a similar Kd of 336 +/- 24 pM. The binding capacities calculated by computer analyses (155 +/- 7 fmol/mg protein) and Scatchard analyses (113 fmol/mg protein) were also in close agreement. By all three methods (kinetic, Scatchard, and computer analyses), the data were most compatible with a single (-)[125I]pindolol binding site. Analyses of equilibrium binding data from ten different human lungs revealed values for the Kd ranging from 79 to 360 pM (mean, 136 pM), and for the receptor concentration ranging from 58 to 196 fmol/mg protein (mean, 118 fmol/mg protein). The displacement of (-)[125I]pindolol binding by various agents exhibited stereoselectivity and the expected rank order of potency predicted for interactions with beta-receptors. Isoproterenol induced a rapid and dose-related increase in cyclic AMP that was prevented by specific beta-antagonists. Approximately 70% of the beta-receptors were found to be of the beta 2-subtype by both radioligand binding and biochemical assays. Thus, (-)[125I]pindolol appears to be an excellent ligand for characterizing human lung beta-receptors since accurate and reproducible results can be obtained with this radioligand using limited tissue sample quantities.

Immunohistochemical identification of lung cells responsive to beta-stimulation with a rise in cAMP

To identify specific lung cells possessing functional beta-adrenergic receptors, we developed an immunoperoxidase-staining procedure capable of in situ localization of cells responding to beta-agonist stimulation with a rise in adenosine 3',5'-cyclic monophosphate (cAMP). Isoproterenol was instilled into the airways of excised intact guinea pig lungs for 5 min and resulted in a six to eightfold rise in cAMP. Immediately thereafter, the lungs were washed in and fixed with 10% buffered Formalin. Sections were then stained using immunoperoxidase techniques and monoclonal antibodies directed against cAMP. We found that isoproterenol-stimulated lungs had widespread increased staining for immunoreactive cAMP. The specific cells consistently demonstrating marked increases in staining were airway epithelial cells, airway smooth muscle cells, alveolar and parenchymal macrophages, and alveolar lining cells, including both type I and type II cells, and capillary endothelial cells. Of all tissues, the airway epithelium was the most intensely stained area for beta-agonist-induced immunoreactive cAMP. The techniques employed herein should make possible the in situ localization of cells responding to any stimuli capable of increasing cAMP, thereby allowing the specific identification of cells possessing functional adenylate cyclase-linked receptors.

Airway responses to methacholine in asymptomatic nonatopic cigarette smokers

We prospectively performed methacholine bronchoprovocation challenges on 46 young smokers to examine the effects of cigarette smoking on airway responsiveness. The subjects, ages 18-35 yr, had no past or present history or physical examination findings of asthma or other lung diseases, rhinitis, allergic diseases, or respiratory infections; were skin test negative to 29 common aeroallergens; and had base-line pulmonary function values greater than 80% predicted. Sixteen of 46 (35%) subjects had a 20% or greater drop in forced expiratory volume in 1 s at a provocative methacholine concentration less than or equal to 25 mg/ml. The degree of methacholine responsiveness was not dependent upon base-line pulmonary function values or the amount of cigarettes consumed, and there was no association between the amount of cigarettes consumed and base-line pulmonary function values. These data suggest that many young asymptomatic nonatopic smokers have increased airway responsiveness to inhaled methacholine without clinically significant hyperreactive airway disease.

Elevated bronchoalveolar lavage fluid histamine levels in allergic asthmatics are associated with methacholine bronchial hyperresponsiveness

Using a sensitive single isotope enzymatic assay we measured bronchoalveolar lavage (BAL) fluid histamine in asymptomatic normal (nonallergic), allergic rhinitic, and allergic asthmatic subjects. Normal subjects were found to have little or no detectable amounts of histamine in BAL fluid (11 +/- 11 pg/ml), and few BAL fluid mast cells. In comparison, the allergic rhinitics and allergic asthmatics had much higher amounts of BAL fluid histamine (113 +/- 53 and 188 +/- 42 pg/ml, respectively), and a significantly greater number of BAL fluid mast cells. Furthermore, despite having equivalent baseline pulmonary function values, allergic asthmatics with BAL fluid histamine levels greater than 100 pg/ml required only 7 +/- 2 breath units of methacholine to induce a 20% drop in forced expiratory volume in 1 s (FEV1) (PD20FEV1) while asthmatics with BAL fluid histamine levels less than 100 pg/ml required 49 +/- 19 breath units (P less than 0.05). These data suggest that allergic asthmatics have ongoing lung mast cell degranulation that might contribute to the etiology of airway hyperresponsiveness.

Immunohistochemical localization of histamine-stimulated increases in cyclic GMP in guinea pig lung

A significant number of asthmatic subjects are provoked by allergic reactions. The underlying pathophysiologic event is mast cell degranulation with the release and generation of the mediators of anaphylaxis. Histamine, one of the major mast cell mediators, causes 10- to 50-fold increases in guinea pig lung cyclic 3',5'-guanosine monophosphate (cyclic GMP) through H1 receptor stimulation. Employing monoclonal antibodies directed at cyclic GMP, immunocytochemical techniques were used to identify those specific cells in lung responding to histamine stimulation with increases in cyclic GMP. The most responsive cells were alveolar and parenchymal macrophages, pleural lining cells, and endothelial and epithelial cells. Little or no increases in bronchial or vascular smooth muscle cyclic GMP was noted. At the height of the reaction, a generalized increase in cyclic GMP staining of all alveolar cells was observed. These findings suggest that the lining cells of the lung including macrophages, mesothelial, endothelial, and epithelial cells may be the most responsive cells to histamine released during allergic responses. The absence of muscular staining suggests that cyclic GMP does not participate in histamine-stimulated muscle contraction.

Exercise-induced anaphylactic syndromes. Insights into diagnostic and pathophysiologic features

To differentiate the diagnoses of exercise-induced anaphylaxis and cholinergic urticaria/anaphylaxis, we developed reproducible diagnostic provocative challenges. The data derived from the study of two representative patients, one with cholinergic urticaria and the other with exercise-induced anaphylaxis, suggest approaches to distinguishing these diagnoses. After specific exercise challenges, both patients developed symptoms consistent with anaphylaxis and had associated increases in plasma histamine levels. After passive heat challenges inducing increases in core body temperature more than 0.7 degrees C, only the patient with cholinergic urticaria developed anaphylactic symptoms and had a rise in the plasma histamine level. Neither patient developed symptoms of anaphylaxis when core body temperatures were increased after administration of intravenous endotoxin. Thus, passive heat challenges are extremely valuable in differentiating these two exercise-related syndromes. Although not important in exercise-induced anaphylaxis, specific thermoregulatory mechanisms appear to play an intricate part in the pathophysiology of cholinergic urticaria/anaphylaxis.

Characterization of histamine H-1 receptors on human peripheral lung

Histamine H-1 receptors in human peripheral lung were characterized by radioligand and biochemical assays employing binding of the H-1 receptor antagonist [3H]pyrilamine to plasma membrane preparations. Simultaneous computerized analyses of the data from fourteen separate equilibrium-binding assays indicated the presence of three distinct classes of binding sites with Kd values of 81 +/- 35 pM, 7 +/- 3 microM, and 320 +/- 167 microM and binding capacities of 23 +/- 3 pmoles, 10 +/- 5 nmoles, and 297 +/- 119 nmoles/mg protein respectively. Dissociation kinetics of [3H]pyrilamine binding also supported the presence of three binding sites or states. Further, competition binding curves for histamine receptor agonists and antagonists also indicated the presence of multiple binding sites for the H-1 receptor. The effect of exogenous stimulation of histamine H-1 receptors on human cyclic nucleotides was also examined. Both histamine and the H-1 agonist 2-methyl histamine caused dose-related increases in the cyclic guanosine monophosphate (GMP) content of human lung. The effects of 2-methyl histamine were selective for cyclic GMP. The histamine-induced increase in cyclic GMP peaked within 1.0 min and was effectively prevented by the H-1 antagonist pyrilamine. Thus, human lung possesses a large number of H-1 receptors which exhibit three binding states and produce cyclic GMP, but not cyclic adenosine monophosphate (AMP), when stimulated.

Characterization of histamine H-1 receptors on human mononuclear cells

Histamine H-1 receptors on peripheral human mononuclear cells were characterized by radioligand binding of the H-1 receptor antagonist [3H]pyrilamine to lymphocyte-rich preparations. Simultaneous computerized analyses of sixteen separate equilibrium-binding assays indicated the presence of two distinct classes of binding sites with dissociation constants (Kds) of 4 +/- 1 nM and 55 +/- 9 microM and binding capacities of 21 +/- 7 fmol and 117 +/- 15 pmol/million cells, respectively. Competition binding curves for displacement of [3H]pyrilamine binding by histamine receptor agonists and antagonists also indicated the presence of multiple binding sites for the H-1 receptor. Further, the ED50 values determined for histamine receptor agonists and antagonists were entirely consistent with the expected rank order of potency for interactions with H-1 receptors. Thus, human mononuclear cells have a large number of H-1 receptors that exhibit two distinct binding sites, and the Kds for these sites are within the range of histamine concentrations achieved either in physiologic states or after mast cell (or basophil) degranulation.

Demonstration that circulating human blood cells have no detectable alpha 1-adrenergic receptors by radioligand binding analysis

The mechanisms underlying the autonomic nervous system abnormalities reported in allergic asthma have not been defined. In order to determine if these abnormalities reflect abnormal alpha-adrenergic receptor numbers or drug affinities, we attempted to identify alpha-receptors on circulating human blood cells. Platelets, red blood cells, polymorphonuclear leukocytes, and mononuclear cells were examined by use of radioligand binding techniques with the [3H]antagonists, dihydro-alpha-ergocryptine, prazosin hydrochloride, and yohimbine as ligands. The presence of alpha 2-receptors was confirmed on platelets, but no detectable alpha-receptors were identified on red blood cells or polymorphonuclear leukocytes. Preliminary observations suggested the presence of specific alpha-receptor binding to mononuclear cells; however, this binding was determined to reflect directly the presence of contaminating platelets. By use of a newly developed isolation technique to obtain platelet-depleted mononuclear cells, no alpha-adrenergic receptors could be identified on platelet-depleted mononuclear cells. Therefore, since no alpha 1-receptors could be identified on circulating human blood cells, these cells are not a suitable model for the study of the mechanisms underlying abnormal alpha-adrenergic responsiveness, and it may be necessary to reanalyze previous reports of alpha-adrenergic responsiveness on human blood cells with the use of platelet-depleted cell preparations.