Journal of Pharmacology And Experimental Therapeutics Fast Forward
First published on April 3, 2007; DOI: 10.1124/jpet.107.120709
0022-3565/07/3221-214-221$20.00
JPET 322:214-221, 2007
NEUROPHARMACOLOGY
Pharmacological Effects of Ephedrine Alkaloids on Human 
1- and 2-Adrenergic Receptor Subtypes
Guoyi Ma,
Supriya A. Bavadekar,
Yolande M. Davis,
Shilpa G. Lalchandani,
Rangaswamy Nagmani,
Brian T. Schaneberg,
Ikhlas A. Khan, and
Dennis R. Feller
The National Center for Natural Products Research (G.M., R.N., B.T.S., I.A.K., D.R.F.), Department of Pharmacology (S.A.B., Y.M.D., S.G.L., D.R.F.), and Department of Pharmacognosy (I.A.K.), School of Pharmacy, University of Mississippi, University, Mississippi
Received for publication
January 29, 2007
Accepted
April 2, 2007.
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Abstract
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Ephedra species of plants have both beneficial and adverse effects primarily associated with the presence of ephedrine alkaloids. Few reports have appeared that examine the direct actions of ephedrine alkaloids on human subtypes of adrenergic receptors (ARs). In the present study, ephedrine alkaloids were evaluated for their binding affinities on human 
1A-, 1B-, 1D-, 2A-, 2B-, and 2C-AR subtypes expressed in HEK and Chinese hamster ovary cells. Cell-based reporter gene assays were used to establish functional activity of ephedrine alkaloids at 1A-, 2A-, and 2C-ARs. The data showed that ephedrine alkaloids did not activate 1- and 2-ARs and that they antagonized the agonist-mediated effects of phenylephrine and medetomidine on 1- and 2-ARs, respectively. As in the binding studies, 1R,2R- and 1R,2S-ephedrine showed greater functional antagonist activity than the 1S,2R- and 1S,2S-isomers. The rank order of affinity for the isomers was 1R,2R > 1R,2S > 1S,2R > 1S,2S. The rank order of potencies of alkaloids containing a 1R,2S-configuration was norephedrine 
ephedrine >> N-methylephedrine. These studies have demonstrated that orientation of the β-hydroxyl group on the ethylamino side chain and the state of N-methyl substitution are important for -AR binding and functional activity of the ephedrine alkaloids. In conclusion, the ephedrine isomers and analogs studied did not exhibit any direct agonist activity and were found to possess moderate antagonist activities on cloned human -ARs. The blockade of presynaptic 2A- and 2C-ARs may have a pharmacological role in the direct actions of Ephedra alkaloids.
The Ephedra or Ma huang species of plants are widely used for their medicinal properties. The principal active constituent in the Ephedra species is ephedrine, which possesses two chiral centers, and can exist as four isomers designated as 1R,2S- and 1S,2R-ephedrine and 1R,2R- and 1S,2S-pseudoephedrine (Griffith and Johnson, 1995
). Naturally occurring ephedrine alkaloids include mainly 1R,2S-ephedrine, 1S,2S-pseudoephedrine, 1R,2S-norephedrine, 1R,2S-N-methylephedrine, and 1S,2S-norpseudoephedrine. These Ephedra/ephedrine alkaloids have been used as nasal decongestants, bronchodilators, and CNS stimulants (Kalix, 1991; Hoffman, 2001), and Ephedra has also been used for the treatment of obesity (Arch et al., 1984; Liu et al., 1995). In recent years, there have been numerous reports of adverse reactions resulting from intake, especially in excessive doses, of herbal products containing extracts of Ephedra as weight loss aids (Josefson, 1996). Other effects such as hypertension, tremors, myocardial infarction, seizures, and stroke have resulted in fatalities (Chua and Benrimoj, 1988; White et al., 1997; Haller and Benowitz, 2000). The United States Food and Drug Administration has recently prohibited the sale of dietary supplements containing Ephedra. However, herbal products containing Ephedra remain in use in other countries.
The beneficial and adverse effects of ephedrine and related analogs are known to be mediated via the - and β-adrenergic receptors (ARs) and can be elicited by either direct interactions with the receptors as agonists or antagonists or indirectly by either causing a release of endogenous catecholamines and/or by preventing their neuronal reuptake (Trendelenburg, 1963; Trendelenburg et al., 1963; Patil et al., 1967; Vansal and Feller, 1999; Rothman et al., 2003; Wellman et al., 2003). The relative contribution of these direct and indirect interactions to the pharmacological effects of the ephedrine isomers in vivo on ARs has remained controversial. Ephedra alkaloids of the 1R,2S-configuration have generally been shown to exert direct AR effects, whereas isomers of the 1S,2S- and 1S,2R-configuration have indirect actions (Patil et al., 1965, 1967; LaPidus et al., 1967; Tye et al., 1967; Patil, 1968; Waldeck and Widmark, 1985; Kawasuji et al., 1996; Vansal and Feller, 1999; Liles et al., 2006). In studies using ephedrine isomers, indirect actions were predominantly demonstrated by depletion of norepinephrine tissue stores using reserpine, and more recently, Rothman et al. (2003), using a battery of in vitro tests, reported that the most potent action of ephedrine and norephedrine analogs and isomers is as substrates for norepinephrine transporters.
Although there are reports of the direct effects of ephedrine isomers on human β-ARs (Vansal and Feller, 1999), no comprehensive information exists on the direct interactions (agonist and antagonist potencies) of the ephedrine isomers and related Ephedra alkaloids at human -ARs. Rothman et al. (2003) found that the ephedrine isomers and analogs were not agonists on human 1A-or 2A-AR, but they did not test compounds for either antagonist activity or on other -AR subtypes. In the present study, the direct effects of the ephedrine isomers and naturally occurring ephedrine alkaloids have been systematically examined as agonists and antagonists on cloned human 1 (1A, 1B, and 1D)- and 2 (2A, 2B, and 2C)-AR subtypes expressed in host cells. A preliminary report of our work has appeared (Ma et al., 2004).
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Materials and Methods
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Materials. Human embryonic kidney (HEK293) cells stably expressing homogeneous populations of the human 1A-, 1B-, and 1D-AR subtypes were obtained from Dr. Kenneth Minneman (Emory University, Atlanta, GA). Human 2A-, 2B-, and 2C-AR subtypes stably expressed in CHO cells were obtained from Drs. Marc Caron, Robert Lefkowitz (Duke University, Durham, NC), and Stephen Liggett (University of Cincinnati, Cincinnati, OH). The ephedrine alkaloids used in this study were provided by Dr. Popat N. Patil (The Ohio State University, Columbus, OH), or purchased from Sigma-Aldrich (St. Louis, MO). The compounds were dissolved in water or in a 1:5 mixture of dimethyl sulfoxide and water. Stock solutions of 10 mM were prepared fresh daily and diluted in water to appropriate concentrations for the studies. [3H]Rauwolscine and [3H]prazosin were obtained from PerkinElmer Life and Analytical Sciences (Wellesley, MA). The phorbol ester (12-O-tetradecanoylphorbol-13-acetate)-response element-luciferase gene (TRE-LUC) and cAMP-response element-luciferase reporter gene (6 CRE-LUC) were kindly provided by Dr. A. Himmler (Boehringer Ingelheim Research and Development, Vienna, Austria). All cell culture reagents were obtained from Life Technologies (Carlsbad, CA). Other chemicals were purchased from Sigma-Aldrich.
Cell Culture. HEK293 cells stably expressing 1A-, 1B-, and 1D-AR subtypes were grown in 150 cm2 Corning culture flasks (Acton, MA) with Dulbecco's modified Eagle's medium supplemented with 10% fetal bovine serum, 2 mM glutamine, penicillin G (100 U/ml), streptomycin (100 µg/ml), and G418 (Geneticin, 100 µg/ml). Upon confluence, the cells were detached by gentle scraping. CHO cells stably expressing 2A-, 2B-, and 2C-AR were grown in 150 cm2 Corning culture flasks with Ham's F12 medium supplemented with 10% fetal bovine serum, 2 mM glutamine, penicillin G (100 U/ml), streptomycin (100 µg/ml), and G418 (100 µg/ml). All cells were cultured at 37°C in an atmosphere of 5% CO2 and 95% humidity. Media were changed every 48 h until the cells were confluent. Upon confluence, the cells were detached by trypsinization (0.25% trypsin EDTA) for 2 min.
Competitive Radioligand Binding Assays. Radioligand binding assays were carried out on intact HEK293 cells stably expressing 1A-, 1B-, 1D-AR subtypes and intact CHO cells stably expressing 2A-, 2B-, and 2C-AR subtypes as described by Lalchandani et al. (2002). The detached cells were washed and centrifuged with Tris-EDTA buffer, pH 7.4, containing 50 mM Tris, 20 mM disodium EDTA, and 154 mM NaCl, in which they were finally suspended. The radioligand was used at a fixed concentration of 0.1 µCi in the absence and presence of various concentrations (the range was 10–10–10–3 M or 10–11–10–4 M) of competing drugs. The drugs were added to the cells (50,000) in 50 mM Tris-EDTA buffer to a total volume of 2.0 ml and allowed to incubate at 37°C for 1 h. Nonspecific binding was determined in the presence of 10 µM phentolamine. Reactions were terminated by rapid filtration through Whatman GF/C filters using a Brandel 12R cell harvester followed by washes with ice-cold buffer twice. Radioactivity on the dried filter discs was measured using a liquid scintillation analyzer (Tri-Carb 2900TR; PerkinElmer Life and Analytical Sciences). The displacement curves were plotted and the Ki values of the test ligands for the receptor subtypes were determined using GraphPad Prism (GraphPad Software Inc., San Diego, CA). The percentage specific binding was determined by dividing the difference between total bound (disintegrations per minute) and nonspecific bound (disintegrations per minute) by the total bound (disintegrations per minute).
Functional Assays. A recently developed sensitive reporter gene assay (Lalchandani et al., 2002) was used to elucidate the effects of the ephedrine alkaloids on human 2-AR subtypes. In 1-AR studies, a TRE-LUC plasmid provided by Dr. A. Himmler (Stratowa et al., 1995) was used to study the functional effects of the ephedrine alkaloids. HEK293 cells stably expressing 1A-AR were transfected with the TRE-LUC plasmid (40 µg/ml) using electroporation (70 ms, single pulse, 150 V). The transfected cells were seeded at a density of 50,000 cells/well in microtiter plates (Cultureplate; PerkinElmer Life and Analytical Sciences) in 200 µl of media and allowed to grow for 24 h with incubation at 37°C (5% CO2). After 24 h, the cells were treated with varying drug concentrations for a period of 20 h, which was found to be optimum during time course analyses performed earlier (data not presented). When antagonist studies were performed, the compounds were added 15 min before the addition of an agonist, L-phenylephrine. After drug exposure, the cells were lysed, and luciferase activity was measured using the Luclite assay kit (PerkinElmer Life and Analytical Sciences).
In 2-AR studies, the cell-based CRE-LUC was conducted as described previously by Lalchandani et al. (2002). The CHO cells stably expressing 2A- and 2C-ARs were transfected with the 6 CRE-LUC plasmid (40 µg/ml) using electroporation (70 ms, single pulse, 150 V). The transfected cells were seeded at a density of 50,000 cells/well in microtiter plates (Cultureplate; PerkinElmer Life and Analytical Sciences) in 200 µl of media and allowed to grow for 24 h with incubation at 37°C (5% CO2). After 24 h, the cells were treated with varying drug concentrations for 4 h. When antagonist studies were performed, the compounds were added 15 min before the addition of the direct adenylate cyclase activator forskolin (3 µM). After drug exposure, the cells were lysed, and luciferase activity was measured using the Luclite assay kit (PerkinElmer Life and Analytical Sciences).
For both 1A- and 2A- and 2C-AR cells, the luciferase activity (counts per second, changes in light production) was determined using a TopCount Microplate Scintillation and Luminescence Counter (model B9904; PerkinElmer Life and Analytical Sciences). Data were normalized relative to luciferase changes of L-phenylephrine (3 x 10–4 M = 100%) and forskolin (3 x 10–6 M = 100%) in 1A- and 2A-/2C-AR expressed in HEK293 and CHO cells, respectively.
Data Accumulation and Analysis. For binding studies in cell lines, varying concentrations of each drug were added in duplicate within each experiment, and the individual IC50 values were determined using GraphPad Prism software. The Ki value of each ligand was determined according to the equation described by Cheng and Prusoff (1973), and final data are presented as pKi ± S.E.M. of n = 6 experiments. The concentration-dependent reversal of forskolin-induced (3 µM) luciferase activity changes in CHO cells by medetomidine and selected ephedrine analogs was used to assess agonist activity, and data for medetomidine were expressed as EC50 values ± S.E.M. of n = 6 experiments. Antagonist activities of the ephedrine analogs were determined by their addition before incubation with a fixed concentration of the 2-AR agonist, medetomidine (10 µM). Data were expressed as IC50 values ± S.E.M. of at least n = 6 experiments. Differences between means of binding affinities and functional responses of individual drugs were analyzed using a paired t test. Values were considered to be statistically significant when P < 0.05.
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Results
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Receptor Binding. To obtain a better understanding of the direct effect of ephedrine isomers on -ARs, the binding affinity of ephedrine isomers and closely related analogs were compared on human 1- and 2-AR subtypes. The structures of all of the ephedrine isomers are provided in Fig. 1. With selected Ephedra alkaloids of the 1R,2S-configuration, the rank order of competitive displacement of radioligands on both 1A- and 2A-AR subtypes was norephedrine ephedrine > N-methylephedrine (Fig. 2). The comparative affinities of these Ephedra alkaloids as displacing ligands were much lower those for than the standard agonist analogs, L-phenylephrine and medetomidine. Calculated affinity values of the standard agonists and of all of the ephedrine alkaloids on 1A- and 2A-AR subtypes are given in Tables 1 and 2. 1R,2S-Norephedrine, lacking an N-methyl group as in 1R,2S-ephedrine, showed increased binding affinities at the subtypes tested. The presence of an additional N-methyl group, as in 1R,2S-N-methylephedrine, showed decreased binding affinities compared with 1R,2S-ephedrine at the 1A-, 2A-, and 2C-AR. The rank order of affinity (Ki values) of ephedrine isomers on human 1-AR in HEK293 cells was 1R,2R-pseudoephedrine > 1R,2S-ephedrine > 1S,2R-ephedrine > 1S,2S-pseudoephedrine at 1A-AR; 1S,2R-ephedrine > 1R,2R-pseudoephedrine > 1R,2S-ephedrine > 1S,2S-pseudoephedrine at 1B-AR; and 1R,2R-pseudoephedrine > 1R,2S-ephedrine > 1S,2R-ephedrine > 1S,2S-pseudoephedrine at 1D-AR. The affinities of 1R,2S-norephedrine and 1S,2S-norpseudoephedrine were similar to 1R,2S-ephedrine at the 1A-AR. The rank order of affinity (Ki values) of ephedrine isomers on human 2-AR in CHO cells was 1R,2R-pseudoephedrine > 1R,2S-ephedrine > 1S,2R-ephedrine > 1S,2S-pseudoephedrine at 2A-AR; 1R,2R-pseudoephedrine > 1R,2S-ephedrine = 1S,2R-ephedrine > 1S,2S-pseudoephedrine at 2B-AR; and 1R,2R-pseudoephedrine > 1R,2S-ephedrine > 1S,2R-ephedrine > 1S,2S-pseudoephedrine at 2C-AR. As shown in Tables 1 and 2, the 1R,2R- and the 1R,2S-ephedrine isomers had greater affinities than the 1S,2R- and 1S,2S-isomers at all -ARs, except at the 1B-AR. 1S,2S-Pseudoephedrine displayed the lowest binding affinity at all -ARs. Furthermore, the binding affinities of 1R,2S-norephedrine and 1S,2S-norpseudoephedrine were similar to that of 1R,2S-ephedrine on these subtypes.
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Fig. 2. Binding displacement curves of ephedrine isomers for 1A (a)-, 2A (b)-, and 2C (c)-ARs expressed in HEK293 or CHO cells. Data are expressed as means ± S.E.M. (n = 6 experiments). , L-phenylephrine;  , medetomidine;  , 1R,2S-ephedrine; , 1R,2S-norephedrine;  , 1R,2S-N-methylephedrine.
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Direct Agonist Effects of Ephedrine Alkaloids on -AR Subtypes. Cell-based reporter gene assays were used to establish functional activity at the 1A-, 2A-, and 2C-ARs. The direct agonist effects of ephedrine isomers on 1-ARs were studied. As shown in Fig. 3, luciferase assay studies using the TRE-LUC plasmid showed that 1R,2S-ephedrine, 1S,2R-ephedrine, 1R,2R-pseudoephedrine, 1S,2S-pseudoephedrine, 1R,2S-norephedrine, 1R,2S-N-methylephedrine and 1S,2S-norpseudoephedrine had little effects on 1A-AR at the highest concentration tested (0.3 mM). These isomers gave a response that was <12% of the L-phenylephrine maximum. L-Phenylephrine activated 1A-AR, giving an EC50 value of 2.01 ± 0.39 µM.
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Fig. 3. a, direct effects of L-phenylephrine and ephedrine alkaloids on 1A-AR in HEK 293 cells. Control and L-phenylephrine (300 µM) luciferase activities (counts per second, mean ± S.E.M., n = 4–6) were 1154 and 7560, respectively. Normalized data (300 µM L-phenylephrine = 100%) are expressed as the mean ± S.E.M. of n = 4–6 experiments. A, 1R,2S-ephedrine (300 µM); B, 1S,2R-ephedrine (300 µM); C, 1R,2R-pseudoephedrine (300 µM); D, 1S,2S-pseudoephedrine (300 µM); E, 1R,2S-norephedrine (300 µM); F, forskolin (3 µM); G, 1R,2S-N-methylephedrine (300 µM); H, 1S,2S-norpseudoephedrine (300 µM). b, concentration-dependent effects of medetomidine and ephedrine alkaloids for the reversal of effects on forskolin-induced cAMP elevation on the 2A- and 2C-ARs expressed in CHO cells. Control and forskolin-induced luciferase activities (counts per second, mean ± S.E.M., n = 4–6) were 1560 and 12,200 for 2A and 3069 and 34,576 for 2C, respectively. Normalized data (3 µM forskolin = 100%) are expressed as the mean ± S.E.M. of n = 4–6 experiments. A, 1R,2S-ephedrine (300 µM); B, 1S,2R-ephedrine (300 µM); C, 1R,2R-pseudoephedrine (300 µM); D, 1S,2S-pseudoephedrine (300 µM); E, 1R,2S-norephedrine (300 µM); F, forskolin (3 µM); G, 1R,2S-N-methylephedrine (300 µM); H, 1S,2S-norpseudoephedrine (300 µM).
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The agonistic effects of the ephedrine isomers on 2A- and 2C-AR subtypes were examined for their abilities to reverse forskolin-induced cAMP elevation measured using the 6 CRE-LUC reporter gene assay. The results in Fig. 3 show that none of the ephedrine alkaloids (1R,2S-ephedrine, 1S,2R-ephedrine, 1R,2R-pseudoephedrine, 1S,2S-pseudoephedrine, 1R,2S-norephedrine, 1R,2S-N-methylephedrine, and 1S,2S-norpseudoephedrine) reversed forskolin-induced cAMP elevations in the 2A- and 2C-AR subtypes. However, the agonist medetomidine significantly reversed forskolin-induced cAMP elevations in the 2A- and 2C-AR subtypes with EC50 values of 79.8 ± 3.5 and 78.3 ± 7.3 nM (Fig. 3). These data indicate that the ephedrine alkaloids do not act as direct agonists on these AR subtypes.
Antagonistic Effects of Ephedrine Alkaloids on -AR Subtypes. Studies were undertaken to examine the antagonistic effects of ephedrine alkaloids on 1A-, 2A-, and 2C-AR subtypes in HEK293 and CHO cells (Table 3; Fig. 4). The results showed that ephedrine isomers and analogs antagonized the effects of the agonists, L-phenylephrine on 1-ARs (Fig. 4a) and medetomidine on 2-ARs (Fig. 4b). In Table 3, it was noteworthy that the antagonistic potencies of ephedrine isomers on the 2A- and 2C-AR subtypes were considerably higher (5- to 24-fold) than those for the inhibition of the 1-AR. Similar to the rank order found in binding studies, 1R,2S- and 1R,2R-pseudoephedrine showed greater antagonist activity than 1S,2R- and 1S,2S-pseudoephedrine (compare data in Tables 1 and 3). No difference was noted in the antagonist potencies of the primary and secondary amines of 1R,2S-norephedrine and ephedrine, respectively. However, 1S,2S-norpseudoephedrine (the primary amine analog) was more potent than 1S,2S-pseudoephedrine as an antagonist on the three -AR subtypes, and its potency was the same as that of the 1R,2S-isomers of ephedrine and norephedrine on these ARs. It appears that the orientation of the 1R-hydroxyl substituent and the methylation state (primary versus secondary amine) in the ephedrine alkaloids are important for -AR activity. The antagonistic activities of all ephedrine isomers were consistent with their binding affinities.
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TABLE 3 The pKB or pIC50 values of antagonist effects of ephedrine alkaloids on human 1A-, 2A-, and 2C-ARs in HEK293 and CHO cells
Values were determined as the effective concentration (IC50) and negative log (pIC50) of each ephedrine analog that reversed the effect of medetomidine on the maximum cAMP response of forskolin. Each value is the mean ± S.E.M. of data from three experiments performed in duplicate.
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Fig. 4. a, displacement curves of antagonist effects of ephedrine alkaloids on L-phenylephrine-mediated agonistic effects on 1A-AR expressing in HEK 293 cells. Data are expressed as the mean ± S.E.M. of n = 4–6 experiments. , L-phenylephrine (0.1–300 µM); , 1R,2S-ephedrine (300 µM) + L-phenylephrine;  , 1R,2S-norephedrine (300 µM) + L-phenylephrine; , 1R,2S-N-methylephedrine (300 µM) + L-phenylephrine. Control and L-phenylephrine (300 µM) luciferase activities (disintegrations per second, mean ± S.E.M., n = 4–6) were 1269 and 7980, respectively. Normalized data (300 µM L-phenylephrine = 100%) are expressed as the mean ± S.E.M. of n = 4–6 experiments. b, reversal of medetomidine inhibition of forskolin-induced cAMP elevation by ephedrine alkaloids on human 2A-AR versus 2C-AR (B). cAMP changes were assessed by measurement of luciferase activity. Control and forskolin-induced measurements of luciferase activity (counts per second, mean ± S.E.M., n = 4–6) were 1483 and 12,780 for 2A and 3069 and 32576 for 2c, respectively. pIC50 and IC50 values (Table 3) were determined as the concentration of ephedrine isomers that reversed the inhibition effect of medetomidine on the luciferase response to forskolin). A, 1R,2S-ephedrine (1, 10, and 100 µM); B, 1R,2S-norephedrine (1, 10, and 100 µM); C, 1R,2S-N-methylephedrine (1, 10, and 100 µM); M, medetomidine (0.01 µM); F, forskolin (3 µM).
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Discussion
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Chemically, ephedrine possesses two chiral centers, and 1R,2S-ephedrine is a long-studied stimulant available both as a prescription and over-the-counter medication and is a major ingredient in widely marketed herbal preparations. Another isomer, 1S,2S-pseudoephedrine is used as a nasal decongestant and precursor for the illicit synthesis of methamphetamine. Standard pharmacology textbooks emphasize the fact that ephedrine is both a direct and indirect acting drug for the activation of ARs (Hoffman, 2001). Previous reports attributed the effects of ephedrine alkaloids to direct agonist activity and by release of norepinephrine from presynaptic nerve terminals via a carrier-mediated exchange mechanism (Trendelenburg, 1963; Trendelenburg et al., 1963; Patil et al., 1967; Vansal and Feller, 1999; Rothman et al., 2003). In the present study, we characterized the direct receptor-mediated effects of the stereoisomers of ephedrine and closely related naturally occurring compounds (1R,2S-isomers of ephedrine, norephedrine, and N-methylephedrine and 1S,2S-isomers of pseudoephedrine and norpseudoephedrine).
Radioligand binding studies of the ephedrine alkaloids showed that the 1R,2R- and the 1R,2S-isomers generally had greater affinities than their corresponding 1S,2R- and 1S,2S-isomers on all of the -AR subtypes. An exception was at the 1B-AR. 1S,2S-Pseudoephedrine displayed the lowest binding affinity at all -ARs. 1R,2S-Norephedrine, lacking an N-methyl group as in 1R,2S-ephedrine, showed increased binding affinities at the subtypes tested. The presence of an additional N-methyl group, as in 1R,2S-N-methylephedrine, showed decreased binding affinities compared with 1R,2S-ephedrine at the 1A-, 2A-, and 2C-AR. In summary, the steric orientation of the hydroxyl group in the 1R-configuration and the presence of a primary or secondary amine on the side chain were associated with the highest binding affinities to -AR subtypes.
Rothman et al. (2003) reported that the ephedrine isomers, at 10 µM, lacked agonist activity in functional studies of intracellular calcium changes in cells expressing human 1A- and 2A-ARs. No further studies were done to determine whether ephedrine analogs were antagonists in these cells. In our study, using cell-based reporter gene assays to measure functional effects at the 1A-, 2A-, and 2C-ARs, we demonstrated that ephedrine isomers and related analogs were also inactive as direct agonists and showed that the ephedrine alkaloids antagonized the effects of the agonists, L-phenylephrine on 1- and medetomidine on 2-ARs. Similar to the rank order found in binding studies, 1R,2S- and 1R,2R-ephedrine showed greater antagonist activity than 1S,2R- and 1S,2S-pseudoephedrine, which indicated that the β-hydroxyl substituent on the side chain in the R-configuration is important for -AR antagonist activity. The antagonistic potencies of the isomers were consistent with their binding affinities, both of which were in the micromolar range. Taken collectively, these findings suggest that the ephedrine isomers possess only direct antagonist activity in cloned human -AR systems.
Early reports by Patil and colleagues (Lapidus et al., 1967; Patil et al., 1967; Tye et al., 1967) have demonstrated that the 1R,2S- and 1R,2R-isomers of ephedrine possess agonist or antagonist activity (dependent upon the tissue examined), whereas the remaining ephedrine isomers exhibit antagonist activities or possessed an indirect mechanism of action on isolated tissues (lung, ileum, vas deferens, and vascular preparations) that contained - and/or β-ARs. In another study by Lee et al. (1974) using rat epididymal fat tissue and measurement of lipolysis, the ephedrine isomers were devoid of β-AR activity on lipolysis, and they reported that 1R,2S-ephedrine is more potent as an antagonist than 1S,2R-ephedrine. In this regard, Wellman et al. (2003) recently reported that 1R,2S-ephedrine-induced hypophagia in rats was attenuated by prazosin, which suggested that this in vivo ephedrine action was mediated via the 1-AR. The 1-AR subtype that mediated this CNS effect of ephedrine on hypophagia was not established. It is apparent that the mechanism of action for Ephedra alkaloids is dependent upon the stereochemistry of the ephedrine analog(s) used, the abundance and distribution of AR subtypes present, and whether nerves remain intact in the target tissue under study.
To date, only a few studies have been completed using human ARs. On the three human β-AR subtypes expressed in CHO cells, the ephedrine isomers showed weak agonist activities (Vansal and Feller, 1999), and the 1R,2S-ephedrine isomer was the most active agonist on the three subtypes, exhibiting the most potent activity on the β2-AR and weakest agonist activity on the β3-AR subtype. In contrast, Rothman et al. (2003) reported that 1R,2S-ephedrine was not active as an agonist on the human β-AR subtypes (data not presented) and suggested that the results of the earlier study by Vansal and Feller (1999) may have been due to the use of cells with higher receptor densities. Other work using fluorescent labeling techniques to evaluate direct β2-AR interactions with 1R,2S-ephedrine (Gether et al., 1995) have yielded more insight into conformational changes. In this study, only full agonists but not weaker partial agonists such as ephedrine, produced significant reductions in fluorescence, providing an interesting approach to investigate the direct AR effects (agonist or antagonist changes). Additional studies with the isomers of Ephedra alkaloids and related analogs may be useful in probing ligand-specific sites of interaction on AR subtypes.
Taken collectively, our present studies show only a weak partial agonist activity (at >10–4 M) by the 1R,2S-isomers of ephedrine and norephedrine and that all isomers of tested Ephedra/ephedrine alkaloids produce only moderate antagonist activities on -AR subtypes. Based upon our findings, it is likely that the in vivo actions of these Ephedra alkaloids are mediated principally by an indirect action on the subtypes of human -ARs. However, a mechanism of direct action for 1R,2S-ephedrine on ARs has been proposed to explain the observed in vivo effects for the pharmacological treatment of asthma (Griffith and Johnson, 1995; Hoffman, 2001) and for its CNS stimulatory and/or cardiovascular (heart and stroke) adverse effects observed with its abuse and misuse in humans (Haller and Benowitz, 2000). Interestingly, our findings contrast with recent in vivo reports on pressor effects (Liles et al., 2006) and hypophagic activity in rats (Wellman et al., 2003), in which they demonstrated that 1R,2S-ephedrine exhibits its agonist activity via direct effects on the 1-AR receptors. In this regard, an in vitro study involving mutational changes (Waugh et al., 2000) in the 1A-AR demonstrated a 4-fold increase in functional potency for the 1R,2S-ephedrine isomer, whereas potency for epinephrine was decreased by 36-fold. Thus, the presence of the aromatic catechol group in epinephrine was more affected by the mutation than a molecule lacking the catechol functionality (ephedrine). Waugh et al. (2000) demonstrated partial agonist activity for only this ephedrine isomer on wild-type and mutated cloned rat and hamster 1A-ARs. In summary, it is apparent that the 1R,2S-ephedrine isomer has direct agonist activity, in vitro and in vivo. Our studies and those of Rothman et al. (2003) failed to demonstrate significant agonist activity for 1R,2S-ephedrine on human 1-ARs. The observed agonist responses to 1R,2S-ephedrine, as reported by Waugh et al. (2000), may be explained in part by an increased density of receptors in the cells used in their experiments. Overall, these results provide evidence in support of both in vivo and in vitro agonist activities for 1R,2S-ephedrine on -ARs.
Ephedra alkaloids are expected to produce their pharmacological actions by mixed mechanisms, involving indirect and direct actions on -AR subtypes. In conclusion, our studies have shown that the ephedrine isomers and analogs investigated did not exhibit any significant direct agonist activity on human 1- and 2-AR subtypes and can be classified as antagonists. We propose that the observed in vivo functional effects of Ephedra/ephedrine alkaloids may be produced, in part, by blocking the action of norepinephrine at 1A- and /or 2A-AR. The results of our studies, particularly with the findings on human 2-AR, suggest that an inhibition of AR subtypes by ephedrine analogs may be related to their in vivo effects. In particular, it is plausible to suggest that naturally occurring ephedrine alkaloids may act as antagonists of presynaptic 2A/2C-ARs present in nerve terminals. As such, ephedrine analogs may interfere with presynaptic norepinephrine uptake and enhance synaptic concentrations and response to this neurotransmitter. In this regard, Rothman et al. (2003) used in vitro assays to measure the effect of phenylpropanolamines on the release or reuptake of norepinephrine in rat whole brain (minus caudate and cerebellum) preparations and proposed that ephedrine analogs act via an indirect mechanism leading to an enhanced release of norepinephrine. They also showed that ephedrine isomers and related analogs produce a similar pattern but with less potency for dopamine release and that ephedrine analogs also have affinity for interaction with serotonergic receptors. The study of Ephedra alkaloid effects on dopamine and serotonin receptor subtypes may also be worthwhile but is beyond the scope of the present work. Thus, it is also clear that interpretation of the in vivo and in vitro results of the pharmacology of ephedrine alkaloids may be further complicated by combination and relative abundance of - and/or β-AR subtypes and other pharmacological receptors and their signaling pathways that are present in the target tissue. More studies will be required to establish the importance of these and other receptors to assess the overall actions of these sympathomimetic amines in vivo. Irrespective of the underlying differences between in vitro and in vivo actions of Ephedra or its alkaloids, it is apparent that their pharmacological effects on ARs will be dependent upon the dose and may involve both direct and/or indirect mechanisms on noradrenergic-dependent pathways.
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Footnotes
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This work was supported in part by Grant R21AT00510 from the National Center for Complementary and Alternative Medicine, and its contents are solely the responsibility of the authors and do not necessarily represent the official views of the National Center for Complementary and Alternative Medicine, National Institutes of Health. This study was also supported in part by United States Department of Agriculture-Agricultural Research Service Agreement 58-6408-2-0009) and the National Heart, Lung, and Blood Institute Undergraduate Short-Term Training Grant 5T35HL07926 (to Y.D.).
Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
doi:10.1124/jpet.107.120709.
ABBREVIATIONS: CNS, central nervous system; AR, adrenoceptor; HEK, human embryonic kidney; CHO, Chinese hamster ovary; TRE-LUC, phorbol ester-response element-luciferase gene; CRE-LUC, cAMP-response element-luciferase.
Address correspondence to: Dr. Dennis R. Feller, Department of Pharmacology, School of Pharmacy, The University of Mississippi, University, MS 38677. E-mail: dfeller{at}olemiss.edu
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References
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Abstract
Materials and Methods
