Effect of antenatal glucocorticoids on sympathetic nerve activity at birth in preterm sheep.

Renal sympathetic nerve activity (RSNA) increases rapidly after delivery of term fetal sheep and parallels the rise in heart rate (HR) and arterial pressure. To examine the RSNA response at birth in immature lambs, experiments were performed in chronically instrumented preterm fetal sheep (118- to 125-day gestation, term 145 days) before and after delivery by cesarean section. HR remained unchanged from fetal values at 1 and 4 h after birth, whereas mean arterial blood pressure (MABP) decreased significantly (P < 0.05) by 4 h after delivery. RSNA significantly decreased after premature birth in all animals studied (n = 6), achieving only 39 +/- 17% of fetal RSNA (P < 0.05; all results are mean +/- SE). Because cardiovascular function after premature birth is improved by the use of antenatal corticosteroids, we also tested the hypothesis that corticosteroid administration would evoke a more pronounced sympathetic response in prematurely delivered lambs (n = 7, 118- to 125-day gestation). After maternal administration of dexamethasone (5 mg i.m., 48 and 24 h before delivery), RSNA increased after birth in six of seven fetuses to 166 +/- 32% of the fetal RSNA value. Dexamethasone treatment also decreased the sensitivity of baroreflex-mediated changes in HR in response to increases in MABP. Because the sympathetic response at birth is depressed in preterm compared with term lambs, we performed an additional study (n = 8) to determine if immature sheep are capable of mounting a sympathetic response to cold. In utero cooling produced rapid and sustained increases in MABP (20 +/- 4%), HR (26 +/- 6%), and RSNA (282 +/- 72%) (all P < 0.05), consistent with a generalized sympathoexcitation. These results suggest that sympathoexcitation is absent after premature delivery despite the presence of functional descending autonomic pathways. Furthermore, exogenous corticosteroids appear to have a maturational effect on the sympathetic response at birth, which may be one mechanism by which maternal steroid administration improves postnatal cardiovascular homeostasis.