Thermodynamic study of iodocyanopindolol beta-adrenoceptors interactions with rat lung and cerebral cortex

(+/-)125 I-cyanopindolol ((+/-) I CYP) was used to characterize beta-adrenoceptors on rat lung and cerebral cortex membranes. The affinity of (+/-) ICYP was higher for lung (Kd = 64.3 pM) at 37 degrees C. The association reaction of (+/-) ICYP was faster with lung (k+1 = 1.52 X 10(9) M-1.min-1) than with cerebral cortex beta-adrenoceptors (k+1 = 1.75 X 10(8) M-1.min-1). In both tissues, the dissociation reaction followed a biphasic process with a fast (t 1/2 = 15.4 min and 5.6 min for lung and cerebral cortex respectively) and a slow component (t 1/2 = 474 min and 255 min for lung and cerebral cortex respectively). The thermodynamic parameters for (+/-) ICYP - beta-adrenoceptors binding have been determined from kinetics and equilibrium studies, for the two tissues, at several temperatures between 0 degrees and 44 degrees C. For lung and cerebral cortex, Arrhenius plots were linear with different energies of activation. Van't Hoff plot was not linear for lung and the standard enthalpy and entropy changes of (+/-) ICYP - beta-adrenoceptors interaction decreased linearly with temperature: the binding occurred with a negative heat capacity change (delta Cp degrees = -368.9 cal. moles-1.K-1) at 25 degrees C. Thermodynamic and kinetic results show that binding of (+/-) ICYP to lung beta-adrenoceptors could involve two successive equilibria with a conformational change of the beta-adrenergic receptor.