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Biomedical subjects

M G Ross

Publications and source records attributed to M G Ross.

At least 73 records · Page 4Linked to original sources
PubMed1996

Fetal gastrointestinal composition: implications for water and electrolyte absorption.

In the late-gestation human or ovine fetus, daily swallowed volumes (100-300 ml/kg body weight) are markedly greater than adult values (40-60 ml/kg). This volume of amniotic fluid water, electrolytes and cellular debris is almost completely resorbed within the fetal gastrointestinal (GI) tract, resulting in the formation of meconium within the distal fetal colon. Despite this capacity for fetal GI water resorption, little is known regarding the sites or processes by which absorption occurs. We examined the composition of GI contents of 13 ovine fetuses (121-148 days). Fetuses were operatively delivered and killed and the Gi tract (rumen to sigmoid colon) was ligated in sections. Luminal samples were obtained and analysed for water and organic content and osmolality and electrolyte composition. The water content significantly decreased (98.7% to 70.1%) while the organic content significantly increased (55.0% to 95.3% of dry weight) from the rumen to colon. Osmolality significantly increased from the rumen to the distal small intestine (303 +/- 3 to 596 +/- 50 mOsm/kg) and returned to near isotonic values in the sigmoid colon. These results indicate significant water absorption within the fetal GI tract, with a pattern similar to that of the adult. However, fetal colonic sodium absorption is either suppresses or immature near term.

Aging↗
PubMed1996

Ovine fetal adaptations to chronically reduced urine flow: preservation of amniotic fluid volume.

Adequate amniotic fluid (AF) volume is maintained by a balance of fetal fluid production (lung liquid and urine) and resorption (swallowing and intramembranous flow). Because fetal urine is the principle source of AF, alterations in urine flow and composition directly impact AF dynamics. Intra-amniotic 1-desamino-8-D-arginine vasopressin (DDAVP) is rapidly absorbed into fetal plasma and induces a marked fetal urinary antidiuresis. To examine the effect of intra-amniotic- DDAVP-induced fetal urinary responses on AF volume and composition, six chronically prepared ewes with singleton fetuses (gestation 128 +/- 2 days) were studied for 72 h after a single intra-amniotic DDAVP (50-microgram) injection. After DDAVP, fetal urine osmolality significantly increased at 2 h (157 +/- 13 to 253 +/- 21 mosmol/kg) and remained elevated at 72 h (400 +/- 13 mosmol/kg). Urinary sodium (33.0 +/- 4.5 to 117.2 +/- 9.7 meq/l) and chloride (26.0 +/- 2.8 to 92.4 +/- 8.1 meq/l) concentrations similarly increased. AF osmolality increased (285 +/- 3 to 299 +/- 4 mosmol/kg H2O), although there was no change in fetal plasma osmolality (294 +/- 2 mosmol/kg). Despite a 50% reduction in fetal urine flow (0.12 +/- 0.03 to 0.05 +/- 0.02 ml.kg-1.min-1 at 2 h and 0.06 +/- 0.01 ml.kg-1.min-1 after 72 h), AF volume did not change (693 +/- 226 to 679 +/- 214 ml). There were no changes in fetal arterial blood pressures, pH, PCO2, or PO2 after DDAVP. We conclude the following. 1) Intra-amniotic DDAVP injection induces a prolonged decrease in fetal urine flow and increases in urine and AF osmolalities. 2) Despite decreased urine flow, AF volume does not change. We speculate that, in response to DDAVP-induced fetal oliguria, reversed intramembranous flow (from isotonic fetal plasma to hypertonic AF) preserves AF volume.

Adaptation, Physiological↗
PubMed1995

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.

Animals↗
PubMed1995

A randomized multicenter trial assessing a home uterine activity monitoring device used in the absence of daily nursing contact.

OBJECTIVE: To evaluate the effectiveness of home uterine activity monitoring in the early detection of preterm labor among women with a history of preterm delivery. STUDY DESIGN: Two hundred eighteen women from four centers were prospectively randomized to routine high-risk prenatal care alone (not monitored) or to the same prenatal care with twice-daily home uterine activity monitoring without daily nursing support (monitored). All women had a history of preterm delivery. The primary study end point was cervical status as measured by cervical dilatation at the time of diagnosis of preterm labor. RESULTS: The two study group populations at entry into the study were similar in medical and demographic characteristics. Of 187 women completing the trial, 21 (24.4%) of the women in the monitored group (n = 86) and 22 (21.8%) of the women in the unmonitored (control) group (n = 101) experienced preterm labor (not significant). Mean cervical dilatation at the time of diagnosis of preterm labor was 1.7 cm in the monitored group and 2.8 cm in the unmonitored group (p = 0.004). A total of 52.4% of the women in the monitored group had a cervical dilatation of < 2 cm when preterm labor was detected, compared with 18.2% of the women in the unmonitored group (p = 0.019). The median duration of gestation after diagnosis of preterm labor was 21.0 days for the monitored group and 3.0 days for the unmonitored group (p = 0.016). CONCLUSION: The diagnosis of preterm labor for women using home uterine activity monitoring without daily nursing contact was detected with less cervical dilatation than found in those women not monitored. This earlier detection of preterm labor demonstrates the utility and effectiveness of home uterine activity monitoring devices and may lead to improved neonatal outcomes.

Adult↗
PubMed1995

An objective definition of shoulder dystocia: prolonged head-to-body delivery intervals and/or the use of ancillary obstetric maneuvers.

OBJECTIVE: To generate an objective definition of shoulder dystocia by timing the events of the second and third stages of labor, and to define the true incidence of shoulder dystocia. METHODS: In 34 arbitrarily selected 24-hour time periods, a nonparticipating observer prospectively timed intervals of the second stage of labor in all vaginal deliveries and recorded the use of obstetric maneuvers (McRoberts, episiotomy after delivery of the fetal head, intentional extension of initial episiotomy after delivery of the fetal head, suprapubic pressure, posterior arm rotation to an oblique angle, rotation of the infant by 180 degrees, delivery of the posterior arm, and general anesthesia) and whether the obstetric attendant identified a delivery with shoulder dystocia. All data are reported as mean +/- standard error of the mean. RESULTS: Two hundred fifty deliveries were timed and recorded prospectively. Mean intervals (in seconds) in nonmaneuver patients were as follows: head to anterior shoulder 14.8 +/- 1.0, anterior to posterior shoulder 3.9 +/- 0.6, posterior shoulder to body 5.4 +/- 0.8, and total head-to-body time 24.2 +/- 1.3. Three groups of patients were defined after delivery. The maneuver group consisted of 27 patients requiring any of the aforementioned obstetric maneuvers, although the obstetric attendant identified only 16 of these as shoulder dystocia. The prolonged delivery group included 29 patients with the head-to-body delivery interval exceeding the mean plus two standard deviations (60 seconds) of nonmaneuver patients. Sixteen of the 27 maneuver patients were identified as prolonged. The 210 not identified as maneuver or prolonged were considered to be normal. Normal patients had a significantly lower newborn birth weight (3269 +/- 38 g), and a lower proportion of 1-minute Apgar scores of 7 or less (11%) than did the maneuver (4247 +/- 86 g, 41%) and prolonged groups (3952 +/- 118 g, 34%). Defining shoulder dystocia as a prolonged head-to-body delivery time and/or the use of obstetric maneuvers identified 40 patients who had birth weights and 1-minute Apgar scores significantly different from the normal patients. CONCLUSION: The incidence of shoulder dystocia, as defined by the use of ancillary obstetric maneuvers, is higher than that reported previously, and the reporting of shoulder dystocia appears to be unreliable. The interval from head-to-body delivery is delayed significantly in patients with shoulder dystocia, despite the lack of recognition of shoulder dystocia. We propose defining shoulder dystocia as a prolonged head-to-body delivery time (eg, more than 60 seconds) or the need for ancillary obstetric maneuvers.

Adult↗
PubMed1995

DDAVP-induced maternal hyposmolality increases ovine fetal urine flow.

Fetal urine flow is influenced by fetal intravascular volume, glomerular filtration rate, tubular reabsorption, and fluid regulatory hormones. As maternal-to-fetal fluid transfer is dependent on hydrostatic and osmotic gradients, we postulated that a chronic decrease in maternal plasma osmolality would promote transplacental fluid transfer and increase fetal urine flow. Six pregnant ewes and singleton fetuses (131 +/- 2 days; term = 150 days) received bladder and hindlimb arterial and venous catheters. After 5 days, plasma and urine composition, urine flow rate (Uvol), and plasma arginine vasopressin (AVP) levels were measured during a 2-h control period. At 2 h, tap water (2 liter, 38 degrees C) was administered to the ewe. At 3 h, ewes received a 20-micrograms bolus of 1-desamino-[D-Arg8]vasopressin (DDAVP), followed by continuous infusion (4 micrograms/h). In response to water loading, maternal urine osmolality decreased (761 +/- 158 to 339 +/- 13 mosmol/kgH2O), and Uvol increased. After DDAVP, maternal urine osmolality increased (1,270 +/- 89 mosmol/kgH2O), and Uvol, hematocrit, plasma osmolality (304 +/- 1 to 284 +/- 4 mosmol/kgH2O), and protein concentration decreased. Five hours after maternal DDAVP infusion, fetal plasma osmolality decreased (300 +/- 1 to 281 +/- 3 mosmol/kgH2O), and Uvol increased (0.4 +/- 0.1 to 1.3 +/- 0.2 ml/min) and remained elevated at 24 h. There was no change in fetal plasma DDAVP (immunoreactive AVP) levels or fetal urine osmolality. Controlled changes in maternal plasma osmolality may prove useful in modulating fetal urine flow and, ultimately, amniotic fluid volume.

Animals↗
PubMed1995

Ovine fetal swallowing response to intracerebroventricular hypertonic saline.

In the near-term ovine fetus, systemic hyperosmolality stimulates dipsogenic responses. Putative systemic dipsogens (hypertonicity, angiotensin II) may initiate responses by stimulation of select cerebral circumventricular nuclei lacking a blood-brain barrier. To investigate whether central osmotic-dipsogenic mechanisms are functional in utero, fetal swallowing responses to intracerebroventricular (i.c.v.) hypertonic saline were examined. Five pregnant ewes with singleton fetuses (128 +/- 1 days gestation) were prepared with fetal lateral cerebral ventricle and vascular catheters, electrocortical (ECoG) electrodes, and electromyogram wires on the fetal thyrohyoid muscle, nuchal and thoracic esophagus, and diaphragm and studied for a minimum of 5 days postoperatively. After a 2-h basal period, fetuses received an i.c.v. infusion of artificial cerebrospinal fluid followed by successive 30-min infusions of hypertonic NaCl in artificial cerebrospinal fluid (500 and 700 mosmol/kgH2O). In response to the i.c.v. hypertonic NaCl infusions, fetal swallowing significantly increased (1.4 +/- 0.4 to 3.9 +/- 1.4 and 2.9 +/- 0.5 swallows/min low-voltage ECoG, respectively). Plasma arginine vasopressin levels increased, although the change was not statistically significant (9.1 to 24.2 pg/ml; P = 0.07), and there was no change in fetal plasma osmolality, sodium concentration, or ECoG activity. Together with previous studies, these results indicate that both central and systemic osmotic dipsogenic mechanisms are functional in utero.

Animals↗
PubMed1994

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.

Animals↗
PubMed1994

Prophylactic amnioinfusion for meconium-stained amniotic fluid.

OBJECTIVE: Previous studies have demonstrated reduced perinatal morbidity in patients receiving amnioinfusion for meconium-stained amniotic fluid compared with control patients receiving no amnioinfusion. Because amnioinfusion for variable fetal heart rate decelerations has become accepted care, we sought to determine the benefit of prophylactic amnioinfusion for meconium compared with standard care, incorporating therapeutic amnioinfusion for variable decelerations. STUDY DESIGN: Ninety-three term patients with moderate to heavy meconium and no variable fetal heart rate decelerations were randomized to immediate prophylactic amnioinfusion (600 ml saline solution bolus followed by 3 ml/min) or to standard care (including therapeutic amnioinfusion for variable decelerations developing later). All babies had DeLee suctioning on delivery of the head. Laryngeal cords were visualized and tracheal suctioning performed when meconium was seen below the cords. Statistical comparisons were performed using Student t test, Fisher's exact test, or chi 2 analysis. RESULTS: There were no significant differences in the incidence of operative delivery, fetal distress, or meconium below the cords or in newborn Apgar scores and umbilical artery gas values between the amnioinfusion (n = 43) and control (n = 50) patients. There were four cases of meconium aspiration, three in the amnioinfusion group, one in the standard care group. The rate of endometritis-chorioamnionitis was higher (p = 0.3) in the amnioinfusion (16%) than in the control group (8%), although time from ruptured membranes to delivery (8.5 hours vs 7.3 hours) and duration of intrauterine monitoring (6.1 hours vs 5.3 hours) were not different. CONCLUSIONS: Although amnioinfusion does dilute amniotic meconium, prophylactic amnioinfusion for meconium in the absence of variable decelerations remains controversial. Prophylactic amnioinfusion in term pregnancies did not improve perinatal outcome and increased the risk for chorioamnionitis-endometritis. Together with recent reports, the current data suggest that the benefit of amnioinfusion for meconium-stained amniotic fluid is a result of the alleviation of variable fetal heart rate decelerations rather than meconium dilution.

Adult↗
PubMed1994

The West Los Angeles Preterm Birth Prevention Project. I. Program impact on high-risk women.

OBJECTIVE: The primary objective of this prospective study was to test whether preterm birth prevention education plus increased clinic visits and selected prophylactic interventions reduce preterm birth. STUDY DESIGN: Eight West Los Angeles prenatal county clinics, comparable with respect to selected demographics, were randomized to be either experimental or control clinics. High-risk patients in all clinics were identified with a risk scoring system derived from a similar population. High-risk patients (N = 1774) in experimental clinics were offered a program of education and more frequent visits and were randomized to receive various secondary intervention protocols in addition to the basic interventions of education and more frequent visits. Control clinic patients (N = 880) received standard county care. RESULTS: Preterm birth rates were 19% lower among the experimental high-risk patients (7.4% vs 9.1%), and differences were significant (p < 0.05) when preterm risk was taken into account. There was no evidence to suggest that the secondary interventions provided added benefit over the primary intervention protocol of preterm birth prevention education and increased visits. CONCLUSION: The 19% reduction in preterm birth rate observed in the experimental clinics suggest an overall program benefit from a protocol that offered education, more frequent visits, and greater attention given to patients while the selected interventions were applied.

Adult↗
PubMed1994

The West Los Angeles Preterm Birth Prevention Project: II. Cost-effectiveness analysis of high-risk pregnancy interventions.

OBJECTIVE: To examine the cost-effectiveness of the West Los Angeles Preterm Birth Prevention Project. METHODS: Maternal and neonatal care data were collected on all preterm deliveries (150) and a random sample of term deliveries (140) from high-risk patients at both experimental and control clinic sites. Costs were determined for prenatal care, inpatient preterm labor, delivery and postpartum care, and newborn care. Cost calculations, weighted by the actual proportions of term and preterm births, were confirmed with square-root transformation and trimmed mean (2%) values. RESULTS: When compared to control clinic high-risk patients, experimental clinic high-risk patients had an average cost savings of $2196 for newborn care (P = .02), resulting in a net savings of $1768 per high-risk mother-infant pair. Births before 32 weeks' gestation accounted for the greatest mean cost. CONCLUSION: Programs of comprehensive prenatal care and patient education may be highly cost-effective for prevention of prematurity.

Adult↗
PubMed1994

Vascular effects alter early-gestation fetal renal responses to vasopressin.

Distinct receptors mediate the vascular (V1) and renal (V2) effects of arginine vasopressin (AVP). Although ovine fetal AVP-induced antidiuresis can be demonstrated in early gestation (< 120 days; term 150 days), the early-gestation fetal renal responses to AVP are variable, including increases in urine flow and glomerular filtration rate (GFR). AVP V1 receptor predominance and/or V2 receptor system immaturity may contribute to variable early-gestation renal responses to AVP. To differentiate these possibilities, we assessed early-gestation fetal V2 receptor function in the presence and absence of V1 receptor-mediated effects by comparing the responses to AVP (a combined V1-V2 receptor agonist; n = 10; 112 +/- 2 days) with the selective V2-receptor agonist 1-desamino-8-D-arginine vasopressin (DDAVP) (n = 5; 111 +/- 2 days). AVP infusion increased fetal mean arterial pressure (MAP; 36 +/- 1 to 44 +/- 2 mmHg) and decreased heart rate (197 +/- 2 to 171 +/- 3 beats/min); DDAVP infusion had no effect on MAP or heart rate. Free water clearance decreased in response to AVP (0.13 +/- 0.02 to 0.02 +/- 0.01 ml.min-1.kg-1) and DDAVP (0.21 +/- 0.04 to 0.04 +/- 0.02 ml.min-1.kg-1), and urine osmolality increased in response to both analogues (AVP: 145 +/- 4 to 283 +/- 15 mosmol/kgH2O; DDAVP: 146 +/- 5 to 244 +/- 32 mosmol/kgH2O).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1994

Ovine fetal swallowing and renal responses to oligohydramnios.

Amniotic fluid (AF) volume regulation is dependent on a balance between fluid production and fluid resorption. We examined the effects of reduced AF volume on AF production by fetal urine and resorption by fetal swallowing and the response of these parameters to AF volume replacement. Eight time-dated pregnant ewes (125 +/- 1 days gestation) were studied before (day 1) and after (day 3) AF and fetal urine drainage. Drainage resulted in a significant decrease in AF volume (415 +/- 89 to 157 +/- 36 ml). Fetal urine osmolality increased (139 +/- 10 to 286 +/- 33 mosmol/kgH2O), while urine flow did not change significantly (0.31 +/- 0.04 to 0.23 +/- 0.06 ml/min), resulting in nonsignificant increases in osmolar, sodium, and chloride excretions. Fetal electromyographic swallowing activity decreased 30% (1.0 +/- 0.1 to 0.7 +/- 0.1 swallows/min; P < 0.05), while net esophageal flow decreased 74% (0.31 +/- 0.12 to 0.07 +/- 0.04 ml/min; P < 0.05). On day 4, 0.15 M NaCl (500 ml; 37 degrees C) was administered into the AF over 30 min. During the 2 h after reinfusion, urine flow (0.29 +/- 0.07 to 0.40 +/- 0.09 ml/min) and osmolar sodium and chloride excretion significantly increased, though fetal swallowing activity and esophageal flow did not change. Thus the ovine fetus responded to reduced AF volume by maintaining AF production and decreasing AF resorption. In response to AF replacement, urine flow increased while fetal swallowing activity did not change, consistent with an intramembranous pathway for fetal AF resorption.

Amniotic Fluid↗
PubMed1994

Central angiotensin II stimulation of ovine fetal swallowing.

In the late-gestation ovine fetus, spontaneous swallowing occurs primarily during fetal low-voltage electrocortical (ECoG) activity in association with fetal breathing movements. Fetal swallowing activity may be stimulated in response to systemic or carotid plasma hyperosmolality, although not to increased plasma angiotensin II (ANG II) levels. In view of the potent dipsogenic effects of central, but not peripheral, ANG II in adult sheep, the present study investigated the effect of intracerebroventricular (ICV) ANG II on fetal swallowing activity. Six ovine fetuses (127 +/- 1 days) were chronically prepared with electromyogram and cortical electrodes and with vascular and lateral ventricle catheters. After a 2-h control period, fetuses received ICV injections of artificial cerebrospinal fluid and increasing concentrations of ANG II (0.1, 1.0, 10, 100, and 500 ng/kg). Fetal ECoG activity did not change, although fetal swallowing activity significantly increased in response to the 100- and 500- ng/kg ANG II doses (1.20 +/- 0.14 to 3.34 +/- 0.59 and 3.46 +/- 0.81 swallows/min of low-voltage ECoG, respectively). In response to the highest ANG II dose, fetal plasma arginine vasopressin levels significantly increased (5.7 +/- 1.2 to 17.2 +/- 4.6 pg/ml). ICV ANG II stimulation of fetal swallowing and arginine vasopressin secretion demonstrates that central ANG II dipsogenic mechanisms are intact by 0.9 of ovine gestation.

Amniotic Fluid↗
PubMed1994

Prophylactic amnioinfusion for oligohydramnios: a reevaluation.

OBJECTIVE: To compare the effects of prophylactic amnioinfusion to standard care plus indicated therapeutic amnioinfusion (for variable decelerations) in term patients with oligohydramnios. METHODS: One hundred sixteen term gestations with oligohydramnios (amniotic fluid index less than 5.0 cm) were randomly assigned to receive prophylactic saline amnioinfusion (600-mL bolus followed by 3 mL/minute) or standard obstetric care (control). Control patients who subsequently developed moderate or severe variable decelerations received therapeutic amnioinfusion. RESULTS: There was no significant difference in overall cesarean delivery (21 versus 17%; P = .68), cesarean delivery for fetal distress (7 versus 10%; P = .83), or umbilical gas values between the prophylactic amnioinfusion group (N = 56) and control group (N = 60), respectively. The rate of intrapartum fever was significantly increased among the prophylactic amnioinfusion patients (23 versus 7%; P = .02), although the duration of intrauterine monitoring (8.8 versus 6.5 hours; P = .06) and time from ruptured membranes to delivery (12.3 versus 14.3 hours; P = .51) were not different. Only 22% of the controls developed moderate or severe variable decelerations in the first stage of labor and received therapeutic amnioinfusion. Thus, in approximately four of five term patients with oligohydramnios, neither prophylactic nor therapeutic amnioinfusion would be indicated. CONCLUSIONS: Compared to indicated amnioinfusion, prophylactic amnioinfusion did not improve perinatal outcome. Amnioinfusion should be reserved for term laboring patients with variable fetal heart rate decelerations, rather than all patients with oligohydramnios.

Adult↗
PubMed1993

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.

Animals↗