Specific non-beta-adrenergic binding sites for 125I-iodocyanopindolol in myocardial membrane preparations: a comparative study between human, rat, and porcine hearts.

STUDY OBJECTIVE

The aim was to investigate observed differences in beta adrenergic and apparent non-beta-adrenergic binding of (-)[125I]-iodocyanopindolol (125I-ICYP).

DESIGN

Binding parameters for several beta adrenergic agonists and antagonists were examined in radioligand binding assay, using 125I-ICYP as radioligand, in membranes prepared from myocardial tissue.

SUBJECTS

Human right auricular myocardium was obtained from patients undergoing open heart surgery. Ventricular myocardium was from Norwegian landrace pigs and Wistar rats.

MEASUREMENTS AND MAIN RESULTS

Specific binding of 125I-ICYP was observed. This was only partially competed for with high affinity by isoprenaline, noradrenaline, adrenaline, and atenolol. Considerable interspecies variations in the magnitude of specific non-beta-adrenergic (NBA) binding of 125I-ICYP were shown. The equilibrium constant of dissociation (Kd) of the specific NBA binding sites for 125I-ICYP was 0.3-0.4 nmol.litre-1, and the binding capacities were 20, 106, and 192 fmol.mg-1 protein in rat, human, and porcine myocardium, respectively. The NBA sites were heat sensitive and destroyed by trypsin. Association to NBA sites occurred more rapidly than to beta adrenoceptors. Dissociation of bound 125I-ICYP from NBA sites and beta adrenoceptors at 30 degrees C revealed first order kinetics with t1/2 of 19 min from NBA, as compared to 120 min from beta adrenoceptors. In all three species the ligand specificity for NBA sites was very similar and various adrenergic agonists and antagonists competed with 125I-ICYP binding with the following potencies: timolol greater than propranolol greater than ICI 118 551 greater than pindolol greater than Sandoz 204 545 greater than terbutaline greater than noradrenaline and adrenaline much greater than isoprenaline and atenolol. Of agonists and antagonists for other receptor systems, only the serotoninergic 5-HT2 antagonist ritanserin could displace 125I-ICYP from the NBA sites with relatively high affinity.

CONCLUSIONS

125I-ICYP and several beta adrenoceptor antagonists interact specifically with receptor like proteins other than beta adrenoceptors, and remarkable interspecies difference in the levels of myocardial NBA sites was observed.