Is uterine blood flow controlled locally or systemically in the pregnant rabbit?

Developmental and reproductive physiology of small mammals at high altitude: challenges and evolutionary innovations

High-altitude environments, characterized by low oxygen levels and low ambient temperatures, have been repeatedly colonized by small altricial mammals. These species inhabit mountainous regions year-round, enduring chronic cold and hypoxia. The adaptations that allow small mammals to thrive at altitude have been well studied in non-reproducing adults; however, our knowledge of adaptations specific to earlier life stages and reproductive females is extremely limited. In lowland natives, chronic hypoxia during gestation affects maternal physiology and placental function, ultimately limiting fetal growth. During post-natal development, hypoxia and cold further limit growth both directly by acting on neonatal physiology and indirectly via impacts on maternal milk production and care. Although lowland natives can survive brief sojourns to even extreme high altitude as adults, reproductive success in these environments is very low, and lowland young rarely survive to sexual maturity in chronic cold and hypoxia. Here, we review the limits to maternal and offspring physiology - both pre-natal and post-natal - that highland-adapted species have overcome, with a focus on recent studies on high-altitude populations of the North American deer mouse (Peromyscus maniculatus). We conclude that a combination of maternal and developmental adaptations were likely to have been critical steps in the evolutionary history of high-altitude native mammals.

Hemodynamic, vascular, and reproductive impact of FMS-like tyrosine kinase 1 (FLT1) blockade on the uteroplacental circulation during normal mouse pregnancy

To investigate the role of FMS-like tyrosine kinase 1 (FLT1, also known as VEGFR1) signaling during pregnancy, mice were injected with anti-FLT1 neutralizing antibody (Ab) beginning on Gestational Day 8 or 12 and every other day thereafter until Day 18; vehicle-only injected mice served as controls. Uterine artery blood flow was measured with ultrasound on Days 13 and 18, and morphometric measurements of the uterine arcade were carried out on Day 19 to provide a measure of gestational vascular remodeling; reproductive performance was evaluated by determining litter size, resorption rates, and pup and placental weights. Ab injections beginning on Day 8 or Day 12 resulted in significant reductions of uterine artery peak systolic and diastolic flows at Days 13 and 18. In addition, normal reproductive function was compromised, as evidenced by a significant reduction in average number of viable pups along with enhanced resorption rates. Reproductive performance was also significantly compromised in this group, although less severely. There was no evidence of a reduction in main uterine artery diameters, though arterial distensibility was reduced, and the diameter of the main uterine vein was significantly smaller in the Ab-injected mice. Significant reductions in main uterine artery and segmental artery length were also noted. Placental and pup weights were similar in all the groups. FLT1 inhibition during murine pregnancy impaired blood flow to the fetal-placental unit, compromised several indices of vascular remodeling, reduced fecundity, and increased fetal reabsorptions. The effects of FLT1 inhibition are most pronounced when targeted during early pregnancy.

Maternal uterine vascular remodeling during pregnancy

Sufficient uteroplacental blood flow is essential for normal pregnancy outcome and is accomplished by the coordinated growth and remodeling of the entire uterine circulation, as well as the creation of a new fetal vascular organ: the placenta. The process of remodeling involves a number of cellular processes, including hyperplasia and hypertrophy, rearrangement of existing elements, and changes in extracellular matrix. In this review, we provide information on uterine blood flow increases during pregnancy, the influence of placentation type on the distribution of uterine vascular resistance, consideration of the patterns, nature, and extent of maternal uterine vascular remodeling during pregnancy, and what is known about the underlying cellular mechanisms.

Predominance of local over systemic factors in uterine arterial remodeling during pregnancy

This study used a rat model in which pregnancy was surgically restricted to one uterine horn to differentiate between local (fetoplacental) and systemic (endocrine) influences on uterine vascular remodeling during pregnancy. Sprague-Dawley rats with single-horn pregnancies were studied on day 20/22 of gestation and compared to age-matched nonpregnant and late-pregnant controls. The morphology (axial length, lumen diameter, wall thickness) of the main uterine artery and of smaller arcuate vessels showed that vascular growth was dramatically increased in the pregnant versus nonpregnant horn, (P < .05). Arcuate artery wall thickness increased in the nonpregnant horn (compared to nonpregnant controls, P < .05), suggesting a limited role for systemic hormonal influences on vascular remodeling. Notably, animals with only one functional horn compensated by increasing the average number of implantation sites per horn from 7.6 to 12.9, thereby maintaining essentially normal litter size without any reduction in fetal or placental weights. These results demonstrate unequivocally that local rather than systemic influences play the dominant role in uterine vascular gestational remodeling of both large and small uterine arteries, and reveal a significant adaptive process that maintains relatively normal fecundity in spite of surgical restriction of normal bilateral pregnancy.

Uterine artery blood flow and renal sympathetic nerve activity during exercise in rabbit pregnancy

The uterine artery blood flow (UtBF) and renal sympathetic nerve activity (SNA) responses to treadmill exercise were evaluated in 12 nonpregnant (NP) and 17 term pregnant (P) rabbits. UtBF was monitored continuously with a Transonic flowprobe. Rabbits underwent three exercise trials (5-min duration) that varied in absolute workload. The rise in renal SNA with exercise was intensity related. Pregnancy did not affect the average steady-state renal SNA response expressed relative to maximum activity (P 24 +/- 1% vs. NP 23 +/- 2% of maximum smoke-elicited activity) and increased the average renal SNA response expressed relative to resting activity (P +155 +/- 19% vs. NP +84 +/- 23% from rest, P = 0.03) At rest, UtBF (P 13 +/- 3 vs. NP 1.9 +/- 0.3 ml/min) and uterine artery conductance (UtC; P 22 +/- 5 vs. NP 2.8 +/- 0.5 ml. min-1.mmHg-1 x 10-2) were elevated in the P rabbits. The average exercise-related decreases in UtBF (P -16 +/- 4% vs. NP -48 +/- 4%) and UtC (P -27 +/- 4% vs. NP -54 +/- 4%) were attenuated in the P rabbits. Pregnancy does not impair the ability to raise renal SNA but attenuates the uterine artery constrictor response to moderate to heavy dynamic exercise in rabbits. Under normal conditions, the pregnant uterine circulatory bed may be relatively protected from exercise-related redistribution of blood flow.

Structural remodeling of rat uterine veins in pregnancy

OBJECTIVE

This study characterized the cellular remodeling that contributes to uterine venous growth during gestation and evaluated the effect of pregnancy on the mechanical properties of the uterine vein.

STUDY DESIGN

Diameter and distensibility were calculated in pressurized uterine veins from virgin and pregnant (19-20 days of gestation) rats; the rates of cellular division (endothelial, vascular smooth muscle) were quantified with an intraperitoneal bromodeoxyuridine injection and immunohistochemistry. Elastin content, adrenergic nerve density, and wall thickness were determined in fixed uterine veins and quantified with an imaging program.

RESULTS

There was a statistically significant increase in the diameter and mitotic indices (endothelial, vascular smooth muscle) in uterine veins from late pregnant versus nonpregnant animals. Adrenergic nerve density was diminished significantly during gestation. Venous distensibility increased during pregnancy, with an associated reduction in elastin content.

CONCLUSION

Pregnancy results in coordinated and multifaceted structural remodeling of uterine veins with subsequent changes in caliber, mechanical properties, rates of cellular division, adrenergic innervation, and matrix composition.