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The efferent mechanisms mediating the well-known diurnal cardiovascular rhythms in the late-gestation fetus are only partially understood. In the present study, we evaluated the contribution of the parasympathetic and sympathetic nervous systems (SNS) to these rhythms. Chronically instrumented fetal sheep at a mean (SE) of 122 (1) days gestation (term is 147 days) underwent either chemical sympathectomy with 6-hydroxydopamine the day after surgery (n = 8), vagotomy at surgery (n = 8), or were sham controls (n = 8). Fetal heart rate (HR), fetal HR variability (HRV), mean arterial blood pressure (MAP), carotid blood flow (CaBF), electrocorticogram (ECoG) activity, and nuchal activity were measured continuously for 24 h. Changes between sleep states were determined in a 6-h interval. Control fetal sheep showed consistent diurnal rhythms in fetal HR, HRV, MAP, and CaBF, with maximal activity in the evening, but not in nuchal activity. Sympathectomy was associated with a significant reduction of both fetal HR and HRV, while vagotomy was associated with a fall in fetal HRV (P < 0.05) but no change in HR. Despite this, most animals in the two intervention groups still showed diurnal rhythms for fetal HR, HRV, MAP, and CaBF, although peak HR may have been delayed in the sympathectomy group (mean 02:22 vs. 23:54 h in controls, P = 0.06). There was no effect of either intervention on sleep state cycling, although state-related cardiovascular rhythms were significantly modulated. These data indicate that, neither the SNS nor vagal activity, in isolation at least, is essential for generating cardiovascular diurnal rhythms in the late-gestation fetus. fetal heart rate variability; diurnal rhythm; fetal sheep; sleep state; autonomic nervous system doi: 10.1152/ajpregu.90979.2008.

The arterial baroreflex is a fundamental reflex that buffers rapid changes in arterial blood pressure (BP) via regulation of the heart rate and sympathetic nerve activity to the vasculature. In adults a sigmoidal relationship between BP and both heart rate and sympathetic nerve activity is well documented. Its role in blood pressure control before birth is unclear. Preterm babies have a high incidence of low BP, especially in the first few days of life, which could be related, in part, to immaturity of the baroreflex. In the present study, we investigated the baroreflex control of fetal heart rate and renal sympathetic nerve activity (RSNA) in preterm fetal sheep in utero (102 [+ or -] 1 days of gestation; term 140 days). Phenylephrine was associated with a significant increase in BP from 38 [+ or -] 2 to 58 [+ or -] 3 mmHg and a decrease in heart rate (HR) from 177 [+ or -] 4 to 116 [+ or -] 8 beats per minute (bpm). Sodium nitroprusside was associated with a significant fall in BP from 38 [+ or -] 2 to 26 [+ or -] 1 mmHg and an increase in HR from 182 [+ or -] 4 to 274 [+ or -] 8 bpm. However, the time between the 50% changes in BP and HR was significantly greater after hypotension than hypertension (31 [+ or -] 8 s vs. 14 [+ or -] 5 s, P < 0.05). No significant changes in RSNA occurred with either stimulus. This suggests that there are different maturational tempos for the components of the central autonomic response to altered blood pressure. telemetry; renal sympathetic nerve

Clinically and experimentally male fetuses are at significantly greater risk of dying or suffering injury at birth, particularly after premature delivery. We undertook a retrospective cohort analysis of 60 female and 65 male singleton preterm fetal sheep (103-104 days, 0.7 gestation) with mean arterial blood pressure (MAP), heart rate, and carotid and femoral blood flow recordings during 25 min of umbilical cord occlusion in utero. Occlusions were stopped early if fetal MAP fell below 8 mmHg or if there was asystole for >20 s. Fetuses that were able to complete the full 25-min period of occlusion showed no differences between sexes for any cardiovascular responses. Similar numbers of occlusions were stopped early in males (mean: 21 min, n = 16) and females (mean: 23 min, n = 16); however, they showed different responses. Short-occlusion males (n = 16) showed a slower initial fall in femoral vascular conductance, followed by greater bradycardia, hypotension, and associated organ hypoperfusion compared with full-occlusion fetuses. In contrast, short-occlusion females (n = 16) showed a significantly more rapid early increase in femoral vascular conductance than the full-occlusion fetuses, followed by worsening of bradycardia and hypotension that was intermediate to the full-occlusion fetuses and short-occlusion males. Among all fetuses, MAP at 15 min of occlusion, corresponding with the time of the maximal rate of fall, was correlated with postmortem weight in males ([R.sup.2] = 0.07) but not females. In conclusion, male and female fetuses showed remarkably similar chernoreflex and hemodynarnic responses to severe asphyxia, but some males did show impaired hemodynamic adaptation within the normal weight range. cardiovascular chemoreflex

Extensive studies in the adult have demonstrated that the sympathetic nervous system plays a central role in cardiovascular control. The maturation of the sympathetic nervous system before birth is poorly understood. In the present study, we directly recorded renal sympathetic nerve activity (renal SNA) in five preterm fetal sheep (99 [+ or -] 1 days gestation; term is 147 days). Recordings were performed in utero using a telemetry-based technique to alleviate movement artifact without anesthesia or paralysis. The preterm fetuses exhibited a coordinated discharge pattern in renal SNA, indicating many individual neurons active at approximately the same time. This is consistent with that observed previously in adult animals, although the frequency of the bursts was relatively low (0.5 [+ or -] 0.1 Hz). The discharges in renal SNA were entrained to the cardiac cycle (average delay between diastolic pressure and maximum renal SNA 319 [+ or -] 1 ms). The entrainment of the sympathetic discharges to the cardiac cycle indicates phasic baroreceptor input and that the underlying circuits controlling SNA within the central nervous system are active in premature fetuses. telemetry; renal sympathetic nerve activity doi:10.1152/ajpregu.00891.2006