Abstract
Benzodiazepines, a class of drugs commonly used to induce anesthesia and sedation, can attenuate intracellular calcium oscillations evoked by α1-adrenergic receptor (α1-AR) stimulation in pulmonary artery smooth muscle cells. We postulated a direct action of benzodiazepines in modulating α1-AR function at the receptor level. Benzodiazepines bound to each of the cloned α1-AR subtypes (α1a-, α1b-, or α1d-AR) on COS-1 cell membranes transiently transfected to express a single population of α1-AR subtype. The ability of benzodiazepines to alter α1-AR signal transduction was investigated by measuring total inositol phosphate generation in rat-1 fibroblast cells, stably transfected to express a single α1-AR subtype. By themselves, benzodiazepines displayed partial agonism. At α1b-ARs and α1d-ARs, the maximal inositol phosphate response to phenylephrine was potentiated almost 2-fold by either midazolam or lorazepam (100 μM). At α1a-ARs, diazepam, lorazepam, and midazolam all increased the maximal response of the partial agonist clonidine at these receptors, whereas the response to the full agonist phenylephrine was unaltered or inhibited. The potentiating actions of midazolam and its partial agonism at α1-ARs was blocked by the addition of 1 μM prazosin, an α1-AR antagonist, and not by a γ-aminobutyric acidA-receptor antagonist. These studies show that benzodiazepines modulate the function of α1-ARs in vitro, and this is the first report of a potential allosteric site on α1-ARs that may be therapeutically useful for drug design.
Footnotes
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Send reprint requests to: Dianne M. Perez, Department of Molecular Cardiology, NB5, Cleveland Clinic Foundation, 9500 Euclid Ave., Cleveland, OH 44195. E-mailperezd{at}ccf.org
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↵1 This work was supported in part by National Institutes of Health Grants R01 HL52544 (to D.M.P.) and HL38291 (to P.A.M.) and an unrestricted research grant from Glaxo Wellcome, Inc. (to D.M.P.). This work was performed under the tenureship of an Established Investigator Award of the National American Heart Association. This work was previously presented at the International Congress of Pharmacology Meeting, Munich, Germany, July 1998.
- Abbreviations:
- AR
- adrenergic receptor
- IP3
- inositol triphosphate
- GABA
- γ-aminobutyric acid
- IP
- inositol phosphate
- DMEM
- Dulbecco's modified Eagle's medium
- [125I]HEAT
- 2-[β-(4-hydroxyl-3-[125I]iodophenyl)ethylamineomethyl]tetralone
- FBS
- fetal bovine serum
- HEM buffer
- 20 mM HEPES, pH 7.4, 1.4 mM ethylene glycol bis(β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid, and 12.5 mM MgCl2
- Received May 12, 1999.
- Accepted August 17, 1999.
- The American Society for Pharmacology and Experimental Therapeutics
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