Corticosteroid binding by fetal rat and rabbit lung in organ culture

To further characterize glucocorticoid action in fetal lung cells, we investigated corticosteroid metabolism and binding in explants of fetal rat and rabbit lung. Cortisone (E) was concerted to cortisol (F) and bound by receptor with a time course only somewhat slower than for F. Production of F (0.243 pmol/min/mg DNA) was the same in male and female rabbits and was not affected by prior exposure to glucocorticoid in utero or in culture. The t 1/2 for dissociation of nuclear-bound [3H]F was 84 min on changing the culture medium and 21 min on addition of excess non-labeled dexamethasone. Dissociation of [3H]dexamethasone was approx 5-fold slower by both procedures. The KD for nuclear binding of dexamethasone, F, E, and corticosterone in rabbit lung were 0.7, 7.3, 6.8 and 70.6 nM, respectively. In rat lung, the KD for dexamethasone was 6.8 nM. The concentrations of dexamethasone and F required for half-maximal stimulation of phosphatidylcholine synthesis were similar to the KD values. Dexamethasone binding capacity (sites/mg DNA) increased with age in both rat (+103% increase from day 16 to 22) and rabbit (+47% between day 23 and 30). Receptor concentration was the same in both sexes, and there were no developmental changes in non-specific binding, nuclear:cytoplasmic distribution, or KD. In 27-day rabbit fetuses, the rate of choline incorporation was higher in lungs with greater binding capacity. We conclude that (1) E is rapidly converted to F in rabbit lung to become an active glucocorticoid, whereas corticosterone probably has little physiologic activity, (2) there is a species difference in the affinity of dexamethasone binding which is reflected in responsiveness (3) there is no difference between sexes in E conversion, receptor capacity, or phosphatidylcholine synthesis, and (4) the concentration of binding sites per lung cell increases during fetal development. We suggest that developmental increases in both F production and receptor may be important factors in the expression of endogenous glucocorticoid effects.