Linda K. Kullama

1-Deamino-[8-d-arginine] vasopressin - induced maternal plasma hypoosmolality increases ovine amniotic fluid volume

OBJECTIVE Maternal 1-deamino-[8-D-arginine] vasopressin (a selective antidiuretic agonist) and oral water loading decrease maternal and fetal plasma osmolality and markedly increase fetal urine flow in sheep over 24 hours. We hypothesized that a sustained reduction in plasma osmolality would increase the amniotic fluid volume. STUDY DESIGN Pregnant sheep (130 +/- 1 days, n = 6) were given oral water loading (2000 ml) and intravenous 1-deamino-[8-D-arginine] vasopressin (20 microgram injection followed by 4 micrograms/hr) to induce maternal urinary antidiuresis and to maintain a maternal plasma osmolality decrease of approximately 20 mOsm/kg for 48 hours. Fetal plasma osmolality, electrolytes, atrial natriuretic factor, and arginine vasopressin levels, fetal swallowing activity, and fetal urine flow and composition were measured each study day, and amniotic fluid volume was measured by tracer dilution before and at 48 hours of plasma hypoosmolality. RESULTS In response to 1-deamino-[8-D-arginine] vasopressin and water loading, maternal plasma osmolality decreased (298 +/- 1 to 275 +/- 3 mOsm/kg), mirrored by fetal plasma hypoosmolality (293 +/-1 to 271 +/- 3 mOsm/kg). Fetal urine osmolality (155 +/- 16 to 117 +/- 10 mOsm/kg) and amniotic fluid osmolality significantly decreased (281 +/- 3 to 269 +/- 5 mOsm/kg). Fetal urine flow rate significantly increased (0.33 +/- 0.06 to 0.51 +/- 0.04 ml/min), whereas fetal swallowing activity decreased (56 +/- 7 to 33 +/-5 swallows per hour). In response to alterations in fetal fluid dynamics, amniotic fluid volume significantly increased from 493 +/- 80 to 839 +/- 160 ml. CONCLUSION Chronic maternal 1-deamino-[8-D-arginine vasopressin-induced plasma hypoosmolality induces a sustained reduction in fetal swallowing activity and an increase in fetal urine production, which results in a marked increase in amniotic fluid volume. Use of maternal 1-deamino-[8-D-arginine] vasopressin and oral fluid loading has the potential for manipulation of amniotic fluid volume homeostasis and may prove useful in the treatment of oligohydramnios.

Maternal Plasma Hypo-Osmolality: Effects on Spontaneous and Stimulated Ovine Fetal Swallowing

Fetal swallowing is a major route of amniotic fluid resorption, and thus swallowing activity may alter amniotic fluid volume. Near-term ovine fetal swallowing increases in response to plasma and/or cerebrospinal fluid hypertonicity. As maternal hydration status alters amniotic fluid volume, we hypothesized that maternal plasma hypotonicity may alter fetal swallowing activity. Pregnant ewes (130 +/- 1 d; n = 6) were chronically prepared with maternal and fetal vascular catheters, a fetal esophageal flow probe, and fetal thyrohyoid and nuchal and thoracic esophagus electromyogram electrodes. Spontaneous fetal swallowing and hypertonic saline thresholds for stimulated swallowing were determined prior to and following maternal hypotonicity induced with water loading and intravenous DDAVP (arginine vasopressin V2 agonist). Fetal swallowing thresholds were determined with intracarotid injections (0.15 ml/kg) of increasing sodium chloride concentrations (0.15-1.2 M) at 2-min intervals. Maternal DDAVP infusion significantly decreased mean (+/-SEM) maternal and fetal plasma osmolalities (298 +/- 2-284 +/- 3; 295 +/- 2-278 +/- 3 mOsm/kg, respectively) and sodium concentrations (147.3 +/- 0.4-137.5 +/- 0.9; 142.7 +/- 0.8-133.5 +/- 1.0 mEq/l, respectively), suppressed spontaneous swallowing activity and volume (1.1 +/- 0.2-0.6 +/- 0.1 swallows/min; 0.7 +/- 0.2-0.5 +/- 0.1 ml/min, respectively) and significantly increased the osmotic threshold for swallowing stimulation (0.77 +/- 0.08-1.03 +/- 0.09 M NaCl). We conclude that: (1) maternal, and thus fetal, plasma hypotonicity results in suppression of spontaneous fetal swallowing activity and a decrease in volume swallowed, suggesting that spontaneous fetal ingestive behavior results, in part, from tonic dipsogenic stimulation, and (2) under hypotonic conditions, the intracarotid NaCl injection concentration for swallowing stimulation increases. These results suggest that the reset (lower) maternal plasma osmolality during human pregnancy may serve to minimize fetal ingestive and perhaps arginine vasopressin-mediated antidiuretic responses to acute maternal hypertonicity.

Fetal endocrine and renal responses to in utero ventilation and umbilical cord occlusion

OBJECTIVE Fetal to neonatal transition involves a myriad of endocrine and renal adaptive changes triggered by multiple simultaneous stimuli. We examined the extent to which ventilation and umbilical cord occlusion have an impact on the many endocrine and renal function changes in fetal sheep at 133 +/- 1 day of gestation. STUDY DESIGN Nine fetuses were chronically prepared with an endotracheal tube, vascular and bladder catheters, and an inflatable umbilical cord occluder. After a 2-hour control period fetuses were treated with commercially prepared surfactant and ventilated with 100% oxygen. One hour after the onset of stable ventilation the umbilical cord was occluded and the animals were monitored for 3-hours. RESULTS In response to ventilation fetal arterial PO2 increased (18 +/- 1 to 86 +/- 29 mm Hg) and remained significantly above control values after cord occlusion. Fetal arterial pH, hematocrit, and mean arterial pressure and heart rate did not change during the study. Ventilation alone evoked significant increases in epinephrine and norepinephrine concentrations. Renal responses to ventilation included significant decreases in urine flow rate, fractional sodium excretion, and fractional water excretion. Neither ventilation nor cord occlusion resulted in significant changes in plasma concentrations of atrial natriuretic factor, arginine vasopressin, and angiotensin II or in glomerular filtration rate, urine osmolality, free water, and osmolar clearances. CONCLUSIONS (1) Mechanical ventilation and oxygenation alone increase circulating fetal catecholamine levels. (2) The addition of umbilical cord ligation has minimal impact on fetal endocrine and renal adaptive responses.

Ovine maternal and fetal renal vasopressin receptor response to maternal dehydration

OBJECTIVE Arginine vasopressin secretion increases in response to increased plasma osmolality or hypovolemia. Dehydration-induced increases in plasma arginine vasopressin levels have been shown to down-regulate arginine vasopressin V2 receptors in adult rat kidneys. Our study determined ovine maternal and fetal renal arginine vasopressin receptor characteristics and receptor response to maternal dehydration. STUDY DESIGN Eight pregnant ewes (113 +/- 1 days) were dehydrated for 72 hours; eight animals served as controls. Renal medullary tissue was isolated from maternal and fetal kidneys, and arginine vasopressin receptor characteristics determined with saturation and competition assays using tritiated arginine vasopressin, arginine vasopressin, and arginine vasopressin analogs. RESULTS Euhydrated maternal and fetal renal medullary arginine vasopressin receptor dissociation constant (3.0 +/- 0.3 and 1.9 +/- 0.3 nmol/L) and maximal binding capacity (149 +/- 15 and 111 +/- 33 fmol/mg protein) values were similar. Pharmacologic profiles with selective agonists indicated a predominance of V2 receptors. Dehydration significantly increased maternal and fetal plasma osmolalities (304 +/- 2 to 320 +/- 2; 296 +/- 1 to 319 +/- 3 mOsm/kg water, respectively) and arginine vasopressin levels (3.8 +/- 1.4 to 29.3 +/- 4.6; 4.4 +/- 1.0 to 16.9 +/- 5.0 pg/ml, respectively) but had no effect on arginine vasopressin receptor binding. CONCLUSION Specific, saturable, single-site tritiated arginine vasopressin binding is present in ovine maternal and fetal renal medullary membranes. Ovine maternal and fetal renal arginine vasopressin receptors do not down-regulate in response to dehydration-induced elevations in plasma arginine vasopressin levels.

Development of Concentration Thresholds for Fetal Swallowing and Arginine Vasopressin Stimulation in the Preterm Ovine Fetus

We previously demonstrated that fetal swallowing was stimulated by intracarotid saline injections in the near term (130 +/- 1 days gestation) ovine fetus. In order to examine the ontogeny of the regulation of thirst-mediated swallowing behavior and arginine vasopressin (AVP) secretion, the present study examined fetal swallowing and AVP secretory responses to hypertonic NaCl and sucrose in preterm (115 days gestation) ovine fetuses. Five fetuses were chronically prepared with thyrohyoid, nuchal and thoracic esophagus, and diaphragm electromyogram (EMG) electrodes, an esophageal flow probe, and vascular catheters. Fetuses received intracarotid injections (0.15 ml/kg) of increasing concentrations of NaCl (0.15-2.55 M), administered at 2-min intervals. The threshold saline concentration for swallowing was defined as the minimum NaCl dose eliciting swallow responses (within 20 s) following 4 of 5 injections at each dose. In 2 fetuses, the threshold dose was 0.75 M (confirmed by 4 of 5 injections resulting in swallows at the next higher dose). In the other 3 animals, no swallowing threshold was observed (i.e., no dose through 2.55 M elicited swallows to 4 of 5 injections). There were no significant increases in AVP secretion. On a subsequent day, equimolar sucrose injections did not affect fetal swallowing activity or AVP secretion. Together with previous reports, these studies indicate that plasma osmolality dipsogenic mechanisms become functional in some ovine fetuses by 115 days gestation, while all fetuses exhibit intact osmotic dipsogenic responses at 130 days gestation. These results further suggest an acute process of development of dipsogenic responsiveness, rather than by gradually increasing sensitivity to osmotic stimuli.

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.

Assessments of Fetal Swallowred Volume: Tracer Disappearance Versus Esophageal Flow

OBJECTIVE: We sought to compare fetal swallowed volume determinations simultaneously by two techniques—amniotic fluid (AF) tracer disappearance and esophageal flow probe measure ments. METHODS: Six ovine fetuses (129 ± 1 days) were chronically prepared with a thoracic esophageal flow probe and vascular and two AF catheters. 125I-labeled albumin was injected into the AF cavity, and samples were withdrawn at timed intervals for 8 hours. The AF volume was calculated by time-0 extrapolation of the semilog of 125I disappearance. Tracer-determined swal lowed volume was calculated as the product of AF volume and the slope of isotope disappearance. Flow probe measurement of swallowed volume was determined by computer integration of cali brated flow probe velocity recordings. RESULTS: The AF volume averaged 805 ± 168 mL. The isotope disappearance rate from the AF was 2.8 ± 0.4%/hour. Average tracer-determined swallowed volume (547 ± 113 mL/day) was greater than flow probe volume (366 ± 81 mL/day), although these values were not significantly different. However, when corrected for estimated swallowed lung fluid, tracer- determined volume was significantly greater than flow probe volume (P < .05). CONCLUSION: Ovine fetal swallowed volume determinations by AF tracer techniques are greater than those determined by esophageal flow probe measurements. (J Soc Gynecol Invest 1994;1:37-44)

Ovine Fetal Laryngeal Chemoreflex Thresholds and Respiratory Effects

In newborn infants, laryngeal contact with solutions of low chloride concentration or pH evokes swallowing, laryngeal adduction, and respiratory inhibition (laryngeal chemoreflex). To determine whether the laryngeal chemoreflex is present during fetal life and its effect on fetal respiratory activity, eight time-bred ewes (128 +/- 2 days) were prepared with fetal electrocortical diaphragm and esophageal electrodes and a nasopharyngeal catheter. After a 60-minute control period, increasing volumes (0.1 to 1.0 ml/kg) of 0.15 mol/L NaCl or distilled water (0.05 to 1.0 ml/kg) and decreasing concentrations of NaCl (0.15 to 0.02 mol/L) at a fixed volume (0.3 ml/kg) were sequentially administered through the nasopharyngeal catheter (38 degrees C). The minimum water volume that stimulated swallowing was significantly less than the minimum 0.15 mol/L NaCl volume (0.10 +/- 0.02 vs. 0.70 +/- 0.05 ml/kg). The maximum NaCl concentration that stimulated swallowing was 0.04 +/- 0.01 mol/L During the control period, respiratory activity averaged 14.6 +/- 0.7 breaths/minute and did not change during absent swallow responses or isotonic saline-induced swallows. However, respiratory activity significantly decreased during water (4.7 +/- 0.6 breaths/minute) and hypotonic saline-induced swallow responses (3.7 +/- 0.7 breaths/minute). Fetal electrocortical activity did not change during absent or stimulated swallows. We conclude that laryngeal water or hypotonic saline solution may stimulate fetal swallowing and suppress fetal respiratory activity, similar to the newborn laryngeal chemoreflex. We speculate that an exaggeration of the laryngeal chemoreflex apnea response in the newborn may predispose to sudden infant death syndrome.

Cocaine stimulation of ovine fetal swallowing

Cocaine acts to block reuptake of neurotransmitters, resulting in elevated intra-synaptic norepinephrine levels. As monoaminergic systems may contribute to central regulation of swallowing motor activity as well as dipsogenic responses, and cocaine is known to alter fetal behavior, we examined the effect of intravenous cocaine on fetal swallowing. Six ovine fetuses (130 +/- 1 days) were chronically prepared with esophageal electrodes and an esophageal flow probe. Following a 1-h control period, fetuses received an intravenous injection of 1.0 mg/kg cocaine over 30 s. Fetal blood samples were withdrawn at timed intervals and fetal swallowing activity was monitored for 180 min after the cocaine bolus. Basal fetal swallowing activity during the control period was 0.6 +/- 0.1 swallows/min, with esophageal flow of 0.3 +/- 0.1 ml/min. At 10 min following cocaine, fetal swallowing rate increased to 1.4 +/- 0.4 swallows/min though esophageal flow did not change significantly (0.4 +/- 0.2 ml/min). Swallowing rate then returned to basal levels. Fetal heart rate increased in response to cocaine injection (149 +/- 14 to 174 +/- 24 bts/min at 180 min) though there was no change in fetal mean blood pressure. These results demonstrate acute stimulation of fetal swallowing activity in response to intravenous cocaine injection. Together with previous reports, these data further illustrate the diversity of fetal behavioral effects in response to in utero cocaine exposure.