Blood volume restitution after hemorrhage was investigated in lambs in the first week of life. Two groups of nonsplenectomized lambs were bled 10 and 20% of their blood volume at 2%/min while being suspended horizontally in a sling with their legs dependent, and a third group was bled 20% while lying down. Blood pressure fell 8% in the lambs bled both 10 and 20% while lying down and 44% in those bled 20% while being suspended. Blood volume was completely restored in all three groups by 5 h after the hemorrhage, the rate of restitution being equal among the groups. The initial phase of restitution was slower when the lambs were bled while lying down. Vasopressin levels were increased only in the lambs bled 20% of their blood volume while being suspended. Plasma renin activity increased similarly in all groups. Hemorrhage increased plasma glucose but did not change plasma protein and serum osmolality. We conclude that lambs bled up to 20% of blood volume restitute relatively quickly at a rate independent of the volume shed. The position of the animal affects the degree of hypotension, the levels of vasopressin, and the rate of the initial phase of volume restoration.
ABSTRACT. The hypothesis that central a-1 adrenoceptors are inhibitory to the hypotension-induced secretion of vasopressin was tested by subjecting lambs that were instrumented for a long term to varying degrees of hypotension after intracerebroventricular injections of prazosin or placebo. Eight lambs in the 1st wk of life treated with intracerebroventricular injections of placebo had their mean arterial blood pressures decreased 14 and 21% by i.v. infusion of nitroprusside. Arginine vasopressin levels rose to 7.3 1 2.4 pmol/L only with the greater degree of hypotension. When the lambs were treated with intracerebroventricular injections of 1 pg/kg of prazosin, the blood pressures were decreased 13 and 23%, and the vasopressin levels were 15.4 2 16.6 and 27.5 2 20.3 pmol/L, respectively. A relationship was shown between the degree of hypotension and the plasma arginine vasopressin levels with both the placebo and prazosin, the slope being much steeper for the prazosin treatment (-1.11) than for the placebo treatment (-0.31). Plasma renin activity was increased a similar amount in both groups, and there was no change in plasma cortisol levels. We conclude that a-1 adrenoceptors in the brain are inhibitory to the secretion of arginine vasopressin. These results differ from observationsh adult rats and dogs and may be accounted for by developmental or species differences. (Pediatr Res 30: 50-54,1991) Abbreviations AVP, arginine vasopressin CSF, cerebrospinal fluid HR, heart rate ICV, intracerebroventricular MABP, mean arterial blood pressure PE, phenylephrine PRA, plasma renin activity AVP has been shown to have a role in fetal and neonatal cardiovascular homeostasis as a "stress" hormone (1-3). Neonatal and fetal AVP secretion may be induced by hypoxia, hypotension, and acidosis. Levels of this hormone are particularly high around the time of delivery, especially in the case of fetal asphyxia (3).Although there is an increasing body of knowledge concerning the control of AVP secretion in adult animals, little is known about the neonate. CNS a-1 and a-2 adrenoceptors are involved in the regulation of AVP secretion in adult animals (4-6). We have previously reported that lamb fetuses treated with i.v. prazosin, an a-1 adrenoceptor blocker, and subsequently subjected to hypotensive hemorrhage had excessive levels of AVP when compared with similar hemorrhagic hypotension without prazosin (7). This observation led us to hypothesize that hypothalamic a-1 receptors exert an inhibitory influence on the secretion of AVP in response to hypotension in the near-term lamb fetus. In the neonatal lamb, AVP levels after hemorrhage were similar with prazosin and with vehicle, but the relative decrease in blood pressure was less in the prazosin-treated lambs and may have been subthreshold for the secretion of AVP (2). To prove our hypothesis, we designed an experiment in which CNS a-1 adrenoceptor blockade was achieved in the newborn lamb by ICV injection, rather than i.v. injection of prazosin. In so doing, we hoped to avoid t...
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