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

The dissociation constant of binding (KD) of 125I-labelled 3-(4-iodophenoxy)-1-isoproplyaminopropan-2-ol (IIP) to guinea-pig myocardial membrane preparations was 2.2 x 10-8M. 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 x 10-8M on atria (pA2 7.54) and 3.3 x 10-8M on trachea (pA2 7-48). These values indicate that IIP is not a selective beta-adrenoceptor blocking drug. In addition, agreement was found between the affinity constant of this antagonist for beta-adrenoceptors as determined by a direct binding study and an indirect pharmacological study.