Beta-adrenergic receptors in the urinary bladder of adult and developing rats

Specific beta-adrenergic receptors were demonstrated in the urinary bladder of adult and developing rats, by direct tissue binding with LD [125I]-cyanopindolol (CYP). The maximum number of binding sites (Bmax) was 167 +/- 25 fmol/mg membrane protein and the dissociation constant (KD) equalled 61 +/- 33 pM. The Hill slopes of the LD [125I]-CYP binding showed a single class of noncooperative receptor sites. The rank order of potency of agonist competition for LD [125I]-CYP binding suggests that the receptors are mostly of the beta 2 subtype. Beta-adrenergic receptor density was approximately half in the first 10 days of life (Bmax 63 to 77 fmol/mg protein) compared with the older age-groups studied (Bmax 91 to 167 fmol/ng protein). On the 1st day after delivery, the calculated beta-adrenergic receptor number/bladder was 9.4, and it increased significantly with age to 2,496 in the adult rat. This is a 250-fold increase in the number of receptors/bladder, while only a 20-fold increase in membrane protein and a five- to sixfold increase in the bladder weight was observed. Thus, an age-dependent increase of beta-adrenergic receptors on the cell membrane surface area occurred in the developing urinary bladder of the rat.