A comparison of the binding constant (Kd) of 125I-labelled 3-(4-iodophenoxy)-1-isopropylamino-propan-2-ol obtained on β-adrenoceptors in guinea-pig myocardial membranes, with its dissociation constants (Kb) obtained on guinea-pig isolated atria and trachea

The dissociation constant of binding (K) of I‐labelled 3‐(4‐iodophenoxy)‐1‐isopropyl‐aminopropan‐2‐ol (IIP) to guinea‐pig myocardial membrane preparations was 2·2 × 10m. In pharmacological experiments with the non‐labelled material and 60 min contact time, IIP produced a parallel shift in the orciprenaline concentration‐response line on guinea‐pig isolated tracheal and atrial preparations. The dissociation constant (Kb) of IIP was 2·9 × 10m on atria (pA 7·54) and 3·3 × 10m on trachea (pA 7·48). These values indicate that IIP is not a selective β‐adrenoceptor blocking drug. In addition, agreement was found between the affinity constant of this antagonist for β‐adrenoceptors as determined by a direct binding study and an indirect pharmacological study. 1978 Royal Pharmaceutical Society of Great Britain