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Rat Pineal ₁-Adrenoceptors: Identification and Characterization Using [¹²⁵I]Iodo-2-[β-(4-Hydroxyphenyl)-Ethylaminomethyl]Tetralone

Section on Neuroendocrinology, Laboratory of Developmental Neurobiology, National Institute of Child Health and Human Development, National Institutes of Health Bethesda, Maryland 20205

Address correspondence and requests for reprints to: Dr. David Sugden, Section on Neuroendocrinology, Laboratory of Developmental Neurobiology, Building 10, Room 8D42C, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20205.

Abstract

[¹²⁵I]Iodo-2-[β-(4-hydroxyphenyl)-ethylaminomethyl]tetralone ([¹²⁵I]HEAT), a selective, high affinity, high specific activity ₁-adrenoceptor ligand, is used to characterize -adrenoceptors in the rat pineal gland. Binding of [¹²⁵I]HEAT to membranes is rapid [association rate constant (K_on) = 3.1 nM^–1 min^–1] and readily reversible either by 100-fold dilution or by addition of excess unlabeled HEAT [apparent dissociation rate constant (K_off) = 0.153 min^–1). Saturation experiments indicate a single class of noncooperative binding sites with anequilibrium binding constant (K_D) of 41 ± 9 pM and a B_max of 399 ± 63 fmol/mg protein. The relative potency of a number of adrenoceptor agonists and antagonists in competing with [₁₂₅I] HEAT indicates the receptor is an ₁-subtype. In addition, inhibition of binding is stereospecific; (–)epinephrine and norepinephrine are more than 100-fold more potent than their (+)isomers. The identification of ₁-adrenoceptors in the pineal gland is consistent with evidence indicating a role for these receptors in the regulation of melatonin synthesis and phosphatidylinositol turnover. (Endocrinology 114: 435, 1984)

Received April 28, 1983.

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