AJP - Regulatory, Integrative and Comparative Physiology, Vol 262, Issue 4 651-R658, Copyright © 1992 by American Physiological Society
Effects of volume expansion on renal sympathetic nerve activity and cardiovascular and renal function in lambs
F. G. Smith, J. M. Klinkefus and J. E. Robillard
Department of Pediatrics and Cardiovascular Center, University of Iowa, Iowa City 52242.
To assess the cardiopulmonary baroreflex in the immature animal, effects of
volume expansion on changes in right atrial pressure, renal sympathetic
nerve activity, and renal function were measured in chronically
instrumented newborn (4-8 days; n = 13) and older lambs (4-5 wk; n = 14).
Studies were carried out for 30 min before and 2 h after volume expansion
with 6% Dextran 70 (25 ml/kg). Right atrial pressure increased by 4.0 +/-
0.5 mmHg in newborns and by 8.8 +/- 0.6 mmHg in older lambs within 15 min
of volume expansion (P less than 0.001). After volume expansion, heart rate
decreased in newborns from 237 +/- 6 beats/min to a nadir of 211 +/- 7
beats/min 2 h later (P less than 0.001) but remained constant at control
levels of 148 +/- 9 beats/min in older lambs. Maximal inhibition of renal
sympathetic nerve activity was achieved at 15 min in older lambs (-50.1 +/-
7.5%) and at 60 min (-58.3 +/- 10.9%) in newborns in which there was a
prolonged sympathoinhibition (P less than 0.001). There was also a
significant diuretic response in both groups but a limited natriuretic
response to volume expansion in newborns (P greater than 0.05) compared
with older lambs (P less than 0.001). These data demonstrate the presence
of the cardiopulmonary baroreflex in the first week of life in lambs. The
sustained sympathoinhibition and bradycardia seen in response to volume
expansion in newborns but not in older lambs support the hypothesis that
the reflexes controlling arterial pressure and blood volume change with
postnatal maturation.
