Changes of plasma osmotic pressure during lactation in rats.

It is known that blood and plasma volume increase during lactation. The present paper examines whether an increase in plasma volume is accompanied by the change in plasma composition or attributed to hydro-dilution. Six dam-nursed pups and six dam-removed pups housed individually were designated as lactating rats and control rats, respectively. The plasma osmotic pressure and hematocrit value (Ht) were measured in the rats on days 3, 5, 7, 10, 13 and 18 of lactation. The total plasma protein (TP) and serum sodium concentration were also measured as they are factors affecting the plasma osmotic pressure. In addition, milk yield was estimated by the Morag technique. On day 5 and after day 10, the osmotic pressure of the lactating rats was found to be significantly lower than that of the control rats. The serum sodium concentration (days 5 and after day 10) and Tp values (days 3, 10 and 18) of the lactating rats were significantly lower than those of the control rats. Except on day 5, the Ht values of the lactating rats were significantly lower than those of the controls. During the period between days 3 and 10, milk yield was increased and it become steady (18 g/12 hr) on days 10 and 18. On and after day 10 when rats secreted a large amount of milk, it is considered that a decrease in the plasma osmotic pressure was mainly attributed to the reduction of sodium concentration by hydro-dilution. The Ht values indicate that an increase in blood volume is mainly through plasma volume rather than blood cell volume in lactating rats.