Postnatal cardiovascular function after manipulation of fetal growth by embryo transfer in the horse

Abstract

This study used between-breed embryo transfer in the horse to investigate the effects of maternal size and uterine capacity on fetal growth and postnatal cardiovascular and neuroendocrine functions. Equine embryos were transferred to establish eight Thoroughbred-in-Thoroughbred (TinT), seven Pony-in-Pony (PinP), five Thoroughbred-in-Pony (TinP) and eight Pony-in-Thoroughbred (PinT), pregnancies. Maternal and foal weights and placental microscopic area were measured at birth. At 6 days of postnatal life, arterial blood pressure and heart rate were monitored and blood samples were taken for hormone analysis before, during and after a 10 min period of nitroprusside-induced hypotension. Values for maternal and foal weights and placental area at birth were larger in TinT than in PinP pregnancies (P < 0.05). PinT pregnancies resulted in larger placentae and heavier foals relative to PinP (P < 0.05). TinP had smaller placentae and lighter foals relative to TinT (P < 0.05). Growth-enhanced (PinT) foals showed elevated basal arterial blood pressure and baroreflex threshold, reduced baroreflex sensitivity, diminished plasma catecholamine responses to acute stress, and increased cortisol responsiveness to ACTH. Conversely, growth-restricted (TinP) foals showed no change in basal arterial blood pressure, baroreflex threshold or adrenocortical responsiveness to ACTH, but had enhanced baroreflex sensitivity and augmented plasma catecholamine responses to acute stress. The data show that fetal growth acceleration as well as fetal growth restriction, resulting from between-breed embryo transfer in the horse, leads to altered postnatal regulation of blood pressure and the circulating concentrations of cortisol. These findings suggest that deviations in the pattern and rate of fetal growth both above and below the normal trajectory may influence cardiovascular function in postnatal life.

Figure 1. Measurements at birth

Values are means ± s.e.m. for the weights of the mare and foal, the total microscopic area of the placenta, and for the basal mean arterial blood pressure (MAP), heart rate and baroreflex threshold in the foal at 6 days of age. Significant differences are (P < 0.05): a, PinP vs. TinT; b, vs. fetal genotype control; c, vs. all (two-way repeated measures ANOVA).

Figure 2. Cardiovascular responses to acute hypotension in the newborn foal

Values are means ± s.e.m. for minute averages of arterial blood pressure and heart rate before, during and after a 10 min period of acute hypotension (bar) induced by i.v. treatment of the foal with sodium nitroprusside.

Figure 3. Baroreflex function in the newborn foal

Values are means ± x and ys.e.m. for minute averages of mean arterial blood pressure and heart rate (A) or mean arterial blood pressure and pulse interval (B) before and during the first 5 min of acute hypotension in the newborn foal. Trend lines were plotted using third order polynomial (A) or linear (B) regression. Comparisons between the slopes and intercepts of linear regressions were conducted according to Armitage & Berry (1994).

Figure 4. Plasma catecholamine response to acute hypotension in the newborn foal

Values are mean ± s.e.m. change from baseline for plasma noradrenaline and adrenaline concentrations before, during and after acute hypotension (bar) in PinP and TinT foals (A) and PinT and TinP foals (B). Significant differences are: a, P < 0.05: baseline vs. hypotension; b, P < 0.05: PinP vs. TinT or PinT; c, P < 0.05: TinP vs. PinT (two-way repeated measures ANOVA).

Figure 5. Pituitary-adrenocortical response to acute hypotension in the newborn foal

Values are mean ± s.e.m. change from baseline for plasma ACTH and cortisol concentrations before, during and after acute hypotension (bar) in PinP and TinT foals (A) and TinP and PinT (B) foals. C, the relationship between plasma ACTH and cortisol (means ± x and ys.e.m.) for all groups. Significant differences are: a, P < 0.05: baseline vs. hypotension or recovery; b, P < 0.05: PinT vs. all (two-way repeated measures ANOVA).