Em. Koehler et al., RESPONSE OF THE MAGNOCELLULAR SYSTEM IN RATS TO HYPOVOLEMIA AND CHOLECYSTOKININ DURING PREGNANCY AND LACTATION, The American journal of physiology, 266(4), 1994, pp. 180001327-180001337
To define changes in the magnocellular neuroendocrine system during la
ctation and pregnancy, we compared plasma levels of oxytocin (OT) and
vasopressin (VP) after polyethylene glycol (PEG)-induced hypovolemia a
nd cholecystokinin (CCK) stimulation. Conscious virgin, pregnant (day
20), and lactating (day 6) Sprague-Dawley rats were injected with eith
er PEG (70-600 mg/ml; 35 or 70 ml/kg sc), CCK (100 mu g/ml; 4 ml/kg ip
), or vehicle and decapitated 4 h (PEG) or 5 min (CCK) later. Changes
in thresholds for release of hormone and the responsiveness (slopes re
lating [hormone] to blood volume depletion or to plasma osmolality) of
the OT and VP systems were determined using an iterative nonlinear th
reshold regression model. After PEG, plasma osmolality increased coinc
ident with a decrease in blood volume, with both stimuli contributing
to the rise in plasma VP and OT. Compared with virgin rats, neither th
e threshold nor the responsiveness of the VP system was altered by the
combined stimulus, whereas the oxytocinergic system of pregnant rats
was more responsive to osmotic component. Lactating rats, however, had
a higher threshold for VP release and an apparent elevation of the OT
threshold beyond 25% volume depletion. Regardless of the reproductive
state, the threshold for VP release was always lower than that for OT
. Intraperitoneal CCK elevated plasma [OT] in each reproductive state,
although the response in lactating animals was attenuated. In virgin
and lactating rats, plasma levels of VP also increased slightly but si
gnificantly in response to CCK. During gestation when cardiovascular v
olume is expanded, both the VP and OT neuroendocrine systems were rese
t, enabling secretion of both hormones in response to hypovolemia with
hypertonicity. During lactation, both neuroendocrine systems are rese
t such that greater changes in fluid balance are needed to stimulate h
ormone release. Regardless of the reproductive state, the threshold fo
r VP release was always lower than that for OT, indicative of preferen
tial release of VP with less than a 5% (virgin, pregnant) or a 20% (la
ctating) loss in blood volume.