Plasma Vasopressin, Renin, and Catecholamines during Nitroprusside-Induced Maternal and Fetal Hypotension in Sheep

Abstract

ABSTRACT. The release of vasopressin, renin, and catecholamines by the fetus during either maternal or fetal hypotension was examined in chronically catheterized fetal lambs. Nitroprusside was infused intravenously for 1 h into seven pregnant ewes (maternal hypotension) or nine fetal lambs (fetal hypotension); the rates were adjusted to achieve a 15 to 30% decrease in mean blood pressure. During maternal hypotension, mean f S E vasopressin in maternal plasma increased from 1.2 f 0.2 pg.ml-' to 208 f 153 pg.ml-' and plasma renin activity increased from 1.5 f 0.3 ng.ml-'. h-' to 6.6 f 1.6 ng.ml-'. h-'. Fetal vasopressin and plasma renin activity also increased during the same interval from 1.1 f 0.3 to 16.9 +. 7.5 pg.ml-' and 3.7 f 1.1 to 10.5 f 2.85 ng.ml-'.h-', respectively; but no changes were observed in fetal blood pressure, heart rate, or acid base status. During fetal hypotension, mean vasopressin in fetal plasma increased from 4.3 f 3.4 pg. ml-' to 1054 f 772 pg.ml-', plasma renin activity increased from 5.7 f 2.2 ng . ml-' to 22.2 f 7.1 n g ml-' . h-', and total catecholamines from 174 f 58 pg . ml-' to 810 f 416 p g ml-'. There was no change in fetal heart rate, acid base status, osmolality, or sodium concentration. The fetus became and remained hypertensive for at least 1 h after the end of infusion. This prolonged hypertension was associated with elevated levels of vasopressin and plasma renin activity. Peak vasopressin levels were proportional to the total nitroprusside dose in both the ewe and fetus (maternal r = 0.796, fetus r = 0.870). These experiments indicate that as in the adult, vasopressin and the reninangiotensin system in the fetus are stimulated by euvolemic hypotension and may play an important role in the maintenance of fetal blood pressure during an hypotensive challenge. We speculate that the prolonged rebound hypertension represents immaturity of the fetus to fine tune the regulation, release, or metabolism of vasoactive mediators. More likely, the hypertension and elevated hormone levels represent homeostatic mechanisms to combat the nitroprusside-induced diminished cardiac output. Lastly, fetal release of vasopressin, renin-angiotensin, and norepinephrine during maternal hypotension suggests that they are sensitive indicators of early fetal compromise. 7The fetus may be compromised by episodes of maternal hypotension (postural changes, regional anesthesia, placenta previa, etc.) or fetal hypotension (cord occlusion, fetal blood loss secondary to abruptio placentae, etc.) (1). In adults of several species, hypotension is known to stimulate several vasoactive systems which act to restore blood pressure towards normal values (2-8).The ability of the fetus to similarly withstand hypotension may have a significant influence on subsequent short-term and longterm outcome. The present studies were undertaken to determine the fetal response to both maternal and fetal hypotension with respect to fetal release of vasopressin, renin, and catecholamines in the maintenance of fetal blood pressur...

Plasma vasopressin, renin and catecholamines during nitroprusside-induced hypotension in the newborn lamb

“…In the present experiments on the newborn lamb, significantly elevated levels of renin and catecholamines were found during this period. The duration of hypertension (post nitroprusside infusion) in the newborn lamb was less than that observed in the fetal lamb [40] but longer than in the ewe [39]. These elevated values thirty minutes after the end of hypotension could be due to low rate of elimination as a result of prolonged effect of nitroprusside on mesenteric and renal blood flows [20,30,38].…”

“…A total of 25 ml of blood (6ml -kg" 1 ) per study was withdrawn and acutely replaced with normal saline. Blood samples for assay of vasopressin and renin activity were collected in chilled glass tubes containing EDTA, catecholamine levels in chillled tubes containing EGTA and glutathione, and pH, blood gases, sodium and osmolality determination in a heparinized syringe as described previously [40]. The volume of blood withdrawn over the two hour period of these experiments has been shown not to affect vasopressin levels [36].…”

“…The recovery of the fetus from nitroprusside induced hypotension, is associated with prolonged high levels of vasoactive mediators and a rebound hypertension that is longer than that observed in the adults [39,40]. The present experiments were designed to investigate a possible mechanism for this difference by examining the simultaneous response of the neurohypophyseal, sympathetic and renin-angiotensin systems and the pattern of recovery of the newborn from nitroprusside induced hypotension in comparison to the fetus and adult.…”

“…The fetus and neonate respond to experimantal hypovolemia or hypotension with increased release of vasoactive mediators in a similar fashion to the adult [10,16,21,22,24,39,40]. The recovery of the fetus from nitroprusside induced hypotension, is associated with prolonged high levels of vasoactive mediators and a rebound hypertension that is longer than that observed in the adults [39,40].…”