Atypical characteristics of frog and chick erythrocyte β-adrenoceptors

Abstract

The characteristics of β-adrenoceptors on frog and chick erythrocytes have been compared to those present on rat erythrocyte, lung and cerebral cortical membranes. The receptors have been examined using the specific ligand [ 3H]dihydroalprenolol ([ 3H]DHA) with conventional filtration techniques. The equilibrium dissociation constant (K D) of [ 3H]DHA and the affinities of the non-selective β-adrenoceptor antagonists propranolol, timolol and alprenolol, were very similar in all preparations examined. The relative order of potency of the catecholamines, isoprenaline > adrenaline > in rat lung, rat erythrocyte and frog erythrocyte; isoprenaline > noradrenaline ⩾ adrenaline in chick erythrocyte and rat cortex, suggese an overall β 2-classification for the former and β 1-classification for the later tissues. However, the use of highly selective β 1- or β 2-agents and computer-assisted iterative curve fitting revealed that whereas rat cortex and lung possess both receptor subtypes (lung 80% β 2, 20% β 1; cortex 65% β 1, 35% β 2) only β 2-sites were observed in rat erythrocytes. On the other hand, chick and frog erythrocytes possess a homogeneous population of receptors that do not strictly correspond either to β 1- or β 2-adrenoceptors. The atypical nature of these non-mammalian erythrocyte β-adrenoceptors is discussed in relation to their previous extensive use as model β-adrenoceptor systems.