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MODEL ORGANISMS AND COMPARATIVE FUNCTIONAL GENOMICS

Activity of the unique -adrenergic Na⁺/H⁺ exchanger in trout erythrocytes is controlled by a novel ₃-AR subtype

Department of Biology, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5

Submitted 21 March 2003 ; accepted in final form 23 May 2003

-Adrenoceptors (-ARs) are seven-transmembrane domain, G protein-coupled receptors that transduce the cellular effects of epinephrine and norepinephrine and play a pivotal role in the vertebrate stress response. This study reports the cloning and characterization of two previously unreported -ARs from the rainbow trout (Oncorhynchus mykiss). Phylogenetic analysis of amino acid sequences indicates that both -ARs are homologs of the mammalian ₃-AR. Analysis of tissue expression patterns indicates that one of these trout ₃-adrenoceptors (_3a-AR) is highly expressed in gill and heart, whereas the second (_3b-AR) is highly expressed by red blood cells (RBC). Expression of the _3b-AR in the RBC coupled with the finding of a single category of -AR binding sites on RBC membranes provides strong evidence for the control of the trout RBC -AR Na⁺/H⁺ exchanger (-NHE) activity by signaling through this _3b-subtype and not through a ₁-subtype as previously proposed. The RBC-specific trout _3b-AR exhibits binding characteristics that distinguish this receptor from each of the three pharmacologically defined categories of mammalian -ARs (₁-, ₂-, and ₃-AR). This study is the first to report the presence of a ₃-AR subtype in a fish species, and the proposal that the _3b-AR controls RBC -NHE activity represents a novel role for the ₃-AR subtype in vertebrates.

molecular sequence; tissue expression; evolutionary analysis; binding kinetics; -adrenoceptors

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