Richard E. Weitzman

The Dynamics of Vasopressin Release and Blood Volume Regulation During Fetal Hemorrhage in the Lamb Fetus

Summary: Experiments were performed on 11 long-term fetal lamb preparations (103–138 days of gestation) to investigate the sensitivity and relative responsiveness of the fetal volume receptors in modulating fetal plasma arginine vasopressin (pAVP) secretion and plasma renin activity (PRA) secretion during fetal hypovolemia and after fetal blood volume replacement. During fetal hemorrhage there were significant decreases (P < 0.05) in fetal hematocrit (34.7 ± 2.58 to 27.0 ± 1.64%), plasma proteins (3.14 ± 0.15 to 2.78 ± 0.19 g/100 ml), mean arterial blood pressure (MABP) (58.1 ± 2.59 to 52.2 ± 2.60 mmHg) and fetal arterial pH (7.38 ± 0.01 to 7.35 ± 0.01). A significant increase in fetal pAVP concentration from 0.73 ± 0.21 to 34.9 ± 10.04 μU/ml (P < 0.01) and fetal PRA from 4.78 ± 2.22 to 40.4 ± 18.31 ng/ml/hr (P < 0.05) was demonstrated at the peak of fetal hemorrhage. Two hr after correction of the fetoplacental blood volume, these values were back to base line levels. No change in either maternal pAVP or PRA was seen during fetal hemorrhage. When individual values for log pAVP and log PRA were plotted as a function of percent of fetoplacental blood volume removed the correlation coefficients were 0.82 and 0.60, respectively. A multiple regression analysis showed a high correlation of log pAVP and log PRA with the volume of blood removed and a low partial correlation with the fetal MABP. This suggests the decrease in fetal MABP was not the primary factor explaining the increase in pAVP and PRA during fetoplacental blood volume depletion. The data indicate that the fetal volume receptors for control of arginine vasopressin secretion are fully functional in the last trimester of gestation and suggests that fetal pAVP and PRA are released as an exponential function of the percent of fetoplacental blood volume depletion. Finally, an isosmotic water shift from the fetal interstitial space to the fetal vascular space is described during fetal hemorrhage.Speculation: It is suggested that a change in the equilibrium between the forces regulating fluid movement through the fetal capillary membranes, in accordance with Starlings principle, activates isosmotic water fluxes from the fetal interstitial space to the fetal vascular compartment counteracting the effects of fetal blood volume depletion. Therefore, the role of arginine vasopressin (AVP) release during fetal hemorrhage, if any, will be to act as a pressor substance helping to maintain fetal blood pressure. No major effect of AVP on placental membranes was demonstrated in vivo.

Arginine Vasopressin Response to an Osmotic Stimulus in the Fetal Sheep

Summary: Baseline plasma osmolality (pOsm) and plasma arginine vasopressin (pAVP) were measured in chronically catheterized fetal sheep. Mean baseline pAVP in fetuses from 101–120 days was 1.9 ± 0.46 μU/ml (mean ± SEM) with a pOsm of 293 ± 1.8 mOsm/kg. In fetuses of 121–141 days of gestation, mean pAVP was significantly lower, 0.77 ± 0.19 μU/ml (P < 0.05), with a similar pOsm (293 ± 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.Speculation: The 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.

Maternal fetal osmolar homeostasis: fetal posterior pituitary autonomy.

Summary: After the infusion of a bolus of 225 mEq NaCl (HS) to maternal ewes, we studied fetal plasma sodium, osmolality, total serum solids, plasma arginine vasopressin (AVP) and plasma renin activity (PRA) responses in 16 chronically catheterized, 112–139 days of gestation fetal lambs.To examine the degree to which this might have represented transplacental passage of AVP, we infused a large amount of synthetic AVP into the fetal circulation (protocol 3) and detected no change in maternal plasma AVP. The protocol was designed to allow multiple, frequent blood sampling not possible by infusing the synthetic AVP into the maternal circulation.In order to compare the fetal AVP response elicited by HS in the maternal ewe and that after direct HS into the newborn lamb and fetus, we calculated stimulus response ratio (SRR) as:Log (AVP)1 - Log (AVP)2, divided by Δosmolality.The SRR of lamb fetuses after maternal HS was significantly greater (0.16 ± 0.02) than that after direct fetal HS (0.04 ± 0.01). In comparison, the newborn lamb has a SRR of 0.02 ± 0.01, and the ewe has a SRR of 0.02 ± 0.01 after HS. These data suggest that after maternal HS infusion, both a volume and, to a lesser extent, an osmolar stimulus for AVP secretion occurs after an induced water flux from the fetal to maternal compartments. Maternal plasma sodium concentration rose promptly from a base line of 146 ± 2.2 (mean ± SEM) to 157 ± 2.8 mEq/liter by 1–20 min where it remained throughout the hour observation period. Fetal plasma sodium concentration rose more slowly from base line of 143 ± 1.8 to 149 ± 1.8 mEq/liter by 1 hr. When 85 mCi22Na was additionally infused with the HS, fetal 22Na constituted only 10% of maternal 22Na counts by the end of 1 hr.During the same period fetal PRA rose from a base line of 12.9 ± 3.8 to 32.0 ± 3.6 ng/ml/hr, while maternal renin remained unchanged. Maternal AVP rose modestly, 11 min after the HS, but promptly returned to base line. There was a rapid and sustained rise by fetal AVP from a base line of 0.7 μU/ml to a peak of 8.2 μU/ml by 22 min post HS.A fetal SRR, Log (AVP)1-Log (AVP)2/Δosmolality after maternal HS was greater than that after direct fetal HS or HS to the ewe or newborn lamb. In an additional experiment, using five chronically catheterized fetuses, 10 million μU AVP injected in the fetal circulation failed to produce an increase in maternal AVP.These results demonstrate that: 1) AVP does not cross the placenta; 2) the fetal sheep neurohypophysis is autonomous and responsive to both direct and indirect (maternal) osmolar stimulation; and 3) the relatively slow rate of maternal to fetal sodium transfer, the augmented SRR after maternal HS, and the elevated fetal PRA and AVP concentration suggest that there is a rapid fetal to maternal flow of water after maternal HS and a combined volume and osmolar stimulus to the fetus.Speculation: Infusions of hypertonic saline in pregnant ewes resulted in rapid increases of fetal plasma sodium due to transfer of water from fetus to mother. A marked rise in fetal renin activity and AVP was also observed. The increase in AVP exceeded that seen in the mother and that produced by infusing hypertonic saline in the fetus. Fetal secretion of AVP is stimulated by hyperosmolality and volume contraction. Elevated AVP may help the fetal kidney retain water and, thereby, minimize hypotonic urine formation and volume depletion.

Radioimmunoassay of vasotocin, vasopressin, and oxytocin in human neonatal cerebrospinal and amniotic fluid

Arginine vasotocin (AVT) has been measured in neonatal cerebrospinal fluid (CSF) and human amniotic fluid using a newly developed specific radioimmunoassay system. There were significant amounts of AVT in all samples. Vasopressin and oxytocin also were measured in the samples and could not account for the levels of vasotocin found. The source and function of these neurohypophyseal peptides in CSF and amniotic fluid remains speculative.

Effects of Furosemide and Acute Salt Loading on Vasopressin and Renin Secretion in the Fetal Lamb

Summary: Circulating arginine vasopressin (AVP) and plasma renin activity responses to furosemide (2 mg/kg) and acute hypertonic saline (10 mEq/kg) were studied in the fetal lamb from 100 days gestation to term. The baseline to peak plasma AVP response (Δ3.7 ± 1.2 uU/ml) and area under the response curve (209 ± 57 uU/ml/65 min) in the fetal lambs > 123 days were greater than in those <106 days gestation (Δ1.8 ± 1.1 and (171 ± 61, respectively), P <0.02. The plasma renin activity/AVP ratio after furosemide was similar in the two gestational groups.The log plasma AVP responses corrected for rise in plasma osmolality (0.090 ± .01 uU/ml) 30 min after infusion, and the area under the response curve (253 ± 49 uU/ml/30 min) was greater (P < 0.02) in the fetal lambs > 120 days than in those under 115 days gestation (.035 ± 0.01 and 88 ± 29, respectively), P < 0.02. These results confirm that the fetal lamb responds to an osmotic stimulus with increased plasma AVP levels and documents that this response significantly matures during the last trimester of gestation. The fetal lamb also manifests a hypothalamus-posterior pituitary AVP response to furosemide that is proportional to the maturing renal renin response.Speculation: There is a significant maturational plasma arginine vasopressin response to acute hypertonic saline and furosemide in the fetal lamb during the last trimester of gestation.

Control of vasopressin secretion in the newborn lamb.

Summary: The plasma sodium, osmolality, and arginine vasopressin (AVP) responses to phlebotomy, hypertonic saline, water loading and fluid restriction were studied in 2–49 day old lambs. Phlebotomy of 10 and 20% of the lambs estimated blood volume produced 37-and 44-fold increments in plasma AVP, without a concomitant change in plasma sodium or osmolality. The infusion of 10 mEq/kg sodium chloride produced a 12% rise in plasma sodium concentration accompanied by a 7-fold rise in plasma AVP. Water loading with 100 ml/kg hypotonic fluid produced a significant fall in plasma sodium concentration (10.7%) and a decrease in plasma AVP. Eighteen hr of water deprivation evoked a 7-fold increase in AVP.These results indicate that the newborn lamb is capable of responding appropriately to known stimuli for AVP secretion. The stimulus response ratio (SRR): of newborn lambs was nearly identical after hypertonic saline and water loading and was also quite similar to that of the adult ewe after a saline challenge. The SRR of water deprived lambs was greater than that after the other stimuli, presumably reflecting combined volume and osmolar stimuli. We conclude that the neurohypophysis and the volume receptor systems of the newborn lamb are capable of appropriate, mature AVP responsiveness during the first days of extrauterine life.Speculation: The present studies indicate that AVP secretion in the newborn sheep is responsive to both volume and osmolar stimuli. Quantitative responses are equivalent to those of mature ewes. If the newborn human is both osmo- and volume sensitive, as seems likely, AVP secretion may be important in fluid and electrolyte homeostasis in the newborn period. Thus, lack of AVP secretion does not explain the limited ability to concentrate urine demonstrated by the newborn infant.

Endogenous angiotensin stimulation of vasopressin in the newborn lamb.

The effect of furosemide on plasma renin, vasopressin (AVP), and aldosterone concentrations was studied in 10 control and 6 nephrectomized lambs during the 1st 2 wk of life. In a separate study in 10 newborn lambs, 1-sarcosine-8-alanine-angiotensin II (saralasin acetate, 5 μg/kg per min) was infused alone for 40 min, after which furosemide 2 mg/kg i.v. was injected in association with continuing saralasin acetate infusion. Plasma renin activity increased from a mean (±SEM) of 21.3±3.4 ng/ml per h in the 10 control lambs to 39.4±8.2 ng/ml per h at 8 min (P < 0.001) and remained high through 120 min after furosemide. Plasma AVP and aldosterone concentrations increased from respective mean values of 2.1±0.4 μU/ml and 12.8±2.5 ng/dl to 9.8±2.0 μU/ml (P < 0.01) and 23.0±7.7 ng/dl (P < 0.05) at 35 min and 13.8±2.1 μU/ml and 23.0±4.4 ng/dl at 65 min after furosemide (each P < 0.01). There was an insignificant AVP response in the 10 lambs treated with angiotensin inhibitor: from a mean base line of 4.7±0.9 to 8.3±2.0 μU/ml at 35 min, and 7.4±2.0 μU/ml at 65 min after furosemide. There was no increase in AVP in the anephric lambs. The mean increment AVP response from base line in the newborn lambs without saralasin, Δ 10.8±2.0 μU/ml, was greater than in the lambs with saralasin, Δ4.0±1.9 (P < 0.05), and greater than in the anephric lambs, Δ3.3±2.1 μU/ml (P < 0.05). The mean blood pressure fell 6 mm Hg in the 10 control lambs (P < 0.05), 7 mm Hg in the anephric lambs (P < 0.05), and 16 mm Hg in the lambs treated with angiotensin inhibitor (P < 0.05) by 35 min after furosemide. However, the changes in plasma AVP were not related to the fall in blood pressure. These data support the view that the observed AVP response to furosemide in the newborn lamb was mediated through the renin-angiotensin system.

298 METABOLIC CLEARANCE RATE OF OXYTOCIN IN MATERNAL AND FETAL SHEEP

Simultaneous maternal and fetal plasma oxytocin (OXY) concn. (μU/ml) were measured by radioimmunoassay before and during continuous infusion of synthetic OXY to steady state conditions into ewe or fetus (gestational age 124-140 days; at least 5 days post-surgery; estimated fetal wt. 3 kg).Fetal metabolic clearance rates (MCR in ml/kg/min) were calculated to be 18.1 ± 1.1 and 15.0 ± 1.3 at the two infusion rates; maternal MCR were 12.7 ± 2.8 and 13.4 ± 2.3, respectively. Examination of simultaneous fetal and maternal baseline OXY concentrations revealed that fetal levels were significantly higher than maternal: 1.9 ± 0.2 vs 0.7 ± 0.1 (p<.05). Continuous monitoring of uterine pressure revealed that uterine contractions were induced by maternal infusion of 800 μU/kg/min; no uterine contractions were induced by fetal infusion. Conclusions based on these data are: 1) plasma OXY levels exceed maternal levels in fetuses of 124-140 days gestation; 2) transplacental passage of OXY is minimal in both M-F and F-M directions; 3) maternal and fetal MCR of OXY are similar and unrelated to plasma OXY levels.

CONTROL OF VASOPRESSIN (AVP) SECRETION IN THE LAMB

The newborn is capable of secreting AVP, but there are no data regarding relative responsiveness to the several known physiological stimuli. We examined AVP, plasma sodium (Ma) and osmolality (Osm) in response to phlebotomy, water loading, hypertonic saline and mild dehydration in 7-11 lambs, 2-49 (mean = 19) days of age. AVP was measured by radioimmunoassay.Repeated phlebotomies (total 20 ml/kg) raised AVP from 4.8 ± 2.1 (mean ± SEM) to 74 ± 19 μU/ml (p < .01), while Na and Osm remained unchanged. When 100 ml/kg 2.5% dextrose/water was infused over 60 minutes, AVP fell from 3.,4 ± 1.2 to 0.7 ± 0.6 μU/ml by 60 minutes (p < .05). Na and Osm fell from 140 to 125 mEq/L and 283 to 262 mOsm/kg respectively (p < .05) Hypertonic (23%) sodium chloride infusion (10 mEq/kg) increased Na from 142 to 159 mEq/L and Osm from 271 to 318 mOsm/kg over a 30 min period (p < .05). In response to this stimulus, AVP increased from 2.9 ± 0.7 to 22.2 ± 9 μU/ml (p < .05). After 18 hours of dehydration, AVP rose from 0 6 ± 0.1 to 34 ± 1 8 μU/ml, Na from 134 to 140 mEq/L (p < 05) and Osm from 293 to 306 mOsm/kg (p < .05).Thus the newborn lamb is capable of responding to both volume and osmolar stimuli. The quantitative stimulus-response ratios (SRR = Mog AVP/ΔOsm) were similar for water loaded and saline stimulated newborns and similar to responses in the adult. The dehydration SRR was not accountable by osmolar change alone but also reflected volume change.

213 FUROSEMIDE STIMULATION OF THE RENIN-ANGIOTENSIN-VASOPRESSIN SYSTEM IN THE FETAL LAMB

We have shown previously that furosemide (FU) stimulates secretion of renin and arginine vasopressin (AVP) in the newborn lamb. Nephrectomy abolished the plasma renin activity (PRA) and AVP responses, and saralasin acetate, an angiotensin II inhibitor, blocked the AVP response, suggesting that the AVP stimulation by FU is mediated by angiotensin II. The present study was designed to assess whether FU influences the renin-angiotensin-AVP system in the fetus. Seven fetal lambs 120-142 days gestational age and 6 newborn lambs were studied (term = 145 days). FU (2 mg/kg estimated fetal weight) was infused over 1-2 min. Blood samples were drawn at 8, 20, 35, and 65 min. post infusion. In the fetal lambs the M and SEM PRA (ng/ml/hr) increased from 10.0 ± 3.3 to 12.5 ± 3.6 at 8 min. (p<.05), 16.8 ± 4.4 at 20 min. (p<.01) and 24.3 ± 5.1 (p<.01) at 35 min. post FU. In the newborn lambs PRA increased from a base of 16.7 ± 5 to 41.8 ± 6 (p<.01) 20 min post FU and remained high. The M and SEM plasma AVP (μU/ml) increased 30 min. after PRA from a baseline of 2.1 ± 0.5 to 5.7 ± 1.8 (p<.05) 35 min. and 6.1 ± 1.4 (p<.05) 65 min. post FU in the fetal lambs. In the newborns AVP increased from a baseline of 2.7 ± 0.5 to 9.9 ± 3.4 (p<.05) 35 min and 13.9 ± 4.4 (p<.05) 65 min post FU Plasma sodium, Hct, and osmolality did not change. Conclusions: 1) FU stimulates the renin-angiotensin-AVP system in the fetal lamb after 120 days gestation; 2) fetal responsiveness is less than in the newborn animal.