AJP - Regulatory, Integrative and Comparative Physiology, Vol 253, Issue 1 31-R38, Copyright © 1987 by American Physiological Society
Inhibition of VP and OT release by water in hypovolemia is independent of opioid peptides
L. M. Rosella-Dampman, R. D. Hartman and J. Y. Summy-Long
Overhydration inhibits release of vasopressin (VP) and oxytocin (OT) from
the hypothalamo-neurohypophysial system during hypovolemia. We investigated
whether opioid peptides mediate the inhibitory effect of water on secretion
of these hormones. Conscious male rats were made hypovolemic by hemorrhage
(HEM, 0.51 ml/min) of 20 and 35% of the blood volume or by injection of
either subcutaneous polyethylene glycol (PEG, 20,000 mol wt, 35 ml/kg) or
intraperitoneal histamine (HIS, 15 mg/kg, 1 ml/kg). Animals were intubated
orally 1-4 min (HEM, HIS) or 6.75 h (PEG) later with or without
administration of water (40 ml/kg). Four to seven min after intubation rats
were injected with saline (1 ml/kg) or naloxone (2 or 5 mg/kg) and then
decapitated 6-10 min later. Control animals were treated similarly but were
not stimulated by hypovolemia. VP and OT were extracted from plasma and
quantified by radioimmunoassay. Data were analyzed by analysis of variance.
In HEM animals blood pressure fell and plasma osmolality increased, both of
which correlated positively with the rise in plasma [VP] and [OT].
Overhydration lowered the plasma osmolality, attenuated the fall in blood
pressure, and reduced [VP] and [OT] in plasma of HEM animals. The opiate
receptor antagonist, naloxone, did not alter these changes in blood
pressure or plasma osmolality, or the plasma [VP] after HEM in rats treated
with or without water. Plasma [OT] was, however, increased by naloxone in
both normally hydrated and overhydrated rats. Thus, regardless of the
hydrational state of the animal, opioid peptides inhibited release of OT
but not VP during hemorrhage. Data consistent with this interpretation were
also obtained from rats made hypovolemic with PEG or HIS.(ABSTRACT
TRUNCATED AT 250 WORDS)
