Although the α- and β-adrenergic receptors have opposing effects on insulin secretion, the inhibitory influence of α-receptors appears to predominate. To determine if this was due to differences in number and affinity of receptors, isolated rat pancreatic islet cells were incubated with [3H]-dihydroalprenolol and [3H]-di-hydroergocryptine as ligands for β- and α-adrenergic binding sites. It was found that the number of β-adrenergic binding sites was 143 fmol/mg islet protein with a Kd = 0.57 nM. The number of α-adrenergic binding sites was 53 fmol/mg protein with a Kd = 0.26 nM. Thus, there are 2.7 times as many β-adrenergic binding sites as α-binding sites, and neither binding site number nor affinity is responsible for the predominant influence of the α-adrenergic receptors.

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