Arginine Vasopressin Response to an Osmotic Stimulus in the Fetal Sheep

Abstract

Summary MATERIALS AND METHODSBaseline plasma osmolality (pOsm) and plasma arginiie vasopressin (pAVP) were measured in chronically catheterized fetal sheep. Mean baseline pAVP in fetuses from 101-120 days was 1.9 + 0.46 pU/ml (mean * SEM) with a pOsm of 293 rt 1.8 mOsm/kg. In fetuses of 121-141 days of gestation, mean pAVP was significantly lower, 0.77 0.19 pU/ml (P < 0.05), with a similar pOsm (293 2 1.9 mOsm/kg). The logarithmic baseline pAVP values were linearly correlated with pOsm for both groups. Hypertonic saline infusion resulted in a similar increase in the log pAVP corrected for the rise in pOsm in the 101-120-day fetuses and in 121-141-day fetuses. The slope of this response was similar to that of the steady state relationship. The data indicates that the fetal osmoreceptor system for control of arginine vasopressin secretion is fully functional in the last trimester of pregnancy. SpeculationThe fetal hypothalamic posterior pituitary system appears to show a relatively heightened response to increases in plasma osmolality during the last trimester of gestation. This may reflect the heightened hypothalamic activity postulated to explain the high circulating growth hormone levels at this time. The higher baseline plasma vasopressin levels in the younger fetuses would support this view. Alternatively, the augmented responsiveness may represent an adaptive response to the immaturity of the fetal kidney with respect to free water clearance or an adaptive response to some extrarenal function of the hormone on salt and water homeostasis in utero.Vasopressin secretion is regulated by two systems of r e c e p tors: the osmoreceptors located in or near the supraoptic nuclei in the hypothalamus and a series of volume receptors, including left atrial stretch receptors and carotid sinus and aortic arch baroreceptors. Available information suggests that the newborn is capable of secreting vasopressin at birth and that both the osmoreceptor and volume control systems are functional (3-6). Data regarding fetal vasopressin physiology, however, are meager. Vasopressin has been identified in the human fetal pituitary at 10-12 weeks of gestation (9, 12) and in the fetal pituitary of other mammalian species, including the guinea pig, sheep, and seal, during the last half of gestation (1,2,10,12,14). In addition, in the sheep and monkey, significant concentrations of circulating vasopressin have been detectable in fetal blood during the last half of gestation (1, 1 l ) , and increases in circulating levels have been evoked either by osmolar or volume (hemorrhage) stimuli (1, 11). However, little is known about the sensitivity and relative responsiveness of the fetal osmoreceptor control system. The present studies were designed to provide this information. 5Pregnant Columbia or Columbia-Suffolk ewes were obtained from a local source and the gestational age based on the induced ovulation technique (7). The animals were maintained in the vivarium and fasted 4 8 hr before surgery. The pregnant ewes were anesthetized using a m...

Vasopressin and Catecholamine Secretion during Metabolic Acidemia in the Ovine Fetus

“…The fetus responds to various forms of intrauterine stimuli by releasing AVP from the neurosecretory granules of nerve endings located in the posterior pituitary gland as early as the middle third of pregnancy (23). Among the numerous nonosmotic stimuli that result in increased fetal AVP secretion are hemorrhage (7-9), parturition (3,4,14), vaginal delivery (2, 5), and episodes of hypoxemia/asphyxia (10)(11)(12)(13)(14)26).…”

“…The residue was suspended in 400 p1 of 0.15 M phosphate buffer (pH 7.2), and 50 or 100 p1 of the solution were assayed for AVP. AVP in extracts was measured using a radioimmunoassay procedure described by Skowsky et al (22), as modified by Weitzman et al (23). The use of this assay in our laboratory has been described and validated by DeVane and associates (5,12).…”

Maternal Fetal Osmolar Homeostasis: Fetal Posterior Pituitary Autonomy

“…The fetal SRR after maternal HS (SRR = 0.16) is greater than that in any of the other animals including direct fetal HS injection (SRR = 0.04) (9). The augmented fetal response after maternal HS represents a combined volume and osmolar stimulus to the fetus after the rapid transfer of water from the fetal to maternal compartments.…”

Effects of Acute Hypercapnia on Maternal and Fetal Vasopressin and Catecholamine Release

“…22). parturition (12,23), cord occlusion (9), and increases in plasma osmolality (24) are potent stimuli resulting in substantial increases in plasma AVP concentrations. Although the effects of hypovolemia and hyperosmolality on hormone secretion are better understood, it is unclear how those stresses associated with changes in arterial blood gases and pH result in increased fetal AVP or catecholamine release.…”