β-Adrenoceptors in Equine Trachea and Heart

Abstract

The density and subtype pattern of β-adrenoceptors in equine tracheal epithelium, tracheal smooth muscle and heart from 6–9 horses were investigated by radioligand binding studies using the non-selective β-adrenoceptor antagonist 125I-cyanopindolol (ICYP). The specific binding of ICYP was 341±162 fmol/mg protein (mean±SD) for epithelium, 42±13 fmol/mg for smooth muscle and 124±39 and 101±19 fmol/mg for the cardiac atrium and ventricle, respectively. The Kd value of ICYP was 6.7–10.2 pmol/L.

In competition studies, different concentrations of either the β2-selective drug ICI 118551 or the β1-selective CGP 20712A competed with 25 pmol/L ICYP for the binding sites. The competition curves for tracheal smooth muscle and epithelium were monophasic with an approximate Kd value for ICI 118551 of 1 nmol/L and for CGP 20712A of 10 000 nmol/L. This corresponds to known Kd values for these substances binding to β2-adrenoceptors. β2-Adrenoceptors were also found in the heart, most pronounced in the atrium, where the density was 29%±6% (mean±SD) of the total receptor density. CGP 20712A and ICI 118551 bound to the dominating binding site of β1-adrenoceptors in the heart with Kd values of approximately 1 nmol/L and 100 nmol/L, respectively.