β₁-Adrenergic Receptors Maintain Fetal Heart Rate and Survival

Abstract

β-Adrenergic receptor (βAR) activation has been shown to maintain heart rate during hypoxia and to rescue the fetus from the fetal lethality that occurs in the absence of norepinephrine. This study examines whether the same subtype of βAR is responsible for survival and heart rate regulation. It also investigates which βARs are located on the early fetal heart and whether they can be directly activated during hypoxia. Cultured E12.5 mouse fetuses were treated with subtype-specific βAR antagonists to pharmacologically block βARs during a hypoxic insult. Hypoxia alone reduced heart rate by 35–40% compared to prehypoxic levels. During hypoxia, heart rate was further reduced by 31% in the presence of a β₁AR antagonist, CGP20712A, at 100 nM, but not with a β₂ (ICI118551)- or a β₃ (SR59230A)-specific antagonist at 100 nM. Survival in utero was also mediated by β₁ARs. A β₁ partial agonist, xamoterol, rescued 74% of catecholamine-deficient (tyrosine-hydroxylase-null) pups to birth, a survival rate equivalent to that with a nonspecific βAR agonist, isoproterenol (87%). Receptor autoradiography showed that β₁ARs were only found on the mouse heart at E12.5, while β₂ARs were localized to the liver and vasculature. To determine if the response to hypoxia was intrinsic to the heart, isolated fetal hearts were incubated under hypoxic conditions in the presence of a βAR agonist. Heart rate was reduced to 25–30% by hypoxia alone, but was restored to 63% of prehypoxic levels with 100 nM isoproterenol. Restoration was completely prevented if β₁ARs were blocked with CGP20712A at 300 nM, a concentration that blocks β₁ARs, but not β₂- or β₃ARs. Our results demonstrate that β₁ARs are located on the heart of early fetal mice and that β₁AR stimulation maintains fetal heart rate during hypoxia and mediates survival in vivo.

High oxygen prevents fetal lethality due to lack of catecholamines

“…In the early fetus, hypoxia slows heart rate to a greater extent in catecholamine-deficient fetuses than in wild-type (wt) siblings. This reduction is completely reversed by ␤-AR agonists (6,41), consistent with the view that released NE increases heart rate during hypoxia in wt animals. Because cardiac output in the fetus is primarily dependent on heart rate (reviewed in Ref.…”

“…Hypoxic wt fetuses are bradycardic (6,41) and show signs of congestive heart failure, myocardial thinning, and epicardial detachment (44).…”

“…In NE-deficient (Th Ϫ/Ϫ or Dbh Ϫ/Ϫ ) mice, genetic replacement of NE or agonists that activate NE receptors (adrenergic receptors) rescue survival (6,41,53,60,61). Treatment of the dam with a non-subtype-specific ␤-adrenergic receptor (␤-AR) agonist rescues 90% of Th null fetuses to birth, whereas a ␤ 1 -specific agonist rescues 74%, suggesting that survival is largely mediated by ␤ 1 -ARs (6,41).…”

“…Treatment of the dam with a non-subtype-specific ␤-adrenergic receptor (␤-AR) agonist rescues 90% of Th null fetuses to birth, whereas a ␤ 1 -specific agonist rescues 74%, suggesting that survival is largely mediated by ␤ 1 -ARs (6,41). ␤ 1 -ARs are detected only in the heart at E12.5, suggesting that the heart may be the primary target of NE in the early fetus (6).…”

Moderate Global Reduction in Maternal Nutrition Has Differential Stage of Gestation Specific Effects on β1- and β2-Adrenergic Receptors in the Fetal Baboon Liver

“…RNA samples were reverse transcribed in the presence of 5X First strand buffer, 10 mmol/ L DTT, random hexamer primers, and SuperScript RTIII. Complementary DNA (cDNA) synthesis was carried out in a thermocycler at 25 C/10 min, 42 C/50 min, 72 C/10 min, and 4 C. Real time reverse transcriptionÀpolymerase chain reaction (RT-PCR) assay was then performed by the DDCt method 17 using the TaqMan probe set for human b 1 -AR (Hs00265096_s1; Applied Biosystems, California) and b 2 -AR (Hs00240532_s1; Applied Biosystems) on an ABI 7900 Sequence Detection System. In each experiment, mRNA levels were normalized to 18S ribosomal RNA (rRNA; measured using TaqMan Gene Expression Assay for 18S, #Hs99999901_s1, from Applied Biosystems), which did not change under experimental conditions (data not shown).…”