Specific binding sites for prolactin and growth hormone in the adult rabbit lung

Specific prolactin (PRL) and growth hormone (GH) binding sites were identified and characterized in lung membranes from male and female adult rabbits. The binding of iodinated human GH ([125I]iodo-hGH) and iodinated ovine PRL ([125I]iodo-oPRL) was time, temperature and protein dependent and was found to conform to the requirements defining a physiological receptor, in terms of hormonal and immunological specificities as well as kinetic properties. [125I]Iodo-hGH was displaced from lung membranes by hGH, oPRL, ovine GH and rat GH, while [125I]iodo-oPRL was effectively displaced only by oPRL and hGH. Scatchard plots of the competition curves of [125I]iodo-hGH and [125I]iodo-oPRL were both linear, suggesting, in each case, a single class of binding sites with affinity constants (Ka) of 1.74 +/- 0.64 X 10(9) M-1 and 0.78 +/- 0.28 X 10(9) M-1 and binding capacities of 6.43 +/- 0.53 and 4.16 +/- 0.69 fmol/mg protein, respectively. Anti-PRL-receptor antiserum significantly inhibited the binding of the [125I]iodo-oPRL to rabbit lung membranes, while it was less potent in preventing the binding of [125I]iodo-hGH, which has both lactogenic and somatogenic activity. Removal of endogenous ligand by treating lung membranes with 4 M MgCl2 increased specific binding of hGH about 2.5-fold, exposing additional specific binding sites without significantly changing the binding affinity. The level of binding of hGH and oPRL to rabbit lung did not show a pronounced sex differentiation. In summary, PRL and GH binding sites have been demonstrated for the first time in adult rabbit lung membranes, and they support the possibility of a physiological role for PRL and GH in the lung.