The development of alpha 2-adrenoceptors in the rat kidney: correlation with noradrenergic innervation.

The development of the renal alpha 2-adrenoceptors in the rat was studied by binding renal membranes of prenatal and postnatal rats with p-[3,5-3H]aminoclonidine (PAC), a selective alpha 2-adrenoceptor agonist. The results demonstrate that prior to birth and on the day of birth [3H]PAC binding is present and reflects a single linear binding affinity (Kd approximately equal to 1.0 nM). In contrast, data from postnatal day 3 demonstrate the addition of a second, lower affinity binding site (Kd approximately equal to 7.0 nM) that develops rapidly during the ensuing weeks of life. Both high and low affinity alpha 2-adrenoceptor binding sites continue to increase in receptor density through 30 weeks of age; however, the binding affinity of each site remains relatively stable. The sympathetic noradrenergic innervation of the rat kidney is present prenatally and rapidly develops during the first 3 weeks of life with peak noradrenaline concentrations being reached at approximately prenatally and rapidly develops during the first 3 weeks of life with peak noradrenaline concentrations being reached at approximately postnatal day 21. Immunohistochemical staining demonstrates that tyrosine hydroxylase-positive axons rapidly develop in the kidney during the same period, indicating that the increase in noradrenaline is related to expansion of renal sympathetic innervation.