Thermodynamic Study of Iodocyanopindolol β-Adrenoceptors Interactions with Rat Lung and Cerebral Cortex

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Abstract
(±)125 I-cyanopindolol (±) I CYP) was used to characterize β-adrenoceptors on rat lung and cerebral cortex membranes. The affinity of (±) ICYP was higher for lung (Kd = 64.3 pM) at 37°C. The association reaction of (±) ICYP was faster with lung (k+1 = 1.52 × 109 M−1.min−1) than with cerebral cortex β-adrenoceptors (k+1 = 1.75 × 108 M−1.min−1). In both tissues, the dissociation reaction followed a biphasic process with a fast (t ½ = 15.4 min and 5.6 min for lung and cerebral cortex respectively) and a slow component (t ½ = 474 min and 255 min for lung and cerebral cortex respectively). The thermodynamic parameters for (±) ICYP - β-adrenoceptors binding have been determined from kinetics and equilibrium studies, for the two tissues, at several temperatures between 0° and 44° 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 - β-adrenoceptors interaction decreased linearly with temperature : the binding occured with a negative heat capacity change (ΔCp° = -368.9 cal. moles−1. K−1) at 25° C. Thermodynamic and kinetic results show that binding of (±) ICYP to lung β-adrenoceptors could involve two successive equilibria with a conformational change of the β-adrenergic receptor.

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