Diurnal rhythms in fetal urine flow, vascular pressures, and heart rate in sheep

Conflicting indirect data exist as to whether diurnal variations occur in fetal urine flow rate. In addition, the extent of diurnal rhythms in fetal venous pressure or arterial pressure is unknown, although 24-h rhythms do exist in fetal heart rate. In the present study, we used on-line computer techniques to continuously monitor these variables in chronically catheterized ovine fetuses. Fetal urine flow rate and vascular pressures were successfully recorded in 6 of 11 animals over a 24-h period on 21 days out of a total of 45 days of monitoring. We found highly significant diurnal variations in fetal urine flow rate (P less than 10(-6). Hourly means displayed a maximum at 2130 h and a minimum at 1330 h with a maximum amplitude of 28 +/- 5% of the 24-h mean. A secondary maximum (at 0630 h) and minimum (at 0330 h) of smaller amplitude also occurred. There were simultaneous and highly significant (P less than 0.0001) diurnal rhythms in fetal arterial pressure (+/- 2%), venous pressure (+/- 7%), and heart rate (+/- 5%). The maxima in arterial pressure and heart rate occurred within 1 h of the maximum in urine flow, while venous pressure changes were opposite those in arterial pressure. Hourly mean urine flow correlated significantly with arterial pressure but not venous pressure or heart rate, suggesting that the observed 24-h variations in fetal urine flow rate may be partially mediated by a pressure diuresis.(ABSTRACT TRUNCATED AT 250 WORDS)