Oral oestrogen reverses ovariectomy-induced morning surge hypertension in growth-restricted mice

Abstract

Perinatal growth restriction (GR) is associated with heightened sympathetic tone and hypertension. We have previously shown that naturally occurring neonatal GR programmes hypertension in male but not female mice. We therefore hypothesized that intact ovarian function or post-ovariectomy (OVX) oestrogen administration protects GR female mice from hypertension. Utilizing a non-interventional model that categorizes mice with weanling weights below the tenth percentile as GR, control and GR adult mice were studied at three distinct time points: baseline, post-OVX and post-OVX with oral oestrogen replacement. OVX elicited hypertension in GR mice that was significantly exacerbated by psychomotor arousal (systolic blood pressure at light to dark transition: control 122 ± 2; GR 119 ± 2; control-OVX 116 ± 3; GR-OVX 126 ± 3 mmHg). Oestrogen partially normalized the rising blood pressure surge seen in GR-OVX mice (23 ± 7% reduction). GR mice had left ventricular hypertrophy, and GR-OVX mice in particular had exaggerated bradycardic responses to sympathetic blockade. For GR mice, a baseline increase in baroreceptor reflex sensitivity and high frequency spectral power support a vagal compensatory mechanism, and that compensation was lost following OVX. For GR mice, the OVX-induced parasympathetic withdrawal was partially restored by oestrogen (40 ± 25% increase in high frequency spectral power, P<0.05). In conclusion, GR alters cardiac morphology and cardiovascular regulation. The haemodynamic consequences of GR are attenuated in ovarian-sufficient or oestrogen-replete females. Further investigations are needed to define the role of hormone replacement therapy targeted towards young women with oestrogen deficiency and additional cardiovascular risk factors, including perinatal GR, cardiac hypertrophy and morning surge hypertension.

Keywords: autonomic; developmental origins; echocardiography; heart rate variability.

Figure 1. Impact of Litter Size on Neonatal Growth

Female C57BL/6 mice were alternatively kept with their birth dam (A, N=255) or cross-fostered into litters of 6 versus 11 or 12 mice (B, N=324). In both cases, weaning weight was inversely proportional to litter size (R = −0.36 and −0.66, respectively; both P<0.001). Neonatal growth restriction, defined by a weanling weight below 6.8g, was more likely to occur when mice were fostered in litters of 11 or 12 pups, but it also occurred in 11% of the pups fostered in smaller litters.

Figure 2. Ambulatory Blood Pressures

Systolic blood pressure was measured by radiotelemetry in adult female control mice (solid symbols) and neonatal growth restricted mice (GR, open symbols) at baseline (A, triangles), after bilateral ovariectomy (B and C, circles), and 3 weeks after post-OVX estradiol therapy (D, squares). Compared to control-OVX mice (B) and GR-baseline mice (C), GR-OVX mice had increased arterial pressures in the absence of estrogen replacement. *P<0.05 versus control-OVX and †P<0.05 versus GR-baseline

Figure 3. Hemodynamic Effects of Sympathetic Blockade

Control (gray bars) and GR mice (white bars) were injected with the α1-adrenergic receptor antagonist prazosin (1 mg/kg ip). Systolic blood pressure (left column) and heart rate (right column) were recorded over the subsequent 60 minutes. *P<0.05 versus corresponding control and #P<0.05 versus GR-baseline

Figure 4. Hemodynamic Effects of Pharmacologic Denervation

Control (gray bars) and GR mice (white bars) were injected with the nicotinic receptor antagonist chlorisondamine (2.5 mg/kg ip) to induce ganglionic blockade (GB). Systolic blood pressure (left column) and heart rate (right column) were recorded over the subsequent 60 minutes. *P<0.05 versus corresponding control

Figure 5. Effect of Neonatal Growth Restriction on Baroreceptor Reflex Activity and Sensitivity

During the light cycle that followed the radiotelemetry recordings, spontaneous baroreceptor reflex events (A) and sensitivity (B) were recorded in adult female control (solid bars) and GR mice (open bars) both before and after bilateral ovariectomy (OVX) +/− estrogen therapy (E2). *P<0.05 versus corresponding control and #P<0.05 versus baseline

Figure 6. Impact of Ovariectomy of Heart Rate Variability

Spectral power was calculated from the same high fidelity radiotelemetry recordings utilized for baroreceptor reflex analysis. Heart rate variability was analyzed at both low frequency (LF HRV, A) and high frequency (HF HRV, B) before and after bilateral ovariectomy (OVX) +/− estrogen therapy (E2). *P<0.05 versus control, #P<0.05 versus GR-baseline and †P<0.05 versus GR-OVX

Figure 7. Plasma Estradiol Levels

Estradiol levels were determined by ELISA with linear values obtained utilizing standards ranging from 3 to 300 pg/ml (A, R²=0.97). Plasma was obtained two weeks after bilateral ovariectomy, 3 weeks after starting estradiol therapy, and again 5 weeks after starting estradiol therapy (B and C; N=6). Comparative plasma was obtained from third trimester pregnant dams that were not receiving exogenous estrogen (C, N=3). *P<0.05 versus OVX and OVX-E2.