Inhibition of [3H]-dihydroalprenolol binding to rat cardiac membranes of various beta-blocking agents.

Binding of [3-H]-dihydroalprenolol ([3-H]-DHA) to rat cardiac membranes was rapid and reversible (k1 = 0.633-0.701 x 10(6) M(-1) S(-1) And k(-1) = 0.0017-0.0043 s(-1). 2 [3-H]-DHA bound to a single class of binding sites with an equilibrium dissociation constant (Kd25degreesc) of 5.7+/-1.1 x 10(-9) M. 3 This binding was specific and the order of potency of adrenoceptor agonists in competing for the binding sites was (-)-isoproterenol greater than (+/-)-isoproternol greater than (+)-isoproterenol greater than (-)-adrenaline greater than (-)-noradrenaline. This was in agreement with the beta1 nature of the cardiac beta-receptors. 4 Cardioselective beta-blockers (i.e. metoprolol, acebutolol and practolol) were shown to have lower binding site affinities, when compared to other blockers. This may be related to steric hindrance by the side-chain at the aromatic end of these molecules.