Carotid baroreceptors in fetal and newborn sheep.

The responses of single carotid baroreceptor afferents were determined in anaesthetized fetal lambs at 88-113 and 131-144 days gestation, and in newborn lambs 1-8 and 30-40 days old. The baroreceptors discharged in synchrony with the arterial pressure pulse and increased their discharge rate as pressure was raised by compression of the abdominal aorta. When step increases in pressure were applied to the vascularly isolated carotid sinus, baroreceptor discharge increased abruptly and then showed adaptation to a steady state level. Basal mean arterial pressure increased from 49.1 in the young fetuses to 87.5 mm Hg in the 30- to 40-day-old lambs without an accompanying increase in basal baroreceptor discharge expressed absolutely or as a function of maximum discharge for each unit. The slope of the steep portion of the stimulus-response curve decreased with gestational age from 7.89 +/- 1.57 (mean +/- SE) at 88-113 days gestation to 1.82 +/- 0.37% nerve activity.mm Hg-1 in the 30- to 49-day-old lambs. Dynamic and steady state response curves were determined using step increases in carotid sinus pressure in two fetal lambs of 135 days gestation and two lambs 8 days old. Both the dynamic and the steady state curves were less steep in the older lambs. We conclude that the sensitivity of the carotid baroreceptors is reset as arterial pressure increases throughout the last third of gestation and the first postnatal month. This resetting is seen as a shift to the right of the response curve and a decrease in its slope.(ABSTRACT TRUNCATED AT 250 WORDS)