Maternal nursing behaviour and the delivery of milk to the neonatal spontaneously hypertensive rat.

Fostering spontaneously hypertensive rat (SHR) pups to Wistar-Kyoto (WKY) dams permanently lowers their adult blood pressure. SHR dams show increased nursing behaviour and the SHR pup displays an exaggerated pressor response to milk ingestion. We have therefore measured pup feeding rates and dam milk secretion rates in SHR and WKY dams nursing natural and foster litters. Maternal behaviour displayed by SHR and WKY dams nursing natural or foster litters and the milk delivery rate to non-fasted pups were measured over postnatal days 1-21. Total milk yield by SHR and WKY dams at postnatal day 6 was measured by comparing the relative weights of milk filled and empty mammary tissue. Fostering SHR pups to WKY dams significantly lowered their mean arterial blood pressure compared with naturally reared SHR pups. SHR dams nursing their natural litters spent more time nursing and less time away from their litters than WKY dams. This difference in behaviour was reduced when dams nursed pups of the opposite strain. SHR dams delivered less milk compared with WKY dams, regardless of pup strain. Total milk yield by SHR dams was significantly lower than that of WKY dams at postnatal day 6, despite comparable mammary tissue mass, suggesting that milk intake by the SHR pup is limited by milk availability, rather than an attenuated maternal response to pup suckling stimuli. The antihypertensive effect of fostering SHR pups to WKY dams may therefore reflect an increase in milk intake during a critical period of growth and development.