Demonstration of ?-adrenoceptors in the rabbit heart by [3H]-dihydroergocryptine binding

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Abstract
For direct identification of α-adrenoceptors in a membrane fraction of the rabbit heart the potent α-adrenoceptor antagonist [3H]-dihydroergocryptine ([3H]-DHE) was used. 1. The binding of [3H]-DHE was saturable with 80 fmol of [3H]-DHE bound/mg protein and of high affinity with an equilibrium dissociation constant (KD) of 11.5 nM. Binding of [3H]-DHE (6 nM) was rapid (t1/2=2 min) and readily reversible. From the ratio of the rate constants for forward (K1=1.97×107 M−1 min−1) and reverse (K2=0.206 min−1) reactions a KD-value of 10 nM was calculated, which is in good agreement with that obtained by equilibrium studies. 2. Adrenergic agonists compete for [3H]-DHE binding in an order of potency: (−)adrenaline > (−)phenylephrine≫ (−)isoprenaline and adrenergic antagonists in the order: phentolamine > yohimbine≫ (−)propranolol. Binding is stereospecific as indicated by the greater potency of (−)adrenaline than (±)adrenaline in displacing [3H]-DHE from the binding sites. 3. For comparison the binding of the potent β-adrenoceptor antagonist (−)[3H]-dihydroalprenolol ((−)[3H]-DHA) was measured in the same membrane fraction. The number and affinity of β-adrenoceptors amounted to 115 fmol of (−)[3H]-DHA bound/mg protein at saturation and KD=7.9 nM. Adrenergic agonists compete for (−)[3H]-DHA binding in an order of potency: (−)isoprenaline > (−)adrenaline > (−)phenylephrine; and adrenergic antagonists in the order: (−)propranolol≫ phentolamine. 4. It is concluded that in a membrane fraction of the rabbit heart there exist binding sites for [3H]-DHE which have characteristics indistinguishable from α-adrenoceptors. Thus the present results are in agreement with previously reported data on the existence of cardiac α-adrenoceptors in the rabbit heart (Schümann et al., 1974; Endoh et al., 1976b).