Glucocorticoids increase the fluidity of the fetal-rat liver microsomal membrane in the perinatal period.

Dexamethasone, a synthetic glucocorticoid, was administered to pregnant rats during the last week of pregnancy in order to examine its effects on the fluidity of the developing fetal-rat liver microsomal membrane. This early prenatal exposure to dexamethasone, which preceded the natural appearance of fetal corticosteroids, markedly accelerated the normal perinatal course of fluidization of this membrane. The lipid apparent microviscosity, which was determined by measurement of fluorescence polarization, decreased in 21-days-old treated fetuses to values that were indistinguishable from those of untreated newborn rats. This dexamethasone-mediated acceleration of membrane fluidization was associated with an increase in the index of unsaturation of the fatty acyl moiety of microsomal lipids. Dexamethasone caused a significant increase in the microsomal content of polyunsaturated fatty acids (arachidonic and linoleic acid), which was accompanied by a decrease in content of monoenoic fatty acids (oleic and palmitoleic acid). This early exposure in utero to dexamethasone precociously induced the changes in fatty acid composition of fetal-rat liver microsomal lipids that normally occur between the last day of pregnancy and the first day of extra-uterine life. These results suggest that endogenous glucocorticoids play a major role in the perinatal fluidization of the rat liver microsomal membrane.