Circulating levels of nitric oxide and vascular endothelial growth factor throughout ovine pregnancy

Abstract

Nitric oxide (NO) production has been shown to increase uterine blood flow and be elevated in ewes carrying multiple fetuses during late gestation. Vascular endothelial growth factor (VEGF) has been reported to increase eNOS expression and NO production in endothelial cell cultures. As angiogenesis and vasodilatation of the uterine and placental vascular beds are important at all stages of pregnancy, it is important to understand how VEGF and NO change throughout gestation in circulation. Therefore the objectives of the current study were to evaluate the systemic levels of VEGF and NO metabolite (NOx) throughout ovine gestation and to determine if there was an effect of sheep carrying singletons versus multiple fetuses. NOx and VEGF concentrations were analysed in systemic blood from pregnant ewes starting on day 27 of pregnancy and at multiple intermittent intervals throughout pregnancy until term. Blood samples from non-pregnant and postpartum ewes were also analysed. NOx concentrations in maternal blood expressed a biphasic pattern with NOx concentrations increasing (P < 0.05) over non-pregnant values on days 40-69 of gestation, returning to non-pregnant concentrations from days 70-100, and again increasing (P < 0.05) until term. Postpartum NOx concentrations were similar to non-pregnant values. While ewes carrying multiple fetuses had increased (P < 0.05) concentrations of NOx on days 60-69, there were no differences in NOx concentrations in ewes carrying singletons or multiples from day 70-99 of gestation. Starting on day 100 and continuing throughout the duration of pregnancy, ewes carrying multiple fetuses had increased (P < 0.05) concentrations of NOx compared to ewes carrying singletons. Concentrations of VEGF showed a different pattern from NOx with VEGF decreasing (P < 0.05) from day 20-69 of pregnancy compared to non-pregnant ewes. Concentrations of VEGF returned to non-pregnant levels by day 70 and remained constant throughout the duration of pregnancy. On days 20-39, ewes carrying singleton fetuses had an increased VEGF concentration (P < 0.05), whereas ewes carrying multiple fetuses demonstrated elevated VEGF concentrations from day 90-109 of gestation. Concentrations from non-pregnant and postpartum ewes did not differ (P > 0.1). While there was no effect of fetal number on circulating VEGF concentrations, circulating levels of NOx were substantially increased (P < 0.05) in ewes carrying multiple fetuses, compared to ewes carrying singletons. The pattern of the rise in NOx in circulating plasma was not directly associated with changes in VEGF regardless of the number of fetuses present. However, circulating concentrations of NOx and VEGF appear to, respectively, follow patterns of uterine blood flow and angiogenesis of the uterus. An understanding of these circulatory patterns may have important implications for fetal size, birth weight and fetal/developmental origins of adult disease.

Figure 1. The effect of single or multiple pregnancy on concentrations of nitric oxide metabolite (NOx) throughout ovine pregnancy

*Mean ± s.e.m. differs from non-pregnant value; P < 0.05. +Mean ± s.e.m. single differs from multiple pregnancy; P < 0.05.

Figure 2. The effect of single or multiple pregnancy on the concentrations of VEGF throughout ovine pregnancy

*Mean ± s.e.m. differs from non-pregnant value; P < 0.05. +Mean ± s.e.m. single differs from multiple pregnancy; P < 0.05.

Figure 3. Effect of fetal numbers on circulating maternal NOx and VEGF concentrations during early (days 20–59), middle (days 60–99) and late (day 100–term) gestation

Triplets += 3,4, or 5 fetuses combined. ^a,b,cMeans ± s.e.m. with different letter superscripts within a period differ significantly; P < 0.05.