Vol. 87, Issue 3, 882-888, September 1999
Venous hydrostatic indifference point as a marker of postnatal
adaptation to orthostasis in swine
Phillip S.
Buckner,
Anthony W.
Quail,
David B. F.
Cottee, and
Saxon W.
White
Discipline of Human Physiology and Neuroscience Group, Faculty of
Medicine and Health Sciences, University of Newcastle, Callaghan,
New South Wales 2308, Australia
The postulate that venous adaptation assists postural baroreflex
regulation by shifting the hydrostatic indifference point (HIP) toward
the heart was investigated in eight midazolam-sedated newborn piglets.
Whole body head-up (+15, +30, and +45°) and head-down (
15
and 30°) tilt provided a physiological range of orthostatic strain. HIP for all positive tilts shifted toward the heart
(P < 0.05), +45° HIP shifted
most [6.7 ± 0.3, 5.9 ± 0.5, and 3.6 ± 0.3 (SE) cm
caudal to right atrium on days 1,
3, and
6, respectively]. HIP for
negative tilts (3.0 ± 0.2 cm caudal to right atrium) did not shift
with postnatal age. Euthanasia on day
6 caused 2.1 ± 0.3-cm caudal displacement of HIP
for positive and negative tilts (P < 0.05). HIP proximity
to right atrium was not altered by 
-, 
-adrenoceptor and
cholinoceptor blockade on day 5. It is
concluded that early HIP migration reflects enhancement of venous
pressure control to head-up orthostatic strain. The effect is
independent of baroreflex-mediated adrenoceptor and cholinoceptor mechanisms.
postural baroreflex; tilt; newborn; swine; autonomic blockade
