Beta-adrenoceptors in equine trachea and heart.

The density and subtype pattern of beta-adrenoceptors in equine tracheal epithelium, tracheal smooth muscle and heart from 6-9 horses were investigated by radioligand binding studies using the nonselective beta-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 beta2-selective drug ICI 118551 or the beta1-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 10000 nmol/L. This corresponds to known Kd values for these substances binding to beta2-adrenoceptors. beta2-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 beta1-adrenoceptors in the heart with Kd values of approximately I nmol/L and 100 nmol/L, respectively.