Characterization of [3H]yohimbine binding to putative alpha-2 adrenergic receptors in neonatal rat lung.

In contrast to the adult rat lung, which lacks alpha-2 adrenergic receptor binding sites, the neonatal rat lung had a high density of [3H]yohimbine binding sites. The binding of this alpha-2 antagonist was saturable, reversible and stereospecific. Steady-state binding was reached by 15 min at 23 degrees C, and the association and dissociation rate constants were 0.17 min-1nM-1 and 0.30 min-1, respectively. From saturation experiments, a Bmax of 304 fmol/mg of protein and a KD of 1.53 nM were calculated. Surprisingly, no appreciable specific binding was observed in this tissue for the alpha-2 agonists [3H]clonidine, [3H]paraminoclonidine and [3H]epinephrine. Inhibition experiments indicated that the binding site had the characteristics of an alpha-2 adrenergic receptor although yohimbine was only 5 times more potent than prazosin. The Ki for epinephrine was decreased 4-fold by 100 microM guanyl-5'-6'-imidodiphosphate suggesting that the receptor binding site may be coupled to adenylate cyclase. Ethanol inhibited the [3H]yohimbine binding with an IC50 of 200 mM. This inhibition was "competitive-like" as increasing concentrations of ethanol resulted in an increased KD, but not a decrease in Bmax. The presence of high density [3H]yohimbine binding sites in early life may have a developmental importance which is not yet understood.