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

Abstract

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.

Perinatal development and adult blood pressure

“…Though attractive, this hypothesis is not supported by our subsequent study of the maternal behaviour of foster dams (40). WKY dams rearing SHR foster litters increased their nursing behaviour, whereas SHR dams rearing WKY foster litters reduced the time spent in nursing postures, suggesting that the pup strain may influence the behaviour of the dam.…”

“…We (39,40) and others (41) have subsequently confirmed that cross-fostering SHR pups to WKY dams consistently lowers adult blood pressure by 20-30 mmHg. In the reverse experiment, cross-fostered normotensive WKY or Sprague-Dawley pups suckled by SHR dams did not develop elevated blood pressure at maturity (37,38).…”

“…SHR dams rearing their natural litters spend more time in nursing postures (pups attached to teats, dam either lying over pups or with arched back and legs extended) and in general contact with their pups than WKY dams both during the light (40,44,45) and dark phases (44). This increased nursing behaviour has been positively correlated with offspring adult blood pressure (45), which has led to the suggestion that nursing may stimulate long-term changes in the cardiovascular system.…”

“…Milk yield by SHR dams at postnatal day 6 was approximately 50% lower than the yield of WKY dams. Accordingly, SHR pups ingested all of the available milk during a test feed period, whereas WKY pups ceased feeding before the milk supply was exhausted, but not before ingesting more milk than the SHR pups (40). The consequence of this difference in milk intake is that SHR pups ingested significantly less sodium and calcium over the first 2 postnatal weeks compared with both WKY pups and cross-fostered SHR (39).…”

“…We (40) and others (53) have shown that naturally reared SHR pups ingest significantly less milk over the first 7-10 days of lactation. This reduction in milk intake is not the result of an inability of SHR pups to suckle adequately and stimulate a milk ejection reflex, as SHR pups suckling a WKY dam ingested more milk than their naturally reared counterparts (40). Rather, it appears that SHR dams secrete less milk during the early stages of lactation than WKY dams.…”