SPECIFIC NON-BETA-ADRENERGIC BINDING-SITES FOR I-125 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 (-)[I-125]-iodocyanopindolol (I-125-ICYP). Design - Binding parameters for several beta-adrenergic agonists and antagonists were examined in radioligand binding assay, using I-125-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 I-125-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 I-125-ICYP were shown. The equilibrium constant of dissociation (Kd) of the specific NBA binding sites for I-125-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 I-125-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 I-125-ICYP binding with the following potencies: timolol > propranolol > ICI 118 551 > pindolol > Sandoz 204 545 > terbutaline > noradrenaline and adrenaline >> isoprenaline and atenolol. Of agonists and antagonists for other receptor systems, only the serotoninergic 5-HT2 antagonist ritanserin could displace I-125-ICYP from the NBA sites with relatively high affinity. Conclusions - I-125-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.