Characterization of α-Adrenergic Binding Sites on Rodent Leydig Cells1

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Abstract
A radioligand binding technique was used to study .beta.-adrenergic binding sites on rodent Leydig cells. .beta.-Adrenergic binding sites were found on Leydig cells in both the rat and mouse. Binding of [3H] CGP-12177 [4-(3-t-butylamino-2-hydroxypropoxy)-[5,7-3H]benzimidazole-2-one] to purified rat Leydig cells was found to be saturable, temperature and time dependent, stereospecific, and readily reversible by the .beta.-adrenergic antagonist propranolol. Scatchard analysis revealed the presence of high-afffinity sites with an apparent dissociation constant (Kd) of 0.79 .+-. 0.22 nM and maximal binding capacity (Bmax) of 1716 .+-. 245 sites per rat Leydig cell. Competition of various .beta.-adrenergic agonists and antagonists with [3H] CGP indicates an order of potency of L-isoproterenol > epinephrine = salbutamol > norepinephrine > D-isoproterenol and dl-propranolol = ICI 118,551 >> atenolol, respectively. These observations suggest that the binding sites are predominantly of the .beta.2-receptor subtype. Incubation of freshly isolated rat Leydig cells with luteinizing hormone (100 ng/ml) caused consistent stimulation of androgen production, but only occasional stimulation by the .beta.-agonist isoproterenol (10 .mu.M) was observed. However, these cells consistently responded to the .beta.-agonist after 3 h in primary cultures. These findings indicate that rodent Leydig cells possess .beta.-adrenergic binding sites and point out a possible dissociation between receptor recognition and physiologic response.

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