RESPONSE OF THE MAGNOCELLULAR SYSTEM IN RATS TO HYPOVOLEMIA AND CHOLECYSTOKININ DURING PREGNANCY AND LACTATION

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.