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The Journal of Nuclear Medicine Vol. 32 No. 1 76-80
© 1991 by Society of Nuclear Medicine
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In vitro and In vivo Characterization of 4-[125I]Iododexetimide Binding to Muscarinic Cholinergic Receptors in the Rat Heart

Kaname Matsumura, Yoshihiro Uno, Ursula Scheffel, Alan A. Wilson, Robert F. Dannals and Henry N. Wagner, Jr.

Division of Nuclear Medicine and Radiation Health Sciences, The Johns Hopkins Medical Institutions, Baltimore, Maryland

Correspondence: For reprints contact: Ursula Scheffel, ScD, Division of Nuclear Medicine and Radiation Health Sciences, The Johns Hopkins Medical Institutions, 615 North Wolfe St., Baltimore, MD 21205-2179.

ABSTRACT

4-[125I]iododexetimide binding to muscarinic cholinergic receptors (mAChR) was evaluated in the rat heart. 4-[125I] iododexetimide displayed high in vitro affinity (Kd = 14.0 nM) for rat myocardial mAChR. In vivo, there was high accumulation of 4-[125I]iododexetimide in the rat atrium and ventricle which could be blocked by ~60% by preinjection of atropine. In contrast, accumulation of the radiolabeled stereoisomer, 4-[125I]iodolevetimide, was 63% lower than 4-[125I]iododexetimide and was not blocked by atropine. The blood clearance of 4-[125I]iododexetimide was rapid, providing heart-to-blood ratios of up to 14:1; however, heart-to-lung and heart-to-liver ratios were below unity. The data indicate that 4-[125I]iododexetimide binds potently to rat mAChR. However, since nonspecific binding is relatively high, it is not clear whether iododexetimide labeled with 123I will be useful in SPECT imaging studies of myocardial mAChR. Further studies in humans are indicated.






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Copyright © 1991 by the Society of Nuclear Medicine.