| Abstract: | 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. |
