Alpha 1-adrenoceptors in guinea-pig heart ventricles. [3H]prazosin and [125I]IBE 2254 are bound specifically to a receptor population with high affinity and low density.

In membrane preparations of guinea-pig heart ventricles [3H]prazosin ([3H]PRAZ) and [125I]-2-[beta-(4-hydroxy-3-iodophenyl)ethylaminoethyl]tetralone ([125I]IBE 2254) were used to determine the quantity and the characteristics of the small number of alpha 1-adrenoceptors. The amount of alpha 1-adrenoceptors was compared with the quantity of beta-adrenoceptors which were determined by [3H]dihydroalprenolol ([3H]DHA) binding. Equilibrium binding of all radioligands was found to be of high affinity, high specificity and saturable. A single homogeneous population of binding sites was labelled with the radioligands. The results of kinetic experiments with [3H]PRAZ and [125I]IBE 2254 confirmed the equilibrium binding results. In competition experiments, the rank order of potency for antagonists and agonists to compete with both radioligands was consistent with the well-established rank order at the alpha 1-adrenoceptor. In competition experiments, (-)-noradrenaline was more potent in inhibiting specific binding of [3H]PRAZ and [125I]IBE 2254 than (+)-noradrenaline by a factor of more than 50. The number of beta-adrenoceptors exceeded those of alpha 1-adrenoceptors by a factor of 4.4. According to our results in ventricles of the guinea-pig heart there exists a small population of alpha 1-adrenoceptors. Therefore, the determination of their number even by an adapted method and by means of radioligands with high specific activity and high selectivity as [3H]PRAZ and [125I]IBE 2254 is difficult.