The role of neuropeptide Y in the ovine fetal cardiovascular response to reduced oxygenation

Abstract

This study investigated the role of neuropeptide Y (NPY) in mediating cardiovascular responses to reduced oxygenation in the late gestation ovine fetus by: (1) comparing the effects on the cardiovascular system of an exogenous infusion of NPY with those elicited by moderate or severe reductions in fetal oxygenation; and (2) determining the effect of fetal I.V. treatment with a selective NPY-Y(1) receptor antagonist on the fetal cardiovascular responses to acute moderate hypoxaemia. Under general anaesthesia, 14 sheep fetuses (0.8-0.9 of gestation) were surgically prepared with vascular and amniotic catheters. In 5 of these fetuses, a Transonic flow probe was also implanted around a femoral artery. Following at least 5 days of recovery, one group of fetuses (n = 9) was subjected to a 30 min treatment period with exogenous NPY (17 microg kg(-1) bolus plus 0.85 microg kg(-1) min(-1) infusion). In this group, fetal blood pressure and heart rate were monitored continuously and the distribution of the fetal combined ventricular output was assessed via injection of radiolabelled microspheres before and during treatment. The second group of fetuses instrumented with the femoral flow probe (n = 5) were subjected to a 3 h experiment consisting of 1 h of normoxia, 1 h of hypoxaemia, and 1 h of recovery during a slow I.V. infusion of vehicle. One or two days later, the acute hypoxaemia protocol was repeated during fetal I.V. treatment with a selective NPY-Y(1) receptor antagonist (50 microg kg(-1) bolus + 1.5 microg kg(-1) min(-1) infusion). In these fetuses, fetal arterial blood pressure, heart rate and femoral vascular resistance were recorded continuously. The results show that fetal treatment with exogenous NPY mimics the fetal cardiovascular responses to asphyxia, and that treatment of the sheep fetus with a selective NPY-Y(1) receptor antagonist does not affect the fetal cardiovascular response to acute moderate hypoxaemia. These results support a greater role for NPY in mediating the fetal cardiovascular responses to acute asphyxia than to acute moderate hypoxaemia.

Figure 1. Efficacy of the NPY-Y₁ receptor antagonist

Values are means ± s.e.m. for absolute values of femoral blood flow and femoral vascular resistance responses to increasing i.a. bolus doses of NPY in fetuses before (○) and after (•) i.v. pre-treatment with the NPY-Y₁ receptor antagonist for 5 min (25 µg kg⁻¹bolus + 0.75µg kg⁻¹ min⁻¹ infusion). Antagonist treatment was stopped 5 min after each dose and cardiovascular variables were allowed to return to baseline before starting the antagonist treatment prior to the next highest dose of exogenous NPY. Significant differences (P < 0.05): * differences by post-hoc analysis indicating a significant main effect of time compared with baseline; † differences by post-hoc analysis indicating a significant main effect of treatment (two way ANOVA + Tukey test).

Figure 3. Cardiac output and regional blood flows (A) and regional vascular resistances (B) during NPY infusion

Values are expressed as the mean percentage change ± s.e.m. from baseline prior to treatment (0 min) and at +10 and +30 min following the onset of fetal intravenous treatment with NPY (17 µg kg⁻¹bolus + 0.85µg kg⁻¹ min⁻¹ infusion). Significant differences are: *P < 0.05, differences by post-hoc analysis indicating a significant main effect of time (one-way repeated measures ANOVA + Student-Newman-Keuls test).

Figure 4. Blood flow (A) and vascular resistances (B) in peripheral circulations during NPY infusion

Values are expressed as the mean percentage change ± s.e.m. from baseline prior to treatment (0 min) and at +10 and +30 min following the onset of fetal intravenous treatment with (17 µg kg⁻¹bolus + 0.85µg kg⁻¹ min⁻¹ infusion). Significant differences are: *P < 0.05, differences by post-hoc analysis indicating a significant main effect of time (one-way repeated measures ANOVA + Student-Newman-Keuls test).

Figure 5. Fetal cardiovascular responses to acute hypoxaemia with and without the selective NPY-Y₁ receptor antagonist

Values are means ± s.e.m. for cardiovascular variables of five fetuses calculated every minute of the experimental protocol (A) and the statistical summary for these cardiovascular data (B). Fetuses were infused with either saline (open horizontal bar) or with the NPY-Y₁ receptor antagonist (filled horizontal bar) dissolved in saline. Treatment started 20 min prior to the onset of acute hypoxaemia (box) and ran continuously until the end of the hypoxaemic challenge. 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), early hypoxaemia (first 15 min of hypoxaemia), late hypoxaemia (remaining 45 min of hypoxaemia), early recovery (first 15 min of recovery), and late recovery (remaining 45 min of recovery). Significant differences were assessed using a two-way ANOVA followed by Tukey or Student-Newman-Keuls tests. Siginificant differences are: *P < 0.05, differences by post-hoc analysis indicating a significant main effect of time. There were no significant differences between groups.

Figure 2. Fetal arterial blood pressure and heart rate responses to NPY infusion

Values are means ± s.e.m. at 5, 10, 15, 30, 45, 60, 75 and 90 min after the onset of fetal intravenous treatment with NPY (17 µg kg⁻¹bolus + 0.85µg kg⁻¹ min⁻¹ infusion). Significant differences are: *P < 0.05, differences by post-hoc analysis indicating a significant main effect of time (one-way repeated measures ANOVA plus Student-Newman-Keuls test).