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Department of Physiology, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853-6401, USA.

1. Chronically instrumented, late-gestation fetal sheep were prepared to: (1) characterize cardiovascular, endocrine and behavioural effects of fetal treatment with clinical doses of betamethasone and dexamethasone; (2) define specific differences, if any, in the actions of betamethasone and dexamethasone of measured fetal responses; and (3) assess the contribution of changes in peripheral vascular resistance to the glucocorticoid-induced hypertension. 2. Following baseline, either saline (n = 9), betamethasone (n = 9), or dexamethasone (n = 6) was infused for 48 h in fetal sheep commencing at 125 days of gestation. A pronounced increase in fetal blood pressure occurred following both betamethasone and dexamethasone treatment. The nature and magnitude of this increase was similar following treatment with either glucocorticoid. 3. To address possible mechanisms contributing to the glucocorticoid-induced fetal hypertension, fetal plasma catecholamine levels and changes in fetal femoral haemodynamics were assessed following fetal glucocorticoid treatment. A fall in fetal plasma noradrenaline and adrenaline concentrations occurred during betamethasone and dexamethasone treatment. In contrast, a progressive femoral vasoconstriction occurred during betamethasone treatment. 4. A modest fall in the incidence of fetal breathing movements occurred during fetal treatment with either betamethasone or dexamethasone. The magnitude of this reduction was similar with treatment of either glucocorticoid. The fall in fetal breathing during betamethasone and dexamethasone treatment was not associated with a fall in the incidence of fetal low voltage electrocortical activity. 5. Our results indicate that prenatal betamethasone and dexamethasone treatment of late-gestation fetal sheep, in doses similar to those employed clinically, is associated with fetal cardiovascular, endocrine and behavioural effects. Both betamethasone and dexamethasone induce similar increases in fetal blood pressure and similar falls in the incidence of fetal breathing movements. The pronounced betamethasone-induced fetal hypertension is associated with an increase in fetal femoral vascular resistance.

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