Milking procedure alters the electrolyte composition of spontaneously hypertensive and normotensive rat milk

A number of groups, including our own, have shown that the severity of hypertension expressed by the adult spontaneously hypertensive rat (SHR), which is primarily considered to be a genetic model of hypertension, can be reduced as a result of exposure to the behavioural and nutritional environment provided by a normotensive foster mother. It has been suggested that the hypertensive influence of the SHR dam may involve increased sodium delivery to the pups and there have been some reports of elevated sodium concentrations in the milk of SHR dams. However, these studies used either a long (≤6 h) dam-pup separation period before collecting milk or repeated milking of the same dams, both of which have been shown to alter the trace element content of rat milk. Therefore, we have compared the electrolyte content of milk collected by these methods with milk derived from SHR and normotensive Wistar-Kyoto (WKY) dams separated from their litters for 2 h prior to a single-milking session. Long separation and repeated milking resulted in variable effects on the electrolyte content of both SHR and WKY dams' milk, compared with milk collected after 2 h from dams which had not previously been milked. The most notable effects were the abolition of significant strain differences, observed following 2-h separation, for milk sodium (WKY 22.1 ± 1.4 vs. SHR 27.5 ± 2.1 mmol/liter, p <0.05) and calcium (WKY 92.3 ±4.3 vs. SHR 69.4 ± 2.9 mmol/liter, p <0.05) when dams were separated for 6 h or were serially milked. These data suggest that the electrolyte content of SHR and WKY dams' milk can be altered by the collection procedure and it is recommended that dams be milked on only one occasion following a short separation period from their litter.