Ps. Buckner et al., Venous hydrostatic indifference point as a marker of postnatal adaptation to orthostasis in swine, J APP PHYSL, 87(3), 1999, pp. 882-888
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 degrees) and head-down (-15 and -30 degrees) tilt provi
ded a physiological range of orthostatic strain. HIP for all positive tilts
shifted toward the heart (P < 0.05), +45 degrees 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 clay 6 cause
d 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 alpha-, beta-
adrenoceptor and cholinoceptor blockade on day 5. It is concluded that earl
y HIP migration reflects enhancement of venous pressure control to head-up
orthostatic strain. The effect is independent of baroreflex-mediated adreno
ceptor and cholinoceptor mechanisms.