RELEASE OF VASOPRESSIN IN RESPONSE TO ALTERED PLASMA-VOLUME AND SODIUM CONCENTRATIONS FOLLOWING PINEALECTOMY IN THE RAT

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, a nd pinealectomized animals during extracellular fluid hypertonicity pr oduced 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 hypert onic saline produced a marked increase in plasma vasopressin concentra tions from 2.18 +/- 0.28 to 7.2 +/- 1.24 pmol/liter, but the response was attenuated in pinealectomized animals, concentrations increasing t o only 3.4 +/- 1.2 pmol/liter. Similarly, following infusion of hypert onic saline, the increase in plasma vasopressin per unit increase in p lasma sodium was lower in pinealectomized animals than the pineal inta ct controls. The response to hypovolaemia was also attenuated, plasma hormone concentrations following reduction in blood volume of approxim ately 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 d ifferences in pituitary vasopressin content in any of the groups studi ed. Thus, the pineal may influence the vasopressin response to physiol ogical stimuli.