Arginine vasotocin in ovine fetal blood, urine, and amniotic fluid.

Arginine vasotocin ([8-arginine]-oxytocin) (AVT), the primary antidiuretic principle in submammalian vertebrates, has been reported to be present in mammalian pituitary and pineal glands. Although the most phyletically ubiquitous of the known neurohypophysial peptides, AVT is still not recognized as a mammalian hormone. We examined plasma, urine, and amniotic fluid from fetal lambs by radioimmunoassay (RIA) for evidence of AVT to assess the possibility of its being such a hormone. Measureable quantities of AVT-like immunoreactivity (irAVT) were observed in fetal plasma (2.4 +/- 0.2 pg/ml), urine (1.4 +/- 0.2 pg/ml), and amniotic fluid (1.9 +/- 0.2 pg/ml). Since the AVT antiserum shows minimal cross-reactivity with arginine vasopressin (AVP) and oxytocin (OT), measurements of AVP and OT concentrations in the same biological fluids also were conducted with specific antisera. The results suggest that the irAVT was not accountable on the basis of cross-reaction. To further verify the identity of the irAVT, a high pressure liquid chromatography system using RIA as a means of detection was developed. This system is sufficiently sensitive to allow the separation and quantitation of picogram quantities of the synthetic peptides AVT, AVP, and OT. In this system, the irAVT in fetal plasma, urine, and amniotic fluid appeared as a single peak coeluting with synthetic AVT. These results indicate that AVT is present in ovine fetal plasma and support the view that the fetus secretes this peptide. The physiological significance of circulating AVT remains to be defined.

Measurement of "true" glucose production rates in infancy and childhood with 6,6-dideuteroglucose.

"New" glucose production has been measured in 54 infants and children for the first time by continuous three-to-four-hour influsion of the safe, nonradioactive tracer 6,6-dideuteroglucose. The use of combined gas chromatography--mass spectrometry with monitoring of selected ions allowed deuterium enrichment in blood glucose to be measured on microliter samples with an error of less than 2 per cent. In the young child, glucose production increased in a slightly curvilinear manner from 1 kg. to 25 kg. body weight, when it reached 140 mg. per minute, almost the adult value of 173 mg. per minute (2.28 +/- 0.23 mg./kg.-min., mean +/- S.E.). Normalized for weight, glucose production in premature infants was 5.46 +/- 0.31 mg./kg.-min., in term neonates averaged 6.07 +/- 0.27 mg./kg.-min., in children below the age of six years was 7.1 +/- 0.27 mg./kg.-min., and in late childhood averaged 5.4 +/- 0.28 mg./kg.-min. Relative to estimated brain weight, however, glucose production was essentially linear from the 1-kg. premature infant to the 80-kg. adult. These data, the first measurements of "new" glucose production in childhood, suggest that brain size may be a principal determinant of those factors that regulate hepatic glucose output throughout life.

Changes in steady state plasma arginine vasotocin levels affect ovine fetal renal and cardiovascular function.

The neuropeptides arginine vasopressin (AVP) and arginine vasotocin (AVT) are present in ovine fetal plasma in similar concentrations. Although AVP appears to be an important factor in the regulation of fetal water metabolism, the significance of AVT in the fetus is not clear. In the present study we assessed the effects of changes in steady plasma AVT levels on renal and cardiovascular function of fetal lambs (mean gestation, 128 days) maintained with vascular and bladder catheters. Successive 1-h infusions of synthetic AVT (n = 8) at rates of 1.0 and 2.0 ng/min X kg resulted in steady state plasma AVT levels of 30 +/- 7 and 59 +/- 12 pg/ml, respectively. The AVT infusions increased both urinary AVT concentrations and urinary AVT excretion. Plasma and urinary AVT clearance rates (45 +/- 8.4 ml/min X kg and 0.38 +/- 0.13 ml/min, respectively) remained constant over the observed range of plasma AVT levels. Urinary AVT clearance accounted for less than 1% of the total plasma AVT clearance and was nonsaturable. Increases in plasma AVT augmented urinary osmolar excretion and osmolar clearance, and evoked a significant natriuresis. AVT at either 1 or 2 ng/min X kg decreased fetal heart rate and tended to increase blood pressure. These results suggest that AVT, like AVP, may be important in modulating ovine fetal electrolyte homeostasis.

Plasma oxytocin concentrations in men, nonpregnant women, and pregnant women before and during spontaneous labor.

Baseline plasma oxytocin (OT) concentrations were measured in 25 healthy men, 102 nonpregnant women, and 59 pregnant women from 15-42 weeks gestation. In addition, plasma OT levels were measured at the onset, peak, and immediately after a single uterine contraction in 6 women in the latent phase and 14 women in the active phases of labor, as well as in 19 women at initial presentation of the fetal head on the perineum (+3 station) and 11 women at the time of delivery of the head during a normal vaginal delivery. Baseline plasma OT concentrations did not vary significantly among men (1.5 +/- 0.2 microunits/ml), nonpregnant women (1.4 +/- 0.2 microunits/ml), or pregnant women before labor (1.3 +/- 0.1 microunits/ml) and did not differ in an additional subgroup of 20 women receiving oral contraceptive medication (1.8 +/- 0.7 microunits/ml). In studies conducted during labor, plasma OT concentrations did not correlate with uterine pressure measurements and did not increase significantly over baseline pregnancy concentrations during the latent (1.3 +/- 0.2 microunits/ml) or active (1.6 +/- 0.2 microunits/ml) phases of labor. There was a significant increase in plasma OT levels from the time of initial visualization of the fetal head to the time of delivery of the head (1.1 +/- 0.1 to 4.2 +/- 1.1 microunits/ml, respectively; P less than 0.05). These data support the view that maternal plasma OT levels remain low during pregnancy until late in the second stage of labor.

Radioimmunoassay of arginine vasopressin in urine: development and application.

A sensitive and specific RIA system for measurement of urinary arginine vasopressin (AVP) was developed. RIA sensitivity was 0.4 microU/l (approximately 0.8 pg/ml). Urine samples were extracted using columns of octadecasilyl-silica. The mean (+/- SEM) recovery of P after extraction (77.7 +/- 1.4%) was independent of urinary osmolality. The extracted immunologically active material migrated similarly to synthetic AVP on high pressure liquid chromatography. AVP immunoreactivity was found to be stable in urine stored for 24 h at room temperature and stable for 3 months when acidified promptly and stored at -20 C. Using the RIA system, we measured urinary AVP excretion as a percentage of total body AVP clearance. For this study, AVP (3.7 microU/min . kg) was infused in into four healthy nonsmoking adults in whom endogenous AVP was suppressed by oral water loading. The mean urinary AVP excretion rate was 14.7 +/- 2.1%. There was a highly significant positive correlation between log urinary AVP concentration and urinary osmolality (r = 0.97).

Development of a circadian variation of plasma oxytocin concentration in the late gestation rhesus monkey.

The 24-h pattern of oxytocin (OT) concentrations in maternal plasma was investigated serially from 112-168 days gestation in four chair-restrained pregnant rhesus monkeys. No change in the mean daily plasma OT concentration was observed with advancing gestational age; there was a change in the pattern of plasma OT secretion throughout the period, however. About 21 days from delivery (150.8 +/- 1.8 days gestational age), plasma OT levels showed occasional fluctuations, distributed throughout the day. About 8 days from delivery (163.2 +/- 2.4 days gestational age), a clear circadian pattern of OT was detected, with an acrophase at 2200 h. These results suggest that there is a relationship between the pattern of OT secretion and advancing pregnancy. This may account partially for the increase in uterine activity known to occur in term rhesus monkeys.

The effect of nursing on neurohypophyseal hormone and prolactin secretion in human subjects.

The effect of nursing on plasma levels of oxytocin (OT), arginine vasopressin (AVP), and PRL was studied in six normal women 2-3 days post partum. Maternal blood samples were obtained for measurement of OT, AVP, PRL, sodium, and osmolality 3 and 0 min before suckling, at 3-min intervals for 15 min during suckling, and 5 min after completion of suckling. Plasma OT rose during suckling from a mean (+/- SEM) baseline value of 1.1 +/- 0.2 to 3.6 +/- 0.6 microU/ml by 3 min (P < 0.001), reached a peak level of 6.4 +/- 1.5 microU/ml by 6 min (P < 0.005), and remained elevated for the entire 15-min period of suckling. Serial measurements of plasma OT during suckling failed to show a pattern consistent with episodic secretion. The baseline plasma AVP concentration was 0.4 +/- 0.1 microU/ml and was not significantly altered by suckling. Plasma sodium and osmolality remained unchanged during the suckling period. The baseline serum PRL level was 268 +/- 24 ng/ml and rose to 362 +/- 31 ng/ml after 15 min of suckling (P < 0.05). The data suggest that suckling is a specific stimulus for OT and PRL secretion but has no effect on AVP release.

A novel oxytocin-like and vasotocin-like peptide in human plasma after administration of estrogen.

Using three antisera to oxytocin (OT Pitt Ab-1, OT Pitt Ab-2, and TOR OT Ab), we found comparable levels of OT in response to infant suckling and during infusion of synthetic OT, and identical standard curves with biological and synthetic standards of OT. Pitt Ab-1, but not Pitt Ab-2 or TOR OT Ab, measured increased OT in response to estrogen. Using an arginine vasotocin RIA (TOR AVT Ab), we found an increase in AVT immunoreactivity after estrogen treatment. Mean basal OT levels measured with OT Pitt Ab-2 in plasma of men [0.75 +/- 0.06 (+/- SEM) microU/ml] and women (0.8 +/- 0.09 microU/ml) were lower than OT measured with Pitt Ab-1 (1.7 +/- 0.09 microU/ml in men and 1.7 +/- 0.07 microU/al in women; P less than 0.001). Mean OT measured with Pitt Ab-2 in the plasma of women given estrogen chronically (0.8 +/- 0.04 microU/ml) and acutely (0.6 +/- 0.15 microU/ml) were not significantly different from basal levels. OT levels measured with Pitt Ab-1 in the same samples were 4.6 +/- 0.5 and 4.3 +/- 0.5 microU/ml, respectively, both significantly increased from basal levels (P less than 0.001) and significantly higher than OT measured with Pitt Ab-2 (P less than 0.001). Mean OT measured with Pitt Ab-1 in the plasma of pregnant women was 8.6 +/- 1.02 microU/ml, significantly higher than OT measured with Pitt Ab-2 (1.0 +/- 0.3 microU/ml; P less than 0.001). Men given 25 mg diethylstilbestrol had significant increases in OT measured with Pitt Ab-1 and in AVT measured with TOR AVT (P less than 0.01), but not in OT measured with Pitt Ab-2. Plasma from a man given diethylstilbestrol was prepared for high performance liquid chromatography and applied to a C18 muBondapak reverse phase column. The plasma contained two peaks of immunoreactivity detected as OT with Pitt Ab-1 and as AVT using TOR AVT Ab. The material was not detected by Pitt Ab-2 or TOR OT Ab and did not coelute with standards of OT, AVT, or AVP. Pregnancy plasma, thioglycolic acid, chymotrypsin, and trypsin reduced Pitt Ab-1, Pitt Ab-2, and TOR OT immunoreactivity of synthetic OT. The percent recovery of OT immunoreactivity was not significantly different with Pitt Ab-1 vs. Pitt Ab-2. A novel peptide, which is increased in response to administered estrogen, is present in human plasma and is detected by some antisera to OT and AVT. The observation explains the wide variability in OT levels in the estrogen-primed state and provides a new mechanism to study estrogen-related physiology and pathophysiology.

High pressure liquid chromatographic separation of an oxytocin/arginine vasotocin-like peptide from the plasma of patients with chronic renal failure.

Levels of a novel oxytocin (OT)- and arginine vasotocin (AVT)-like peptide detected by one antiserum to OT (Pitt Ab-1) and one antiserum to AVT (Tor AVT) were recently found to rise in human plasma in response to administration of estrogen. The novel peptide rose in parallel with the estrogen-stimulated neurophysin (ESN). The mean level (+/- SEM) of ESN in plasma of 11 individuals with altered renal function (nondialyzed) was significantly higher than the level in individuals with normal renal function (4.2 +/- 0.9 vs. 1.1 +/- 0.04 ng/ml; P less than 0.01). In patients treated with hemo- or peritoneal dialysis, mean (+/- SEM) levels of ESN were 18.1 +/- 3.2 and 16.8 +/- 3.7 ng/ml, respectively. Levels of estradiol and estrone were not elevated and did not correlate with high levels of ESN. Levels of OT Pitt Ab-1, AVT, and ESN immunoreactivity were measured in plasma form nine patients undergoing hemodialysis and eight patients undergoing peritoneal dialysis. Mean (+/- SEM) levels of all three peptides were elevated (12.9 +/- 1.5 microU/ml, 32.1 +/- 6.7 pg/ml, and 13.5 +/- 4.0 ng/ml, respectively). ESN was significantly correlated with OT Pitt Ab-1 and AVT (R2 = 0.80; P less than 0.001). Plasma samples from the same patients were pooled, treated, and separated by reverse phase HPLC. The plasma contained a peak of immunoreactivity detected by Pitt Ab-1 and Tor AVT Ab. The position of the material was distinct from that of synthetic OT, AVT, or AVP and corresponded to the position of the novel OT-like peptide found in plasma of individuals given estrogen. The findings support parallel secretion of the OT-like peptide with ESN and represent the first disease state characterized by high levels of this OT- and AVT-like peptide.

Vasopressin levels and pediatric head trauma.

The syndrome of inappropriate secretion of antidiuretic hormone is associated with head trauma; however, there are no reports concerning vasopressin levels in pediatric patients with head trauma. Urine vasopressin in eight children (mean +/- SEM, age 7.5 +/- 1.6 years, range 1 to 15 years) was measured by radioimmunoassay during their hospitalization for head trauma. Urine vasopressin values for ten healthy children (mean age 5.4 +/- 1.3 years) and for eight children hospitalized for systemic antibiotic treatment of infections (age 5.9 +/- 1.8 years) also were obtained. Urine vasopressin, urine and serum sodium concentration and osmolality, urea nitrogen, creatinine, and fluid intake were measured within 24 hours of admission and daily for the following two days. For the first three days following head trauma, mean urine vasopressin levels in pediatric patients with head trauma were increased (P less than .05) compared with those of healthy children. Despite fluid restriction to 85% of maintenance level, 25% of patients with head trauma exhibited the clinical syndrome of inappropriate secretion of antidiuretic hormone (hyponatremia, increased urinary sodium, diminished serum osmolality, and urine osmolality greater than serum osmolality). Urine osmolality greater than 800 mosm/kg was associated with markedly increased urine vasopressin levels (200 to 1,650 pg/mL); children with this finding may be at particular risk for the syndrome of inappropriate secretion of anti-diuretic hormone without restrictive water intake.

Tocopherol efficacy and safety for preventing retinopathy of prematurity: a randomized, controlled, double-masked trial.

To test the efficacy and safety of vitamin E in preventing retinopathy of prematurity, 287 infants with birth weights of less than 1.5 kg or gestational ages of less than 33 weeks were enrolled within 24 hours of birth in a randomized, double-masked trial of IV, followed by oral, placebo v tocopherol (adjusted to plasma levels of 3 to 3.5 mg/dL). In the 196 infants completing ophthalmic follow-up, tocopherol did not prevent retinopathy of prematurity of any stage (28% placebo treated v 26% tocopherol treated) or moderately severe retinopathy of prematurity (8% placebo treated v 11% tocopherol treated). Cicatricial sequelae were not significantly different (1/97 placebo treated v 3/99 tocopherol treated), with one placebo-treated infant and one tocopherol-treated infant having retinal detachments. Among all 232 infants examined, those treated with tocopherol had more retinal hemorrhage than placebo-treated infants (8/121 placebo treated v 16/111 tocopherol treated), and retinal hemorrhage correlated positively (P less than .01) with plasma levels of tocopherol after the first 2 weeks of age. Prospective monitoring of morbidity including late-onset sepsis, necrotizing enterocolitis, etc revealed no differences between groups except that grades 3 and 4 intraventricular hemorrhage occurred more frequently in infants weighing less than 1 kg at birth who had received tocopherol (14/42, 33%) v those who had received placebo (4/43, 9%) (P less than .02). Our data do not support the use of tocopherol for prophylaxis against retinopathy of prematurity in premature infants and suggest that IV tocopherol treatment starting on day 1 may increase the incidence of hemorrhagic complications of prematurity, particularly in infants with birth weights of less than 1 kg.

Immunologic response to early and routine DTP immunization in infants.

The effect of early immunization, prior to discharge from the newborn nursery, on subsequent immunity as determined by enzyme-linked immunosorbent assay (ELISA) immunoglobulin (Ig) M and IgG antibody titers to filamentous hemagglutinin and lymphocytosis-promoting toxin (LPT) of Bordetella pertussis and by standard pertussis agglutinin titers was investigated. Eighteen infants received routine diphtheria-tetanus-pertussis (DTP) immunization at 2, 4, and 6 months of age; 17 other infants received routine immunization and an additional DTP immunization in the newborn nursery. Antibody was determined on samples of cord blood and whole blood obtained at 4, 6, and 9 months of age. IgM anti-filamentus hemagglutinin was significantly higher at 4 and 6 months of age in the group that received early immunization (P less than .05). There was no significant difference in IgM anti-LPT, IgG anti-filamentus hemagglutinin, IgG anti-LPT, or pertussis agglutinin antibodies. Six control infants had high cord IgG anti-LPT titers. These six infants had significantly lower antibody titers to LPT at 6 and 9 months of age when compared with control with control infants with lower cord titers. Thirteen infants in the early immunization group with lower cord IgG anti-LPT titers achieved significantly lower titers at 9 months of age than the 12 comparable infants in the control group.

Multiple receptor contributions to ovine fetal cardiovascular responses to vasopressin.

Selective AVP V1 receptor antagonist and V2 receptor agonist treatments were used to study V1 and V2 receptor contributions to AVP-induced effects on ovine fetal blood pressure, heart rate, and renal function. The results indicate that V1 receptors do not contribute to late gestation fetal renal responses to AVP. The pressor response to AVP was abolished by V1 receptor blockade while the heart rate response was not affected (Fig. 1). This separation of the blood pressure and heart rate responses to AVP has been noted previously, and indicates that AVP-induced fetal bradycardia is not a reflex response to increased blood pressure. A direct V1 receptor-mediated effect on the heart also appears unlikely. The absence of a dDAVP effect on fetal heart rate indicates that AVP-induced bradycardia is not a V2 receptor-mediated event. An AVP effect on oxytocin receptors appears unlikely due to the antioxytocic action of the Manning Compound. Demonstration of AVP V1a receptors in and around the brainstem cardiovascular control centers supports the view that circulating AVP can modulate vagal outflow via effects in the area postrema. However, if Manning Compound infusion blocks fetal V1a receptor-mediated vascular responses and V1b receptor-mediated ACTH release, then blockade of the heart rate response to AVP also would have been expected. In summary, the fetal heart rate response to AVP is not dependent on AVP-induced increases in blood pressure, V1 receptors blocked by the Manning Compound, or V2 receptors stimulated by dDAVP. We conclude that AVP contributes to fetal heart rate regulation, and the effect is not mediated by any known population of AVP receptors.

The plasma of pregnant women contains a novel oxytocin-vasotocin-like peptide.

Basal levels of immunoreactive oxytocin (OT) were measured in plasma of healthy pregnant women using two antisera to OT, Pitt Ab-1 and Pitt Ab-2, and an antiserum to arginine vasotocin, Tor AVT Ab. The mean (+/- SEM) level of immunoreactive OT Pitt Ab-1 was significantly higher, 7.7 +/- 0.9 microU/mL, than immunoreactive OT Pitt Ab-2, 0.9 +/- 0.2 microU/mL, P less than .001 measured in the same samples. AVT immunoreactivity in plasma of nonpregnant individuals was 0.8 +/- 0.16 pg/mL and in plasma of women in late pregnancy was 5.0 +/- 0.4 pg/mL. In four pregnant women receiving an infusion of synthetic OT (Pitocin, Parke-Davis, Morris Plains, NJ) a linear correlation was found between the dose of OT infused and the concentration of OT in plasma in samples measured with Pitt Ab-2, but no correlation was found in the same samples measured with Pitt Ab-1. Immunoreactive OT Pitt Ab-1 in plasma was not destroyed by a 60-minute incubation with pregnancy plasma. Pooled plasma from pregnant women was separated by reverse phase high pressure liquid chromatography (HPLC). OT Pitt Ab-1 and Tor AVT immunoreactivities in pregnancy plasma eluted in a position separate from synthetic OT. The differences found in levels of immunoreactive OT in the same samples of plasma measured with two antisera to OT illustrate an important reason why levels of OT in pregnant women may be reported to be variable among laboratories.

Pharmacokinetics of oxytocin in the human subject.

Oxytocin (OT) metabolic clearance rates (MCR) were measured during the constant infusion of 4 mU/min synthetic oxytocin (Pitocin) in 22 healthy adult subjects: 6 men, 6 nonpregnant women, and 10 pregnant women. Pregnant subjects were at term and undergoing OT induction of labor. Mean (+/- SEM) baseline plasma OT concentrations were similar for men, pregnant women, and nonpregnant women. OT MCR was 27 +/- 1.8 ml/kg/min in men; 20.6 +/- 2.8 ml/kf/min in nonpregnant women; and 23.1 +/- 2.6 ml/kg/min in pregnant women. These values were statistically similar. The OT MCR corrected to prepregnancy weight was 25.4 +/- 2.0 ml/kg/min. This value is also statistically similar to the values in men and nonpregnant women. OT degradation was studied in vitro in pooled plasma of men, nonpregnant women, pregnant women, and cord blood. No significant degradation was observed in men, nonpregnant women, or cord plasma. There was an 85% per hour decrease in OT concentration in plasma of pregnant women, confirming earlier reports. These results suggest that the cystine amino peptidase enzyme mediating OT degradation in pregnancy plasma is preferentially secreted by trophoblast cells into maternal plasma and does not seem to cross the placental barrier.

Oxytocin and prolactin responses in long-term breast-feeding.

Plasma levels of oxytocin and prolactin were measured before and during 12 minutes of breast pump stimulation in five healthy, lactating, amenorrheic women on three occasions: ten to 90 days post partum, 90 to 180 days post partum, and 180 days to one year post partum. Baseline mean (+/- SEM) plasma oxytocin levels were similar in the three study periods. Mean stimulated plasma oxytocin levels increased in the three study periods (each P less than .001; mean baseline versus stimulated). Stimulated plasma oxytocin values were significantly greater at ten to 90 than at 90 to 180 days (P less than .05; analysis of variance). Baseline serum prolactin levels were 61 +/- 9.5, 36 +/- 8.6, and 33 +/- 10.8 ng/ml, respectively (not significant; one-way analysis of variance). Mean stimulated prolactin levels were 71 +/- 8.1, 43 +/- 4.5, and 43 +/- 2.8 ng/ml, respectively (not significant). Thus, the oxytocin secretory reflex continues in long-term lactation for the first year post partum. In addition, breast stimulation in long-term lactating women continues to produce a slight increase in serum prolactin levels.

Povidone-iodine for ophthalmia neonatorum prophylaxis.

PURPOSE: The agents currently used to prevent ophthalmia neonatorum are less than optimal, with reports indicating evidence of bacterial resistance, ineffectiveness, and toxicity. Povidone-iodine ophthalmic solution, which has been shown to be effective in the preoperative preparation of the eye, generates no resistance, is an effective antimicrobial agent, and has low toxicity. We evaluated the effectiveness and safety of povidone-iodine for ophthalmia neonatorum prophylaxis. METHODS: A bacterial culture was taken from the conjunctiva of each eye of 100 infants within 30 minutes of birth. A drop of 2.5% povidone-iodine solution was then placed on one eye, while the other eye received either one drop of silver nitrate 1% ophthalmic solution or 0.5% erythromycin ointment. Conjunctival bacterial cultures were again taken two to four hours after birth. At each culture and at 24 hours after birth, the eyes were examined for toxic changes. To measure the effectiveness of the medications, the number of bacterial colony-forming units and species from each culture was compared. RESULTS: All three agents significantly reduced the number of colony-forming units, but povidone-iodine caused the most significant decrease. The number of species was reduced significantly by povidone-iodine (P = .00051) and silver nitrate (P = .007), with povidone-iodine yielding the most significant decrease. Erythromycin did not significantly reduce the number of species. Silver nitrate demonstrated more ocular toxicity at the 24-hour determination point than did either of the other two medications (P < .001). CONCLUSIONS: Povidone-iodine 2.5% ophthalmic solution is an effective antibacterial agent on the conjunctiva of newborns and causes less toxicity than silver nitrate.

Ontogeny of ovine fetal renal atrial natriuretic factor receptors.

Ovine fetal renal responses to ANF decrease during the last third of gestation, although circulating fetal plasma atrial natriuretic factor (ANF) levels are higher than in the maternal circulation, and do not change with gestation. This study examined whether previously reported maturational changes in fetal renal responses to ANF are due to changes in renal ANF receptor numbers and/or affinity during gestation. ANF receptor numbers (Bmax) and dissociation constants (Kd) were measured in isolated renal glomeruli from early (95 and 110 day; mean 103 +/- 2) and late gestation (131 and 145 day; mean 138 +/- 2) fetal and maternal sheep. Fetal renal ANF receptor Bmax values significantly increased between 103 and 138 days gestation (13 +/- 3 to 29 +/- 4 fmol/mg protein) but were significantly lower than maternal values (60 +/- 13 fmol/mg protein). Fetal ANF receptor Kd values also increased significantly (245 +/- 34 to 370 +/- 36 pM), with early gestation values significantly lower than maternal values (470 +/- 69 pM). Thus, the blunted fetal renal response to ANF in late as compared to early gestation is not due to reduced ANF receptor numbers. Rather, an increased proportion of ANF clearance receptors, reduced post-receptor function and/or altered intrarenal hemodynamics may contribute.

Diurnal variations in plasma arginine vasotocin (AVT) concentrations in the ovine fetus.

Previously we have demonstrated the presence of AVT in the blood of fetal sheep. The source is not clear, but AVT has been identified in fetal pineal and pituitary glands. In view of the circadian secretory pattern of the pineal gland, we questioned whether fetal plasma AVT levels might vary diurnally. Plasma samples from five chronically catheterized ovine maternal ewes and fetal lambs 129-135 days' gestation were obtained at 3-19 hourly intervals for 1-2 days (mean +/- S.E.M. = 35 +/- 6 hours. Plasma AVT levels were determined by radioimmunoassay. Results were analyzed by nonlinear curve fitting procedures to relate hormone levels with time of day. Plasma AVT values for maternal ewes did not vary during the day in response to light/dark periods. The curve for mean fetal plasma AVT plotted against time showed oscillations with a period of about 25 hours (p less than 0.05). Peak fetal AVT levels were observed at 1600 hours and minimal levels at 0400 hours. These results indicate that ovine fetal AVT secretion varies diurnally. The site of AVT secretion may be the pineal gland; however, confirmation of this and identification of the physiological stimuli for secretion of fetal plasma AVT require further information.

Fetal lung fluid response to maternal hyperosmolality.

Lung fluid production in utero, a significant source of amniotic fluid, may be regulated by the fetus in response to environmental stress. We monitored changes of maternal and fetal plasma osmolality, plasma arginine vasopressin level, and fetal lung fluid production in response to mannitol-induced change in maternal osmolality in five chronically catheterized ovine fetuses. Four additional ewes and their fetuses did not receive mannitol and served as controls. Maternal infusion of 20% mannitol resulted in maternal and subsequent fetal hyperosmolality (changing from 303 +/- 2 to 322 +/- 2 mOsm and 300 +/- 5 to 319 +/- 8 mOsm, respectively), Mean fetal lung fluid production significantly decreased (from 2.6 +/- 0.4 to 1.6 +/- 0.4 ml/10 min), whereas lung fluid osmolality significantly increased (from 298 +/- 2 to 303 +/- 3 mOsm) in the study animals. No changes in lung fluid sodium or potassium concentrations were observed in study or control fetuses. Fetal plasma arginine vasopressin level significantly increased in the study fetuses (2.2 +/- 0.3 to 5.3 +/- 1.6 microU/ml), though not in control animals. These results indicate that the fetal lamb responds to increased osmolality by decreasing lung fluid production, and that there is a concomitant increase in lung fluid osmolality. This response appears to be mediated via increased fetal plasma vasopressin levels.