Evidence that presynaptic beta-receptors account for the low affinity site in Schild plots for beta-adrenergic antagonists in the field stimulated rat vas deferens.

The low affinity site in the Schild plot of (-)-propranolol as an antagonist of isoproterenol in the field stimulated rat vas deferens was absent after reserpinization or treatment with rauwolscine. The apparent dependence of intact catecholamine stores and functional alpha 2-adrenoreceptors for demonstration of the low affinity site for propranolol in this preparation suggests that the low affinity site reflects the ability of beta-adrenergic blockers to antagonize an action of isoproterenol upon presynaptic beta-receptors.