Blood pressure and heart rate in the ovine fetus: ontogenic changes and effects of fetal adrenalectomy

Ontogenic changes in baseline and 24-h rhythms of fetal arterial blood pres sure (FABP) and heart rate (FHR) and their regulation by the fetal adrenal were studied in 18 fetal sheep chronically instrumented at 109-114 days ges tation (GA). In the long-term study, FABP and FHR were continuously recorde d from 120 days GA to spontaneous term labor (>145 days GA) in five animals . Peak times (PT) and amplitudes (Amp) of cosinor analysis were compared at 120-126, 127-133, and 134-140 days GA. Consistent, significant linear incr eases in FABP and linear decreases in FHR were observed in all fetuses. Sig nificant 24-h rhythms in FABP and FHR were observed during all the time win dows. In the adrenalectomy study, to test the hypothesis that fetal cortiso l plays a key role in cardiovascular maturation, fetal adrenals were remove d in eight animals (ADX); sham fetal adrenalectomy was performed on five an imals (Con). Cortisol (4 mu g/min) was infused intravenously in four ADX fe tuses from day 7 postsurgery for 7 days (ADX+F). No significant changes in PT and Amp in FABP and FHR were observed. Plasma cortisol levels remained l ow in Con and ADX fetuses (<4.9 ng/ml). Cortisol infusion increased fetal p lasma cortisol to 22.3 +/- 3.2 ng/ml (mean +/- SE) on day 13 in ADX+F fetus es. FABP increased in control and ADX+F but not ADX fetuses; FHR decreased in control and ADX but rose in ADX+F fetuses. These results suggest that, i n chronically instrumented fetal sheep at late gestation, 1) increases in F ABP and decreases in FHR are maintained consistently from 120 to 140 days G A, with distinct 24-h rhythms, the PT and Amp of which remain unchanged, an d 2) the physiological increase in FABP is dependent on the fetal adrenal; bilateral removal of the fetal adrenals does not prevent the ability of cor tisol to produce a sustained increase in FABP.