Response of the magnocellular system in rats to hypovolemia and cholecystokinin during pregnancy and lactation.

To define changes in the magnocellular neuroendocrine system during lactation and pregnancy, we compared plasma levels of oxytocin (OT) and vasopressin (VP) after polyethylene glycol (PEG)-induced hypovolemia and cholecystokinin (CCK) stimulation. Conscious virgin, pregnant (day 20), and lactating (day 6) Sprague-Dawley rats were injected with either PEG (70-600 mg/ml; 35 or 70 ml/kg sc), CCK (100 micrograms/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 relating [hormone] to blood volume depletion or to plasma osmolality) of the OT and VP systems were determined using an iterative nonlinear threshold regression model. After PEG, plasma osmolality increased coincident with a decrease in blood volume, with both stimuli contributing to the rise in plasma VP and OT. Compared with virgin rats, neither the 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 significantly in response to CCK. During gestation when cardiovascular volume is expanded, both the VP and OT neuroendocrine systems were reset, enabling secretion of both hormones in response to hypovolemia with hypertonicity. During lactation, both neuroendocrine systems are reset such that greater changes in fluid balance are needed to stimulate hormone release. Regardless of the reproductive state, the threshold for VP release was always lower than that for OT, indicative of preferential release of VP with less than a 5% (virgin, pregnant) or a 20% (lactating) loss in blood volume.