Release of vasopressin in response to altered plasma volume and sodium concentrations following pinealectomy in the rat

Abstract: Pinealectomy has been shown to alter daily rhythms of neurohypophysial hormone release, with plasma hormone concentrations being elevated in the morning, as compared to intact rats. To determine whether pineal removal also altered the response to known stimuli of hormone release, vasopressin concentrations were measured in control, sham‐operated, and pinealectomized animals during extracellular fluid hypertonicity produced by an intraperitoneal (i.p.) injection of hypertonic saline or hypovolaemia produced by an i.p. injections of polyethylene glycol. In the combined sham‐operated and unoperated groups, injection of hypertonic saline produced a marked increase in plasma vasopressin concentrations from 2.18 ± 0.28 to 7.2 ± 1.24 pmol/liter, but the response was attenuated in pinealectomized animals, concentrations increasing to only 3.4 ± 1.2 pmol/liter. Similarly, following infusion of hypertonic saline, the increase in plasma vasopressin per unit increase in plasma sodium was lower in pinealectomized animals than the pineal intact controls. The response to hypovolaemia was also attenuated, plasma hormone concentrations following reduction in blood volume of approximately 10% increasing to only 3.6 ± 0.6 pmol/liter as compared to 7.3 ± 2.2 pmol/liter in the control groups. There were no significant differences in pituitary vasopressin content in any of the groups studied. Thus, the pineal may influence the vasopressin response to physiological stimuli.