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H J Motulsky

Publications and source records attributed to H J Motulsky.

At least 37 records · Page 2Linked to original sources
PubMed1988

In vivo regulation of beta-adrenergic receptors on mononuclear leukocytes and heart. Assessment of receptor compartmentation after agonist infusion and acute aortic constriction in guinea pigs.

In animals injected with a bolus of isoproterenol, beta-adrenergic receptors in both mononuclear leukocytes (MNL) and heart were sequestered away from the cell surface, and the time course (0-120 min) and dose-response patterns were similar in the two tissues. In guinea pigs given a constant infusion of isoproterenol, 0.15 mg/(kg.h), down-regulation of total receptor number occurred more quickly and to a greater extent in the MNL than in the heart. We also compared receptor sequestration after aortic constriction-induced acute heart failure. Negligible sequestration (9%) of beta-adrenergic receptors occurred in the MNL of animals treated in this manner, whereas the number of receptors in the sarcolemmal fraction decreased 61%. This selective sequestration of cardiac receptors may result from the action of high concentrations of norepinephrine (which is selective for beta 1 over beta 2 receptors) present at sympathetic nerve-cardiac cell synapses. We conclude that although receptor redistribution occurs similarly in MNL and heart in response to a circulating nonselective agonist, beta-adrenergic receptor redistribution may occur selectively in the heart in response to such stimuli as aortic constriction-induced acute heart failure that activate the sympathetic nervous system.

Adenylyl Cyclases↗
PubMed1988

The potency and kinetics of the beta-adrenergic receptors on human neutrophils.

The binding to catecholamines to the beta-adrenergic receptors on human polymorphonuclear leukocytes rapidly inhibits cell responses stimulated by chemoattractant ligands. As a first step in understanding the mechanism of the inhibition, we investigated the number of beta-receptors required to optimally block superoxide anion production, a response that is measured kinetically by a convenient spectrophotometric assay for the reduction of cytochrome c. We found that after blockade of 50-60% of the beta-adrenergic receptors with an irreversible antagonist, maximal inhibition of the response was still elicited by isoproterenol (ISO). Next we investigated the kinetics with which superoxide generation is inhibited. We found that half-maximal inhibition was observed at 3 x 10(-8) M ISO, which approximates the Kd because many of the receptors are involved. Cell responsiveness recovered when propranolol was added between the time of ISO and chemoattractant addition. From the recovery we estimated that the half-time for ISO dissociation is less than 10 sec. Finally, we examined the rate at which cell responses decay following ISO administration after chemoattractant. Optimal rates of inhibition, turning off oxidant production in seconds, occur at ISO concentrations greater than or equal to 3 x 10(-7) M. Taken together, these observations are consistent with an association rate constant for ISO estimated to be greater than or equal to 10(8) M-1min-1 and a dissociation rate constant greater than or equal to 4 min-1. These results are discussed in terms of the available data concerning the binding of agonists to beta-adrenergic receptors.

Adrenergic beta-Agonists↗
PubMed1988

Characterization of a bromoacetylated derivative of pindolol as a high affinity, irreversible beta adrenergic antagonist in cultured cells.

We have utilized cultured cell lines to test the utility of N8-(bromoacetyl)-N1-[3-(4-indolyoxy)-2-hydroxypropyl]-(Z)-1,8-diam ino- p-menthane, BIM, a recently synthesized, irreversible beta adrenergic antagonist. Previously available irreversible antagonists of beta adrenergic receptors have generally exhibited low affinity (typical IC50 values greater than or equal to 1 microM). By contrast, S49 lymphoma cells incubated with 10 nM BIM for 120 min and then washed extensively showed a 70% loss in beta adrenergic receptors, as measured by [125I]iodocyanopindolol binding. This loss, which could be prevented by propranolol, represented a decrease in receptor number without a change in affinity of the remaining receptors for [125I]iodocyanopindolol. The BIM-induced decrease in binding sites was persistent in membranes incubated for several hours after BIM treatment. BIM did not inactivate alpha-1 adrenergic receptors on Madin Darby canine kidney cells, alpha-2 adrenergic receptors on human erythroleukemia cells, nor did BIM treatment alter guanyl-5'-yl-imidodiphosphate-mediated regulation of agonist binding to the beta adrenergic receptors in S49 cell membranes. BIM treatment decreased cyclic AMP (cAMP) accumulation in S49 cells in response to the beta adrenergic agonist isoproterenol, but increased prostaglandin E1-stimulated cAMP accumulation (P = .09) without altering cAMP production in response to forskolin. The inactivation of beta receptors in S49 cells by BIM (IC50 = 0.30 nM) correlated closely with the loss in beta adrenergic receptor-mediated cAMP accumulation in these cells (IC50 = 0.59 nM), implying the absence of substantial receptor reserve for this response. We conclude that BIM is a potent, irreversible, selective beta adrenergic antagonist for the study of beta adrenergic receptors in cultured cells.

Adrenergic beta-Antagonists↗
PubMed1987

Receptor redistribution does not accompany terbutaline-induced down regulation of beta-adrenergic receptors on human mononuclear leukocytes.

Mononuclear leukocytes are easily accessible cells for investigating the regulation of beta-adrenergic receptors in humans. We have previously shown that brief incubations with agonists redistribute (? internalize) most of the beta-adrenergic receptors on mononuclear leukocytes away from the cell surface without changing total receptor number. However, negligible redistribution occurred after exercise or an infusion of isoproterenol. The current study was designed to ask whether receptor redistribution occurs over a longer time course after administration of terbutaline, a beta 2-adrenergic agonist that is known to cause a decrease in receptor number. Normal volunteers were given terbutaline, 5 mg t.i.d. for 6 days. As expected, the number of beta-adrenergic receptors decreased. However, the remaining receptors were not redistributed. Redistribution also did not occur after 1 or 2 days of terbutaline, at which time down regulation was minimal. We also found that terbutaline did not alter the ability of the receptors to be redistributed or desensitized by a preincubation with isoproterenol.

Administration, Oral↗
PubMed1987

Ischemia- and agonist-induced changes in alpha- and beta-adrenergic receptor traffic in guinea pig hearts.

We have used radioligand binding techniques and subcellular fractionation to assess whether changes in expression of myocardial alpha 1- and beta-adrenergic receptors are mediated by a redistribution of receptors between various membrane fractions. Three fractions were prepared from the left ventricles of guinea pigs that underwent either 1 h of ischemia or injection of epinephrine (0.25 mg/kg ip): a crude membrane, a purified sarcolemma, and a light vesicle fraction. In control animals alpha 1-adrenergic receptors ([3H]prazosin binding) in light vesicles was only 25% of the total alpha 1-receptor density found in sarcolemmal and light vesicle fractions as compared with 50% for beta-adrenergic receptors ([125I]iodocyanopindolol binding sites). Although ischemia was associated with a 53% decrease in the number of light vesicle beta-adrenergic receptors and a 42% increase in the number of sarcolemma beta-receptors (P less than 0.05), there was no change in the number of light vesicle alpha 1-receptors, even though the number of sarcolemmal alpha 1-receptors increased 34%. Epinephrine treatment promoted internalization of beta-adrenergic receptors; sarcolemma beta-receptors decreased 37% and light vesicle beta-receptors increased 28% (P less than 0.025). For alpha 1-receptors, epinephrine treatment decreased the number of sarcolemmal receptors 41% (P less than 0.025) but failed to increase the number of receptors in the light vesicle fraction. The changes in receptor binding to beta-adrenergic receptors in sarcolemmal fractions were mirrored by parallel changes in isoproterenol-stimulated adenylate cyclase activity. These results indicate that alpha 1- and beta-adrenergic receptors may undergo a different cellular itinerary in guinea pig myocardium.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1987

Propranolol treatment externalizes beta-adrenergic receptors in guinea pig myocardium and prevents further externalization by ischemia.

Purified sarcolemmal and light vesicle (intracellular) fractions of beta-adrenergic receptors were used to examine the effects of propranolol on receptor translocation in guinea pig heart. Guinea pigs were given propranolol (0.15 mg/kg/hr) via minipumps for 7 days and either killed or made ischemic for 1 hour via a coronary ligature. Propranolol treatment led to an externalization of beta-receptors from light vesicle to sarcolemmal fractions. This externalization increased the number of surface beta-adrenergic receptors that were functional, as assessed by isoproterenol-stimulated adenylate cyclase activity. After chronic propranolol treatment, ischemia did not further alter receptor distribution. These results suggest that externalization of beta-adrenergic receptors from a light vesicle fraction to the sarcolemma contributes to up-regulation of beta-receptors that occur in response to both propranolol treatment and ischemia. Because propranolol-treated animals show blunting in externalization after myocardial ischemia, propranolol treatment and myocardial ischemia appear to access the same pool of intracellular beta-adrenergic receptors. Depletion of this pool of receptors along with receptor blockade may thus contribute to the mechanism by which the drug is efficacious in preventing some adverse effects of ischemia.

Adenylyl Cyclases↗
PubMed1987

Interactions of amiloride with alpha- and beta-adrenergic receptors: amiloride reveals an allosteric site on alpha 2-adrenergic receptors.

Interaction of amiloride with adrenergic receptors was studied using radioligand binding techniques. Amiloride competed for [3H]prazosin binding to alpha 1-adrenergic receptors on rat renal cortical membranes and BC3H-1 muscle cell membranes. Non-linear regression analysis of radioligand binding isotherms showed that amiloride increased Kd without a change in Bmax, suggesting the drug binds competitively in a mutually exclusive manner with the radioligand at the receptor-binding site. Similarly, amiloride competitively blocked [125I]iodocyanopindolol binding to beta-adrenergic receptors on both tissues. The addition of guanylyl 5'-imidodiphosphate or sodium chloride did not alter the interaction of amiloride with alpha 1- or beta-adrenergic receptors. The interaction of amiloride with alpha 2-adrenergic receptors was more complex and revealed an allosteric site. In both rat renal cortical membranes and intact human platelets, amiloride increased the Kd for [3H]rauwolscine binding, as well as decreasing the apparent Bmax. In binding experiments where amiloride competed for [3H]rauwolscine-binding sites, pseudo-Hill slopes of less than 1.0 were obtained for both platelet and renal alpha 2 receptors. In addition, amiloride increased the rate of [3H]rauwolscine dissociation from renal alpha 2 receptors. In the presence of 100-120 mM sodium chloride, the Ki for amiloride competition was decreased an average of 54% in renal membranes; in contrast, sodium increased the Kd of the agonist epinephrine. Taken together, these data support the hypothesis that alpha 2-adrenergic receptors, but not alpha 1- or beta-adrenergic receptors, have an allosteric site to which amiloride binds and which we propose to be a cation-binding site.

Allosteric Regulation↗
PubMed1987

Compartmentation of alpha 2-adrenergic receptors in human erythroleukemia (HEL) cells.

We have identified alpha 2-adrenergic receptors on human erythroleukemia (HEL) cells, a suspension-grown cell line related to human platelets. properties of receptors were assessed in intact cells by binding of the antagonist [3H]yohimbine and by inhibition of cAMP accumulation. [3H]Yohimbine labeled 5900 +/- 2100 receptors/cell with a Kd of 3.6 +/- 0.9 nM (n = 7). alpha 2-Adrenergic receptors were potently coupled to inhibition of adenylate cyclase, with EC50 values for epinephrine, UK-14,304, and p-aminoclonidine in the low nM range. Treatment of cells with pertussis toxin abolished this response. In radioligand binding studies with membrane preparations [3H]yohimbine and [3H]UK-14,304 bound to the same number of sites (71 versus 69 fmol/mg of protein), and epinephrine competed for [3H]yohimbine binding in a biphasic manner. After addition of GTP, no high affinity [3H]UK-14,304 binding was detected, and epinephrine competed for [3H]yohimbine binding with lower affinity at both 4 degrees and 37 degrees. In studies with intact cells, we detected no specific binding of [3H]UK-14,304 at either 37 degrees or 4 degrees. At 37 degrees, epinephrine competed for all [3H]yohimbine binding sites with a low apparent affinity (Ki = 21 microM), whereas at 4 degrees epinephrine (up to 1 mM) was able to compete for only 59 +/- 13% of [3H]yohimbine-binding sites. The potency of epinephrine in competing for [3H]yohimbine sites in intact cells at 4 degrees was greater than at 37 degrees (Ki = 1 microM) and was similar to that observed with membranes in the presence of GTP. We hypothesize that sites not detectable by epinephrine at 4 degrees are sequestered within the cell. Treatment of HEL cells with pertussis toxin reduced the proportion of receptors on the surface from 51 +/- 12% to 23 +/- 7% (n = 3, p less than 0.05) of the total sites. Treatment of HEL cells with epinephrine (100 microM, 1 hr) reduced the cell surface component to 25 +/- 8% (n = 3) of the total sites. This treatment was not accompanied by significant desensitization of the ability of epinephrine to inhibit cAMP accumulation. We conclude that alpha 2-adrenergic receptors exist in more than one compartment in HEL cells and that interaction of receptors with a guanine nucleotide-binding protein or with agonist may regulate this compartmentation. These cells provide a new model system for the study of expression and metabolism of alpha 2-adrenergic receptors.

Adenylyl Cyclases↗
PubMed1987

Fitting curves to data using nonlinear regression: a practical and nonmathematical review.

Many types of data are best analyzed by fitting a curve using nonlinear regression, and computer programs that perform these calculations are readily available. Like every scientific technique, however, a nonlinear regression program can produce misleading results when used inappropriately. This article reviews the use of nonlinear regression in a practical and nonmathematical manner to answer the following questions: Why is nonlinear regression superior to linear regression of transformed data? How does nonlinear regression differ from polynomial regression and cubic spline? How do nonlinear regression programs work? What choices must an investigator make before performing nonlinear regression? What do the final results mean? How can two sets of data or two fits to one set of data be compared? What problems can cause the results to be wrong? This review is designed to demystify nonlinear regression so that both its power and its limitations will be appreciated.

Data Interpretation, Statistical↗
PubMed1986

Temperature-dependent binding of hydrophilic beta-adrenergic receptor ligands to intact human lymphocytes.

Important differences in binding characteristics between agonists and antagonists of the beta-adrenergic receptor have been described. However, these observations have been complicated since most available antagonists are much more lipophilic than agonists. In order to separate out those binding characteristics of agonist vs. antagonist from those characteristics of lipophilic vs. hydrophilic ligands, we have studied competition of the hydrophilic ligands isoproterenol (agonist) and CGP-12177 (antagonist) with [125I]iodopindolol binding in intact human lymphocytes. Analyzing competition curves from assays performed at 13 degrees C, 25 degrees C and 37 degrees C we demonstrated that at lower temperatures there was a decrease in IC50 for isoproterenol but not for CGP-12177. Using cells preincubated with isoproterenol then extensively washed, competition curves with both isoproterenol and CGP-12177 were biphasic, and characterized by the appearance of a population of receptors with a low affinity for both hydrophilic ligands. Furthermore, at lower temperatures the biphasic nature of these curves was accentuated and was characterized by a 6-fold and 40-fold increase in the apparent KD of a population of low affinity sites for isoproterenol and CGP-12177, respectively.

Adolescent↗
PubMed1986

Transient catecholamine modulation of neutrophil activation: kinetic and intracellular aspects of isoproterenol action.

Modulation of neutrophil activation by catecholamines may reflect regulatory mechanisms that couple beta-adrenergic and N-formyl peptide receptors to antagonistic biochemical pathways. We examined kinetic and mechanistic aspects of the inhibition by catecholamines of neutrophil activation by formyl peptides. Inhibition of oxidant production by isoproterenol (ISO) was detected as low as 3 nM, had an ID50 of 10(-7) M, and could be blocked and reversed by propranolol. Recovery of cell function occurred over a period of minutes when the concentration of ISO was less than 10(-6) M. These observations are discussed in terms of the interaction of ISO with the adrenergic receptors. The site of catecholamine action is addressed. ISO neither influences formyl peptide-receptor interaction nor does it inhibit oxidant production by phorbol ester. These results suggest an impairment of intracellular signalling processes that couple the formyl peptide-receptor binding to cell activation. We observed inhibition of intracellular Ca++ elevation by ISO only at low formyl peptide concentrations. This inhibition is consistent with a partial inhibition of phosphoinositide metabolism, which was observed. Several other cell responses, including actin polymerization and right angle light scatter, are minimally inhibited by 10(-6) M ISO indicating that the cell activation process is not entirely obliterated. The presence of catecholamine and formyl peptide results in a synergistic elevation of cAMP. The intracellular targets of ISO action may be regulated by cAMP dependent kinases and could follow a branchpoint in the activation sequence that leads distinctly to oxidase activation and cytoskeletal activation.

Actins↗
PubMed1986

Desensitization and redistribution of beta-adrenergic receptors on human mononuclear leukocytes.

We have used intact human mononuclear leukocytes (MNL) to examine desensitization of beta-adrenergic receptors in normal mammalian cells. MNL were prepared and radioligand binding experiments were performed at 4 degrees C. At this temperature the ligand [125I]iodocyanopindolol ([125I]ICYP) identified the same number of receptors as at 37 degrees C, and the agonist isoproterenol competed for this binding with high affinity (dissociation constant, Ki = 20 nM). At 37 degrees C, results were similar when the binding incubation was terminated after 1 min, but the apparent affinity of the receptors for isoproterenol was several 100-fold lower when the incubation was allowed to reach steady state. In desensitized MNL (prepared by incubating whole blood with 10 microM isoproterenol at 37 degrees C for 10 min, and then isolating and washing the MNL at 4 degrees C), isoproterenol-stimulated cAMP accumulation was reduced 63 +/- 4%. After desensitization, the total number of beta-receptors was unchanged, but isoproterenol and the hydrophilic antagonist CGP-12177 were able to compete with [125I]ICYP for binding to only 18 +/- 6% of these sites. Direct binding with [3H]CGP-12177 yielded similar results. These results demonstrate that isoproterenol promotes a rapid desensitization of beta-adrenergic receptors on MNL and a concomitant redistribution of receptors into a cellular compartment to which some ligands (including catecholamines) have restricted access. The findings demonstrate that redistribution of beta-receptors may be a mechanism mediating desensitization to catecholamines in normal mammalian cells.

Adolescent↗
PubMed1986

In vivo regulation of beta-adrenergic receptors on human mononuclear leukocytes: assessment of receptor number, location, and function after posture change, exercise, and isoproterenol infusion.

We studied the regulation of beta-adrenergic receptors in human mononuclear leukocytes (MNL). Total receptor number was determined as specific binding at 4 C of [3H] dihydroalprenolol or [125I]iodopindolol, and redistributed receptors were defined as those binding sites to which the hydrophilic antagonist CGP-12177 did not have access. Receptor function was assessed as cAMP accumulation stimulated by isoproterenol. In in vitro experiments, high concentrations of isoproterenol desensitized receptor function and promoted redistribution of about 80% of the receptors away from the cell surface. However, three in vivo protocols (upright posture for 3 h, moderate exercise, and infusion of isoproterenol for 30 min) redistributed few beta-adrenergic receptors on MNL. The 30-min isoproterenol infusion did not alter later cAMP accumulation, but posture change and exercise increased isoproterenol-stimulated cAMP accumulation in intact MNL. Infusion of isoproterenol for 120 min redistributed 9 +/- 2% (+/- SEM) of the receptors and decreased isoproterenol-stimulated cAMP accumulation by 19 +/- 6%. Isoproterenol-stimulated adenylate cyclase activity in membranes isolated from MNL previously was found to be decreased with upright posture, and we confirmed these findings in assays that did not include exogenous GTP, but instead relied upon guanine nucleotides retained in the membrane preparation. However, when excess GTP was included, isoproterenol-stimulated adenylate cyclase activity in MNL membranes was not altered by posture change. We conclude that substantial receptor redistribution of beta-receptors on MNL does not readily occur in physiological situations.

Adenylyl Cyclases↗
PubMed1986

Desensitization of epinephrine-initiated platelet aggregation does not alter binding to the alpha 2-adrenergic receptor or receptor coupling to adenylate cyclase.

Several investigators have shown that incubating unstirred platelets with epinephrine blunts subsequent aggregation when the platelets are stirred. Using aspirin-treated platelets, we further characterized this desensitization of alpha 2-adrenergic receptor-initiated aggregation. Desensitization occurred with a t1/2 of 3-6 min and was maximal at 20-30 min, at which time the initial rate of aggregation and its maximal extent were about half that of control platelets. When we preincubated platelets with epinephrine, and then added phentolamine to block the alpha 2-receptors, ADP-initiated aggregation occurred normally. Thus, the desensitization of epinephrine-initiated aggregation was not associated with a generalized impairment of aggregation. At concentrations too low to initiate aggregation, epinephrine is known to potentiate aggregation initiated by other agents. Clonidine also acts at alpha 2-receptors to potentiate aggregation initiated by other agents, but it does not initiate aggregation by itself. Preincubating clonidine with platelets for 30 min abolished its potentiating effect on ADP-initiated aggregation. Thus, the ability of alpha 2-receptors to both potentiate and initiate aggregation desensitizes after a brief preincubation with agonist. We performed several types of experiments to investigate the mechanism of this desensitization. Platelet alpha 2-receptors are coupled to an inhibition of adenylate cyclase. We found, however, that alpha 2-mediated inhibition of prostaglandin E1-stimulated cAMP accumulation occurred normally in desensitized platelets. Similarly, epinephrine inhibited basal adenylate cyclase activity normally in membranes prepared from desensitized platelets. In membranes prepared from desensitized platelets, epinephrine competed normally for [3H]rauwolscine binding, and this competition was modulated normally by guanine nucleotides. Thus, the properties of the alpha 2-receptors, as measured in radioligand binding experiments, were unchanged by densensitization. In conclusion, desensitization of alpha 2-adrenergic receptor-mediated aggregation occurs without change in the alpha 2-adrenergic receptors or in their coupling to an inhibition of adenylate cyclase.

1-Methyl-3-isobutylxanthine↗
PubMed1986

Regulation of hormone-stimulated cyclic AMP accumulation in intact human mononuclear leukocytes by blood plasma.

Several hormones, including catecholamines, histamine, and prostaglandin E1, regulate the function of human mononuclear leukocytes (MNL) by stimulating the accumulation of cAMP. Isoproterenol-stimulated cAMP accumulation in MNL isolated and washed at 4 degrees is five times greater than in cells prepared at ambient temperature. The current study was aimed at understanding this difference. cAMP accumulation in MNL prepared at ambient temperature could not be increased by chilling the cells for 4 hours. Warming MNL prepared at 4 degrees for 30 min, however, reduced later isoproterenol-, histamine-, and PGE1-stimulated cAMP accumulation by 65-85% without altering forskolin-stimulated cAMP accumulation and without altering cellular viability or ATP content. In broken cell preparations, there was no difference in either adenylate cyclase or phosphodiesterase activities, and no difference in the binding of isoproterenol to the beta-adrenergic receptors. The reduction in isoproterenol-stimulated cAMP accumulation in warmed intact cells was reversed when the MNL were incubated with autologous leukocyte-depleted blood or with plasma. These data suggest the presence of one or more factors in plasma that enhances hormone-stimulated adenylate cyclase activity in intact MNL.

Adenosine Triphosphate↗
PubMed1986

Expression of fibrinogen receptors during activation and subsequent desensitization of human platelets by epinephrine.

Epinephrine causes platelet aggregation and secretion by interacting with alpha 2-adrenergic receptors on the platelet surface. Platelet aggregation requires the binding of fibrinogen to a specific receptor on the membrane glycoprotein IIb-IIIa complex. Although the IIb-IIIa complex is identifiable on the surface of resting platelets, the fibrinogen receptor is expressed only after platelet activation. The current studies were designed to examine the effect of occupancy of platelet alpha 2-adrenergic receptors by epinephrine on the expression of fibrinogen receptors and on the aggregation of platelets. The ability of epinephrine to induce the expression of fibrinogen receptors was studied under two different conditions: acute stimulation (less than 1 min) and prolonged stimulation (50 to 90 min), the latter of which is associated with a reduction or "desensitization" of the platelet aggregation response. Expression of the fibrinogen receptor was monitored with 125I-fibrinogen as well as with 125I-PAC-1 (PAC-1), a monoclonal antibody that binds to the glycoprotein IIb-IIIa complex only after platelets are activated. Epinephrine caused an immediate increase in PAC-1 and fibrinogen binding that was dependent on occupancy of the alpha 2-receptor by epinephrine and on the presence of extracellular free Ca (KCa = 30 mumol/L). By itself, 1 mmol/L Mg was unable to support induction of the fibrinogen receptor by epinephrine. However, it did decrease the Ca requirement by about two orders of magnitude. Prolonged stimulation of unstirred platelets by epinephrine led to a 70% decrease in the aggregation response when the platelets were subsequently stirred. Despite their decreased aggregation response, desensitized platelets bound PAC-1 and fibrinogen normally, indicating that the loss of aggregation was not due simply to a decrease in fibrinogen receptor expression. Although desensitization was not affected by pretreatment of the platelets with aspirin, it was partially prevented when extracellular Ca was chelated by EDTA during the long incubation with epinephrine. These studies demonstrate that once platelet alpha 2-adrenergic receptors are occupied by epinephrine, extracellular Ca is involved in initiating the aggregation response by supporting the induction of the fibrinogen receptor and the binding of fibrinogen. Furthermore. Ca-dependent reactions subsequent to fibrinogen binding may be necessary for maximal platelet aggregation and are impaired when platelets become desensitized to epinephrine.

Antibodies, Monoclonal↗
PubMed1985

Externalization of beta-adrenergic receptors promoted by myocardial ischemia.

beta-Adrenergic receptors were identified in two fractions of guinea pig myocardium: a purified sarcolemmal fraction and a light vesicle (presumably intracellular) fraction. In the light vesicle fraction, which contained approximately 25 percent of the myocardial receptors under control conditions, the receptors appeared to be segregated from the stimulatory guanine nucleotide binding and catalytic components of adenylate cyclase. During myocardial ischemia, beta-adrenergic receptors were redistributed from the intracellular vesicles to the sarcolemmal fraction, where isoproterenol-stimulated adenylate cyclase activity was increased. These findings should facilitate further studies on cellular and molecular mechanisms that regulate adrenergic receptor traffic in the myocardium and may explain the rapid enhancement in adrenergic receptor expression that occurs with myocardial ischemia.

5'-Nucleotidase↗