Enhanced nitric oxide activity offsets peripheral vasoconstriction during acute hypoxaemia via chemoreflex and adrenomedullary actions in the sheep fetus

Abstract

We tested the hypothesis that enhanced nitric oxide (NO) opposes fetal peripheral vasoconstrictor responses to acute hypoxaemia via actions involving the carotid chemoreflex and the adrenal medulla. The hypothesis was tested in the late gestation ovine fetus using a novel NO clamp technique, which involves fetal combined treatment with the NO synthase inhibitor, L-NAME, and the NO donor, sodium nitroprusside. In contrast to treatment with L-NAME alone, combined fetal treatment with L-NAME and nitroprusside prevents generalized vasoconstriction and pronounced hypertension, not only maintaining basal cardiovascular function, but also permitting blockade of the de novo synthesis of NO during hypoxaemia while compensating for the tonic production of the gas. Under general anaesthesia, seven sheep fetuses were surgically prepared with catheters and a femoral Transonic flow probe. Five days after surgery, fetuses were subjected to a 3 h protocol: 1 h normoxia, 1 h hypoxaemia and 1 h recovery. Fetal hypoxaemia was induced during either fetal infusion with saline or treatment with the NO clamp. During saline infusion, fetuses responded to hypoxaemia with transient bradycardia, femoral vasoconstriction and increases in plasma noradrenaline and adrenaline. During fetal treatment with the NO clamp, bradycardia persisted and there were greater peripheral vasoconstrictor and catecholaminergic responses to hypoxaemia. Further analysis showed that NO clamp treatment enhanced the chemoreflex component of the fetal cardiovascular defence to acute hypoxaemia. These data support the hypothesis that enhanced NO synthesis during acute hypoxaemia offsets fetal peripheral vasoconstrictor responses to hypoxaemia via chemoreflex and adrenomedullary actions.

Figure 1. Fetal metabolic responses to acute hypoxaemia

Values are means ± s.e.m. at 15 (N15) and 45 (N45) min of normoxia, at 5 (H5), 15 (H15) and 45 (H45) min of hypoxaemia, and at 15 (R15) and 45 (R45) min of recovery of arterial blood concentrations of glucose and lactate for fetuses exposed to 1 h of hypoxaemia during saline infusion (○; n = 6) or during treatment with the NO clamp (•; n = 5). Box represents period of hypoxaemia. * Significant differences (P < 0.05) by post hoc analysis indicating a significant main effect of time compared with normoxia; † differences by post hoc analysis indicating a significant main effect of treatment (two-way ANOVA + Tukey's test).

Figure 2. Fetal cardiovascular responses to acute hypoxaemia

Values are (A) means ± s.e.m. for change from normoxic baseline calculated every minute during the experimental protocol and (B) the statistical summary of these changes during fetal infusion with saline (○; n = 6) and during fetal treatment with the NO clamp (•; n = 5). Values for the statistical summary represent the means ± s.e.m. for cardiovascular variables calculated for the following time periods: normoxia (1 h of baseline, N), early hypoxaemia (first 15 min of hypoxaemia, H15), late hypoxaemia (remaining 45 min of hypoxaemia, H45), early recovery (first 15 min of recovery, R15) and late recovery (remaining 45 min of recovery, R45). * Significant differences (P < 0.05) by post hoc analysis indicating a significant main effect of time compared with normoxia; † differences by post hoc analysis indicating a significant main effect of treatment (two-way repeated measures ANOVA + Tukey's test).

Figure 3. Fetal functional chemoreflex analysis

A, correlation between absolute change from mean normoxic baseline in fetal P_a,O2 and fetal femoral vascular resistance (FVR) or fetal heart rate (FHR) within the first 15 min of the acute hypoxaemia protocol during saline infusion (○; n = 6) and during fetal treatment with the NO clamp (•; n = 5). Values are means ± s.e.m. at 15 and 45 min of normoxia and at 5 and 15 min of hypoxaemia. B, histograms represent the means ± s.e.m. of the individual slopes. † Significant differences (P < 0.05) indicating a significant main effect of treatment (Student's t test for paired data).

Figure 4. Fetal plasma catecholamines during acute hypoxaemia

Arterial plasma concentrations of noradrenaline and adrenaline in fetuses exposed to 1 h of hypoxaemia during saline infusion (○; n = 5) or during treatment with the NO clamp (•; n = 5). Values are means ± s.e.m. at 15 (N15) and 45 (N45) min of normoxia, 15 (H15) and 45 (H45) min of hypoxaemia, and 45 (R45) min of recovery. Box represents period of hypoxaemia. * Significant differences (P < 0.05) by post hoc analysis indicating a significant main effect of time compared with normoxic baseline; † differences by post hoc analysis indicating a significant main effect of treatment (two-way repeated measures ANOVA + Tukey's test).