Melatonin modulates the fetal cardiovascular defense response to acute hypoxia

Abstract

Experimental studies in animal models supporting protective effects on the fetus of melatonin in adverse pregnancy have prompted clinical trials in human pregnancy complicated by fetal growth restriction. However, the effects of melatonin on the fetal defense to acute hypoxia, such as that which may occur during labor, remain unknown. This translational study tested the hypothesis, in vivo, that melatonin modulates the fetal cardiometabolic defense responses to acute hypoxia in chronically instrumented late gestation fetal sheep via alterations in fetal nitric oxide (NO) bioavailability. Under anesthesia, 6 fetal sheep at 0.85 gestation were instrumented with vascular catheters and a Transonic flow probe around a femoral artery. Five days later, fetuses were exposed to acute hypoxia with or without melatonin treatment. Fetal blood was taken to determine blood gas and metabolic status and plasma catecholamine concentrations. Hypoxia during melatonin treatment was repeated during in vivo NO blockade with the NO clamp. This technique permits blockade of de novo synthesis of NO while compensating for the tonic production of the gas, thereby maintaining basal cardiovascular function. Melatonin suppressed the redistribution of blood flow away from peripheral circulations and the glycemic and plasma catecholamine responses to acute hypoxia. These are important components of the fetal brain sparing response to acute hypoxia. The effects of melatonin involved NO-dependent mechanisms as the responses were reverted by fetal treatment with the NO clamp. Melatonin modulates the in vivo fetal cardiometabolic responses to acute hypoxia by increasing NO bioavailability.

Keywords: cardiovascular; hypoxia; melatonin; nitric oxide; oxidative stress.

Fig. 1

Diagrammatic representation of the experimental protocol. The experimental protocol consisted of a 3-hr period divided into the following: 1.5-hr normoxia, 0.5-hr hypoxia (black bar), and 1-hr recovery, during saline vehicle infusion (n = 6), treatment with melatonin low (0.05 ± 0.01 μg/kg/min; n = 6), treatment with melatonin high (0.5 ± 0.1 μg/kg/min; n = 6), or treatment with melatonin high during nitric oxide (NO) blockade with the NO clamp (n = 6; gray bar). Arrows represent times at which arterial blood samples were collected.

Fig. 2

Fetal plasma concentrations of melatonin. Values represent the mean ± S.E.M. for plasma concentrations of melatonin at 0 (N0) and 75 (N75) min of normoxia, at 15 (H15) and 30 (H30) min of hypoxia, and at 30 (R30) and 60 (R60) min of recovery for fetuses exposed to 0.5-hr hypoxia (dashed box) during saline vehicle infusion (○; n = 6), treatment with melatonin low (0.05 ± 0.01 μg/kg/min; ; n = 6), treatment with melatonin high (0.5 ± 0.1 μg/kg/min; •; n = 6), or treatment with melatonin high during nitric oxide (NO) blockade with the NO clamp (□; n = 6). Significant differences: ^aP < 0.05, versus time period N0; ^bP < 0.05, versus saline vehicle infusion; ^cP < 0.05, melatonin low versus melatonin high (two-way RM ANOVA with post hoc Tukey test).

Fig. 3

Fetal metabolic responses to acute hypoxia. Values represent the mean ± S.E.M. for concentrations of blood glucose and lactate at 0 (N0) and 75 (N75) min of normoxia, at 5 (H5), 15 (H15), and 30 (H30) min of hypoxia, and at 30 (R30) and 60 (R60) min of recovery for fetuses exposed to 0.5-hr hypoxia (dashed box) during saline vehicle infusion (○; n = 6), treatment with melatonin low (0.05 ± 0.01 μg/kg/min; ; n = 6), treatment with melatonin high (0.5 ± 0.1 μg/kg/min; •; n = 6), or treatment with melatonin high during nitric oxide (NO) blockade with the NO clamp (□; n = 6). Significant differences: ^aP < 0.05, versus time period N0; ^bP < 0.05, versus saline vehicle infusion; ^cP < 0.05, melatonin low versus melatonin high (two-way RM ANOVA with post hoc Tukey test).

Fig. 4

Fetal cardiovascular responses to acute hypoxia. Values represent the mean ± S.E.M. calculated every minute for arterial blood pressure, heart rate, femoral blood flow, and femoral vascular resistance during 1.5-hr of normoxia, 0.5-hr of hypoxia (dashed box), and 1-hr of recovery for fetuses during saline vehicle infusion (n = 6), treatment with melatonin low (0.05 ± 0.01 μg/kg/min; n = 6), treatment with melatonin high (0.5 ± 0.1 μg/kg/min; n = 6), or treatment with melatonin high during nitric oxide (NO) blockade with the NO clamp (n = 6).

Fig. 5

Statistical summary of the fetal cardiovascular responses to acute hypoxia. Values for the statistical summary of the cardiovascular responses represent the mean ± S.E.M. for the area under the curve over every 30 min during normoxia (N), hypoxia (H), and recovery (R) for fetuses during saline vehicle infusion (○; n = 6), treatment with melatonin low (0.05 ± 0.01 μg/kg/min; ; n = 6), treatment with melatonin high (0.5 ± 0.1 μg/kg/min; •; n = 6), or treatment with melatonin high during nitric oxide (NO) blockade with the NO clamp (□; n = 6). Significant differences: ^aP < 0.05, versus time period N1; ^bP < 0.05, versus saline vehicle infusion; ^cP < 0.05, melatonin low versus melatonin high (two-way RM ANOVA with post hoc Tukey test).

Fig. 6

Fetal plasma catecholamine concentrations in response to acute hypoxia. Values represent the mean ± S.E.M. for plasma concentrations of epinephrine and norepinephine at 0 (N0) and 75 (N75) min of normoxia, at 15 (H15) and 30 (H30) min of hypoxia, and at 60 (R60) min of recovery for fetuses exposed to 0.5-hr hypoxia (dashed box) during saline vehicle infusion (○; n = 6), treatment with melatonin low (0.05 ± 0.01 μg/kg/min; ; n = 6), treatment with melatonin high (0.5 ± 0.1 μg/kg/min; •; n = 6), or treatment with melatonin high during nitric oxide (NO) blockade with the NO clamp (□; n = 6). Significant differences: ^aP < 0.05, versus time period N0; ^bP < 0.05, versus saline vehicle infusion; ^cP < 0.05, melatonin low versus melatonin high (two-way RM ANOVA with post hoc Tukey test).