Prolactin release in response to infusion of isotonic or hypertonic saline

In ketamine-anaesthetized rats, the infusion of 10 ml either isotonic (0.9%) or hypertonic (3.0%) saline over a 30-min period was associated with a decrease in the secretory activity of the anterior pituitary prolactin cells (assessed morphologically by electron microscopy), and a decrease in the serum prolactin concentration (determined by radioimmunoassay). The hypertonic saline infusion significantly (P less than 0.01) increased the serum osmolality whereas the isotonic infusion did not. The results indicated that prolactin release is reduced in response to volume expansion regardless of whether the fluid load is isotonic or hypertonic, and suggested that, in the rat, the hormone may play a role in volume regulation rather than osmoregulation.