Activity of the unique β-adrenergic Na+/H+ exchanger in trout erythrocytes is controlled by a novel β3-AR subtype

Abstract

β-Adrenoceptors (β-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 β-ARs from the rainbow trout ( Oncorhynchus mykiss). Phylogenetic analysis of amino acid sequences indicates that both β-ARs are homologs of the mammalian β3-AR. Analysis of tissue expression patterns indicates that one of these trout β3-adrenoceptors (β3a-AR) is highly expressed in gill and heart, whereas the second (β3b-AR) is highly expressed by red blood cells (RBC). Expression of the β3b-AR in the RBC coupled with the finding of a single category of β-AR binding sites on RBC membranes provides strong evidence for the control of the trout RBC β-AR Na+/H+ exchanger (β-NHE) activity by signaling through this β3b-subtype and not through a β1-subtype as previously proposed. The RBC-specific trout β3b-AR exhibits binding characteristics that distinguish this receptor from each of the three pharmacologically defined categories of mammalian β-ARs (β1-, β2-, and β3-AR). This study is the first to report the presence of a β3-AR subtype in a fish species, and the proposal that the β3b-AR controls RBC β-NHE activity represents a novel role for the β3-AR subtype in vertebrates.